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Natural Attenuation for Groundwater Remediation (2000)

Chapter: 4 Approaches for Evaluating Natural Attenuation

« Previous: 3 Scientific Basis for Natural Attenuation
Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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4
Approaches for Evaluating Natural Attenuation

Documenting that a contaminant has disappeared or that the concentration has become very low in groundwater samples is an important piece of information for proving that natural attenuation is working, but it is not sufficient, even at simple gas station sites. Contaminants can bypass sampling locations due to the dynamic nature of groundwater systems. Also, some mechanisms can cause apparent loss of the contaminant, when in fact the contaminant has moved to a place or changed to a form that is difficult to detect.

Because of the limitations of monitoring only for the loss of a contaminant, the National Research Council’s (NRC’s) Committee on In Situ Bioremediation proposed that two other types of evidence are needed to prove that in situ bioremediation of any type is working (NRC, 1993). The first is sound scientific documentation (laboratory measurements or literature describing such measurements) that the mechanism claimed as responsible for contaminant destruction or control is scientifically feasible in the type of environment at the site. The second is documentation that the proposed mechanism is actually occurring at the site. The key issue is that an observed disappearance of contaminants has to be linked to the mechanism acting at the site. In short, cause and effect must be supported. This same principle applies to natural attenuation.

This chapter describes a weight-of-evidence approach for demonstrating the mechanisms responsible for observed contaminant losses in natural attenuation. Direct field measurements of mechanisms of contaminant transformation or degradation are difficult or impossible. Several types

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

of field data, combined with models of the subsurface, generally will be needed to link the observed decreases in contaminant concentration to the underlying mechanisms responsible for contaminant losses. As described at the end of this chapter, the level of detail of data and analysis required will vary substantially depending on the complexity of the site. Leaks from gas stations may require only a small fraction of the analysis necessary at large industrial sites with contaminants that are less well understood than gasoline. Nonetheless, the basic principles of analysis described in this chapter apply to all sites.

FOOTPRINTS OF NATURAL ATTENUATION PROCESSES

Although the mechanisms that destroy or sequester contaminants in groundwater cannot be observed directly, they leave “footprints.” Footprints occur because the mechanism controlling contaminant fate also consumes or produces other materials, many of which can be measured in groundwater samples. Thus, an observation of the loss of a contaminant, coupled with observation of one or (preferably) several footprints, helps to establish the cause and effect that is so crucial to documenting natural attenuation in field settings. As examples, Box 4-1 describes briefly some types of footprints produced by different contaminants and attenuation mechanisms. Table 4-1 summarizes the footprints important for documenting the varying degrees to which natural attenuation occurred in the case studies in Chapter 3, as well as in two new case studies (Bemidji, Minnesota, and an unnamed field site) described later in this chapter.

Table 4-1 illustrates two important features of footprints. First, footprints provide evidence for and against attenuation mechanisms. For example, the conversion of organic material (measured as chemical oxygen demand, COD) to methane provided evidence of the reductive dechlorination of trichloroethene (TCE) at the St. Joseph site, but the lack of COD removal indicated that reductive dechlorination of TCE was unlikely at Edwards Air Force Base. Second, the observation of positive footprints does not necessarily mean that the contaminants are fully controlled. Incomplete removal of the original contaminants (as at the Hudson River site) or formation of hazardous products (as at the St. Joseph site) means that contaminant concentrations are still above regulatory levels, even though a natural attenuation mechanism is at work.

Using footprints to link cause and effect is not always straightforward. In some cases, detecting small changes that would prove cause and effect is extremely difficult. As an example, reductive dechlorination of TCE at low concentrations may produce chloride and acid at rates that are overwhelmed by natural background levels. In other cases, footprints can be obscured by reactions that produce or use the footprint materials. One

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-1
Examples of Footprints That Can Indicate Natural Attenuation

Mechanisms that cause contaminants to degrade or transform in the subsurface cannot be observed directly, but they leave footprints that can be detected in groundwater samples. The following examples explain how these footprints can be used to document natural attenuation:

  • The aerobic biodegradation of petroleum hydrocarbons consumes oxygen and produces inorganic carbon in well-established ratios. Estimating the oxygen supply rate and correlating it with increases in inorganic carbon can yield a quantitative estimate of the rate of hydrocarbon biodegradation, if the changes in inorganic carbon concentration can be measured properly.

  • The biodegradation of organic contaminants, including hydrocarbons, under denitrifying or sulfate-reducing conditions consumes nitrate or sulfate and produces inorganic carbon and alkalinity. Estimating the supply rates of sulfate or nitrate and correlating them with changes in inorganic carbon concentration and alkalinity can provide evidence for these anaerobic biodegradation reactions.

  • Reductive dechlorination of solvents such as trichloroethene (TCE) and trichloroethane (1,1,1-TCA) releases the chloride ion (Cl) and strong acid, while it consumes an electron donor. Thus, the release of Cl can be correlated with the supply rate of an electron donor, such as H2 or an H2 precursor, and a decrease in alkalinity. In many cases, only a small fraction of the electron donor is used to reduce TCE or 1,1,1-TCA. In these cases, consumption of the donor can be a large, easily measured rate, even if Cl production and an alkalinity decrease are not easy to detect.

  • Precipitation of uranium as UO2(s) due to the reduction of the mobile uranium species UO22+ requires consumption of an electron donor and produces strong acid. Therefore, loss of UO22+ from solution should be accompanied by corresponding losses of an electron donor and a decrease in alkalinity.

example is the dissolution of calcareous minerals, which adds alkalinity and inorganic carbon to water and therefore can mask the footprints of biodegradation reactions that change the alkalinity or inorganic carbon concentration. Another confounding factor is transfer of contaminants or footprint chemicals to or from another phase, such as exchange of CO2 or O2 with soil gas. Sampling errors also can confound efforts to document footprints.

Because of the possibility of confounding factors, a weight-of-evidence approach, measuring several footprints, generally must be used to document natural attenuation. Even though one type of evidence may be compromised, having several different types can lead to the conclusion that attenuation mechanisms are (or are not) acting based on a weight of

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

TABLE 4-1 Summary of Natural Attenuation Footprints Evaluated in Case Studies

Case Study

Contaminant(s)

Contaminants Controlled?

Footprints

Traverse City

BTEX

Yes

Depletion of O2; formation of CH4 and Fe2+

Vandenberg Air Force Base

MTBE

No

Insignificant O2 and SO42− concentrations; extension of MTBE plume far beyond BTEX plume

Borden Air Force Base

Five chlorinated solvents

Partially

Detection of metabolites of solvent degradation

St. Joseph

TCE

Partially

Formation of CH4; detection of degradation by-products (vinyl chloride and ethene)

Edwards Air Force Base

TCE

No

Documentation of high NO3 and SO42− concentrations; demonstration that TCE moves with water

Dover Air Force Base

TCE, 1,1,1-TCA

Yes

Formation of degradation by-products (cis-1,2-DCE, 1,1-DCA, vinyl chloride, and ethene); CH4 and H2S formation; increase in Cl concentration

Hudson River

PCBs

Partially

Detection of breakdown products; detection of unique transient metabolites; observation of microbial metabolic adaptation and expressed biodegradation genes

South Glens Falls

PAHs

Yes

Depletion of O2; detection of unique metabolic by-product; detection of genes for degrading PAHs in site microorganisms; rapid PAH degradation in soils taken from site

Pinal Creek Basin

Metals, acid

Yes now; may not be sustainable

Observation of carbonate dissolution leading to pH increase coincident with metal precipitation; observation of manganese oxide precipitates in stream sediments

Hanford 216-B-5

Radionuclides

Yes

Sorbed radionuclides observed in site samples

Anonymous Field Site (Borden et al., 1995)

BTEX

Yes

Loss of O2, NO3, and SO42−; formation of Fe2+ and CH4; increase in inorganic carbon concentration; increase in alkalinity

Bemidji

Petroleum hydrocarbons

Partially

Loss of O2; formation of Fe2+, Mn2+, and CH4; formation of intermediate metabolites; observation of selective degradation of petroleum hydrocarbons relative to more stable chemicals

NOTE: BTEX = benzene, toluene, ethylbenzene, and xylene; DCA = dichloroethane; DCE = dichloroethene; MTBE = methyl tert-butyl ether; PAHs = polycylic aromatic hydrocarbons; PCBs = polychlorinated biphenyls; TCE = trichloroethene; TCA = trichloroethane.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

evidence. The greater the degree of uncertainty at a site, the greater will be the need for more and different types of information.

Using footprints to document cause-and-effect linkages in natural attenuation requires three steps. The first step is to create a conceptual model of the site. The conceptual model should include a description of the groundwater flow system, estimated locations of the contaminant source and plume, and a list of reactions that might contribute to natural attenuation. The second step is to analyze site measurements to quantify the attenuation processes. This analysis may take a variety of forms, including identification of trends in concentrations of the contaminants and footprint chemicals, a simple mass budget that attempts to correlate changes in the contaminant mass with changes in footprint materials, or comprehensive computer-based models that use mass-balance equations to track contaminants and footprints. The final step is to establish a long-term monitoring program to document that natural attenuation continues to perform as expected. Data collected during long-term monitoring should indicate whether or not the plume is behaving in a manner consistent with the conceptual and quantitative models of the site. The remainder of this chapter describes in more detail how to carry out each of these three steps.

CREATING A CONCEPTUAL MODEL

The first step in understanding natural attenuation processes at a site involves creating a conceptual model. A conceptual model is an idealized picture of the important features of the flow and transport processes operating at a site. Although the model depicts all of the important features of the system, initially it must be based on simplifying assumptions because data for a more detailed model generally are unavailable in the early stages of site investigation.

Because of the necessity to make assumptions, development of the conceptual model must be an iterative process. In the early stages, the conceptual model can be expressed simply in the form of a block diagram or a picture showing a cross section of the site. Initially, information commonly available about a particular site may include existing large-scale maps, reports conducted in early characterization studies, or expert knowledge. Developing a preliminary model based on such existing information can save costs by helping to identify an optimal plan for gathering more data.

As understanding of the site increases, preliminary calculations often help to identify the dominant attenuation processes. In some cases, preliminary information can be entered in a computer model that simulates the behavior of the site, even with only “best guesses” for missing param-

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

eter values or with a very simplified model. As data are collected, the conceptual model has to be updated to provide a more complete and accurate picture. As part of this process, the calculations used to create the model should be updated and made more sophisticated.

This iterative approach makes full use of all available knowledge at each stage of the process. The purposes are to optimize resources and to systematically document and increase understanding of the system. Benefits include the best possible planning for sampling programs and analyses needed to decide whether natural attenuation is effective at the site. However, numerical answers at each stage of the process should be scrutinized carefully and not be overvalued.

Characterizing the Groundwater Flow System

The foundation of a site conceptual model always is the site’s hydrogeology. In which direction does the groundwater flow? What is its velocity? Is the flow steady or unsteady over time? Is it homogeneous in space or highly varied by location in the subsurface?

Contaminants in the subsurface move with the groundwater. Necessary reactants, such as electron acceptors for bioremediation, are transported with the water. Knowing where and how groundwater flows is therefore essential for tracking contaminants and their footprints. In addition, an observation that contaminants are not moving at the rate expected based on groundwater velocity alone provides a first line of evidence that natural attenuation reactions may be controlling the contamination.

Characterizing a site’s hydrogeology involves determining the following:

  • the geometry of the hydrogeologic units and their hydraulic properties;

  • hydraulic heads (essentially, groundwater elevations at different points in the subsurface); and

  • the locations and types of hydrologic boundaries, including the locations and flow rates of the most important sources and sinks for groundwater.

The distribution of hydrogeologic units is a key aspect controlling the migration of contaminant plumes. Data from surface topography and vegetation, bore hole cuttings, geophysical surveys, regional geologic studies, and concentrations of different chemicals in the groundwater can be used to create an initial three-dimensional concept of the hydrogeologic units. The properties of these units can be estimated initially from their

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

lithology (the types of geologic materials that make up the particular aquifer) and then refined using results from hydrogeologic tests. Measurements of hydraulic heads in all available wells should then be used to create maps in cross section and plan view showing the groundwater elevations at the site. Hydrologic boundaries to be shown in the conceptual model include surface water bodies, flow divides, recharge wells, pumping wells, and evaporation.

Temporal and Spatial Variability in the Flow System

Experience shows that the conceptual model for site hydrogeology must account for temporal and spatial variabilities. Frequently, transient flow conditions occur due to natural phenomena, such as seasons and extreme weather events, and to anthropogenic phenomena, such as pumping or irrigation. These transients mean that water levels measured on one day do not necessarily represent other days or the long-term average (King and Barker, 1996). Another common confounding factor is spatial variability in aquifer properties. Homogeneous systems occur only in the laboratory or in models. Heterogeneity in aquifer properties is more pronounced at some sites than others, but at every site it limits the ability to document contaminant fate in the subsurface. Identifying and incorporating temporal and spatial complexities are difficult tasks that require significant amounts of information.

When the flow direction shifts, the center of the contaminant plume also shifts. Contaminant concentrations for locations normally near the center of the plume may decrease temporarily, only to rise when the flow direction changes again. In addition, a plume may appear to shrink. For example, during drought years the water table may fall below the level of entrapped residual nonaqueous-phase liquid (NAPL) contaminants in the soil, temporarily removing the source of contamination. In subsequent years with higher rainfall, the concentrations in the plume will rise again as the water table comes in contact with the NAPL.

