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Contaminants in the Subsurface: Source Zone Assessment and Remediation (2005)

Chapter: 4 Objectives for Source Remediation

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Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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4
Objectives for Source Remediation

The remedial objectives of interest to the Army and other potentially responsible range from groundwater restoration and plume shrinkage and containment, to mass removal, risk reduction, and cost minimization. A realistic evaluation of the prospects for, or success of, a source remediation action requires the specification of these objectives with clarity and precision. The project manager and other stakeholders must know the full range of site remedial objectives, their relative priorities, and how they are defined operationally as specific metrics, in order to determine whether source remedial actions will contribute to meeting objectives for the site. The primary purpose of this chapter is to describe the many objectives possible at sites for which source remediation is a viable option, many of which have been institutionalized within regulatory, risk assessment, and economic frameworks for site cleanup.

Failure to explicitly state remedial objectives appears to be a significant barrier to the use of source remediation. That is, the vagueness with which objectives for remedial projects are often specified can preclude effective decision making with regard to source remediation. Too often, either data presented on the effects of source remediation are irrelevant to the stated objectives of the remedial project, or the objectives are stated so imprecisely that it is impossible to assess whether source remediation contributes to achieving them.

Evidence supporting the above was received by the committee over the course of its deliberations during numerous briefings on source remediation projects at Department of Defense (DoD) facilities and other sites, supported by extensive documentation on some of these remedial efforts. Other related documents were also reviewed, including many case studies on source remediation

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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efforts available at the U.S. Environmental Protection Agency’s (EPA) Technology Innovation Office. In many of the cases reviewed, remedial project managers (RPMs) appeared unable to articulate a clear a priori rationale for undertaking source remediation at a site or to quantify the extent to which source remediation efforts were contributing to accomplishment of remedial objectives. To a significant extent, these interrelated problems appeared to reflect the absence of unambiguously stated remedial objectives for the sites or of clear operational definitions of those objectives. For example, during a brief report on an attempt to use steam recovery to remove a source area from a relatively homogenous unconsolidated aquifer, the project manager expressed considerable frustration with the effort, not only from a technical point of view but, more important, from the point of view that it was consuming significant resources while not contributing to any reduction in human health risk. There was no complete exposure pathway to the contaminated groundwater at the site, nor any expectation of a complete pathway in the near future. This is illustrative of a situation in which an explicit operational statement of site objectives (e.g., a reduction of human health risk as estimated by the procedures specified in EPA’s Risk Assessment Guidance for Superfund, RAGS), if made prior to the attempt at source remediation at this site, might have led to a decision not to attempt source remediation.

This widespread problem of vaguely formulated remedial objectives, tenuously linked to performance metrics, is neither specific to the issue of source remediation nor reflective of any unusual failure on the part of the specific project managers with whom the committee interacted. Rather, this ambiguity is embodied in long-standing national policy statements (i.e., the National Contingency Plan) and analytical procedures (as embodied in RAGS). It is compounded by the fact that multiple stakeholders at a site not only may have very different objectives, but may also use very similar language to describe those very different objectives. Moreover, a particular performance metric may potentially correspond to a variety of different objectives and accordingly be viewed quite differently by different stakeholders. Finally, both the DNAPL problem and the effects of source remediation efforts raise temporal issues that are very poorly addressed by conventional analytical frameworks for assessing risks to human health and the environment.

This chapter describes a variety of substantive remedial objectives.1 It shows how a stated objective can be defined operationally by several different metrics

1  

Substantive objectives are concerned with the results of a decision process, in terms of a physical change at the site. In contrast, procedural objectives focus not on the outcome of a remedial effort, but on the process by which a decision is reached (e.g., transparency of the decision process, opportunities for public participation). Procedural objectives are often as important to stakeholders as substantive objectives, but they are not considered here because they are outside the scope of this report.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

and how a particular metric may represent the operational definition of several different objectives. This complex relationship between ultimate cleanup objectives and the ways in which they are measured can be the source of considerable uncertainty in the evaluation of remedial alternatives. It can also mask serious differences in stakeholder priorities, which only become apparent when an apparently “successful” remediation fails to satisfy key stakeholders. This is not meant to imply that better specification of objectives and their relationship to metrics of remediation will ensure stakeholder satisfaction. An unambiguous delineation of objectives and metrics will, however, allow the decision on source remediation to be more clearly evaluated. The relevance of source remediation to different stakeholders’ objectives can be identified in advance, and progress can be measured.

FORMULATING OBJECTIVES

One source of ambiguity during site remediation is that various stakeholders may use similar (or identical) language to describe radically different objectives for remediation of a site. Thus, this section provides an approach to clearly describing stakeholder objectives. There are three critical, interdependent elements in the unambiguous specification of a remedial objective for a site: (1) identifying the objective, (2) determining the appropriate metric(s) to measure achievement of the objective, and (3) determining the status of the objective. Although difficulties in specifying each of these elements among the projects reviewed have been noted, the element of status is addressed first because it is often overlooked, followed by discussions of common objectives and the selection of appropriate metrics for an objective.

Status of Remedial Objectives

Status refers to the fact that any identified remedial objective can be seen either as important in itself or as a means to an end. In the former case, the objective is termed absolute or primary, while in the latter, the objective is functional. (For an exposition of the contrast between absolute and functional objectives, see Udo de Haes et al., 1996, and Barnthouse et al., 1997.)

Consider, for example, the objective of reducing contaminant concentrations in groundwater to a specified level at a particular point in time and space. This may be mandated under a particular regulatory framework as a necessary feature of a successful remediation, in which case it represents an absolute objective. Failure to achieve these concentrations represents failure of the remedy. The identical criterion, however, could be selected as a means of ensuring that risks to human health have been reduced to an acceptable level. In this case, the objective is functional, because there may be other objectives that achieve a comparable degree of health protection, such as precluding use of contaminated groundwater.

Confusion about whether an objective is absolute or functional is not uncommon

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

at a wide range of sites, particularly with regard to maximum contaminant levels (MCLs) and how they are viewed by various stakeholders. MCLs are frequently cited as an absolute regulatory objective. Indeed, MCLs (or non-zero maximum contaminant level goals, MCLGs) may be determined to be either an “Applicable” or a “Relevant and Appropriate” requirement under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). However, they can also serve as a functional objective that supports an absolute statutory objective. For example, a state may have determined that all of its groundwater should be protected as a potential source of drinking water. Consistent with state law, a demonstration that concentrations were below MCLs would indicate that the groundwater resource had been adequately protected. The state might, however, be open to other indicators that its requirement of resource protection had been met.2

There is further complexity with the designation of status. For example, MCLs may serve as a functional objective supporting a higher-level functional objective of achieving an acceptable human health risk, which in turn serves the absolute objective of protecting human health. In this case, there are clearly alternative functional objectives, both to meet the risk assessment functional objective and to meet the absolute objective of protecting health. In the first case, it may be that the actual conditions of use would indicate an acceptable level of risk even if the MCL were exceeded. In the second case, there are any number of ways to interrupt the relevant exposure pathway (such as institutional controls or the provision of a public water supply).

The distinction between absolute and functional objectives is important, because trade-offs among different absolute objectives cannot be accomplished at a technical level. Rather, they represent social value judgments that must be made among stakeholders.3 In contrast, trade-offs between functional objectives can be made at a technical level, subject to the requirement that equivalence in meeting the corresponding absolute objective can be demonstrated. Thus, functional objectives are fungible.4 In the above example, the project manager can achieve health protection by precluding use of the contaminated water or by lowering concentrations in groundwater. Similarly, but within the realm of physically specified objectives, if the absolute objective were defined as meeting a concentration at a specified point of compliance (e.g., a fenceline), the project manager could trade off between preventing contaminant migration from the

2  

For example, higher concentrations of contaminants that would then be reduced during routine disinfection of raw water sources might be deemed acceptable.

3  

A considerable body of literature on such judgments among qualitatively different environmental objectives has developed in the context of life cycle assessment (e.g., Udo de Haes et al., 1996).

4  

“Fungible” refers to goods or commodities that are freely exchangeable for or replaceable by another of like nature or kind in the satisfaction of an obligation.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

source or capturing migrating contaminants before they reached the point of compliance.

