Contaminants in the Subsurface Source Zone Assessment and Remediation (2005) / Chapter Skim
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3 Source Zone Characterization
3 Source Zone Characterization
Pages 79-124
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From page 79... ...
Although it is impossible to prescribe a specific step-by-step source zone characterization process because of differing conditions from site to site, there are four broad categories of information that are critical for characterizing all source zones: 79
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From page 80... ...
However, each of the activities is related to the others, and a good deal of iteration between the general categories is not only desirable but critical to the process. Furthermore, iteration between source zone characterization and other site conceptual model building blocks should be employed to constantly reassess site understanding and integrate new data from all facets of the characterization.
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From page 81... ...
KEY PARAMETERS OF SOURCE ZONE CHARACTERIZATION AND THE TOOLS TO MEASURE THEM The four categories of information important for source zone characterization are (1) the nature and presence of the source material, whether it be a DNAPL or chemical explosives, (2)
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From page 82... ...
Furthermore, some of the tool categories are much broader than others, and some may overlap slightly. The tables are meant to be inclusive and provide a broad overview of the array of tools and methods often used in source zone characterization.
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From page 83... ...
Regional Information about fractures, Used for site Site-specific details Geology sink-holes, springs, and conceptual model difficult to infer discharge points. and determining from this hydrogeologic information.
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From page 84... ...
Direct Push Direct push techniques are Used for gaining Direct push used both for retrieving information about techniques are subsurface samples and for the physical generally quicker performing in situ analyses properties of soils, and more mobile of physical and chemical stratigraphy, depth than traditional drill parameters. Two major to the water table, rigs, and there is no techniques include cone pore pressure, and drilling waste.
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From page 85... ...
e) Natural gamma logging Reveals presence measures emissions from of shale or clay isotopes preferentially layers.
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From page 86... ...
cone and rebound can be used conceptual model Not specific for to estimate permeability, development and locating preferential hydraulic conductivity, the remedial activities. paths or highly radius of influence, and flow permeable zones.
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From page 87... ...
, interfacial conceptual model tension (e.g., pendant drop (SCM) and method)
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From page 88... ...
Environmental Protection Agency (EPA) on DNAPL source depletion presents a summary of characterization tools (EPA, 2003a)
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From page 89... ...
Source Presence and Nature Before extensive source zone characterization methods are undertaken, an effort should be made to first determine the nature of the source. Determining the composition of the DNAPL or explosive material is useful for a variety of site management activities.
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From page 90... ...
If the tempera ture dependent solubility limit is used with estimates for rb, Kd, q, a, and KH in the equation above, CT is the maximum total concentration, as determined from a field soil sample, before a separate chemical solid phase must appear. Figure 3-1 is the result of an analysis that shows the maximum total soil con centrations of RDX (a solid phase energetic material)
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From page 91... ...
Historical records may be able to provide some information on the age and identity of the compounds and how they were used on the site. Yet, short of collecting actual chemical samples such as a DNAPL sample, it may not be possible to fully understand the nature of the source material (in terms of the key physical and chemical parameters mentioned above)
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From page 92... ...
. Characterizing the Hydrogeology An accurate depiction of the subsurface and its flow characteristics is critical to the overall site conceptual model and necessary for developing successful remediation strategies.
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From page 93... ...
Understanding the flow system and being able to determine groundwater velocities are important in order for mass flux reduction to be used as a metric for source remediation (see Chapter 4)
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From page 94... ...
Typically used in conjunction with other characterization tools such as soil cores, downhole methods can provide information about stratigraphy, formation density, porosity, fractures, flow paths, and moisture content, depending on the method used. For example, various downhole imaging techniques can provide information about the location of fractures and their orientations, if the fractures intersect the boreholes.
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From page 95... ...
Source delineation is most commonly accomplished by analyzing sediment cores, by measuring dissolved concentrations of specific compounds and, in rare cases where possible, by analyzing the free product found in wells or cores. Obtaining soil and rock cores and dissolved contaminant concentrations is generally accomplished using invasive technologies such as core retrieval via drilling and direct push techniques, as previously described.
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From page 96... ...
. Although sources can be delineated by monitoring contaminant concentrations at sampling points over time, measurements of contaminant mass flux (the amount of contaminant mass migrating through a cross section of the aquifer orthogonal to groundwater flow within a given time)
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From page 97... ...
. Yet, the measurement of mass flux holds promise as a more robust method of quantifying the mass of contaminant loss in cases where information on the groundwater flow system, including hydraulic conductivity and hydraulic gradient, can be obtained.
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From page 98... ...
Source Characterization at Hydrogeologically Complex Sites Not all of the characterization tools discussed above are applicable or appropriate in all hydrogeologic settings. Most of the tools have been developed and utilized at sites with porous media (particularly unconsolidated granular geologic environments)
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From page 99... ...
, hydrogeologic characterization is an essential first step during source zone characterization. A combination of historical chemical-use data and analyses of water samples is then used to determine likely areas of DNAPL occurrence.
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From page 100... ...
A sitewide hydrogeologic investigation of the karst system (Phase I) was performed with the following objectives: establish the significance of karst on groundwater flow and contaminant transport, delineate karst watersheds in order to define potential sourcereceptor flow paths, identify optimal surface and ground water monitoring locations for long-term monitoring and possible remediation per formance assessment, develop a sitewide conceptual model to support decision making, and evaluate the existing perimeter monitoring well network.
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From page 101... ...