This tendency of plumes to shrink and grow in response to hydrologic variations has implications for natural attenuation investigations. To avoid being misled by transient temporal effects, contaminant losses (and other evidence, as well) must be documented over an area that encompasses the longitudinal axis and fringes of the plume over several years. Special attention should be given to potential contaminant migration pathways presenting the greatest risk. These pathways can be identified by careful site characterization. If significant uncertainty remains regarding the location of such pathways, the inescapable conclusion is that the efficacy of natural attenuation cannot be assessed with confidence.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Wide variations in aquifer permeability also complicate the movement of the plume. The most common heterogeneities are discontinuous distributions of sand, gravel, and clay found in aquifers consisting of alluvial or glacial outwash sediments. Alluvial and glacial outwash aquifers are common near the ground surface. In these types of aquifers, the water and contaminants preferentially move through the most permeable zones. For example, a plume may meander as it migrates preferentially through sands and gravels of a buried river channel. Plumes traveling in networks of rock fractures underground are the most difficult to characterize, and methods for characterization are a topic of active research.

Heterogeneities also affect the trapping of NAPLs, creating multiple sources of contamination in zones with entrapped NAPLs. For example, NAPLs may migrate into rock fractures, and contaminants from NAPLs may diffuse into low-permeability zones. In effect, each NAPL source dissolves to form its own plume. Therefore, it is possible that groundwater samples taken from different locations at the site in some cases can come from plumes generated by different NAPL sources.

The greatest effort should be spent documenting the behavior of the largest and fastest-moving plume, which should be along the connected path with the highest permeability. Thus, samples of the plume in the gravels and higher-permeability sands are key to projecting the maximum extent to which a contaminant plume will spread.

Uncertainty in Modeling the Flow System

Although sophisticated equipment and analysis techniques are utilized to characterize the subsurface, uncertainty is inevitable in estimates of contaminant behavior because of temporal and spatial variability. The best approach to accounting for this uncertainty is the formulation of multiple conceptual models, each representing a different hypothesis about how the system behaves. Hydrogeologists refer to the different representations of the site in this set as “realizations.”

Working with multiple realizations and maintaining an open mind with respect to site interpretation until the data are sufficient to support one realization over the others is essential in accurately characterizing the site. Rather than deciding on one conceptual model of the site and then trying to “prove” it is right, this iterative approach involves assessing data needs and gathering new data to discriminate among realizations. Decisions regarding natural attenuation should differ depending on which realization most accurately represents the actual configuration of units in the subsurface. Sometimes, it is not possible to establish that only one realization represents the site, and the modeler must proceed with the evaluation of multiple realizations.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

The act of creating and testing realizations provides clues to possible misunderstandings in the conceptual model. For example, the top of Figure 4-1 shows a hypothetical site having three bore holes that intersect zones of differing hydraulic conductivity. If no other information is available, all four of the realizations shown in Figure 4-1 (and many more that are not shown) are reasonable interpretations of the subsurface. The distributions of hydraulic conductivity in each of the realizations affect predictions of groundwater flow (and subsequent predictions of contaminant transport) in different ways. To resolve which realization is the most accurate, additional information is needed.

Usually, more is known about a site than just the locations of high-and low-hydraulic-conductivity zones in a few bore holes, and this information can be used to rule out some of the realizations. Simultaneous assessment of all available data reduces uncertainty because some of the realizations, while accurately representing some categories of field data, will not represent other data. To illustrate this point, if each circle in

FIGURE 4-1 Several interpretations of the type of connection between zones of different hydraulic conductivity based solely on knowledge of the occurrence of two types of geologic material. Dark zones represent high hydraulic conductivity relative to the white background.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Figure 4-2 represents the suite of possible interpretations, then using all of the information together reduces the suite of possible interpretations and the uncertainty in modeling the groundwater flow (and associated contaminant transport). If the data circles do not overlap, none of the realizations can explain all types of information. Then the project team needs to identify shortcomings in the data or create new realizations.

The Site-Specific Constructed Model of the Flow System

A powerful tool for evaluating realizations in hydrologic systems is the constructed model, which is a set of mathematical equations designed to represent the site’s hydrogeology. In each realization, a different set of numerical parameters is used for different parts of the equations. The foundation of a constructed model is mass balance, which is simply an accounting system to make sure that the mass of a material (such as a contaminant) being modeled in the flow system is neither lost nor created out of nothing. The mass balance is a formal way to set up a budget on a material, and it consists of equations and a means to solve these equations.

FIGURE 4-2 Use of multiple types of data to reduce the number of possible interpretations of a contaminated site. Each circle represents possible interpretations of the specific data set. Model realizations that can represent reasonable interpretations of all data sets are retained for further analysis.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

To set up a mass balance, the modeler defines a domain, which is a volume of the subsurface with specified boundaries. A model domain can be very large (e.g., kilometers on a side) or very small (e.g., meters on a side), depending on the goals of the model. Based on the way the model is to be used, the domain boundaries might be defined by the property lines of the site, the physical extent of the hydrologic system, or an area encompassing a plume. Once the domain is defined, the mass balance states that the mass of the material being modeled changes inside the domain in response to inputs or outputs crossing the boundary and in response to processes that produce, store, or consume material inside the volume.

The different realizations of the site are simulated through changes in the model parameters. For example, a very porous zone has a very large value of hydraulic conductivity, while a nonconducting area has a very small value. Likewise, the presence of sources and sinks of water may be represented differently in different realizations. Each model realization is complete when its optimal parameter values (and their associated statistical confidence intervals) are determined.

Optimal parameter values generally are estimated by calibrating each of the realizations. The calibration process involves forward modeling: that is, substituting estimated parameter values (for example, hydraulic conductivities, heads at boundaries, and recharge rates) in the constructed model and calculating the simulated values (for example, heads, flow rates, and travel times). The simulated values are then subtracted from values observed in the field. The differences, or errors, are called the residuals, and the weighted sum-of-squared residuals is calculated. These weights reflect the certainty associated with each observation. Often, the weights are the inverse of the variance of the measurement that established the value of the observation. Realizations that give reasonable values for the parameters (e.g., conductivity) and have a low value for the weighted sum-of-squared residuals are retained for further consideration, while the others are eliminated. The modeler also should evaluate the residuals to ensure that they have a mean near zero and are not biased with respect to space, time, or simulated value.

The time-consuming nature of the trial-and-error approach limits the number of alternative model realizations that can be considered. Automated techniques now are available for optimizing parameter values.

Delineating the Contaminant Source

After characterizing the groundwater flow system, identifying the sources of contamination is the next critical step in creating a conceptual model. As described in Chapter 3, the source is the subsurface volume

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

containing the concentrated target contaminants, which usually are trapped within the solid matrix. Ideally, the goals of delineating the source include estimating the amount of contaminant mass in the source, the composition and longevity of the plume emanating from the source, and the occurrence of contaminant transformation reactions within the source.

Because the sources are likely to be heterogeneously distributed (in space and time) and variable in composition, they yield plumes that are variable in composition. Sometimes the source cannot be located, even when the plume it creates has been measured. These characteristics of the source create inherent uncertainty in delineating it, just as spatial and temporal variability in aquifer properties create uncertainty in modeling groundwater flow.

The state of practice of subsurface source characterization, especially for NAPL contaminants, is evolving rapidly, reducing—but by no means eliminating—uncertainty in source characterization. The traditional approach to source characterization relies on analyses of discrete samples of solids or water taken from the subsurface. In the traditional approach, samples are collected from various points in three dimensions—for example, by sampling from various vertical intervals in each of a number of wells or by using sampling tools that can be pushed into the subsurface without drilling wells. The tools available for this effort are proliferating, and the rate at which samples can be acquired is increasing dramatically. However, the total number of samples that typically is viewed as affordable is still small compared to the size and extreme heterogeneity of source distribution. The result is that, inevitably, sources (especially NAPLs) are imperfectly delineated. Commonly, discrete sampling is used to define the outermost edges of the contaminated zones, leaving internal detail on distribution and composition unknown. Without detail on the source itself, predicting some of the most important information about natural attenuation—including the flux of contaminant mass into the plume and the mass of contaminant remaining in the source—is not possible.

Two alternate approaches to characterizing NAPL sources are geophysical methods and partitioning tracer tests (Feenstra and Cherry, 1996). Geophysical methods involve using magnetic, radar, seismic, or other techniques to examine large volumes of the subsurface. They offer promise for rough source delineation but do not contribute to understanding the composition of the source. Partitioning tracer tests involve flushing the source area with a tracer that will partition in the NAPL; the amount of tracer recovered indicates how much tracer dissolved in the NAPL and can be used to estimate NAPL mass. While tracer tests, like geophysical surveys, examine large volumes of the subsurface, they do not identify

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

the three-dimensional distribution of source mass or provide much insight into source composition, and they are expensive.

An alternative being used in numerous evaluations of natural attenuation is to search for hot areas in plume transects. Hot areas in the plume are presumed to be downgradient of the hot spots in the source. Multiplying the concentration of the contaminant in the hot areas by the groundwater flow velocity at that spot provides a minimum estimate of the contaminant flux emanating from the source. The estimate is a minimum, because degradation processes may occur between the source and the sampling location. Although the mass flux may not indicate exactly how much contamination is in the source area, information about mass flux is important because it tells how fast contamination is moving away from the source area.

Evaluation of hot areas is a powerful technique for assessing the current status of the source, but it cannot always be used to estimate the long-term performance of natural attenuation. Evaluating time-series data on contaminant concentrations in the plume may provide some insight into the composition of upgradient sources and, thereby, the longevity and composition of the plume in the future (Feenstra and Guiger, 1996). Nonetheless, more work is necessary to refine and/or demonstrate methods for characterizing contaminant sources in order to predict the success and long-term performance of natural attenuation.

Delineating the Plume

As part of developing a conceptual model, the contours of the plume of contamination emanating from source areas must be delineated. The groundwater with the highest concentration of contaminants is referred to as the core or center of the plume, while lower concentration areas comprise the fringes. In theory, the center of the plume follows a flow line along the average flow path of the groundwater. Samples taken across a transect normally show a decrease in concentration from the center to the fringes, whether or not natural attenuation is transforming or sequestering the contaminant.

At many sites, a substantial number of wells will have been installed before the natural attenuation investigation begins. However, existing wells frequently do not track the center of the plume even at highly instrumented research sites, because some plumes have very narrow cores and extensive fringes (e.g., Cherry, 1996). A downgradient sample taken from the fringe will have a lower concentration than an upgradient sample from near the center, whether or not natural attenuation is acting to remove the contaminant. On the other hand, an upgradient sample from the fringe may not have a higher concentration than a downgradient

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

sample from the center, even when natural attenuation reactions are destroying the contaminant. To avoid erroneous conclusions about the rate at which contaminant concentrations decrease along the groundwater flow path, distinguishing the center of the plume from its fringes is essential.

To track the center of the plume, additional sampling beyond existing wells at the site normally is necessary. Available data should be used to determine the optimal locations for additional samples. Contour maps of groundwater elevations in the wet and dry seasons should serve as the basis for estimating the most likely flow direction. A preliminary screening using temporary bore holes and field analyses can provide a relatively low cost method for identifying the center of a shallow plume. Ultimately, a grid of multilevel samplers along planes perpendicular to the axis of the plume is necessary to determine the locus of maximum concentrations vertically and horizontally.

Although delineating the plume never is a simple and inexpensive task, an efficient monitoring network with a minimum number of short-screened wells can be designed if available data on groundwater flow and contaminant concentrations are used from the beginning. A long-term monitoring network based on this strategy also minimizes the number of wells that must be sampled for the indefinite future.

Reactions Contributing to Natural Attenuation

The final step in developing the conceptual model is to postulate which types of reactions are most likely to affect the contaminant, given conditions at the site. Chapter 3 described destruction and immobilization reactions that can cause loss of a contaminant in a natural attenuation setting. The goal at this stage of evaluation is to develop a conceptual model of the reactions based on observations that are connected directly to possible destruction or immobilization mechanisms. In other words, do observations of site conditions match what should occur if the destruction and immobilization reactions were acting?

The general strategy for postulating reactions is to identify reaction footprints. At the early stages of site evaluation, footprints provide excellent screens that indicate whether or not natural attenuation is plausible and worth documenting in detail. When measurements of key footprints are missing, they should be included as part of the monitoring plan.

Reaction Footprints for Petroleum Hydrocarbons

For petroleum hydrocarbons, several key footprints often are measurable. These include

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
  • loss of electron acceptors (mainly O2, NO3, Fe3+, and SO42−);

  • generation of the products of acceptor reduction (such as Fe2+ and CH4);

  • presence of organic acids that are known intermediate products of petroleum hydrocarbon degradation;

  • an increased concentration of dissolved inorganic carbon; and

  • a characteristic change in the alkalinity.

For example, Table 4-2 shows how the footprints of toluene (C7H8) degradation by microorganisms can be determined. In the reactions in Table 4-2, chemicals on the left-hand side are consumed by microbial reactions, while those on the right-hand side are produced:

  • The contaminant, C7H8, is consumed in all of the possible reactions.

  • When O2, NO3, or SO42− is the electron acceptor (as in the first three equations in Table 4-2), each is consumed. Disappearance of these chemicals in parallel with the oxidation of toluene provides a footprint of natural attenuation.

  • For iron reduction, the electron acceptor is the ferric iron (Fe3+) in the solid Fe(OH)3(s). Reduction of ferric iron produces dissolved ferrous iron (Fe2+)—a footprint for this type of reaction.

  • For methanogenesis, the footprint is the formation of CH4.

  • All of the reactions produce inorganic carbon, indicated by CO2 in the equations. Therefore, increases in CO2 concentration are a footprint for these reactions, although the amount is lower for methanogenesis.