It is important to bear in mind that a given functional objective may serve more than one absolute objective, and also that a particular objective may be functional for one stakeholder and absolute for another. For example, limiting the migration of contaminants in groundwater beyond the boundaries of a site may serve the absolute objectives of meeting a state statutory requirement or preventing “chemical trespass” (Gregory, 1993). On the other hand, it may serve the higher-level functional objectives of limiting human health risk to an acceptable level (by reducing exposure potential) or avoiding the effort and uncertainty of applying for an alternative concentration limit under the Resource Conservation and Recovery Act (RCRA). Different stakeholders may all agree that limiting the migration of contaminants in groundwater beyond the boundaries of a site is an important objective, but they would likely have very different responses to any proposals for substituting an alternative objective.

Metrics for Remedial Objectives

Ultimately, accomplishment of (or progress toward) a remedial objective can only be evaluated if there is a measurable value or metric associated with that objective.5 Accordingly, any objective that cannot be stated directly in terms of a metric must be assigned one or more subsidiary functional objectives that can be formulated in terms of a metric. This is illustrated in the following simplified example.

The absolute objective is the protection of human health. This is not directly measurable in most cases, where illness has not been recorded in a site-associated population. Accordingly, a common functional objective is the specification of a Hazard Index < 1.0 and a cancer risk estimate < 10–4 in a quantitative risk assessment. In practical terms, this requires a lower-level functional objective—that “exposure point” concentrations in groundwater be less than a certain value. Two alternatives can be employed to achieve this: (1) change contaminant concentrations at the defined exposure point or (2) change the exposure point, for example, by providing alternate water sources and prohibiting water use near the contaminant source. Each of these functional objectives has an associated metric—a revised concentration in the water at the relevant exposure point.

It is important to bear in mind that not all metrics are as unambiguously specified as is the concentration of a particular chemical at a particular point in

5  

Our use of “metric” differs from that of EPA (2003b), which describes classes of metrics that were not in fact measured but were inferred from measured quantities. In our use, such “Type II and Type III” metrics would be considered functional objectives.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

time or space, as will become clear in the following section, which discusses common objectives during source remediation and their associated metrics.

COMMONLY USED OBJECTIVES

Whether defined by the stakeholder as absolute or functional, there are a set of objectives that have been widely used in site remediation. In many cases examined by the committee, the identification of site objectives has been less than clear, such that metrics appropriate to one objective have been employed for a different objective to which they are not applicable. The following sections distinguish between alternative possible objectives (whether absolute or functional) in four areas. There are obviously other kinds of objectives dealt with at sites, including programmatic and societal concerns. Moreover, the list of objectives within each area is merely illustrative and far from exhaustive. The four areas are

  1. Objectives related to a physical change at the site

  2. Objectives related to risks to human health and the environment

  3. Objectives related to life-cycle and other costs

  4. Objectives related to the time required to reach particular milestones

Physical Objectives

There are a number of physical objectives that may drive the design and performance evaluation of source zone treatment methods. These include mass removal, concentration reduction, mass flux reduction, reduction of source migration potential, plume size reduction, and changes in toxicity or mobility of residuals. The specification of metrics for performance evaluation with respect to these objectives is typically easy since the objectives are related to physical, measurable properties. In some cases, however, considerable inference must be interposed between the available metrics and the physical objective. Each of these objectives and their associated metrics are described briefly below.

Mass Removal

Removal of contaminant mass from a source zone is a common objective at hazardous waste sites and may be either absolute or functional (depending on the stakeholders, the governing regulations, and other factors). Many of the source zone treatment technologies, particularly those that rely upon fluid flushing of the source area (including surfactant/cosolvent flushing, steam flushing, air sparging, and water flushing), are designed to remove contaminant mass from the swept zone. For these technologies, the injected fluids serve as a carrier medium to transport the contaminant mass to the surface. The mass removed is recovered at

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

the surface through the collection and treatment of these flushing fluids. Other source zone treatment methods, such as chemical oxidation/reduction, soil heating, or enhanced bioremediation, are designed to destroy or convert the form or phase of the contaminant mass in situ.

Common metrics for the mass removal objective are the mass of contaminant recovered or destroyed and the percentage of the total contaminant mass present in the subsurface that was recovered or destroyed. The first metric is relatively straightforward for flushing technologies, for which mass removal is quantified by measuring the contaminant mass recovered as a component of the extracted flushing fluid. For destruction or conversion methods, however, mass removal is less easily quantified, and one must rely on indirect metrics. Under some conditions, the measurement of the concentrations of reaction by-products can facilitate inference of mass removed. Accurate mass balances, however, are typically difficult or impossible to achieve in such situations, since mass conversion or destruction methods do not rely upon injected fluid recovery. Finally, the ability to measure the percentage of the total contaminant mass in the subsurface that is recovered or destroyed depends on estimates of the total mass present, which, depending on site characterization data, may be quite poor.

Concentration Reduction

Even more common than the mass removal objective is the objective of reducing contaminant concentration within an affected medium (i.e., soil, sediment, groundwater, etc.) to a desired lower value. The obvious associated metric is contaminant concentration. Like mass removal, concentration reduction can serve as an absolute objective (e.g., meeting MCLs) or as a functional objective for reaching some other absolute objective (e.g., reducing exposure and consequently health risk). The use of reductions in contaminant concentrations as remediation objectives is common because regulations often specify concentration compliance levels.

Concentration is defined as the mass of the target compound per volume (or mass) of the affected medium (pore water, core sample, solid sample). Thus, concentration compliance or target levels can be defined in a number of ways, depending on the sampled medium. For example, groundwater concentration is typically defined as the contaminant mass per volume of pore fluid, while solid-phase concentration is often defined as a contaminant mass per mass of the sampled solid phase. Although concentration is often viewed as a precise metric, it should be noted that it really represents an average value over the volume sampled. If the distribution of contaminant mass within a source zone is highly irregular, local concentrations within a source zone can vary substantially from one another and from the average concentration that would take the entire source volume into consideration. In this report, the term “local” implies that concentrations are sampled over small spatial intervals by extracting small volumes of pore

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

fluid, such that these concentrations are representative of a known physical location within the contaminated zone at the time of sampling.

Remediation technologies seek to reduce concentration levels through mass removal, conversion, or destruction. Thus, it is important to note the connections between mass removal and concentration reduction as remediation objectives. Ideally, if the total mass of the contaminant were removed from the source zone, the concentration in the aqueous phase would be reduced to below detection limits. In practice, however, total mass removal is next to impossible, and may instead be confined to more “treatable” areas. Treatability will depend upon the selected remediation method, but it also depends on permeability (flushing potential), degree of aquifer material affinity for the contaminant, volume and distribution of NAPL present, and composition of the contaminant. For example, aquifer material with high organic content may tend to more strongly sorb contaminants, or the presence of a NAPL pool may limit the degree of contact between the contaminant and the injected flushing fluid. Due to subsurface heterogeneity, treatability typically varies spatially, resulting in a significant spatial variability in the distribution of contaminant mass within the source zone subsequent to treatment. Although some technologies (e.g., steam flushing) may be more robust in their mass removal behavior, it is expected that contaminant concentrations will be locally variable following treatment. Thus, it is very difficult to make generalizations about how removing a certain percentage of mass relates to achieving a certain percent reduction in contaminant concentrations. A number of field studies (e.g., Londergan et al., 2001; Abriola et al., 2003, 2005; EPA, 2003a) have documented that local concentration reductions are achievable with source zone remediation. Such concentration reductions have ranged over one half to two orders of magnitude.

While it is possible that local concentration levels in the treatable zones may be substantially reduced by mass removal, local concentrations in less accessible DNAPL zones will likely remain high (at or near aqueous solubility levels) following treatment. Less accessible zones may also retain significant organic mass on the solid (through sorption) or within stagnant pore fluids (through diffusion). Furthermore, if local groundwater flow rates are low within the source zone under natural conditions, diffusion from less accessible zones may result in increasing contaminant concentration levels in the more accessible areas over time (often termed concentration rebound) once remediation operations have ceased. Thus, the potential effect of mass removal on local contaminant concentrations will be a complex function of source zone properties, including DNAPL distribution, natural groundwater gradients, and the spatial distribution of sorbed mass. Based upon these considerations, the use of local concentration within a source zone as a metric of remedial success is problematic, particularly in the absence of high-resolution sampling. More integrative metrics are discussed below.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×
Mass Flux Reduction

While measurements of local concentrations permit the development of a picture of the spatial distribution of contamination within the pore water in the source zone, mass flux quantifies the potential influence of these concentrations on a downstream receptor. Mass flux is typically defined at some cross-sectional (planar) surface selected downstream of the source zone and roughly perpendicular to the direction of flow. The mass flux at a particular location in this transect is defined as the mass of contaminant moving across the surface per unit area per unit time. The total mass flux (or, more accurately, mass flow rate) is then obtained by integrating the mass flux over the plane. The average mass flux can then be obtained by dividing the total mass flow rate by the area of the cross-sectional plane of interest (it should be noted that these approaches may not translate well to fractured flow systems). A related metric, the flux-averaged concentration, is determined by dividing the average mass flux by the average groundwater velocity in the cross section.