Groundwater screening Refine limits of DNAPL source area. Drive point (DPT)
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From page 102... ...
APPROACH TO SOURCE ZONE CHARACTERIZATION Gaining an understanding of the complexity of a site through source zone characterization before engaging in expensive remediation technologies can save resources and help to define reasonable cleanup goals. Beyond the four primary types of source characterization information and the associated tools mentioned above, there are other factors site managers should consider during source characterization, as discussed below.
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From page 103... ...
Plume-scale observations are needed for the purpose of defining potential exposure pathways. However, it is important to avoid overdelineation of the plume at the expense of more localized source zone characterization efforts.
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From page 104... ...
Furthermore, users may collect data at the wrong time or place, they may misinterpret data if the spatial or temporal scale of the sample is not understood (Sposito, 1998) , or they may use incorrect or oversimplified conceptual models to relate what is directly measured to what needs to be determined (e.g., in pumping or tracer tests, the hydrogeology is usually oversimplified to determine hydrogeologic parameters from head or concentration observations)
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From page 105... ...
in the underlying conceptual models and how they represent physical, chemical, and biological processes and (2) in the implementation of mathematical models.
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From page 106... ...
should become a more routine part of source characterization, especially at complex sites where uncertainties regarding the source composition, distribution, and strength are very large. The following section discusses three approaches to uncertainty analysis classified as statistical, inverse, and stochastic inverse methods.
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From page 107... ...
in particular. They could be used in analyzing data from source zones in order to better estimate contour maps or spatial averages of, say, NAPL in the soil and to provide a better appreciation of the uncertainty associated with these estimates.
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From page 108... ...
involve inverse modeling using a simple conceptual model. At the early stages of a remedial investigation, the presence of a source is usually inferred only indirectly (e.g., from measurement of solute concentrations that are near the solubility limit or from the persistence of the contamination problem even after many contaminated pore volumes have been treated)
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From page 109... ...
, the presently available characterization techniques may be inadequate to support such an action because one cannot identify the source's exact location and strength. Stochastic inverse methods (see reviews by Yeh, 1986; Ginn and Cushman, 1990; McLaughlin and Townley, 1996)
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From page 110... ...
Nevertheless, increased use of stochastic methods is encouraged. They are well suited to the analysis of data from source zones in order to improve understanding of site conditions such as estimates of total mass, to provide a better appreciation of the uncertainty associated with these estimates, and to design monitoring programs.
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From page 111... ...
For example, consider a scenario in which there is a relatively high level of uncertainty associated with the nature and geometry of the source zone, while the vertical and horizontal boundaries of the source zone area are relatively certain. Here, a certain level and type of source zone characterization effort may be sufficient for
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From page 112... ...
The uncertainty resulting from limited data, hydrogeologic variability, and other factors must be taken into account when evaluating and selecting technologies for source remediation. Most source remediation studies circumvent an objective and systematic analysis of data and evaluation of uncertainty with respect to delineating the source zone and predicting the effectiveness of remediation (although there are frequently used statistical
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From page 113... ...
Box 3-4 summarizes the progression and intensity of source zone characterization efforts carried out at the relatively small-scale Camp Lejeune, North Carolina. After the development of the overall site conceptual model, which included a preliminary effort focused on defining the source zone and local hydrogeology, this site was selected as a test site for source remediation via surfactant flushing.
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From page 114... ...
As certainty about the DNAPL distribution above and along the clay unit began to increase, source remediation was presented as a potential objective, and wells were installed in and around the source zone. Using these wells, more specific characterization strategies, such as pump tests and partitioning interwell tracer tests (PITT)
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From page 115... ...
In Case 2, a chemical flushing technology is applied prematurely and results in the migration of a portion of the DNAPL to the lower aquifer. In Case 3, adequate source zone characterization leads to an accurate evaluation of the extent of the source and to more successful execution of the chemical flushing technology, substantial source strength reduction, and subsequent monitored natural attenuation.
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From page 116... ...
However, because the source material is not efficiently reduced by the pump-and-treat remedy, the prognosis for this scenario is that there will be a long-term operation and maintenance horizon with costs continuing indefinitely. Illustrative Case 2: Insufficient Source Zone Characterization.
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From page 117... ...
Detailed source zone characterization can be unappealing to the responsible party since it may pave the way to source remediation regarded as complicated, costly, and perhaps of questionable effectiveness. Indeed, it is worthwhile to note
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From page 118... ...
The Army case studies reviewed by the committee also suggest that the development of site conceptual models is evolving rapidly in parallel with improved Army characterization efforts. At Fort Lewis, for example, a number of nontraditional characterization technologies were used including CPT-LiF, MIP, GPR, resistivity, dye studies, and multilevel wells.
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From page 119... ...
All sites require some amount of source characterization to support the development and refinement of a site conceptual model. In general, successful source remediation requires information on the nature of the source material, on the site hydrogeology, on the source zone distribution, and on the site biogeochemistry.
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From page 120... ...
2001. An analytical quantification of mass fluxes and natural attenuation rate constants at a former gasworks site.
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From page 121... ...
1996. Conceptual models for the behavior or dense non aqueous phase liquids DNAPLs)
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2004. Modeling the benefits of partial mass reduction in DNAPL source zones.
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From page 123... ...
2000. NAPL source zone characterization and remediation technology performance assessment: recent developments and applications of tracer techniques.
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From page 124... ...
Water Resources Research 36(7)
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