  • Consumption of H+ results in an increase in alkalinity; conversely, H+ production indicates a decrease in alkalinity. Aerobic and methanogenic

TABLE 4-2 Complete Biodegradation of Toluene (C7H8) by Five Different Processes

Process

Electron Acceptor

Chemical Representation of Transformation Process

Aerobic

O2

C7H8 + 9O2 → 7CO2 + 4H2O

Denitrification

NO3

C7H8 + 7.2NO3 + 7.2H+ → 7CO2 + 3.6N2(g) + 7.6H2O

Sulfate reduction

SO42−

C7H8 + 4.5SO42− + 9H+ → 7CO2 + 4.5H2S + 4H2O

Iron reduction

Fe(OH)3(s)

C7H8 + 36Fe(OH)3(s) + 72H+ → 7CO2 + 36Fe2+ + 94H2O

Methanogenesis

Fermentation to CH4 and CO2

C7H8 + 5H2O → 2.5CO2 + 4.5CH4

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

biodegradation of toluene does not change alkalinity, while reductions of NO3, SO42−, and Fe3+ increase the alkalinity in differing amounts. An increase in alkalinity therefore is a footprint for some of the reactions.

During biodegradation of petroleum hydrocarbons, the most rapid process is aerobic degradation. However, the solubility of oxygen in water is limited. The maximum concentration of oxygen in water under natural conditions is only about 10 mg/liter, which can allow toluene oxidation of only about 3 mg/liter. Although oxygen can penetrate the fringes of the plume, it often cannot reach the core. Therefore, oxygen is exhausted in the core of the plume, and degradation in the plume core—if it proceeds—must occur via anaerobic reactions.

Some state environmental regulators require minimal data to approve natural attenuation as a remediation strategy for sites with maximum concentrations of petroleum hydrocarbons less than 1 mg/liter and a significant distance to humans or sensitive ecosystems. An underlying assumption of this minimal criterion is that at least 3 mg/liter of dissolved oxygen is likely to be available to sustain aerobic biodegradation of the 1 mg/liter of hydrocarbons present. Although the expectation is reasonable in many situations, loss of the contaminants and correlated footprints still should be documented because of the possibility that the supply of oxygen or other electron acceptors will not be sufficient to prevent further migration of the contamination.

Another minimal criterion that some state regulators use to approve natural attenuation for contaminant management is data showing that a plume is shrinking over the course of one to two years. One serious problem with this criterion is that a plume may initially shrink when efficient electron acceptors (such as O2) are available to drive the degradation process. However, once the supply of favorable electron acceptors is exhausted, the plume may resume growing as degradation ensues via slower reactions such as methanogenesis. Results from the Bemidji, Minnesota, research site, which is contaminated with a large quantity of residual and mobile NAPL petroleum hydrocarbons, show that the plume appeared stable for several years when abundant Fe(III) was available. However, the plume core is now slowly expanding as solid-phase Fe(III) is depleted (Cozzarelli et al., 1999). This example illustrates that sites with higher concentrations of petroleum hydrocarbons (e.g., greater than 3 mg/liter) and residual NAPL sources that will persist for many decades cannot be evaluated adequately with a simple, short-term criterion that does not account for the long-term sustainability of electron acceptors. For estimating the long-term risk, the slowest sustainable degradation rate (which may be methanogenesis) has to be compared to the minimum travel time to humans or sensitive ecosystems.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
Reaction Footprints for Chlorinated Solvents

For chlorinated solvents, reductive dechlorination is the most widely applicable destruction mechanism. This process requires a supply of biologically oxidizable organic matter to maintain reducing conditions in the aquifer and to serve as an electron donor. In some cases, the organic matter is part of a mixed contaminant source and enters the groundwater with the chlorinated solvent. In other cases, the organic matter can come from another contaminant source, such as a petroleum NAPL upgradient of the solvent source. In the absence of contaminant sources of organic matter, natural organic matter present in the aquifer must drive the process. In any case, the presence of electron donors is the foremost screening criterion used to determine the potential for reductive dechlorination of chlorinated solvents.

An example of a reductive dechlorination reaction is given here for TCE (C2Cl3H):

(4-1)

The reaction shows that complete reductive dechlorination of TCE produces these potential footprints: loss of an electron donor, which is represented by toluene (C7H8); production of ethene (C2H4); release of chloride ion (Cl); destruction of alkalinity (H+); and production of inorganic carbon (represented by CO2).

In reality, the footprints for reductive dechlorination normally do not appear in exactly the quantities shown in the example. The actual footprints differ because only a small fraction of the electrons removed from an electron-donor substrate (such as toluene) is used for reductive dechlorination. The large majority of the electron flow is generally used for the normal metabolism of competing microorganisms. Thus, many of the footprints that occur when reductive dechlorination acts are those from the metabolism of organisms that compete for the available electron donor with the organisms responsible for dehalogenation reactions (such as illustrated in Table 4-2, when C7H8 is the electron donor).

The following reaction shows what happens when the total electron flow from the donor (still C7H8) is 10 times that needed for reductive dechlorination of TCE to ethene:

(4-2)

In this reaction, the electrons from C7H8 that are not used to reduce TCE are distributed equally between two types of reactions that the microbes

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

use to produce energy and new cells: (1) sulfate reduction and (2) methanogenesis. For this more realistic scenario, the reliable footprints are

  • a large consumption of an electron donor (C7H8 in this case);

  • results of the normal metabolic reactions used to degrade the donor—in this case production of CH4, consumption of SO42−, and increases in the dissolved organic carbon (CO2) concentration and alkalinity (consumption of H+); and

  • generation of Cl and ethene (C2H4) in proportion to the loss of TCE.

In some cases, intermediates of TCE degradation accumulate and can be used as footprints, too. These include cis-1,2-dichloroethene (cis-1,2-DCE), and vinyl chloride.

Reductive dechlorination requires special conditions at the site: the coexistence of the solvents and the electron donor. One example of a type of site at which these conditions can occur is a landfill, which may generate large concentrations of electron donors. However, reducing conditions and the electron donor might be present only in the immediate vicinity of the landfill, in which case contaminants that escape the reducing conditions in and near the landfill will continue to migrate.

Anaerobic bottom sediments in wetlands, rivers, and ponds provide another favorable environment for reductive dechlorination. At Aberdeen Proving Ground, Maryland, complete reductive dechlorination of TCE and 1,1,1-TCA (trichloroethane) at initial concentrations of up to 2,000 parts per billion (ppb) was documented over a distance of 1 m during discharge to a tidal wetland (Lorah et al., 1997). However, although bottom sediments can provide a suitable environment for natural attenuation, demonstrating natural attenuation in sediments requires the observation of footprints at all locations where groundwater discharges to the sediments. Occasionally, reductive dechlorination occurs in some sediment segments but not others affected by the same plume (Ellis, 1996). For example, in one field investigation, researchers documented reductive dechlorination in some locations but not at others for a plume discharging to a stream (Conant, 1998). A complicating factor in documenting reductive dehalogenation during discharge to tidal systems is that water-level fluctuation can dilute plumes to a depth of the order of 1 m or more (Jon Johnson, U.S. Geological Survey, personal communication, 1998).

When petroleum hydrocarbons provide the electron donors required for chlorinated solvent degradation, natural attenuation of the solvents is tied strongly to the presence and longevity of the petroleum hydrocarbon

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

source. In these cases, characterizing the amount and distribution of petroleum hydrocarbons is as critical as characterizing the chlorinated solvent distribution, even petroleum hydrocarbons are not the target contaminants from the point of view of regulatory criteria. Figures 4-3, 4-4, and 4-5 illustrate this broader view of the contaminant source. For petroleum hydrocarbons to catalyze reductive dechlorination, the petroleum hydrocarbon and chlorinated solvent plumes must occupy the same vertical interval of the aquifer. Furthermore, petroleum hydrocarbons must be present in sufficient amounts during the entire time the solvent plume

FIGURE 4-3 Plan view illustrations of the importance of interactions between petroleum hydrocarbon (PH) and chlorinated solvent (CS) source zones for natural attenuation. (A) Source zones do not interact, and natural attenuation of the solvent is not supported by the petroleum hydrocarbons. (B) Source zones partly interact, since the strongly reduced zone created by petroleum hydrocarbons overlaps a portion of the solvent plume and supports natural attenuation of that portion. (C) Source zones interact completely, leading to complete natural attenuation of the chlorinated solvent. These examples assume that the petroleum hydrocarbon and chlorinated solvent plumes occupy the same vertical interval of the subsurface, which may not be the case.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

is present. Both conditions are met only in the example in Figure 4-3C. Figure 4-4 illustrates a more likely spatial situation: the vertical intervals of the subsurface occupied by the petroleum hydrocarbon and chlorinated solvent plumes are not identical, and complete natural attenuation of the chlorinated solvent plume does not occur. Figure 4-5 illustrates a likely temporal sequence: natural attenuation of the chlorinated solvent plume is initially complete but later slows or ceases. Petroleum hydrocarbon contamination is depleted through its own natural attenuation.

FIGURE 4-4 Vertical schematic illustrations of the importance of interaction of petroleum hydrocarbon (PH) and chlorinated solvent (CS) source zones for natural attenuation. (A) Source zones do not interact, and natural attenuation of the solvent is not supported by the petroleum hydrocarbons. (B) Source zones partly interact since the strongly reduced zone created by petroleum hydrocarbons overlaps a portion of the solvent plume and supports natural attenuation of that portion. Examples assume that the sources and plumes completely overlap in plan view (as in frame C of Figure 4-3).

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

FIGURE 4-5 Plan view illustrations of a probable temporal sequence of conditions for the case in which a petroleum hydrocarbon (PH) source initially creates strong reducing conditions leading to natural attenuation of a chlorinated solvent (CS) plume. As illustrated, natural attenuation of the chlorinated solvent ceases when the petroleum hydrocarbon supply is exhausted.

The examples in Figures 4-3, 4-4, and 4-5 illustrate why the presence of easily oxidized organic material, such as petroleum hydrocarbons, is not sufficient to ensure natural attenuation of chlorinated solvent contamination. The three-dimensional locations of the two sources and their plumes must be delineated well enough to evaluate whether or not the solvents and petroleum hydrocarbons overlap in space and time. Complete and sustainable natural attenuation of a chlorinated solvent plume due to a plume of petroleum hydrocarbons should be considered the exception, rather than the rule.

Some chlorinated solvents also can be transformed by aerobic cometabolism (see Chapter 3), and the footprints of this type of transformation differ from those of reductive dechlorination. For example, the aerobic, cometabolic biotransformation of TCE depends on the presence and activity of bacteria having critical oxygenase enzymes. Possible footprints include

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
  • a significant loss of one of the normal substrates for the oxygenase enzymes (methane, toluene, or phenol);

  • consumption of O2 in proportion to loss of the normal substrate;

  • aerobic conditions at the proposed location of TCE biotransformation reactions; and

  • release of Cl in proportion to TCE loss.

Reaction Footprints for Other Contaminants

For contaminants other than petroleum hydrocarbons and chlorinated solvents, the footprints that can be used to postulate reactions are less well established. In some cases, the scientific basis for the postulation is weak. In other cases, the science is sound, but methods for detecting the footprints in the field are unproven. Nonetheless, knowledge and evaluation techniques are evolving, and new destruction and immobilization reactions are likely to be demonstrated over time. The strategy for evaluating natural attenuation of any contaminant is the same as that for evaluating petroleum hydrocarbons and chlorinated solvents: several footprints of the postulated reaction must be documented.

As an example, immobilization of heavy metals, such as cadmium, can occur by precipitation of sulfide solids:

To create the conditions for the precipitation of CdS(s), sulfide must be present. This normally occurs through microbially driven sulfate reduction, such as is shown in Table 4-2. Therefore, the footprints for Cd immobilization are those for sulfate reduction, as well as loss of Cd2+ due to the precipitation reaction. Footprints of sulfate reduction are

  • loss of an electron donor (normally organic material, as measured by COD),

  • loss of sulfate in proportion to donor loss,

  • formation of sulfide, and

  • increased alkalinity.

When mixtures of contaminants occur, footprints have to be documented for all contaminants that pose risks. Similarly, footprints are needed to document the fate of potentially harmful by-products of contaminant degradation.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

ANALYZING SITE DATA

Once a conceptual model is created for the site, analyzing the data generated by site monitoring is the next step in establishing cause and effect between the loss of contaminants and the processes responsible for this loss. Detecting the presence of footprint materials does not necessarily prove that the postulated reaction is responsible for complete natural attenuation of the contaminant. Instead, the rates at which the footprint materials are generated have to be commensurate with the rate at which the contaminant is removed. Further, the materials necessary to support natural attenuation must be sustainable for the life of the contaminant.

A hierarchical set of approaches can be used for data analysis. Which approaches are appropriate depends on the complexity of the site, the nature of the contaminants, and the anticipated risk associated with spreading of the contamination. In general, there are four levels of complexity in data analysis:

  1. graphical and statistical analyses of trends in concentrations of contaminants and other substances;

  2. mass budgeting to track the fate of the mass of contaminants;

  3. simple modeling of solute transport; and

  4. comprehensive flow and solute transport models.

The intensity of effort increases from the top to the bottom of the list.

Table 4-3 provides guidance in choosing the level of data analysis appropriate for a site. Generally, less intensive analyses are sufficient for sites with low contaminant concentrations and simple hydrogeology, whereas more intensive approaches are necessary for sites with higher contaminant concentrations and complex hydrogeology. In addition, the level of analysis varies with the class of contaminant. Compounds that undergo efficient attenuation reactions under commonly encountered conditions require simpler analyses than compounds for which attenuation reactions are limited to specialized geochemical conditions. Implicit in the hierarchy of Table 4-3 is that whenever a detailed analysis is appropriate, the simpler analyses also are applied to the site. For example, if simple solute transport modeling is needed for a site, then graphical and statistical analyses and mass budgeting are performed to prepare for the transport modeling. The table does not categorize sites according to the level of risk. A higher level of analysis may be necessary for a high-risk site (such as one where exposure to contamination has occurred or is imminent) than Table 4-3 recommends on the basis of hydrogeologic conditions or contaminant type alone.