Mass flux reduction can be a functional objective that may, for example, support the higher-order objectives of reducing exposure to downstream receptors or preventing the growth of a plume downstream of the source zone. Although conceptually attractive as a remediation objective, mass flux reduction is difficult to quantify in practice, as suggested in Chapter 3. Most existing methods typically involve measurement of contaminant concentration at distinct points in the selected transect. Transformation of these measurements to flux estimates requires application of assumptions about the groundwater velocities at the measurement points. Furthermore, computation of average fluxes from such measurements is subject to a high level of uncertainty. More integrative methods for estimating average mass flux are currently under development. These involve alteration of the flow field through downstream pumping or installation of in situ flow-through devices at selected downstream locations.

The relationship between concentration reduction or mass removal and mass flux reduction is as yet poorly understood. Considerable research is currently being directed at developing information and methodologies for the prediction and quantification of mass flux reduction from data on source zone mass removal and on aquifer and contaminant characteristics. Box 4-1 illustrates some of these efforts. A more extensive numerical investigation of this sort suggests that a two-orders-of-magnitude mass flux reduction may be achievable following partial source zone mass removal in Type I media (Lemke et al., 2004).

Reduction of Source Migration Potential

Reducing the potential for the source to migrate into clean subsurface areas is a commonly stated objective of the projects reviewed by the committee. Many source zones are characterized by the presence of DNAPL pools, which are

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

BOX 4-1
Relationship between Partial Source Removal and Mass Flux Reduction

Recent analytical and numerical modeling efforts provide evidence that partial source zone removal may result in significant (several orders of magnitude) reduction in posttreatment contaminant mass flux (Rao et al., 2002; Lemke and Abriola, 2003; Rao and Jawitz, 2003). Consider two simulated DNAPL cross-sectional saturation profiles shown in Figures 4-1(A) and 4-2(A). Here the modeled formation is based upon an unconfined sandy glacial outwash aquifer located in Oscoda, Michigan, at the site of a former dry cleaning business. Aquifer characterization efforts were conducted in support of a Surfactant Enhanced Aquifer Remediation (SEAR) pilot-scale test (see Chapter 5), designed to solubilize and recover residual tetrachloroethylene (PCE) from a suspected DNAPL source zone at the site (Bachman Road). As part of the SEAR design effort, alternative spatial variability models of the unconfined aquifer were developed from formation core data and were used to generate entrapped PCE distributions using the immiscible fluid flow model MVALOR (Abriola et al., 1992; Rathfelder and Abriola, 1998; Abriola et al., 2002). Further details pertaining to the spatial variability models and simulation conditions can be found in Lemke and Abriola (2003) and Lemke et al. (2004). The simulated spill involved the release of 96 liters of PCE over four grid cells at the top of the model domain at a constant flux of 0.24 liter·day–1 for 400 days, with an additional 330 days for subsequent organic infiltration and redistribution.

Examination of the two saturation distributions in Figures 4-1 and 4-2 reveals that while both contain the same total volume of PCE, this mass is distributed more uniformly in Figure 4-1 than in Figure 4-2. Much of the mass in Figure 4-2 is contained within a thin pool, where saturations reach up to 91 percent of the pore space. Alternatively, in Figure 4-1 maximum saturations of PCE do not exceed 31 percent.

The potential influence of source zone mass removal on DNAPL distributions is illustrated in Figures 4-1(B) and 4-2(B), which present saturation mass depletion profiles. Here PCE mass removal was simulated using a lab-validated version of MISER (Taylor et al., 2001; Rathfelder et al., 2001). The initial saturation profiles shown in Figures 4-1(A) and 4-2(A) were flushed with approximately 1.5 pore volumes of surfactant solution and 10 pore volumes of water. Further details pertaining to the simulated surfactant flush can be found in Lemke (2003) and Lemke et al. (2004). Inspection of Figures 4-1(B) and 4-2(B) reveals that different degrees of mass removal are predicted for the different initial mass distributions, even though each was subjected to the same flushing conditions. In Figure 4-1(B), flushing has resulted in 97.8 percent PCE removal, with the remaining PCE distributed in thin of pools of short lateral extent. The mass dissolution for the PCE distribution in Figure 4-2(A) is substantially less, with only 43.2 percent of the mass removed. Here much of the original pooled PCE persists, with maximum concentrations still ranging up to 86 percent (compared to 13 percent in Figure 4-1(B).

Figures 4-1(c) and 4-2(c) illustrate the source zone PCE concentrations evolving from the saturation distributions shown in Figures 4-1(b) and 4-2(b). Notice that despite the substantial mass removal, concentrations are still quite high in the

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

FIGURE 4-1 PCE infiltration and subsequent mass removal behavior: (A) initial PCE saturation, (B) PCE saturation after surfactant flushing, and (C) aqueous PCE concentration after surfactant flushing. SOURCE: Adapted from Lemke and Abriola (2003) and Lemke et al. (2004).

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

FIGURE 4-2 PCE infiltration and subsequent mass removal behavior in a system with significant pooling behavior: (A) initial PCE saturation, (B) PCE saturation after surfactant flushing, and (C) aqueous PCE concentration after surfactant flushing. SOURCE: Adapted from Lemke and Abriola (2003) and Lemke et al. (2004).

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

immediate vicinity of the pooled PCE, reaching 100 mg/L (the aqueous solubility) just above the pools. Thus, if groundwater were sampled at these locations, the results would suggest that little benefit had been gained from mass removal.

If one considers the impact of the source zone mass reduction on mass flux, however, a different picture emerges. Calculation of mass flux at a downstream plane that intersects the entire contaminated plume reveals that mass removal has resulted in a substantial reduction in PCE mass flow rate (approximately 1.5 orders of magnitude in both spill scenarios). In fact, although more mass remains in the second case, the mass flow rate is actually lower, since the remaining contaminated zone represents a smaller fraction of the cross-sectional plane.

A flux-averaged downstream concentration can also be computed for these scenarios. The flux-averaged concentration represents the total mass crossing the plane divided by the total volume of groundwater crossing the plane in the same time period. Notice that initial (pre-dissolution) mass-averaged concentrations are quite high (close to the aqueous solubility of PCE), consistent with the local concentrations within the source zone. However, post-mass removal, flux-averaged concentrations have been reduced by more than 1.5 orders of magnitude. Conceptually, one might view these flux-averaged concentrations as more representative of the risk to downstream receptors, since they incorporate the dilution effect, similar to that which would be measured by a fully screened well directly downstream of the source.

Another way to present the results of these types of analyses is to plot the relationship between mass removal and mass flux reduction. Figure 4-3 shows the

FIGURE 4-3 Potential remediation benefit for the two NAPL distributions shown in Figures 4-1(A) and 4-2(A). SOURCE: Adapted from Lemke and Abriola (2003) and Lemke et al. (2004).

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

potential benefit of mass removal for the two alternative PCE distributions shown in Figures 4-1(A) and 4-2(A). Here the results for Figure 4-1(A) are indicated by the solid squares and those for Figure 4-2(A) by the open squares. Note the very different shapes of these two curves. In the former, mass flux reduction lags mass removal slightly, while in the latter, initial mass removal results in very substantial benefits in terms of flux reduction. The mass flux reduction “plateau” exhibited by the Figure 4-2 scenario is associated with the persistence of a pool that contains much of the PCE mass.

Although limited simulations are available to extrapolate to other aquifer types, release scenarios, and/or remediation technologies, the illustrations given above suggest that source mass removal may offer substantial benefits, if mass flow rate is the metric of choice. In more heterogeneous formations, one might anticipate that even more of the mass would be distributed in pooled areas, leading to reduced mass flow rates following treatment. For example, Figure 4-4 illustrates predicted PCE distributions in three formations that have identical release rates and average permeability values. The formations differ only in the magnitude of the variance [in ln(k)] of the permeability field. Note that the formation with the highest variance has the most extensive pooling. The results shown above are also expected to be representative of a variety of flushing remedial technologies that remove (or destroy) mass preferentially from high-permeability zones (including pump-and-treat, chemical oxidation, and cosolvent flushing).