Regardless of the level of detail of site analysis, the first step is to

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

TABLE 4-3 Recommended Levels of Natural Attenuation Data Analysis for Different Contaminants and Site Conditions

 

Contaminant Characteristics

Hydrogeology

Biodegrades Under Most Conditions (e.g., BTEX)

Immobile Under Most Conditions (e.g., Pb)

Biodegrades Under Limited Conditions (e.g., chlorinated ethenes)

Immobile Under Some Conditions (e.g., Cr)

Mobile and Degrades or Decays Slowly (e.g., tritium, MTBE)

Simple flow, uniform geochemistry, and low concentrations

Graphical and statistical analyses

Graphical and statistical analyses

Mass budgeting

Simple solute transport model

Mass budgeting with simple solute transport model

Simple flow, small-scale physical or chemical heterogeneity, and medium-high concentrations

Mass budgeting

Simple solute transport model

Mass budgeting with simple solute transport model

Simple solute transport model

Comprehensive flow and solute transport models

Strongly transient flow, large-scale physical or chemical heterogeneity, or high concentrations

Mass budgeting or simple solute transport model

Mass budgeting or simple solute transport model

Comprehensive flow and solute transport models

Comprehensive flow and solute transport models

Comprehensive flow and solute transport models

NOTES: In the site descriptions given along the left-hand side, the recommended data analysis strategy applies when all of the conditions are satisfied unless the term “or” is used. Data completeness and consistency are to be evaluated in all cases. All techniques listed in higher rows of the same column are to be applied, along with the methods in the applicable row. Where mixed contaminants are present, the most thorough analysis recommended for any single contaminant should be applied to the entire site. BTEX = benzene, toluene, ethyl benzene, and xylene; MTBE = methy tert-butyl ether.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

examine the data for completeness and consistency. This step is necessary to identify gaps in time or space and to check for possible errors in field data (see Box 4-2). Hydrogeologic data—such as heads, hydraulic conductivities, flow rates, and known boundary conditions—must be consistent with the conceptual model of the flow system. Data on plume concentrations and estimates of source location and strength have to provide a coherent picture of the contaminant migration pathway. Finally, the measurements of the various geochemical parameters should be consistent with the conceptual model of possible reactions. A common example is inconsistency between dissolved oxygen concentration and redox potential. A high dissolved oxygen concentration is inconsistent with a negative redox potential and indicates that at least one measure is inaccurate or unrepresentative. For example, many monitoring wells collect water samples from several depths and mix them. If the samples come from depths with different metabolic environments, the mixture will not represent either environment, and sampling often will show several parts per million (ppm) of dissolved oxygen and a low redox potential. Once the data are self-consistent, they can be used to postulate which reactions might contribute to natural attenuation under the geochemical conditions at the site.

Graphical and Statistical Analyses

The most basic level of data analysis is to create several different types of graphical displays of the data. These include contour plots, time-series plots, and x-y plots of data from well profiles located along different flow lines. Horizontal contour plots show the lateral extent of contamination, but they provide no information about the maximum depth of contamination or the vertical migration of the plume. Thus, vertical contour plots oriented along the hypothesized axis of the plume also are essential. Time-series plots show variation in the data from a particular well over time. They can be used to detect oscillations due to hydrologic changes or trends that show a decrease with time. Plots for wet and dry seasons can be used to assess the degree of seasonal fluctuation in the plume over time. Similarly, data from a line of wells located along a flow line in three dimensions can be displayed on an x-y plot and used to identify spatial trends in the data.

Visual identification of trends is useful for formulating hypotheses, but it often is inadequate for demonstrating the effectiveness of natural attenuation. Humans can see patterns where none actually exist. In addition, a pattern may be present, but it may be quantitatively too small to serve as evidence of natural attenuation. For example, depletion of 1 mg/liter of dissolved oxygen is much too small to explain the disappear-

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

ance of 3 mg/liter of BTEX (benzene, toluene, ethylbenzene, and xylene). To overcome these problems, statistical tests are used to translate numerical results into objective statements of probability and reliability. Measurements are never perfectly certain. Use of statistics establishes limits, or statements in terms of probability. The advantage of this form of analysis is that it quantifies the uncertainty.

Statistical analyses can be used to answer the following types of questions:

  • Is the concentration in a downgradient well truly different from the concentration in an upgradient well?

  • Are the concentrations in a well truly decreasing over time?

The case studies in Box 4-3 describe methods for addressing both of these questions.

By quantifying what is known and the degree of uncertainty in this knowledge, statistical analyses help to avoid “seeing” nonexistent patterns. They also provide important input for the subsequent levels of evaluation, which compare the rates of reactions.

Mass Budgeting

The next level of data analysis is mass budgeting. Mass budgeting involves evaluating whether the rate at which footprint compounds are being produced is commensurate with the rate at which the contaminant is destroyed or sequestered. The relative rates reveal which natural attenuation processes are important and which are not. Mass budgeting does not predict the kinetics (speed) of a reaction. Instead, it involves defining a domain such that the input and output rates can be estimated directly from measurements. For example, a domain can be defined in such a way that it encompasses a plume underneath a NAPL. Then, the rates at which electron acceptors and inorganic carbon transport into and out of the domain can be estimated. When one-dimensional advection dominates the transboundary inputs and outputs and the concentrations are at steady state (not changing at a specific point over time), the net mass-per-time reaction rate of a material within the domain is

(4-3)

in which R stands for a reaction rate, with units of mass per unit volume per time; XYZ is the domain volume for the budget calculation; VD is the Darcy velocity of groundwater flow (also known as the specific discharge); Cup and Cdown are the upstream and downstream concentrations of the

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-2
Common Errors in Field Data

The subsurface is frequently perceived as homogeneous, but it is actually quite inhomogeneous, which complicates collection of field data that are representative of the site. Further, the parameter set required to assess natural attenuation is often new to field crews; many have not yet been thoroughly trained to gather these parameters accurately. Yet the acquisition and handling of samples in the field can have a substantial effect on the accuracy of the conclusions reached. Following are common errors in field data collection:

  • Well purging problems: Much traditional sampling guidance recommends that at least three well bore volumes of water be pumped out of a well before collecting groundwater samples for chemical analysis. Although this technique works well in high-yield, homogeneous aquifers, it has many drawbacks in less permeable and less homogeneous aquifers. In low-permeability zones, wells can be drawn down very far or even dried out completely by purging too vigorously. As the wells recharge, the groundwater is exposed to air as it cascades down the gravel pack for the well or the interior of the well screen. Volatile contaminants and/or metabolic products can escape rapidly into the air and thus not be detected. Dissolved metals that are sensitive to redox will usually oxidize and precipitate. Redox and dissolved oxygen concentrations will not be representative of the surrounding aquifer. The best solution to this problem is to use low-flow groundwater sampling techniques.

  • Turbidity interference: Chemical analysis kits for field use often employ colorimetric analysis. Because colorimetric kits rely on measuring light transmission through the test cell, turbid groundwater samples present a challenge to their use. Turbid samples may have to sit for a brief time before a colorimetric analysis

material; YZ is the area perpendicular to the flow direction; and X is the length of the domain.

The key to mass budgeting is having all rates on a common mass-per-time basis, R(XYZ). By using the input and output rates to estimate R(XYZ) for a contaminant, electron acceptors, inorganic carbon, alkalinity, or any other material of interest, their stoichiometric ratios can be computed and compared to what ought to occur if natural attenuation is acting. R(XYZ) also can be used to estimate the mass rates for reactions or transfers that cannot be measured directly. These include the rate at which the contaminant is entering the groundwater from the source.

Box 4-4 illustrates that a mass budget is analogous to a financial budget, such as that for a family. For a family budget, the unit of “mass” is the dollar, and R(XYZ) is expressed in dollars per year. Transboundary

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
  • is attempted. Such samples must be kept well sealed to avoid contact with air, and storage conditions and time should be included in field notes.

  • Inaccurate dissolved oxygen measurement: Field crews have long used dissolved oxygen as a rough measure of well stabilization during purging before sampling. However, measurement of dissolved oxygen for this purpose does not require nearly the level of accuracy necessary for documenting natural attenuation. As a consequence, field crews may need specific instructions about ensuring the quality of dissolved oxygen data. Crews should use more accurate dissolved oxygen and redox probes, calibrate them by using standard solutions, and check for values (e.g., in general, oxygen readings above 10 ppm) that are not physically possible. One quality check that crews can use is the correlation between redox and dissolved oxygen. Although sites exist at which these values do not correlate, in general they show a regular relationship and can have only a limited range of values.

  • Broken sampling probes: Sampling probes can have a number of problems, ranging from undetected breakage to manufacturing defects. In these cases, the data will be essentially random and impossible to interpret. The only way to recognize this problem is to understand the site and compare field data carefully to the conceptual model.

  • Contact with air: The timing of lab work done in the field can be critical to getting a high-quality analysis, particularly for dissolved oxygen, redox potential, and concentrations of compounds that may react with oxygen. Field analysis of water samples should be done using in-line probes or conducted as soon as practical when field test kits are used. In some cases, field crews store water samples in open containers on the tailgates of their trucks until they have large batches of samples to analyze. This practice allows oxygen to dissolve in the water, invalidating many measurements.

inputs and outputs in a mass budget are analogous to deposits to and debits from a family budget. Reactions mirror changes in the value of the family’s assets, such as a change in the value of the pension plan. In both cases shown in the box, the “balance” is declining over time, because losses exceed gains. If losses just equal gains, the budget—in mass or in dollars—is at steady state, in which the change with time is zero.

Box 4-5 provides an idealized example of how a mass budget can provide evidence that biodegradation is causing the loss of contaminants. (The example illustrates the principles of mass budgeting but does not account for how other geochemical reactions might affect the calculations.) In this case, the advection of O2, NO3, and SO42− into the domain encompassing the plume and the advection of CH4 and Fe2+ out of the domain correspond nearly perfectly to the net generation of inorganic

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-3
Statistical Analyses: Examples

The following examples show how statistical analyses can be used to help analyze data and answer questions important in deciding whether to rely on natural attenuation.

How Representative Are the Measured Contaminant Concentrations?

Consider a site at which five values for contaminant concentration have been measured: 13.55, 6.39, 13.81, 11.20, and 13.88. The mean value is = 11.77, and the estimated standard deviation s = 3.30 (McBean and Rovers, 1998). How “good” is the mean value based on these measurements (i.e., how close is to the true concentration represented by these samples)? An answer can be provided in terms of a “confidence interval,” given by

This computation indicates that the mean value is likely between 11.77–4.10 and 11.77 + 4.10. The value of t reflects a probability (in this case 95 percent) that the true mean falls within this limit; its value can be found in common statistical tests or computer packages. The confidence interval can be narrowed by increasing the number of samples n.

Has a Trend Developed With Time?

The plot in Figure 1 shows a time series of uranium-238 concentrations in groundwater measured at the former St. Louis Airport storage site for the time period 1981-1983 (Clark and Berven, 1984). Examination of the plot indicates a possible upward trend. Statistics can be used to determine whether this increase is real. Gilbert (1987) analyzed these data using the Mann-Kendall test for trend. The Mann-Kendall test compares changes in signs between values collected at each time with all of those collected later. The test is formulated in terms of a hypothesis test: a null hypothesis of no trend is compared to the alternative hypothesis that there is an upward trend. In this case, Gilbert used the test to show that there is a 95 percent probability of a true upward trend.

Is One Set of Measurements Larger Than the Other?

Statistical analyses also can be used to compare data from two monitoring points and evaluate whether one set of values is larger than the other. Consider the data shown in Table 1, providing the maximum oxidant pollution concentrations at two air monitoring stations in California (Gilbert, 1987). Values are given for 20 days. The values are paired because they might be expected to rise or fall together due to overall atmospheric conditions and, hence, correlate through time.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

The data can be analyzed by considering the number of comparisons for which the oxidant concentration was higher at Station 1 than at Station 2 (Gilbert, 1987) or by similar tests for comparing whether one set is larger than the other. “Before” and “after” values can be compared using similar tests. In this example, at a probability level of 95 percent, the hypothesis of no difference in the two populations could not be rejected.

FIGURE 1 Concentration of 238U in groundwater for January 1981 through January 1983 in well E at the former St. Louis Airport storage site. SOURCE: Clark and Berven, 1984.

TABLE 1 Data from Two Air Monitoring Stations

Day

Station 1

Station 2

Day

Station 1

Station 2

1

8

10

11

11

13

2

5

7

12

12

14

3

6

7

13

13

20

4

7

7

14

14

28

5

4

6

15

12

6

6

4

6

16

12

7

7

3

3

17

13

7

8

5

4

18

14

6

9

5

5

19

12

4

10

6

4

20

15

5

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-4
Analogy Between Contaminant Mass Budget and Family Budget

Family Budget

Contaminant Budget

Balance at Start of Year

$10,000

Mass Accumulation at Start of Year

10,000 g

Deposits

Transboundary Inputs

Gross Salary

+$40,000/yr

 

Advection

0 g/yr

 

Interest

+$500/yr

 

Dispersion

0 g/yr

 

 

 

 

NAPL

+50,000 g/yr

 

 

 

 

Dissolution

 

 

 

 

+$40,500

 

 

+50,000 g

Debits

Transboundary Outputs

Taxes

− $10,400/yr

 

Advection

− 10,000 g/yr

 

Rent

− $12,000/yr

 

Dispersion

− 1,000 g/yr

 

Utilities

− $4,000/yr

 

Volatilization

− 5,000 g/yr

 

Food

− $12,000/yr

 

Miscellaneous

− $9,000/yr

 

 

 

− $47,400

 

 

− 16,000 g

Savings

Reactions

Pension plan

+ $4,500/yr

 

Biodegradation

− 36,000 g/yr

 

 

 

+$4,500

 

 

− 36,000 g

Balance at End of Year

Mass Accumulation at End of Year

 

 

$7,600

 

 

8,000 g

carbon and alkalinity. This consistency among three types of evidence (electron acceptor, inorganic carbon, and alkalinity) means that other loss mechanisms or confounding factors are unlikely. Further, in this example, documenting consistent changes for each electron acceptor makes it possible to estimate the rate at which BTEX is being depleted from the NAPL source. The rate at which C7H8 is consumed equals its concentration change (computed from the acceptor changes) multiplied by the product of the groundwater velocity and the cross-sectional area of the domain. Box 4-5 shows the result of this computation: a depletion rate of 2,550 g of C7H8 per year. Long-term tracking of the source depletion rate

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

in this manner can indicate when natural attenuation is complete, which occurs when the source depletion rate becomes very small.

One common problem in applying a straightforward mass budget analysis is that advection may not dominate the inputs and outputs of all the footprint materials. Phase transfers can supply or remove materials independently of water flow. Important examples are

  • transfer of oxygen from the soil air,

  • dissolution of calcareous minerals,

  • transfer of volatile compounds to the gas phase, and

  • adsorption of hydrophobic compounds to aquifer solids.

Although these nonadvective inputs (or outputs) complicate the evaluation, they are not insurmountable obstacles as long as enough different footprint measures are available. When sufficient measures are available, the unknown phase transfers can be computed from the budget analysis.

Box 4-6 describes a field study (Borden et al., 1995) in which the advection of O2, NO3, SO42−, CH4, and Fe2+ was much too small to explain the observed increase in inorganic carbon (54 mg C/liter). As described in Box 4-6, one possible explanation is the transfer of O2 into the plume from the soil gas. The assumption that the rate of aerobic biodegradation is increased due to nonadvective transfer makes the evidence of natural attenuation consistent. It also allows an estimation of the depletion rate from the NAPL (9,000 g C7H8 per year).

As described in Box 4-6, having several types of information allows for identification of important mechanisms, in this case phase transfers of O2, that are not detected directly by initial sampling of the groundwater. When such a mechanism is critical, it must be documented by field measurements. For situations like the example of Box 4-6, nonadvective input rates for O2 would have to be measured to verify that high rates of O2 transfer to the plume are possible. Alternate explanations (in this case, the possibility that degradation is occurring by methanogenesis) also would have to be evaluated. (The example of Box 4-6 considers methanogenesis and concludes that it accounts for a small percentage of BTEX degradation.)

Mass budgeting is a powerful tool for determining the relative importance of different processes and establishing their approximate rates. On the other hand, perfect agreement among all possible footprint measures should not be expected in most cases. Imprecise mass balances can come about due to phase transfers or other confounding reactions, deviation from steady state, and dynamic changes in the flow field. Having several footprint measures helps determine the reasons for inconsistencies, as illustrated in Box 4-6.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-5
Mass Budget Analysis to Determine the Depletion Rate of a NAPL

Consider a scenario in which a gasoline leak has created a small NAPL at the top of an aquifer. BTEX dissolves into the groundwater at an unknown rate, but a series of groundwater monitoring wells establishes that a BTEX plume extends less than 46 m (150 ft) from the NAPL source. BTEX concentrations as high as 10 mg/liter are detected within the plume. Upgradient measurements indicate that O2, NO3, SO42−, and CO2 are available as electron acceptors. At the furthest sampling well, BTEX, O2, NO3, and SO42− are virtually absent, but Fe2+ and CH4 appear. Also, the alkalinity and pH increase across the plume. Hydrogeologic analyses indicate that the advective velocity is 30 m/year and the porosity is 0.25. Table 1 summarizes the upgradient and downgradient values.

TABLE 1 Field Measurements

Constituent

Upgradient

Downgradient

Change

BTEX, mg/liter

0

0

0

O2, mg/liter

8

0.2

−7.8

NO3, mg/liter

7

0.1

−6.9

SO42−, mg/liter

9

1

−8.0

Fe2+, mg/liter

0

40

40

CH4, mg/liter

0

1

1

Alkalinity, mg/liter as CaCO3

10

130

120

pH

4.7

6.1

1.4

Total CO2, mg/liter as C

29

44

15

To assess whether biodegradation is responsible for the loss of BTEX and to estimate the NAPL depletion rate, stoichiometric relationships among the measured species and for the possible reactions can be used. Table 2 shows key stoichiometric ratios, using C7H8 to represent BTEX (see also text, Table 4-2):

TABLE 2 Stoichiometric Ratios

Reaction

g C7H8/g acceptor

g CO2-C/g acceptor

g alkalinity as CaCO3/g acceptor

Aerobic

0.319 g

−0.29 g C/g O2

0 g as

(O2)

C7H8/g O2

 

CaCO3/g O2

Denitrification

0.917 g

−0.83 g C/g N

−3.57 g as

(NO3 as N)

C7H8/g N

 

CaCO3/g N

Sulfate reduction

0.637 g

−0.53 g C/g S

−3.13 g as

(SO42− as S)

C7H8/g S

 

CaCO3/g S

Iron reduction

−0.046 g

0.042 g C/g Fe2+

1.79 g as

(Fe2+ generated)

C7H8/g Fe2+

 

CaCO3/g Fe2+

Methanogenesis

−1.28 g

0.42 g C/g CH4

0 g as

(CH4 generated)

C7H8/g CH4

 

CaCO3/g CH4

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

The ratio in the table of reactions is combined with the observed changes in the electron acceptors to compute the predicted changes in BTEX, CO2, and alkalinity based on the changes in advecting acceptors, as shown in Table 3.

TABLE 3 Computed Changes in BTEX (as C7H8), Inorganic Carbon (as C), and Alkalinity (as CaCO3)

Reaction

Observed Change in Acceptor Concentration (mg/liter)

Computed Changes

Total CO2 (mg C/liter)

Alkalinity (mg as CaCO3/liter)

BTEX (mg C7H8/liter)

Aerobic (O2)

−7.8

2.3

0

2.5

Denitrification (NO3-N)

−6.9

5.7

24.6

6.3

Sulfate reduction (SO42−-S)

−8

4.6

25.0

5.1

Iron reduction (Fe3+)

+40

1.7

71.6

1.8

Methanogenesis (CH4)

+1

0.4

0

1.3

Total

14.7

121.2

17.0

The computed changes in inorganic carbon and alkalinity (Table 3) agree with the changes observed in field measurements (Table 1): gains of approximately 15 mg/ liter and 120 mg/liter as CaCO3, respectively. These results support that biodegradation is responsible for the loss of BTEX, because the intrinsic supply rates for the acceptors are consistent with the observed footprint measures.

The total BTEX biodegradation is 17 mg/liter of C7H8. With a flow velocity of 30 m/year and plume cross section of 10 m wide by 2 m deep, the BTEX depletion rate is then

It is valuable to note that the majority of BTEX degradation and inorganic carbon generation come from denitrification and sulfate reduction. This is important because the continued supply of NO3 and SO42− from upgradient advection is easily monitored. The majority of the alkalinity gain, however, comes from iron reduction. It is possible that the natural source of ferric iron (iron oxide solids) could become depleted over time.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-6
Budget Analysis to Determine the Biodegradation Pathways and Depletion Rate of a NAPL at a Field Site

At a field site studied by Borden et al. (1995), the continued leakage of gasoline from an underground storage tank created a NAPL substantially larger than that described in Box 4-4. The total BTEX concentration is as high as 30 mg/liter in the plume, which extends more than 180 m (600 ft). Hydrogeologic measurements indicate a groundwater flow velocity of 30 m/year and a porosity of 0.25. Table 1 shows measured concentrations upgradient of the plume and at a monitoring well located 180 m downgradient of the NAPL source. The BTEX concentration declines considerably but does not reach zero at the 180-m position. O2, NO3, and SO42− are nearly depleted—signs of biodegradation by aerobic, denitrifying, and sulfate-reducing microorganisms. Ferrous iron and methane also appear—signs of iron reduction and methanogenesis. The pH and alkalinity increase substantially.

TABLE 1 Field Measurements

Constituent

Upgradient

Downgradient (180 m)

Change

BTEX, mg/liter

0

5.4

+5.4

O2, mg/liter

3.1

0.2

−2.9

NO3-N, mg/liter

1.4

0.1

−1.3

SO42−-S, mg/liter

6.3

1.3

−5.0

Fe (II), mg/liter

0.2

52.0

+51.8

CH4, mg/liter

0.0

0.1

+0.1

Alkalinity, mg/liter as CaCO3

6.0

122.0

+116

pH

4.6

6.1

+1.5

Total CO2, mg/liter as C

16

70

+54

Table 2 computes the expected changes in inorganic carbon, alkalinity, and BTEX for the observed changes in advecting electron acceptors. The stoichiometric ratios shown in Box 4-5 are used.

TABLE 2 Computed Changes

Reaction

Observed Changes in Concentration (mg/liter)

Changes

Total CO2 (mg C/liter)

Alkalinity (mg as CaCO3/liter)

BTEX (mg C7H8/liter)

Aerobic

−2.9 O2

0.8

0

0.9

Denitrification

−1.3 NO3-N

1.1

4.6

1.2

Sulfate reduction

−6.0 SO42−-S

3.5

18.8

3.8

Iron reduction

+51.8 Fe2+

2.2

92.7

2.4

Methanogenesis

+1.0 CH4

0.4

0

1.3

Total

8.0

116.1

9.6

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

Although the computed alkalinity increase matches the observed alkalinity increase very well, the computed total CO2 increase is much too small: only about 15 percent of that measured. Therefore, considerably more oxidation of C7H8 must be occurring than that represented in the above table, and it must be occurring by a reaction that does not add alkalinity (since the measured alkalinity change matches the computed change). The most likely candidate is aerobic oxidation. The most likely location for this aerobic reaction is in the vadose zone between the NAPL source and the groundwater. Table 3 increases the aerobic reaction to account for all of the CO2 increase.

TABLE 3 Computed Changes When O2 Is Added via Phase Transfer

Reaction

Acceptor Concentration (mg/liter)

Changes

Total CO2 (mg C/liter)

Alkalinity (mg as CaCO3/liter)

BTEX (mg C7H8/liter)

Aerobic

161 O2

46.8

0

51.4

Denitrification

−1.3 NO3-N

1.1

4.6

1.2

Sulfate Reduction

−6.0 SO42−-S

3.5

18.8

3.8

Iron Reduction

+51.8 Fe3+

2.2

92.7

2.4

Methanogenesis

+1.0 CH4

0.4

0

1.3

Total

54

116.1

60.1

The results with the added O2 transfer, which match the observed changes in total O2 and alkalinity, show that 85 percent of the BTEX biodegradation is aerobic, although 80 percent of the alkalinity increase is due to iron reduction. The BTEX dissolution and degradation amount to 60.1 mg/liter as C7H8. With a Darcian velocity of 30 m/year and a plume cross section of 10 m wide by 2 m deep, the BTEX depletion rate is 9,000 g/year C7H8.

NOTE: The data for this example are taken from Borden et al. (1995), and the analysis was developed by Charla Reignanum, Northwestern University.

Solute Transport Models

The highest level of analysis for natural attenuation site data uses mathematical equations to represent the full suite of processes that can affect the fate of contaminants and other important solutes in the groundwater. Table 4-3 identifies two levels of solute transport models: simple and comprehensive. Both levels have the same objective, which is to quantify each of the mechanisms affecting the fate of contaminants and

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

other materials over space and time. Both solve mass-balance equations to achieve the goal. Box 4-7 shows how the mass balance in a domain is quantified for a simple one-dimensional model. The difference between the two levels of modeling is the degree to which site complexity can be included. Simple models work only for situations in which the reactions and hydrogeology are uncomplicated. Comprehensive models allow for as much complexity as needed to represent the site. For sites at which degradation processes are straightforward and hydrogeology is relatively simple, a budget analysis combined with a simple solute transport model often is adequate. On the other hand, as biogeochemical or hydrogeological characteristics become more complex, analysis of the site requires the rigor of comprehensive models.

Comprehensive models can quantitatively integrate the several processes that occur simultaneously in natural attenuation. This integration is most powerful and essential when a site is complex in terms of its biogeochemical reactions and hydrogeology. Then, human intuition often is unable to make the connections among the processes and prioritize their importance based on geographical and statistical analyses or mass budgeting alone. Comprehensive models are powerful tools for developing and evaluating conceptual models, understanding why natural attenuation should (or should not) be appropriate for remediation of a given site, and designing an effective monitoring program. For complicated situations, a comprehensive mechanistic model is the only tool that can integrate the microbiological, chemical, and physical processes that are active at a site. Combining different types of models (e.g., groundwater flow with contaminant transport and reaction) and attempting to find parameters that represent several different materials (e.g., water flow, a contaminant, and a product of reaction) provide a rigorous test of conceptual understanding of the site. Computer-based models can integrate the otherwise overwhelming amount of information needed to describe a complex field site, and they carry out the extensive computations that are impossible for a human. The process of quantifying a groundwater system also highlights the shortcomings of the available data set.