As pointed out above, local concentrations in the source zone, subsequent to mass removal, may remain high. Thus, if MCLs in the source zone are used as a metric, little benefit may be realized from treatment. If flux-averaged concentration, however, is employed as a metric, substantial benefits may be achieved from even partial mass removal. The reduction in mass flux can reduce concentrations at downstream receptor wells and may reduce average downstream concentrations to levels where microbial transformation of the chlorinated solvents becomes feasible (Nielsen and Keasling, 1999; Yang and McCarty, 2000; Adamson et al., 2003; Sung et al., 2003).

typically formed when the downward migration of the organic liquid has been impeded by the presence of a capillary barrier or low-permeability layer. DNAPL will tend to spread along such interfaces of contrast until a dynamic equilibrium is reached locally between gravitational, capillary, and pressure forces. Although the pools may be stationary under this dynamic equilibrium, the mass of DNAPL present in these pools cannot be considered immobile. Future disruption to the dynamic balance of forces can induce further migration of the DNAPL to previously uncontaminated areas, enlarging the extent of the source zone. For example, such a disruption could occur as a result of a physical breach in the barrier during

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

FIGURE 4-4 Potential effect of formation ln(k) variance on PCE saturation distribution. k = permeability. SOURCE: Reprinted, with permission, from Phelan et al. (2004). © 2004 Elsevier Science.

field characterization efforts or due to aging of the contaminant that leads to alterations in the interfacial properties controlling the capillary forces.

As discussed in Chapter 2, the capillary force that acts to retain DNAPL in a pore is controlled by the pore size, the wettability characteristics of the solid, and the interfacial tension between the water and DNAPL. A DNAPL is truly immobile when this capillary force exceeds the pressure and gravity forces that can act to induce migration. At the larger scale, there is a quantifiable relationship between the average capillary pressure and the DNAPL saturation; in a particular material, capillary forces tend to be smaller at higher saturations. Thus, in a given

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

formation, the mobility of the DNAPL is linked to its saturation, as well as to the porous medium texture. One metric for the mobility of a particular DNAPL, then, is the mass of that DNAPL exceeding a particular local saturation. Such a metric, however, is nearly impossible to quantify in practice, due to the dependence of this saturation threshold on the texture of the medium and the high variability of local source zone saturations. Other metrics that might indicate achievement of this objective include alterations in the DNAPL viscosity or changes in certain soil properties.

Primarily through pool mass removal, source zone remediation activities can result in a reduction in the local saturations of the DNAPL left in place. As noted above, this may result in a reduction of mobility and a reduction in the risk of further migration. It is also possible that chemical oxidation techniques may reduce pool mobility through the formation of reaction “crusts” at the edges of the pool (Li and Schwartz, 2004). This effect has not yet been thoroughly investigated. It should also be cautioned that many source zone remediation technologies are designed to enhance DNAPL mobility during the treatment process (often through a reduction in interfacial tension).

Plume Size Reduction

Another objective of source zone treatment can be to reduce concentration levels within the downstream contaminant plume and/or to reduce the physical extent of the plume. Reductions in contaminant mass within the treated zone and in mass fluxes from this zone (a reduction in the strength of the source) will theoretically result in a reduction in downstream plume size. Every aquifer has a natural capacity to dilute or attenuate the contaminants. Dilution processes include diffusion and dispersion, while attenuation processes include sorption and chemical/ microbial reactions. Such processes act to limit the rate of migration and growth of a plume. For example, for contaminants that are subject to constant reaction rates (a rather crude but illustrative simplifying assumption) and for a continuous source of a fixed size, there is a maximum size to which the plume will grow. If the source strength is reduced, this maximum size will decrease. Thus, it is possible that a reduction in source strength would eliminate problematic plume discharges to surface waterbodies or would permit natural microbial processes to shrink the plume to a size that fails to reach receptor points of concern.

It is important to recognize the potentially significant time lag that will occur between the reduction of source strength and any recorded changes in concentrations in the plume (which are the primary metrics). Initially, the reduction in flux from the source zone will produce a lower concentration in the dissolved phase plume immediately downgradient of the source, but this effect will migrate only at the rate of dissolved phase contaminant migration. Thus, concentrations from a well in a dissolved phase plume located several hundred meters from the source

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

zone may not be affected for many months or even years. In general, few field data exist to document the benefits of source zone treatment on plume size.

Changes in Toxicity and/or Mobility of Residuals

In many contamination scenarios, the entrapped NAPL in the source zone exists as a mixture of many compounds. Common examples of NAPL mixtures include coal tars and combined fuels/solvents from spills from degreasing operations. Usually, there are certain compounds within the mixture that are of greater concern, due to their higher toxicity and/or mobility in the subsurface. Thus, another objective of source zone remediation can be to change the composition of the NAPL in situ, resulting in a reduction in overall contaminant toxicity or mobility. Certain technologies, including air sparging, soil heating, water flushing, and enhanced bioremediation, are designed to selectively extract or destroy NAPL components of concern. These technologies take advantage of contaminant component properties, such as solubility, volatility, and biodegradability, to alter the characteristics of the NAPL. Favorable changes can sometimes be achieved without large reductions in total contaminant mass. Furthermore, reductions in the concentrations of target constituents within the NAPL (which is the primary metric) may also reduce both the toxicity and mobility of the downstream contaminant plume.

Elimination of Barriers to Subsequent Remedial Action

A final physical objective of source zone treatment can be to create a subsurface environment that is conducive to the application of other remediation technologies. For example, in many situations, the high concentration levels or the total mass of a contaminant within a source zone may preclude application of enhanced bioremediation. However, if enough mass is extracted from the zone, the accompanying reduction in concentration levels and mass fluxes may facilitate successful application of bioremediation technologies. Furthermore, some source zone treatment technologies (e.g., surfactant flushing) may leave chemicals in place that alter the biogeochemical environment, making it more conducive to microbial transformation processes. Similarly, concentration reductions may make installation of a reactive barrier a more feasible treatment option. The required thickness of a barrier is a direct function of downstream concentration levels, and reductions in concentrations within the plume will also reduce the risk associated with barrier failure. Under some conditions, where substantial mass removal has been achieved in source zone treatment, monitored natural attenuation may even be a feasible follow-on treatment choice. The metrics associated with this objective are variable, but they commonly include reductions in contaminant concentrations, mass, and mass fluxes.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

Objectives Related to Human Health and Environmental Risk

Risks to human health and/or the environment cannot be directly measured, at least not in any context relevant to the selection and evaluation of remedial technologies for source zones and contaminated groundwater. Accordingly, these objectives inherently involve subsidiary functional objectives and associated metrics, many of which were described in the previous section on physical objectives.

Risk to human health and the environment from contaminants in the subsurface is a function of both the level of exposure sustained and the toxicity of the chemical(s) to which the individual is exposed. Thus, risk reduction can be achieved by reducing or eliminating exposure or by reducing the toxicity of the chemicals present.

Reducing the Level of Exposure

Reducing an individual’s exposure to contaminants is a common functional objective for site cleanup. The level or degree of exposure sustained by a human or ecological receptor to a chemical in the environment is dependent on several factors:

  • The spatial extent of contamination (the area affected by the contaminant)

  • The concentration of the contaminant present at the point or points of contact

  • The frequency and duration a receptor is in contact with the contaminant (e.g., daily, monthly, occasionally)

  • Behavioral characteristics of the receptor (e.g., the ingestion of soil by children, other feeding habits, hand washing frequency, degree of skin covering, etc.)

  • Fate and transport of chemicals from one environmental medium to another (e.g., migration of vapors from subsurface soil or groundwater into buildings), thereby creating exposure pathways from the contaminant to the receptor

Thus, there are myriad ways to reduce exposure to subsurface contaminants. The ways most commonly encountered during site remediation are (1) to reduce the amount of chemical present at a site (e.g., via any of the previously mentioned physical objectives like mass removal or concentration reduction), (2) to interrupt the exposure pathway (e.g., by constructing containment technologies, or by reducing or eliminating access to the site), or (3) to remove/alter the receptor (e.g., relocation of populations). Thus, the metric of success of the overall objective of exposure reduction may be a physical, measurable property, such as reduction of the concentration of a contaminant at the point of contact with the receptor, or it may be very qualitative in nature, such as an evaluation of the long-term success of institutional controls imposed on a site.

Knowing which of the physical functional objectives is most appropriate for achieving exposure reduction at a site is not a trivial undertaking, as illustrated in Box 4-2. The complex interconnections between mass removal, concentration

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

BOX 4-2
Evaluation of Physical Objectives for Achieving Exposure Reduction: The Role of Environmental Setting

This scenario is intended to highlight the importance of site characterization and site-specific conceptual modeling in the selection of appropriate physical objectives. Consider an aquifer contaminated by a mixture of spent chlorinated solvents (released as a DNAPL). The contaminant has penetrated deep in the saturated zone of an unconfined aquifer, used downstream for potable water. Within the source zone, the contaminant is distributed as a separate phase liquid in pools and ganglia. The aquifer formation is composed of alternating sandy and silty layers of contrasting hydraulic conductivity. Core and drivepoint aqueous phase samples within the contaminated zone reveal pockets of extremely high concentrations of chlorinated solvents (100–1,000 ppm). Suppose that the absolute objective in this scenario is to reduce risk to human health, and that the most important exposure pathway is through water consumption from the supply well.

Two physical functional objectives for remediation are being considered at this site: DNAPL mass removal from the source zone and aqueous concentration reduction within the source zone. Selection of the functional objective is a complex task and, in this scenario, is dependent on the chemical and hydrogeologic setting. Indeed, one, both, or neither of these objectives may be linked to the desired absolute objective of risk reduction.

For example, if the spilled chlorinated solvent had been previously used in dry cleaning, it is likely that it would contain additives that would lower its interfacial tension. Under reduced interfacial tension conditions, the spilled DNAPL would likely penetrate and become entrapped within the finer silty layers in the formation. Under these conditions, a small percentage of mass removal (removal from the higher-permeability zones) may achieve substantial reductions in mass fluxes to the receptor well and may reduce health risks (by reducing concentrations at the well). This mass removal, however, is unlikely to lower maximum local source zone aqueous or solid phase concentrations substantially. Alternatively, since the DNAPL is present primarily in low conductivity zones, reduction of aqueous concentrations within the source zone (particularly within these finer-textured materials) will be extremely difficult and would be a poor indicator of downstream concentrations.

In contrast, if the spilled solvent is reagent-grade (few impurities), it will likely remain pooled within the higher-permeability zones of the formation. Under these conditions, substantial (high percentage) mass removal will likely be necessary to achieve downstream concentration and risk reductions, since most flow through the source zone will be exposed to the DNAPL. In this scenario, however, concentration reductions within the source zone would be a better indicator of risk reduction than would DNAPL mass removal.

Another scenario can be envisioned in which the contaminant source is a solvent that has been used in degreasing operations. In this situation, co-contamination of the solvent with oils and aromatic hydrocarbons is likely. Such co-contaminants can serve as substrates for microbial transformation of the solvents. Microbial transformation may be exerting the primary control on downstream concentrations. In this situation, mass removal or concentration reductions within the source zone may have no discernable influence on receptor well concentrations.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

reduction, and mass flux reduction have been previously described. Box 4-2 demonstrates that the appropriateness of these various physical functional objectives is influenced by the hydrogeologic setting and the properties of the contaminant.

Reducing Chemical Toxicity

A change in toxicity was previously discussed as a physical objective of cleanup, relevant to those cases where entrapped DNAPL in the source zone may exist as a mixture of many compounds. Reducing the concentration of those DNAPL constituents of higher toxicity should result in reduced overall toxicity of the downstream contaminant plume, which directly supports the higher-order functional objective of reducing risk. Because the degree and type of toxicity of a contaminant is an inherent property of the interaction of the contaminant with a particular biological system, to affect a change in toxicity, the DNAPL component must be physically altered. While for inorganic contaminants this can be achieved in some cases by changing a contaminant’s chemical form to be less toxic, or by making the contaminant less bioavailable, for DNAPLs the issue is one of changing the proportions of toxic chemicals in a complex mixture. It is worth noting that some transformations (both natural and human-induced) can result in the production of more toxic components, such as when TCE is reductively dechlorinated to produce vinyl chloride.

Financial Objectives

Cost minimization is typically one of the absolute objectives of any cleanup decision. That is, most stakeholders agree that, all else being equal, the lower-cost option should be selected, thereby freeing up funds for other beneficial uses. Typically, therefore, the challenges arise not in stating the objective, but rather in selecting the metrics and estimating the costs of the alternatives.

Cost provides a good example of how a stated objective can be measured by many different metrics, and how use of the same word, that is, “cost,” can mask significant differences in stakeholder values. Examples of the different types of metrics routinely used to evaluate cost include annual cost, capital cost, life-cycle cost, cost to the community, cost to the state, project cost, and cost to the federal government. For example, annual cost sometimes may play a large role in decision making; if the annual cost of implementing a given technology is sufficiently large, it may be difficult to fund, and that technology may be abandoned in favor of one with a flatter cost profile. Representatives of local government must be concerned with impacts on the local economy, such as boom–bust cycles and consequent strains on community resources, impacts on property values, and long-term vitality of the community. Stakeholders may also have different perspectives regarding the appropriate discount factor to use in a present value calculation—an issue that may become quite important when the analysis involves

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

long time periods. Some cleanup alternatives may include transferring responsibilities from one organization to another (e.g., from the federal government to state government for long-term monitoring of a site, or from governments to citizen watchdog groups) (NRC, 2000a). In such cases the perspective from which costs are measured may influence the cost metric and resulting cost estimate. Similarly, government cleanup decisions may produce real economic impacts on local and regional economies by affecting local and regional labor markets, property values, community emergency preparedness costs and insurance premiums, and economic development (NRC, 1996). Decisions on whether to accelerate or delay closure of a site may result in boom–bust cycles for the affected communities. In all of these cases, the perspective may influence the choice of metric and ultimately the cost estimate obtained.

Although a variety of different cost metrics are in use, for government decision making the life cycle cost metric is recommended. Life cycle cost analysis represents an attempt to create a comprehensive accounting of the full range of direct and indirect costs and benefits resulting from a course of action over the entire period of time affected by the action. Thus, life cycle cost typically includes all costs associated with an alternative from start-up through long-term stewardship, and it avoids the problems of suboptimization presented by other cost metrics. Even when life cycle cost is defined as the metric to be used, divergent cost estimates may be obtained due to differences in assumptions regarding the scope and boundaries of the analysis and in projections regarding the future of technology, regulations, human and institutional behavior, and other factors, as discussed below.

Overview of Life Cycle Cost Analysis

Key to life cycle cost analysis is a full cost inventory that includes all direct and indirect costs ranging from project start-up costs (e.g., design, studies to prove a technology or obtain permits), capital and operating costs, through decommissioning, site closure, and long-term stewardship costs. A life cycle cost analysis requires careful consideration of both the scope of the analysis and the time horizon of analysis, in order to ensure that the full long-term costs and liabilities are factored into decision making. For example, costs that may be borne by other entities (e.g., waste management-related costs, or future surveillance and maintenance costs) should be considered in addition to direct project costs. The time horizon of the analysis should be long enough to include all of the impacts of an alternative. Future and long-term costs such as those needed for continued monitoring, reporting, maintenance, other regulatory compliance-related matters, replacement or corrective maintenance of caps, and other infrastructure should be captured in the analysis. If project time span exceeds the design life of support facilities or other items important to the project’s success, the cost of replacing these facilities must be considered. Frequently, a life cycle

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

analysis will reveal that the approach with the lowest initial cost is not the low-cost approach from a life cycle perspective, due, for example, to high long-term operating costs or the need for future replacement of a remedy.

In addition, so-called “hidden costs” should be included in a life cycle cost estimate. Hidden costs are costs not charged to the project actually responsible for incurring them; instead, these costs are charged to indirect or overhead accounts or to other entities. These issues arise frequently in federal facility cleanup decisions because costs associated with a given cleanup may be budgeted for in many separate government accounts. Examples of project-related costs that may not be fully charged to a project include utilities, permitting and regulatory oversight, environmental monitoring, security, long-term surveillance and maintenance, and the full cost of waste disposal. Long-term liability is another form of hidden cost that is sometimes neglected or underestimated; neglecting such a liability may result in a bias in favor of perpetual care alternatives. Finally, economic benefits of an alternative must also be addressed, as, for example, when cleanup leads to beneficial reuse of a building and/or land.