The results of solute transport modeling can be valuable inputs for decision making. Often, the model outputs help define what is likely to occur for the range of conditions that might exist. For example, Figure 4-6 illustrates the level of confidence about how well natural attenuation controls the fate of a contaminant. The three types of outcomes are as follows:

  1. the range of predicted fates is narrow and unambiguously supports that natural attenuation is a viable option (case A);

  2. the range of predicted fates is narrow but shows that natural

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-7
Mass Balance Computations for Simple One-Dimensional Uniform Flow

In modeling the subsurface, the domain is defined as a cube having length X, height Y, and breadth Z. The rate of change of mass in the domain is simply the rate of change of the product of the volume and concentration:

C(XYZ)ε/t

where C = the concentration in the volume; XYZ = the control volume, which is assumed to be small; ε = porosity, or fraction of volume that holds water; t = time; and ∂ stands for the partial differential, or change. When X, Y, Z, and the porosity are fixed, the change in accumulation simplifies to

(∂C/∂t) εXYZ

The rates of inputs and outputs have the same form and represent mass that crosses the boundary of the control volume. A very important way in which a material crosses the volume’s boundary is when it flows with the water that enters or leaves. Crossing the boundary by movement with the water is termed advection, and the rate of advection for the one-dimensional example is represented by

VD(∂C/∂x)XYZ

where (∂C/∂x) is the gradient of concentration along the flow path and VD is the Darcy velocity. The minus sign indicates that advection increases the mass inside the volume when the upstream concentration is higher than the concentration leaving the volume.

A second way in which a material can cross a boundary is by dispersion, which occurs when the concentration gradient differs on either side of the volume. The dispersion rate for the one-dimensional model is

D(∂2C/∂x2XYZ

in which D = the longitudinal dispersion coefficient and (∂2C/∂x2) is the second derivative of the concentration, or the gradient of the concentration slope from one side of the volume to the other. An increasingly positive gradient across a volume causes an increase in the accumulation due to dispersion. A common feature of the rate expressions for advection and dispersion is that they include the product YZ, which is the cross-sectional area through which the input or output occurs.

A third way for mass to cross the system’s boundaries is by a phase transfer. Two examples are (1) the transfer of a volatile organic compound from the water in the control volume to the soil gas in the vadose zone above the water table and (2) the transfer of an organic solvent from a NAPL to the water. The rate of a phase transfer is represented generally by

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

JA

in which J is the flux across the boundary and A is the area through which the transfer takes place.

The rates of consuming and producing reactions describe the phenomena that destroy or sequester contaminants and create the several footprint effects. They can be expressed in the general form

R (XYZ)

in which R stands for a reaction rate with units of mass per unit volume per unit time. R takes a positive sign if the material is produced, or added to the water phase. R has a negative sign if the material is consumed, or removed from the water phase.

Substituting all of the rate expressions for the word expressions converts the mass balance into a mathematical expression of the form

(∂C/∂tXYZ = −VD (∂C/∂x) XYZ + D(∂2C/∂ x2XYZ + JA + R(XYZ)

The constant εXYZ can be divided out, yielding

(∂ C/∂ t) = −VD (∂ C/∂ x)/ε + D(∂2C/∂ x2) + Ja/ε +R

in which a is the specific surface area of the phase transfer, or A/XYZ.

R often depends on concentrations other than C, because the most likely removal mechanisms involve reactions of the contaminant with one or more other reactants. For example, biodegradation involves, at a minimum, reaction of the contaminant with the microorganisms active in its metabolism. Likewise, precipitation reactions require that the contaminant react with a counter ion. Thus, a model that tracks the contaminant often also must track the other reacting materials. In practical terms, mass balances have to be written for all material that must be tracked, and all of the mass balances are then solved simultaneously. Although writing and solving added mass balances increases the complexity of a model, it also is the critical step that integrates—systematically and quantitatively—the fate of the contaminant with the observable measurements.

attenuation is incapable of controlling the contaminants (case B), in which case natural attenuation should be eliminated from consideration and other remediation measures pursued; and

  1. the range of predicted fates is wide and does not lead to any clear-cut decision about the viability of natural attenuation (case C), in which case the stakeholders must consider whether or not more resources should

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

FIGURE 4-6 Various distributions of predicted concentration at a point in space and time resulting from analysis of a range of conceptual models lead to very different decisions about natural attenuation: (A) accept, (B) eliminate, and (C) collect more information.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

be invested to improve site characterization in order to reduce uncertainty.

The scenarios illustrated in Figure 4-6 do not give just one “answer.” As long as multiple model realizations remain, predictions of contaminant fate at the site must be generated for all of the realizations. Decision makers then can see the range of likely fates and the degree of confidence in the predictions of these fates.

The output of a solute transport model, whether simple or complex, can address only items included in the model. Therefore, modeling should be carried out after one or more of the other levels of analysis have identified the key reactions and materials to include in the model.

Simple Solute Transport Models

The most basic level of modeling involves the use of simple solute transport models. Solute transport models are simple when they do not attempt to capture the physical heterogeneity or biogeochemical complexity that occurs at many sites. Examples of simplifications are

  • representing the aquifer properties, such as conductivity, with a single value for the entire aquifer;

  • assuming the flow is steady in time and the direction is along a straight line;

  • representing the dispersion or spreading of the plume with a constant value that does not change with location;

  • describing all reactions by first-order decay, in which the loss rate is proportional to the contaminant’s concentration and a constant; and

  • assuming that two reactants react instantaneously and consume all of the reactant present in the lowest amount.

Simple solute transport models can be divided into two categories, according to the format of the solution. An analytical solution is comprised of one or a small number of mathematical equations. The equations are “closed form,” which means that the predictions, such as the contaminant concentration with distance from the source, can be calculated from a formula. To have mass-balance equations that can be solved analytically, the system must be described in very simple terms. The alternative to an analytical solution is a numerical solution, which requires a computer to make the many repetitive calculations. Even though a computer is used to compute the results, the solute transport model is simple when the mass-balance equation incorporates simplifications such as those listed above.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

The highly idealized conditions described by simple models limit their ability to represent accurately all of the phenomena occurring at most sites. However, they can be useful for exploring a variety of worst-case scenarios for plume migration, especially if site conditions are relatively simple and assessments are preliminary. Thus, if the groundwater at a site flows at a steady rate and mainly in one direction, an analytical solution can provide a sense of how fast the plume is expected to migrate and spread. The effect of uncertainty in the velocity estimates can be explored by trying a range of values for parameters such as hydraulic conductivity.

Many simple analytical models used in assessing natural attenuation assume that the contaminants degrade at a first-order rate. If a contaminant’s first-order degradation rate is estimated from field data, this rate might be useful in roughly estimating the long-term effect of continued biodegradation on the plume migration rate. However, first-order rate constants should be restricted to the range of concentrations used to measure the rate and not extrapolated beyond this range. Even when used in this manner, assuming the decay rate is first-order can produce misleading results. Use of a first-order decay rate assumes that the microbes and reactants that are currently driving biodegradation will remain at all necessary locations into the future. Because conditions frequently change in time and space, a constant first-order rate is almost never accurate.

Simple models can be used to estimate rates of transformation processes, such as biodegradation, but several criteria must be met before beginning such an effort:

  1. Evidence must be clear that the reactions being estimated are actually occurring. This evidence can come from the footprints of attenuation discussed elsewhere in this chapter or from laboratory investigations.

  2. The natural attenuation process must be internally consistent with a site-specific conceptual model. For example, a conceptual model that is based on a site being highly aerobic is the diametrical opposite of postulating that a reductive process is removing contaminants and estimating a rate for this process. Similarly, oxidizing processes should not be postulated if the conceptual model indicates that the site is highly reduced.

  3. The direction and velocity of groundwater flow and effects of retardation must be defined.

  4. Reaction rates should be estimated only for individual contaminants. Groups of related contaminants should not be lumped together. For example, TCE is detoxified through a series of three reductive dechlorination reactions that convert TCE to cis-DCE, cis-DCE to vinyl chloride, and vinyl chloride to ethene. Each reaction in such a sequence has a

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

characteristic rate that must be estimated separately. A combined rate of solvent removal for a TCE site is inaccurate and should not be used to predict the rate of vinyl chloride generation or removal.

In addition, it is important to keep in mind that some simple analysis methods assume that contaminant concentrations are at steady state. If concentrations throughout a plume are declining over time, then the rates estimated by assuming steady state are too slow. Conversely, if contaminants are migrating downgradient and the area of the plume is growing, the estimated attenuation rates will be too rapid. Boxes 4-8 and 4-9 provide additional information on estimating rates with simple models and using laboratory studies to help meet the above criteria.

Comprehensive Solute Transport Models

Comprehensive, computer-based models (known technically as “numerical models”) can account for variations in hydrogeologic proper-

BOX 4-8
Estimating First-Order Rate Constants

First-order rate constants are often estimated from field data by plotting the natural logarithm of concentration versus travel time of the groundwater. If the plot yields a straight line, the slope is equal to k1, the first-order attenuation constant. However, this k1 represents all the processes (e.g., biodegradation, sorption, dispersion, and advection) that affect the contaminant concentration, and therefore it is an “apparent” first-order degradation rate.

Two steps are necessary to overcome this limitation. To separate the effects of sorption, the contaminant’s flow velocity must be computed by dividing the groundwater flow velocity by the contaminant’s retardation factor R. Then, to compensate for the effects of dispersion and nonuniform flow, the contaminant concentration must be normalized to the concentration of a conservative tracer also present. Wiedemeier et al. (1997) provide details on this approach.

Although computing k1 values can be useful (see text), these first-order rate constants have very limited use. One reason is that degradation rates often vary on a site and with time. Changes in subsurface conditions can significantly change rates. Rate changes can occur due to depletion of reactants, changes in the nature or composition of a source area, or changes in the microbiological community. A second reason is that many transformation reactions do not follow first-order kinetics in any case, and first-order decay is irrelevant. Thus, first-order decay rates should be used only when the rate of transformation is likely to be first-order.

One reason first-order rate constants often are used to interpret field data is that the field data are limited in scope and precision. With such limited data, the kinetics for more sophisticated models cannot be estimated.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

BOX 4-9
Uses of Laboratory Studies

At some sites, collecting field evidence adequate to estimate attenuation rates or even to document that key transformation reactions are occurring is not possible. In these cases, laboratory studies are the only options available (Rittmann et al., 1994).

Laboratory microcosm studies can document that key reactions can occur at the site. For example, finding that site water or aquifer samples contain bacteria capable of biodegrading a recalcitrant compound, such as MTBE (methyl tert-butyl ether), offers strong evidence that the biodegradation reaction is feasible in the subsurface. More elaborate laboratory studies can provide the data for defining reaction types and pathways and for estimating kinetic parameters useful for modeling the transformations in the field.

Controlled laboratory studies can provide useful estimates of degradation rates, but the value of the rates is controlled by the specific conditions of the tests. Comparisons of laboratory and field degradation rates often show that the field attenuation rate is slower. How much slower depends on the conditions under which the laboratory and field rates are estimated. For example, a laboratory test often includes all substrates and nutrients in excess to achieve the fastest biodegradation rate possible. However, the supply rates of key materials, such as electron acceptors for BTEX biodegradation, often are limited in the field; thus, a faster degradation rate is expected in the laboratory.

ties and biogeochemical processes in space and time. Comprehensive models are needed when the site is complicated and the complexity has to be captured in the solute transport model. In these cases, the formalism and power of a comprehensive computer model provide the only realistic means for representing processes at the site. These more realistic representations can start with the sediment and rock types that are present at most sites. For example, hydraulic conductivity can vary with location as the lithology changes. More comprehensive models can represent time-varying groundwater flow by changing boundary conditions, such as sources and sinks.

Situations for which comprehensive solute transport models are particularly useful include those in which

  • the reactive materials exist in different chemical forms, such as in a range of acid-base species or complexes;

  • the products of key reactions participate in other reactions (e.g., precipitation or complexation) that affect aqueous-phase concentrations;

  • the contaminant materials or products partition to other phases;

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×
  • the loss reactions occur in multiple steps that produce and consume intermediates; and

  • the hydrogeology is complex and/or dynamic.

Comprehensive models can describe several types of reactions that may contribute to natural attenuation (e.g., growth of one or more microbial types, consumption of electron acceptors, and phase transfers). Mass-balance equations and reaction rates are needed for each component to be simulated (NRC, 1990). Comprehensive predictive modeling of natural attenuation should be undertaken only when the underlying processes are understood well enough that they can be represented by model expressions and when adequate data are available to generate reasonable parameter estimates.

A case study of a crude-oil spill site in Bemidji, Minnesota, presented in Box 4-10, shows how a mass balance and comprehensive modeling of the contaminants and observed footprints can provide an estimate of the relative contributions of aerobic and anaerobic biodegradation processes in a petroleum hydrocarbon plume. A comprehensive model was created for this site because the aquifer is heterogeneous and numerous reactions contribute to the overall biodegradation of the hydrocarbons. Modeling results illustrate how the most favorable electron acceptors, such as oxygen and manganese, are used first. Eventually, even the large supply of solid-phase iron oxide that was initially present in the aquifer is exhausted near the source. Once this occurs, only methanogenic degradation is active in the core of the plume. The time required to use all of the favorable electron acceptors at a distance 36 m downgradient from the center of the oil body is almost 10 years from the time of the spill. Thus, the modeling shows that the geochemical conditions and important degradation reactions have changed slowly since the site became contaminated. However, the quantity of oil in the aquifer is very large, and the oil will continue to be a source of contaminants to the groundwater in the future. This case study provides a particularly good example of how a comprehensive solute transport model can be very useful in predicting the evolution of geochemical conditions and contaminant concentrations for a long-lasting source.

In creating a comprehensive solute transport model, the effort needed to integrate and evaluate the mechanisms of natural attenuation varies depending on the goals of the model. Therefore, modeling goals must be established before modeling begins. Simple goals, such as gaining an order-of-magnitude estimate of the travel time from a contaminated well to an uncontaminated one, may require less effort than more sophisticated goals, such as quantifying all of the processes affecting natural attenuation. The complexity of the modeling effort also depends on site

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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and contaminant characteristics. Relatively homogeneous sites and contaminants transformed by well-defined reactions generally require less effort than do heterogeneous sites and contaminants affected by poorly defined reactions or many interactions with other contaminants or aquifer materials.

Modeling should be an iterative process. As more and better data are collected, the conceptual model of the site may change, and parameter values will have to be refined. This iterative approach is normal and is a key ingredient for good-quality modeling.