Numerous checklists have been developed to help identify cost elements to aid in producing a full cost accounting (see, for example, NRC, 1997; EPA 1995, 2000; Department of the Army, 2002). Table 4-1 provides an example of an expanded cost inventory that may be appropriate to a federal facility cleanup. A life cycle cost analysis would include all labor, equipment, and material costs associated with the cost elements in the table. In practice, analysts typically use a

TABLE 4-1 Example Cost Elements in a Life Cycle Cost Analysis

WBS

Element Name

1.0

Research, Development, Test, and Evaluation

1.01

Design and Engineering

1.02

Prototype

1.03

Project Management

1.04

System Test and Evaluation

1.05

Training

1.06

Data

1.07

Equipment

1.08

Facilities

1.09

Other Research, Development, Test, and Evaluation

2.0

Preparation/Mobilization

2.01

Planning/Engineering

2.02

Site Preparation

2.03

Regulatory Compliance/Permitting

2.04

Mobilization

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

WBS

Element Name

3.0

Capital

3.01

Facilities (e.g., buildings and structures such as onsite labs, health and safety offices, monitoring facilities)

3.02

Equipment (e.g., boiler for steam production, vapor extraction equipment, condenser equipment, pumps, gas–liquid separators, water and gas treatment systems, off-gas treatment equipment, tanks, pumps, blowers, aboveground drainage, containment structures, air or water monitoring equipment)

3.03

Engineering/Manufacturing/Tooling/Quality Control

3.04

Project Management

3.05

System Test and Evaluation

3.06

Other Construction and Installation (e.g., well installation, barrier wall construction)

3.07

Training

3.08

Data

3.09

Start-up

4.0

Operation and Maintenance

4.01

Sampling and Analysis

4.02

Monitoring/Regulatory Compliance

4.03

Materials/Chemicals/Consumables

4.04

Operation

4.05

Water/Gas Treatment

4.06

Equipment Repair and Maintenance

4.07

System Engineering/Project Management/Quality Assurance

4.08

Safety and Health

4.09

Training

4.10

Utilities (electricity, natural gas, water, other utilities)

4.11

Transportation

4.12

Waste Management/Disposal

5.0

Site Restoration

5.01

Demobilization

5.02

Capping

5.03

Decommissioning/Closure

5.04

Restoration

6.0

Long-Term Management

6.01

Institutional Controls

6.02

Sampling/Monitoring

6.03

Remedy Failure/Repair/Replacement

6.04

Natural Resource Damage Liability

6.05

Other Long-Term Liability

 

SOURCE: Adapted from EPA (2000) and Department of the Army (2002).

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

graded approach to life cycle cost analysis, performing an analysis with a level of detail commensurate with the decision to be made and the level of information available to support the analysis.

Consideration of Time and Associated Uncertainty

Remedial action projects typically involve construction costs that are expended at the beginning of a project and costs in subsequent years that are required to implement and maintain the remedy after the initial construction period. Present value analysis is a method used to compare alternatives that produce cash flows in different time periods, by transforming the future stream of benefits and costs to a single number, called the present value. The present-value method is based on the concept that a dollar today is worth more than a dollar in the future because, if invested in an alternative use today, the dollar could earn a return.

A present-value analysis of a remedial alternative involves three key steps: (1) define the scope and period of the analysis, (2) estimate the costs and benefits occurring in each year, and (3) select a discount rate to use in calculating the present value of future benefits and costs.6 The larger the discount rate, the lower is the present value of future cash flows. Discounted values of even large costs incurred far in the future tend to be small. For example, for a 200-year project with a constant annual cost of $500,000 at a 3.2 percent discount rate, 96 percent of the present value cost is incurred in the first 100 years, 79 percent in the first 50 years, and 61 percent in the first 30 years.

Decisions on discount rates can play an important role in remedial action decision making, particularly in determining what remedy to choose to meet the cost objective—for example, contaminant mass removal vs. contaminant isolation or long-term stewardship measures. Such tradeoffs regarding the appropriate discount rate come to the fore when considering sites, like certain DNAPL sites, that would be extremely costly to remediate to a level that would allow unrestricted access, but which would otherwise require indefinite government stewardship. Discount rates may also play a role in decisions related to the time required to close a site, because cleanup costs incurred in the future have a lower present value than the same costs incurred today. In general, differences in discount rates can lead to substantially different conclusions about which of two alternatives is the most cost-effective.

6  

For government decision making, the discount rate is the cost of borrowing, that is, the interest rate on Treasury notes and bonds. Office of Management and Budget Circular A-94 provides guidance on discount rates to be used in the analysis of federal projects. For 2003, and for programs of longer than 30-year duration, Appendix C of Circular A-94 reported a real discount rate of 3.2 percent.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

To illustrate this, Table 4-2 compares the life cycle cost of five remedial alternatives that have different initial capital costs, annual operating and maintenance (O&M) costs, and project durations. The life cycle cost is calculated using real discount rates of 0 percent, 3.2 percent (typical of government projects), 7 percent, and 12 percent (the latter two are typical of real discount rates used in the private sector to evaluate alternative investment options). Alternative E has the highest cost on an undiscounted basis but the lowest present value for discount rates of 3.2 percent, 7 percent, and 12 percent. This is because much of its cost occurs in the future, and the present value of these future costs is small. Setting aside Alternative E, Alternative D has the lowest present value at a 12 percent discount rate, but Alternative A has the lowest present value at a 3.2 percent discount rate. The undiscounted cost of Alternative B is less than that of Alternative C, but its present value at discount rates of 3.2 percent, 7 percent, and 12 percent is higher than that of Alternative C at these rates due to B’s large upfront capital cost. As these examples illustrate, the relative economic benefits of competing alternatives may depend on the choice of discount rate. Low discount rates would tend to make source depletion options appear more attractive, whereas high discount rates would tend to make containment options more attractive. Differences in public sector and private sector discount rates and other financial considerations may lead to different decisions being made at private sector sites and government sites.

Life cycle cost estimation over long timeframes is complex not only due to differences of opinion on the choice of discount rate, but also, and perhaps more important, due to the large uncertainty in projections of future costs given the uncertainties surrounding the site conceptual model, future technology, regulatory policies, societal norms, land use, population density, etc. Major sources of uncertainty in cost estimates relate to the site characterization model and the effectiveness of a selected technology at the specific site. For example, the nature

TABLE 4-2 Effect of Discount Rate on Life Cycle Cost Calculation

Remedial Alternative

Initial Capital Cost ($1000)

Annual O&M Cost ($1000)

Project Duration (Years)

Life Cycle Cost ($1000)

Real Discount Rate

0%

3.2%

7%

12%

A

$3,650

$583

15

$12,400

$10,500

$8,960

$7,620

B

$10,800

$548

30

$27,200

$21,300

$17,600

$15,200

C

$2,850

$696

50

$37,700

$20,100

$12,500

$8,630

D

$5,500

$230

80

$23,900

$12,100

$8,770

$7,420

E

$2,000

$200

220

$46,000

$8,240

$4,860

$3,670

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

and extent of contamination may prove to be greater than anticipated, or the technology may lack sufficient performance history for reliable cost estimation. Furthermore, technology performance may be sensitive to site-specific geologic and contaminant conditions, making cost extrapolations between sites difficult.

There are several instances of wastes that were disposed of decades ago that are now being remediated due to changes in regulatory standards. Cost estimates performed decades ago would not have foreseen the high remediation costs being incurred today. Conversely, development of new technology may result in unforeseen cost reductions. Estimating costs associated with long-term institutional controls adds additional uncertainty because it involves (1) predicting future costs associated with management of the site, (2) predicting potential liabilities (e.g., risk of failure of containment strategies and costs of remedy), (3) making projections regarding the future ability of government or other entities to maintain control, (4) evaluating the ability to maintain both technology and records over long time periods, and (5) predicting the potential costs in the event of institutional control failure at some time in the future.

Cleanup cost estimates are also highly dependent upon decisions regarding the cleanup schedule and the future use of a site. For example, very different cost estimates may result depending on whether the site is to be cleaned up to a level to permit unrestricted residential use or industrial use or whether the site is to be maintained in perpetual government stewardship as, for example, a wildlife preserve. The schedule of cleanup, e.g., the date on which ownership of the site is to be transferred, may also have a substantial effect on the remedial design and on work plans and therefore on cost estimates. Thus, if there is a specific date on which ownership of the site is to be transferred, high annual costs in certain years may be acceptable in order to meet the deadline. Conversely, budgetary pressures and/or a desire to avoid boom–bust cycles and minimize disruption to the local economy may lead to preference for a level funding profile. Either case may have a substantial effect on the remedial design, the work plans, and, consequently, the life cycle cost estimates.

For all these reasons, uncertainty analysis is a critical element of cost estimation, as it is for the characterization of risk (see Box 4-4). Various analytical techniques exist to characterize the uncertainties surrounding cost estimates.7 Probabilistic techniques such as Monte Carlo analysis (EPA, 1997a) may be used to gain a better understanding of the likely range of costs and their probability of occurrence. When a cost analysis is conducted using probabilistic techniques, parameter distributions that represent the uncertainty inherent in each of the parameters are used as inputs to the cost calculations rather than point estimates. The simulation output is the range of possible costs and the probabilities that they

7  

See, for example, NRC (1996), which discusses uncertainty in risk estimation. Much of this discussion applies to cost estimation as well.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

will occur, which provides decision makers with a much more complete picture. These tools are used to answer questions such as “What are the chances of this project finishing under budget?” or “What is the probability that the cost of remediation will exceed X amount?” Box 4-3 provides an example of the use of Monte Carlo simulation for the evaluation of remedial alternatives at a hypothetical site.

Schedule Objectives

Schedule objectives—for example, time to complete cleanup—may vary substantially among stakeholders and often play a significant role in decision making as they relate directly to stakeholders’ visions and objectives for the future of a site. For some stakeholders, particularly the military, the objective may be to finish cleanup and transfer the site within a specified timeframe as mandated by base realignment and closure (BRAC) requirements. For other stakeholders, the objective may be to avoid boom–bust cycles and minimize disruption to the local economy, which may lead to a desire to spread remediation out over time and maintain a more level funding profile.

Schedule objectives are complicated by stakeholder values relative to future land uses and future site ownership. For example, one stakeholder may value accelerating cleanup of a site in order to transfer ownership and reuse the site for commercial purposes, while another stakeholder may value maintaining the site in long-term government stewardship as, for example, a wildlife preserve. Schedule objectives may be absolute objectives or functional objectives. For example, accelerating site cleanup is a functional objective when it is used as a means to achieve the absolute objective of reduction of risk to human health and the environment.

Given the numerous and disparate issues related to schedule objectives, it is not surprising that many different metrics have been devised to measure progress. For example, the volume of material removed in a given year (related to a “get on with it” or “get started” objective), the planned year of completion, or the reduction of the amount of contamination present at a site may all be relevant metrics. Indeed, the military uses several temporal milestones as measures of success in the cleanup of DoD facilities. These include the signing of records of decision (RODs) for particular sites, the placement of the remedy (RIP) at sites, the designation of sites as being “response complete,” and the closeout of sites.

Other Objectives

As alluded to under the discussions of financial and schedule objectives above, stakeholders often have a variety of socioeconomic, institutional, and programmatic objectives ranging from maintaining corporate reputation and goodwill to sustained employment or ensuring that cleanup activities are designed

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

BOX 4-3
Example of a Life Cycle Cost Analysis for Evaluation of Remedial Alternatives

Based on a hypothetical example developed by

Kyle A. Gorder of Hill Air Force Base

This hypothetical example is based on the economic model being used at Hill AFB. The problem is defined as follows: a plume of dissolved-phase volatile organic compound (VOC) contamination underlies a residential area. The area of groundwater contamination is 50 acres (20 hectares). The groundwater table is generally located about 10 feet (3 m) below ground surface, and the average thickness of contamination is 50 feet (15 m). Two scenarios (out of many possibilities) were evaluated for illustrative purposes:

Scenario 1. Based on site investigations and monitoring indicating that the plume is stable, the management strategy for the site is monitored natural attenuation.

Scenario 2. Additional remedial action is taken at the site that results in reduction in the areal extent of groundwater contamination.

Scenario 1

Liabilities (Table 4-3) considered for this site include the cost of long-term monitoring, the possibility of finding indoor air contamination in residences and the consequent costs associated with mitigation system operations, maintenance, and monitoring (OMM), the potential for Natural Resource Damage (NRD) claims, and the potential cost of having to obtain remediation easements.

TABLE 4-3 Parameters Used in Analysis of Scenario 1

Description

Value

Symbol

Annual Long-term monitoring cost ($)

50,000

CLTM

Years of long-term monitoring

30

YLTM

Average number of homes/acre

2.25

 

Probability of indoor air contamination (%)

1 to 20

 

Number of homes with indoor air contamination

1 to 23

N

Mitigation system installation and startup cost ($/home)

10,500

CMS

Annual mitigation system OMM cost ($/home)

1,500

OCMS

Years of mitigation system OMM

30

YMS

Value of groundwater ($/acre-ft)

919

 

NRD liability (value of groundwater x contaminated volume) ($)

689,250

CGW

NRD settlement probability (%)

10

PNRD

NRD settlement year

5 to 30

YNRD

Value of residential land ($/acre)

440,000

 

Easement liability (land value x area of contamination) ($)

22,000,000

CL

Easement settlement probability (%)

0 to 10

PL

Easement settlement year

3 to 30

YL

Discount rate (%)

4

I

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

Values given by ranges in Table 4-3 are variables in the Monte Carlo simulation. Distributions for some of these variables are presented below.

Number of Homes with Indoor Air Contamination

Given the area of contamination and the number of homes per acre, indoor air contamination could affect approximately 113 residences. Assuming that between 1 percent and 20 percent of the homes will be affected and that 10 percent is the most likely number, the number of homes affected was assumed to follow the triangular distribution shown in Figure 4-5. The minimum number of homes affected is 1, the maximum is 23, and the most likely is 11 homes.

Natural Resource Damage

The NRD settlement year was assumed to follow the uniform distribution shown in Figure 4-6. This distribution indicates that the NRD settlement could occur at any time between year 5 and year 30. All years in this range have equal probability of selection during the Monte Carlo simulation.

Remediation Easements

Two variables related to remediation easements (settlement probability and settlement year) were chosen for inclusion in the Monte Carlo simulation. The probability of an easement settlement was assumed to follow the triangular distri-

FIGURE 4-5 Distribution of number of homes potentially requiring indoor air remediation. Vertical axis is relative probability, such that the area under the curve = 1.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

bution shown in Figure 4-7. This distribution sets the minimum probability of an easement settlement to zero percent and the maximum probability to 10 percent, with the most likely probability of 5 percent. The easement settlement year was

FIGURE 4-6 Distribution of NRD settlement year (min = 5 yrs, max = 30 yrs). Vertical axis is relative probability.

FIGURE 4-7 Distribution of easement settlement probability. Vertical axis is relative probability.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

assumed to follow a uniform distribution, based on the assumption that an easement settlement could occur at any time between years 3 and 30. All years in this range have equal probability of selection during the Monte Carlo simulation.

Scenario 2

Scenario 2 examines the potential liability reduction that could be achieved with the implementation of an aggressive source remediation strategy designed to reduce plume size. For this example, it is assumed that plume reduction from 50 acres (20 hectares) to 12.5 acres (5 hectares) could be achieved over a 30-year timeframe and that this reduction would occur linearly. Note that any plume area reduction curve could be used in the analysis and that this should be based on some understanding of the site conceptual model. Ideally, this curve would be based on detailed analyses of site conditions and/or numerical modeling.

The effect of a smaller plume footprint is incorporated into the analysis in three ways: (1) The average number of years of OMM on indoor air vapor mitigation systems is decreased and follows the distribution shown in Figure 4-8, (2) the volume of contaminated groundwater is reduced to account for the reduction in plume area (volume as of the year of NRD settlement is used), and (3) the land area used to determine the easement liability is reduced, again according to the year of easement settlement.

FIGURE 4-8 Average number of years of OMM on indoor air vapor mitigation systems (mean = 20 yrs, standard deviation = 4 yrs, range is from 5 to 35 yrs). Vertical axis is relative probability.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

Calculation of Total Liability

The total liability for each scenario is calculated as follows:

The present value of the liability is calculated as:

where

pa(cost, time, I) = the present value of an annual cost over time at discount rate “I” pf(cost, time, I) = the present value of a future cost at year “time” and discount rate “I.” All other variables are defined in Table 4-3.

Results

Table 4-4 presents the results of the liability calculations for the two scenarios. Note that these results do not include the cost (liability) associated with implementing the aggressive source remediation under Scenario 2. The cost ranges presented in the table represent 90 percent confidence intervals estimated from the Monte Carlo analysis.