Model Quality Issues

Solute transport models should never be used without a proper foundation. Misleading and even irrelevant results are likely when the modeler does not understand the underlying mechanisms, the code has not been properly validated, or the equations and parameters representing the mechanisms or site conditions are inadequate. In 1990 the NRC published comprehensive guidelines on how to ensure quality in model results; the guidelines are summarized briefly here.

The best way to ensure the validity of model results is to employ a competent modeling team. A list of ideal qualifications includes expertise in hydrogeology, low-temperature geochemistry, microbiology, reaction kinetics, applied mathematics, computer programming, statistics, and field-sampling methods. Beyond this basic knowledge, practical experience gained from modeling a variety of sites is highly desirable. Because the scope of knowledge and skills is so large, an individual seldom has all the necessary qualifications. Thus, a team comprised of several experts is desirable to supply all of the technical skills. The team should interact throughout the modeling process.

One trait of a good modeler is a disciplined integrity when describing and assessing the results of model simulations. On the one hand, the modeler must remember and communicate that the model never perfectly describes all processes that occur in the field. For this reason, the purpose of the modeling must be carefully defined, and the use of the results must be limited to this purpose. Further, the modeler should view poor correspondence between model predictions and field measurements as an opportunity to improve understanding, not as a failure of the modeling or the modeler. Modeling is an iterative process, and the poor predictions of an early model open the door for improving the conceptual model, as well as the solute transport code.

Ensuring that the model is properly formulated is also necessary to producing reliable results. Table 4-4 describes the three common categories of problems with the models themselves.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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BOX 4-10
Simulation of the Bemidji, Minnesota, Crude Oil Spill Site

A buried oil pipeline located in a glacial outwash plain near Bemidji, Minnesota, ruptured in 1979, spilling about 11,000 barrels of crude oil. An estimated 3,200 barrels of the spilled oil infiltrated into the subsurface, creating a long-term, continuous source of hydrocarbons that dissolve in and are transported with groundwater. Figure 1 shows the extent of the plume in 1992 (Baedecker et al., 1996). Evidence for microbial degradation of the petroleum hydrocarbons has been documented in several studies (Baedecker et al., 1993; Bennett et al., 1993; Eganhouse et al., 1993). The evolution of redox zones and microbial populations in the groundwater plume were simulated by Essaid et al. (1995).

FIGURE 1 Bemidji plume in 1992.

In the conceptual model of the site, volatile and nonvolatile dissolved organic carbon (VDOC and NVDOC) are transformed by aerobic, Mn4+- and Fe3+-reducing, and methanogenic biodegradation. Aerobic degradation takes place first, and oxygen inhibits anaerobic processes. In addition, iron reduction is inhibited by solid-phase manganese. Thus, as oxygen is consumed and an anoxic zone develops, the Mn-Fe reducers and methanogens begin to grow and release dissolved Mn, dissolved Fe, and methane. The model accounts for the transport and reactions of seven mobile solutes (VDOC, NVDOC, O2, N, Mn2+, Fe2+, and CH4); consumption of two solid-phase concentrations (Mn4+ and Fe3+); and three microbial populations (aerobes, Mn-Fe reducers, and methanogens). A vertical cross section parallel to the direction of groundwater flow along the sampling transect was simulated from the time of the spill in 1979 until September 1992 using the computer code BIOMOC (Essaid and Bekins, 1997). Steady-state flow was assumed. Literature values, theoretical estimates, and field biomass measurements were used to obtain reasonable estimates of the transport and biodegradation parameters used in the simulation. The observed spatial and temporal variations in solute concentrations were used to calibrate the model.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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FIGURE 2 Simulated and measured VDOC and NVDOC concentrations at the Bemidji site.

FIGURE 3 Simulated and measured concentrations at the Bemidji site.

Figures 2 and 3 provide the simulated concentrations and data for a well that is 36 m downgradient from the center of the oil body. The simulation predicts that 46 percent of the total DOC is degraded. Aerobic degradation accounts for 40 percent of the total DOC degraded, and anaerobic processes account for the remaining 60 percent of degradation (5 percent by Mn reduction, 19 percent by Fe reduction, and 36 percent by methanogenesis). Thus, the simulation suggests that anaerobic processes account for more than half of the removal of DOC at this site.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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TABLE 4-4 Common Problems With Models

Type of Problem

Examples

Solution

Model Framework: Poor Assumptions or Input

Applying an inappropriate model or concept to the problem

Using a first-order rate law for biodegradation; simulating reactions that do not occur at the site or assuming a reactant is present in excess

Check that site geochemical data support the model formulations

Relying on parameter values taken from publications unrelated to the site

Sorption coefficients, biodegradation coefficients, hydraulic conductivities

Use site-specific measurements to obtain reasonable values of parameters

Failing to meet conditions assumed in the model

Assuming that climatic conditions and anthropogenic effects will remain the same

Evaluate the uncertainty associated with this assumption and its effect on the results

Weighting observations inappropriately in the calibration

Errors associated with inaccuracy and imprecision of the measuring device and process or human error

Weight the observations using the inverse of the variance of the measurements that established the value of the observation

Model Application: Closed Mind During the Modeling Process

Failing to consider alternate conceptual models

Filling gaps in hydraulic conductivity measurements according to a single conceptual model

Use multiple realizations of conceptual models of a site; combine all available data types to reduce uncertainty

Forcing the model to predict the expected outcome

Changing the input parameter values to match the data

Evaluate whether processes that control the fate of the plume may have been overlooked; constrain parameter values to reasonable ranges

Model Use and Presentation

Extrapolating beyond the model’s capability

Using a flow model calibrated to steady-state conditions to predict transient flow fields

Collect new data for calibration of storage coefficient or other uncalibrated features

Overstating accuracy or reliability

Reporting only a single value for the prediction of interest, with numerous significant figures

Provide a range of possible outcomes, reflecting the range of uncertainty associated with input parameters

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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The first category encompasses problems with the model framework. Solute transport models are only as good as the conceptual models and data on which they are based. The equations, boundary conditions, and parameters used must be appropriate for the conditions at the site now and in the future. Also, when measurements are used to constrain the model, the estimated measurement errors should also be input to the model.

The second category includes problems that arise when the model is applied to evaluate the field data. A common mistake is to accept prematurely only one conceptual model. Then, the model parameters are “adjusted” to make the accepted model give the right answer. Often, unrealistic parameter values must be used, or nonconforming results are simply ignored. A much better strategy is to keep an open mind and revise the conceptual model.

The third category is concerned with the final use and presentation of the model results. The model should not be used for simulations beyond the time frame or conditions for which it is appropriate. The modeler also must describe the uncertainty inherent in the model inputs and how this translates into uncertainty in the results. Also, the model’s conceptual basis, including the mass-balance equation, underlying assumptions, and parameter values, must be fully documented.

In its 1990 review, the NRC recommended two steps for preventing the kinds of problems listed in Table 4-4. First, the computer code must be fully documented. Normally, the model’s developer carries out this step. In addition to providing instructions for using the model, the documentation should describe the mathematical equations solved by the code and the numerical algorithms used to obtain the solutions. The documentation should also describe how the model was verified by testing against known analytical solutions or other codes. The verification process ensures that the model code accurately solves the governing equations.

Second, the foundation of the model’s reactions and equations should be validated by comparison to field or laboratory data. The mathematical equations solved and the parameter values for these equations must be valid for the site to be modeled. For some codes, the mathematical expressions have been tested by many researchers in the past and are accepted as valid. In other cases, the mathematical forms used to represent reactions are novel and not widely accepted. In these cases, the model user must take responsibility for validating that the code’s equations and parameters are appropriate.

Qualified modelers are in short supply (NRC, 1990). Thus, modeling results of poor quality are all too common. Because of this, many decision makers are highly suspicious of using models to simulate natural attenu-

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

ation. Poor-quality modeling is not inevitable, as long as the modeler is competent and problems in the models themselves are avoided.

Exploring Sources of Uncertainty and Assessing Their Effects

Any comprehensive model will have to be calibrated to the particular site. Calibration involves determining the best values of unknown parameters by comparing model results to field data, a process known as inversion. The calibration process can help discriminate between acceptable and unacceptable representations of the site.

Calibration that identifies an acceptable representation of the site involves meeting four criteria. First, the model must provide a reasonable fit to the field data. If the model cannot capture the observed trends no matter how well it is calibrated, then its conceptual basis surely is wrong. Second, residuals should be randomly distributed in space and/or time. Systematic bias in the residuals usually means that the conceptual model or its mass-balance equations are not correct. Third, the estimated parameter values must reasonable. For example, hydraulic conductivity or biodegradation rate coefficients cannot be orders of magnitude larger than normal and acceptable values. Again, unrealistic parameter values usually signal that the conceptual model is flawed. Fourth, the correlation between parameter values should be low enough that the parameters are uniquely estimated. Models that meet these criteria should be retained, while the rest should be rejected or revised. Reporting of uncertainty associated with the parameter values and predictions is also an important step in the calibration process.

Inversion codes are valuable tools for automated calibration and can use uncertainty analysis to discriminate between acceptable and unacceptable representations of the site. Inversion codes developed in recent years (Doherty, 1994; Poeter and Hill, 1998) make it possible to compute best-fit parameters for many models, including those not originally designed for estimating parameters by comparison to field data. Although the objectives of automated inversion are the same as those of trial-and-error calibration, automated inversion codes overcome the lack of rigor and, sometimes, the biases of the trial-and-error approach by systematically searching for optimal parameter values. Thus, truly optimal parameters can be identified. The case study in Box 4-11 demonstrates how automated inverse modeling can be used to differentiate poor model realizations from realizations that provide an acceptable representation of the site. Hill (1998) provides guidance on the practical application of inversion modeling, while Poeter and Hill (1998) describe public domain software for inversion of any combination of model codes. Field applications of inverse modeling concepts are described in the following publications:

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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Anderman et al. (1996); Barlebo et al. (1996); Christensen (1997); Christiansen et al. (1995); D’Agnese et al. (1996a,b, 1998); Cooley (1979, 1983a,b); Cooley et al. (1986); Gailey et al. (1991); Giacinto (1994); Kueper (1994); McKenna and Poeter (1995); Olsthoorn (1995); Tiedeman and Gorelick (1993); and Yager (1993).

Estimating the Sustainability of Natural Attenuation

When analyzing data from a natural attenuation site, a key question often is whether the mechanisms that destroy or immobilize contaminants are sustainable for as long as the source area releases them to the groundwater. More specifically, whether the rates of the protecting mechanisms will continue to equal the rate at which the contaminants enter the groundwater may be a concern. Sustainability is affected by the rate at which the contaminants are transferred from the source area and whether or not the protecting mechanisms are renewable. Unfortunately, most evaluations of natural attenuation to date have not analyzed the sustainability of the reactions.

Sustainability is of the greatest concern when the contaminant release rate is high. This may occur when the source area is large, the contaminant concentration in the source is high, and/or the contaminant transfers readily to the groundwater. A large hydrocarbon NAPL and tailings pond are examples. The presence of a major source often can be detected by high contaminant concentrations in the groundwater near the source or in the center of the plume. Even though the source often cannot be located and quantified precisely, mass-budget analyses such as those illustrated in Boxes 4-5 and 4-6 offer a means to estimate the rate at which a contaminant is released to the groundwater. The mass-budget analysis (or solute transport model in more complex settings) also can be used to estimate the long-term rates of the destruction and immobilization reactions based on characteristics of the groundwater, the mineralogy, and the hydrogeology.

Some protecting mechanisms are continuous and renewable, but others are not. For example, the long-term supply rates of electron acceptors in the upgradient groundwater or from the soil gas often are predictable and reasonably steady. On the other hand, supply rates of electron donors for reductive reactions normally depend on the long-term existence of a hydrocarbon NAPL or a landfill. Electron-donor supply rates might be predictable and stable if the donor source is identified and long-lasting, but they would decline significantly if the donor source were removed or depleted.

Natural attenuation mechanisms that rely on soil minerals to provide sorption sites, electron acceptors, or alkalinity have a finite capacity and

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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BOX 4-11
Use of Inverse Modeling to Select the Most Representative Model

At the U.S. Department of Energy’s Kansas City Plant, researchers used inverse modeling to evaluate alternative conceptual models (Anderman et al., 1998). Characterization activities carried out by multiple consulting firms had resulted in a myriad of inconsistent conceptual models of the site. Fifteen equally plausible models were evaluated, representing not only the different views of the firms, but also different levels of model complexity (i.e., number of parameters included in the models to represent the system).

A two-layer, steady-state MODFLOW (McDonald and Harbaugh, 1988) model was used to represent the major hydrogeologic units of the alluvial aquifer system in all of the alternative conceptual models. The upper layer consists of approximately 9 m (30 ft) of clayey silt, and the lower layer consists of less than 3 m (10 ft) of basal gravel. Nonlinear regression (using UCODE; Poeter and Hill, 1998) was used to estimate optimal parameter values for each conceptual model by matching field observations of 239 head and 13 flow measurements. Statistics resulting from the regression were used to discriminate between the conceptual models and determine which model best represented the site.

Figure 1 illustrates that increasing complexity (i.e., greater number of estimated parameters as displayed on the left panel) of the conceptual models improved the fit (second panel) to observed data for models 2 through 7. However, additional complexity did not improve the fit. Beginning with conceptual model 11, particles were tracked from the source area through the simulated flow field, and the final particle paths were compared to the observed plume movement (third panel). Prior information on the parameter values from aquifer tests and a flow observation representing flow through the entire system were added for conceptual model 15. Although conceptual model 15 did not match the particle paths as well as some of the other models, it was considered the “best” representation of the site because many of the particles followed the observed plume movement; the estimated parameter values were reasonable; and the total flow through the system matched the observed flow better than other models (fourth panel).

are not renewable. If the contaminant source is large, as in the Pinal Creek case study of Chapter 3, the contaminant plume will migrate as the continual release of contaminant exhausts the capacity of the minerals.

In summary, estimating the sustainability of natural attenuation requires identifying active attenuation mechanisms, distinguishing nonrenewable mechanisms from renewable mechanisms, and comparing release rates of contaminants to the potential rates of transformation and immobilization. Mass budgeting is an important tool for assessing sustainability. However, with mass budgeting, uncertainties will remain in predictions of sustainability, due to uncertainties inherent in all site assessments. There-

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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FIGURE 1 Comparison of conceptual models 2 through 15 (model 1 is omitted because it did not adequately represent the site). The first panel shows the number of parameters estimated. The second panel represents the fit of the model to the field data (low values indicate a closer fit). The third panel displays the percentage of simulated particles that followed the observed path of the plume in the field. The fourth panel indicates the deviation from the observed flow through the entire site.

fore, long-term monitoring will be needed to ensure that natural attenuation is continuing to protect public health and the environment.

MONITORING THE SITE

The final step in documenting natural attenuation is to establish a long-term monitoring plan. If the results from the conceptual model and data analysis lead to the decision that natural attenuation is protective, then long-term monitoring must provide assurance that the site’s protective processes continue to operate over time. Monitoring within the plume

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

is necessary to ensure that reactions that destroy or sequester the contaminants continue to be active. Monitoring downgradient from the plume is necessary to ensure that contaminants are not migrating beyond the zone in which natural attenuation is supposed to take place. Monitoring frequency, intensity, and duration will vary with the complexity of the site, groundwater flow direction and velocity, and plume transport speed. Simple sites contaminated with low concentrations of BTEX will not require the intensity or duration of monitoring necessary at sites contaminated with high concentrations and recalcitrant contaminants. Regardless of the site conditions, the monitoring will have to continue until it demonstrates that natural attenuation has succeeded in achieving the required cleanup goals or that natural attenuation has failed in achieving cleanup standards and a contingency plan has to be implemented. Sites at which natural attenuation is a formal remedy should have an exit strategy specifying when long-term monitoring of natural attenuation can stop.

Although long-term monitoring is an essential part of using natural attenuation as a remediation strategy, protocols for long-term monitoring of natural attenuation sites are lacking. Comprehensive data from long-term monitoring of existing natural attenuation sites also are lacking for most sites. Long-term monitoring results from existing natural attenuation sites have to be carefully studied in light of the goals described in the preceding paragraph. Based on these results, guidelines for long-term monitoring of natural attenuation should be developed.

CONCLUSIONS

Processes that degrade and/or transform contaminants in the subsurface leave footprints that often can be measured. Analysis of these footprints with models of the subsurface should form the basis for determining whether natural attenuation can control contamination at a site. The basic steps to document natural attenuation are to

  1. develop a conceptual model of the site’s hydrogeology and biogeochemical reactions;

  2. analyze site measurements to quantify the attenuation processes (looking for changes in contaminants and their footprints);

  3. establish a long-term monitoring program to document that natural attenuation continues to perform as expected.

Although the basic steps are the same for all sites, the level of effort needed to achieve them varies substantially with the complexity of the site and the likelihood that the contaminant is controlled by a natural attenuation process. More uncertainty about site conditions or processes

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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that can control the contaminants increases the level of effort required. Table 4-5 summarizes how characteristics of the site and the contaminants determine the typical level of effort for data gathering and interpretation. The level of effort depends on two factors: (1) the contaminant and (2) the hydrogeology. In general, a higher level of data gathering and analysis is required when the contaminant is less likely to transform (along the rows of the table) and the hydrogeology is more complex (down the columns). The way in which the effort level increases depends on the site and the contaminant. Effort involves a combination of the amount of information that must be gathered and the sophistication of the data analysis (i.e., as summarized in Table 4-3). Table 4-5 offers some general guidelines for levels of data gathering and analysis for different site conditions. The table provides relative indications of effort levels as follows:

  • A level-1 effort is appropriate when all contaminants are in the category of high likelihood of success (as defined in Table 3-6) and the hydrogeology is simple and well understood. In these cases, data gathering and analysis must be sufficient to document that the flow direction is reasonably constant, that contaminant migration is consistent with the flow direction, and that contaminant concentrations decrease with distance from the source. In most cases, at least one footprint should be detected at levels commensurate with the loss of contaminant. For example, a common type of data analysis is to develop contour plots of the hydraulic head in the wet and dry seasons to assess the consistency of flow direction. Contour plots of the contaminant and footprint concentrations often are used to document the principal direction of contaminant migration and that contaminant loss is tied to an attenuation mechanism. A set of two or three vertically nested wells located in the central portion of the plume in plan view can be used to estimate the vertical rate of migration. Even at these relatively simple sites, the sustainability of the attenuation reactions has to be demonstrated through long-term monitoring of contaminants and footprints.

  • A level-2 effort is necessary when the site’s hydrogeology is not simple, the likelihood of success is not high, or the attenuation mechanisms may not be sustainable. If heterogeneities are important, cross-sectional plots of the subsurface geology are needed to show the important lithologic units and their properties. A conceptual model should show how the heterogeneities affect plume migration, and vertical and horizontal plots of concentration data should demonstrate that plume migration is consistent with the conceptual model. If the likelihood of success is not high or the sustainability is uncertain (for example, due to high concentrations from the source), then the postulated reactions that cause contaminant loss have to be documented for the geochemical con-

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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TABLE 4-5 Summary of Typical Effort Required for Site Characterization and Data Interpretation

 

Likelihood of Success of Natural Attenuation of the Contaminant of Concerna

Site Hydrogeology

High (e.g., BTEX, alcohols)

Moderate (e.g., monochlorbenzene, Pb)

Low (e.g., MTBE, TCE, 99Tc)

Simple flow, and uniform geochemistry, and low concentrations

1

2

2

Simple flow, and small-scale physical or chemical heterogeneity, and medium-high concentrations

2

2

3

Strongly transient flow, large-scale physical or chemical heterogeneity, or high concentrations

2

3

3

NOTES: Level of effort refers to the number and frequency of samples taken, parameters analyzed in site samples, and type of data analysis (see text); 1 = low effort; 2 = moderate effort; 3 = high effort. BTEX = benzene, toluene, ethylbenzene, and xylene; MTBE = methyl tert-butyl ether; TCE = trichloroethene.

aLikelihood of success refers to judgments in Table 3-6.

ditions of the aquifer. For example, data should demonstrate that footprints are present in the aquifer or that long-term sorption is occurring. Generally, a mass-budget analysis is needed to show that the postulated natural attenuation reactions are sufficient to destroy or immobilize all of the contaminant and are sustained over time. In some cases, simple mass transport modeling may be needed to interpret whether or not concentrations are decreasing over distance and time.

  • A level-3 effort is needed when the site is highly heterogeneous, flow is strongly transient, the likelihood of success is moderate to low (according to Table 3-5), and/or the potential for sustainability is not high. Extensive effort also may be needed when the site contains complex contaminant mixtures. Extensive effort involves collecting enough data to construct a flow and reactive transport model of the plume. The number and locations of samples and the types of materials assayed (i.e., the contaminants and footprints) must be commensurate with the scope and complexity of the model. The model should simulate the important mass-loss mechanisms, and it should describe the footprints, as well as the contaminants. Outputs from the model should be evaluated with

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

long-term monitoring, and the model should be improved as new data are obtained. Model simulations should show that the mass-loss mechanisms could be sustained for the lifetime of the contaminant source. Model simulations should include cases in which the flow system and the geochemistry are perturbed from the present ones.

Although natural attenuation may be a feasible alternative in many cases, Table 4-5 makes clear that documenting natural attenuation may require a great effort if the site characteristics or the controlling mechanisms are uncertain.

RECOMMENDATIONS

  • At every regulated natural attenuation site, the responsible company or agency proposing the remedy should document the probable processes responsible for natural attenuation. Observing the disappearance of the contaminant is important to prove that natural attenuation is working, but it is not sufficient by itself.

  • Responsible parties should use “footprints” of natural attenuation processes to document which mechanisms are responsible for observed decreases in contaminant concentration in the groundwater. Footprints generally are changes in concentrations of reactants or products of the biogeochemical processes that transform or immobilize the contaminants. Footprints are well established for some biodegradation reactions—for example, for many petroleum hydrocarbons and chlorinated solvents. Footprints for other contaminants should be based on known biogeochemical reactions. Observing several different footprints and correlating them with decreases in contaminant concentration is necessary evidence for or against natural attenuation and helps overcome confounding factors.

  • Responsible parties should have a conceptual model of the site’s hydrogeology and reactions to show where groundwater and contaminants are moving. The conceptual model includes the groundwater flow, the contaminant source, the plume, and the reactions and chemical species relating to natural attenuation at the site. A good conceptual model guides site investigation and decision making.

  • Responsible parties should gather field data in order to evaluate the validity of the conceptual model and quantify the natural attenuation processes. At the beginning of the site investigation, multiple conceptual models will have to be created. Field data should be used to rule out the models that do not adequately represent the site. Field data also should be used to refine the conceptual model that is ultimately chosen as the best site representation.

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
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  • Responsible parties should analyze the field data at a level commensurate with the complexity of the site and the contaminant type. At the most basic level, graphing and statistical analyses are helpful for formulating hypotheses about trends and possible reactions; they may be adequate for documenting cause and effect for sites with simple hydrogeology and when the reactions affecting the contaminant are well understood. At the next level, mass budgeting is a powerful tool for demonstrating whether or not the footprints of the reactions are commensurate with observed contaminant losses; it is valuable for handling sites with moderate levels of uncertainty. Finally, a solute transport model may be needed when uncertainty is high due to site complexity or poorly understood reactions; these models range in complexity from analytical models to comprehensive numerical models that account for variations in aquifer properties, groundwater flow rates, and contaminant reactions (see NRC, 1990, for details).

  • Responsible parties should repeatedly improve the conceptual model and data analysis for their site. The conceptual model represents an evolving understanding of the site. As new data are collected and analyzed, the conceptual model should be refined. As the conceptual model is refined, new data may be needed. Having a new conceptual model and/or new data often requires that analyses be revisited or modified.

  • Responsible parties should provide a higher level of effort to document natural attenuation for sites at which the uncertainty is greater due to site or contaminant characteristics. Table 4-5 summarizes the conditions that lead to increasing level of effort for site characterization and data analysis.

  • When modeling studies are presented as part of a site assessment, the responsible party should present adequate documentation so that the regulator can assess the quality of the model simulations. This documentation should show whether the model accurately represents the processes and is consistent with the data and conceptual model of the site. The uncertainty of the results should be quantified. Other model quality assurance issues are discussed in Groundwater Models: Scientific and Regulatory Applications (NRC, 1990).

  • A long-term monitoring plan should be specified for every site where natural attenuation is approved as a formal remedy for contamination. Monitoring should take place for as long as natural attenuation is necessary to protect public health and the environment. The required monitoring frequency will have to vary substantially depending on site conditions and the degree of confidence in the sustainability of natural attenuation. Simple sites contaminated with low concentrations of BTEX will not require the same degree of monitoring as complex sites with

Suggested Citation:"4 Approaches for Evaluating Natural Attenuation." National Research Council. 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. doi: 10.17226/9792.
×

higher contaminant concentrations and more recalcitrant types of contaminants. Guidelines on long-term monitoring of natural attenuation sites are lacking, and such guidelines have to be developed for different type of sites.

REFERENCES

Anderman, E. R., M. C. Hill, and E. P. Poeter. 1996. Two-dimensional advective transport in groundwater flow parameter estimation. Ground Water 34(6):1001-1009.

Anderman, E. R., A. D. Laase, J. O. Rumbaugh, and J. L. Baker. 1998. The Use of Inverse Modeling to Incorporate Model Uncertainty in Evaluation of Alternative Remedial Actions, Poster Session of the MODFLOW’98 Conference, International Ground Water Modeling Center, Colorado School of Mines, Golden, Colo.


Baedecker, M. J., I. M. Cozzarelli, D. I. Siegel, P. C. Bennett, and R. P. Eganhouse. 1993. Crude oil in a shallow sand and gravel aquifer. III. Biogeochemical reactions and mass balance modeling in anoxic ground water. Applied Geochemistry 8:569-586.

Baedecker M. J., I. M. Cozzarelli, P. C. Bennett, R. P. Eganhouse, and M. F. Hult. 1996. Evolution of the contaminant plume in an aquifer contaminated with crude oil, Bemidji, Minnesota. Pp. 613-620 in U.S. Geological Survey Toxic Substances Hydrology Program. Colorado Springs, Colo.: U.S. Geological Survey.

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In the past decade, officials responsible for clean-up of contaminated groundwater have increasingly turned to natural attenuation-essentially allowing naturally occurring processes to reduce the toxic potential of contaminants-versus engineered solutions. This saves both money and headaches. To the people in surrounding communities, though, it can appear that clean-up officials are simply walking away from contaminated sites.

When is natural attenuation the appropriate approach to a clean-up? This book presents the consensus of a diverse committee, informed by the views of researchers, regulators, and community activists. The committee reviews the likely effectiveness of natural attenuation with different classes of contaminants-and describes how to evaluate the "footprints" of natural attenuation at a site to determine whether natural processes will provide adequate clean-up. Included are recommendations for regulatory change.

The committee emphasizes the importance of the public's belief and attitudes toward remediation and provides guidance on involving community stakeholders throughout the clean-up process.

The book explores how contamination occurs, explaining concepts and terms, and includes case studies from the Hanford nuclear site, military bases, as well as other sites. It provides historical background and important data on clean-up processes and goes on to offer critical reviews of 14 published protocols for evaluating natural attenuation.

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