TABLE 4-4 Estimated Liabilities for the Scenarios

Cost Component

Present Value Cost ($1000)

Scenario 1

Scenario 2

Cost Range

Mean

Cost Range

Mean

Operations, Maintenance, and Monitoring

865

865

 

 

Indoor Air Contamination Mitigation

144 to 688

407

117 to 588

342

Natural Resource Damage

22 to 54

36

6 to 46

23

Remediation Easement

146 to 1,210

597

68 to 1,010

395

Combined Liability

1,370 to 2,570

1,900

312 to 1,420

760

The results shown in Table 4-4 are interpreted as follows. (Note that mean values are used to simplify this discussion. In practice, the entire distributions resulting from the Monte Carlo simulation for each scenario would be compared.) The mean combined liability present value for Scenario 1 is $1,900,000. The mean combined liability present value for Scenario 2 is $760,000. The difference in these two liabilities ($1,140,000) represents the breakeven point for an investment in aggressive source remediation. An investment in aggressive source remediation with a present value of less than or equal to the breakeven point would be considered a cost-effective investment.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

and conducted in a manner that is consistent with community values and long-term vision. Intergenerational equity and long-term land protection are also objectives voiced by some stakeholders. Cost and schedule objectives are sometimes functional objectives used as a means to accomplish some of these larger concerns, as, for example, when communities specify annual funding requests or completion schedules.

Another set of objectives includes those that may be established in order to meet regulatory commitments or avoid the imposition of sanctions or negative consequences. One example of such an objective is the prevention of any offsite contamination that may eventually result from improper handling or disposal of contaminants. Such offsite contamination may result in private or public nuisance lawsuits by owners of neighboring properties, whose use and enjoyment of their land may be adversely affected by the contaminants. It may also result in the imposition of enforcement sanctions by regulatory authorities.

Communication with Stakeholders Regarding Objectives

The absolute objectives for site remediation can differ significantly, partly because they reflect value judgments made by many different stakeholders. In addition, they depend upon the physical and social environment in which remediation takes place. The first five physical objectives described earlier are singled out in Chapters 5 and 6 for the purposes of evaluating source remediation technologies and developing a protocol for source remediation. These physical objectives can and usually do serve multiple absolute objectives (note that some appear in both the risk- and time-related discussions). Whether these physical objectives can be achieved depends heavily on the technology used and the hydrogeologic setting—a major theme of this report.

In general, the Army, DoD, and other institutions charged with hazardous waste remediation should be cognizant of selecting remedial actions, in particular source remediation, that take into account the absolute and functional objectives held by stakeholders. Decisions as to how to manage historical releases occur within a broad social context, involving multiple parties that may each have diverse and dynamic sets of drivers. A hypothetical example of a set of parties and their primary drivers is presented in Table 4-5, but the actual list of stakeholders is generally much larger. In addition to those stakeholders included in the table, a host of other players may be seeking to satisfy various objectives via remedial decisions. These include persons who are ostensibly agents of one or more of the listed stakeholders (consultants, vendors, researchers, etc.) but who have individual objectives that may profoundly influence their contribution to the decision-making process.

Most of the parties involved in these decisions are working against the background of an organizational policy that sets out criteria for decision making. Decisions are not made on the basis of individual preferences except where an

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

individual is the sole responsible party for a site—typically only true at the very smallest of sites. There may be conflicts between the desires of different stakeholder groups (as Table 4-5 suggests). Indeed, the committee is aware of many cases where programmatic objectives may be in conflict with the technical realities of what can be accomplished at sites. Nonetheless, although decisions depend upon both technical and nontechnical factors, once a decision has been made, the focus must be on the technical goals to determine if remediation is successful.

This report does not revisit the discussion of how to build successful programs for stakeholder involvement at a site, since the difficulties and opportunities involved are discussed in previous NRC reports (1999a,b, 2000a), among other references. Nonetheless, it is essential to the success of source remediation for the RPM to effectively capture the results of stakeholder processes. By noting for each stakeholder whether particular remedial objectives are considered absolute or functional, the RPM will be able to make more informed judgments about the evaluation of source remediation. Thus, if the regulatory authority has a preference for “reducing mass or toxicity,” technologies that can be demonstrated to achieve this (and convincing metrics that indicate whether or not they have achieved this) should receive additional consideration. In contrast, if the local community views contaminant migration as “chemical trespass,” reduction in contaminant mass may only be relevant insofar as it effectively (and, often,

TABLE 4-5 Hypothetical Example of Stakeholders and Potential Drivers for Determining Objectives

Stakeholder

Potential Drivers

Responsible Party

Corporate decision policy and protocol. Protect human health and the environment, manage financial impacts to mission or business, manage reputation

Project Manager (typically employees of the Responsible Party)

Corporate decision policy and protocol. Meet schedule and budget commitments, maintain positive relationships with all parties

Federal Regulator

Compliance with regulations, decision mechanisms driven by legislation, meet public expectations, meet schedule commitments, manage reputation

State Regulator

Compliance with regulations, decision mechanisms driven by legislation meet public expectations, minimize economic liabilities that may pass to state during long-term operations and maintenance, manage reputation

Public

Protection of health, preservation of property values, punishment of responsible parties for damages, jobs

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

quickly) reduces concentrations at the fence line and beyond. In this case, mass removal technologies may fare poorly in comparison with a comparable investment in plume capture.

By actually drafting a chain of objectives and metrics and presenting it to stakeholders, the RPM’s ability to satisfy the range of absolute objectives in the community can be clarified, and the areas where policy-level trade-offs are needed can be separated from those where a more technical evaluation of alternatives is possible.

EXISTING FRAMEWORKS, THEIR OBJECTIVES, AND ASSOCIATED METRICS

At all hazardous waste sites, cleanup takes place within one or more decision frameworks that provide structure to the activities that occur, from the initial discovery of contamination to the eventual closure of the site. Two broad classes of frameworks are discussed here because they impact cleanup at a high percentage of sites, and because they have defined objectives with associated metrics of success and thus may represent a significant influence on stakeholder formulation of objectives. The major categories of existing frameworks can be broadly classified as regulatory, which defines the legal goals and objectives of the remedial action, and risk assessment, which defines the existing threat to human health and the environment and the level of remediation required to reduce this risk to acceptable levels.

An understanding of these existing frameworks for site remediation is essential to developing an unambiguous set of absolute objectives for site remediation and corresponding functional objectives that define whether those absolute objectives have been obtained. That is, just as different stakeholders may view the same objective in different ways (e.g., as absolute rather than functional, or as serving different absolute objectives), the accustomed cleanup framework(s) of each stakeholder will also shape their perceptions of alternative remedial objectives for a site. This may in turn influence the ability of the stakeholders to achieve a working consensus on remedial objectives.

Knowledge of these conventional/historical frameworks can also aid in the analysis of the status of a potential objective for a stakeholder. An objective in one of the frameworks may be shared by another framework but serve different purposes in each. For example, the objective of mass removal can be found in multiple regulatory frameworks (and thus has been interpreted by some as an absolute objective) as well as in risk assessment frameworks (for the purpose of reducing exposure potential). Indeed, in a risk assessment framework, mass removal is a purely functional goal, and in fact is generally separated from the absolute goal of reducing or eliminating risk by several inferential steps and corresponding functional goals.

The historical frameworks described below tend to inherently weigh certain

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

absolute objectives, and even particular functional objectives, more heavily. This can lead to conflicts between stakeholders or to balancing problems for the RPM, who must apply several frameworks simultaneously. Therefore, it is important for site-specific objectives to be defined with consideration of all relevant frameworks early in the process of remedy evaluation. Early identification of objectives will allow the information that is needed to support the remediation decision to be made available through site investigation, the potential conflicts between the objectives of different frameworks to be addressed, and the potential for establishing mutually agreed upon cleanup objectives to be maximized.

Regulatory Framework

The cleanup of contaminants at U.S. Army installations takes place within a highly structured, complex regulatory environment. At its core is the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), a statute amended in 1986 by the Superfund Amendments and Reauthorization Act (SARA) (P.L. 99-499) to bring all military facilities under the authority of the Superfund Program. SARA established the Defense Environmental Restoration Program (DERP)—managed by the Department of Defense—which includes an Installation Restoration Program (IRP) that conducts environmental cleanups at military bases. In the case of the most contaminated military facilities—that is, those listed on the National Priorities List (NPL)—the cleanups are directly regulated by officials of the U.S. Environmental Protection Agency (EPA) or their state counterparts.

The general procedures and standards to be followed with respect to contaminated facilities have been set forth by the EPA in its National Contingency Plan (NCP) (40 CFR 300 et seq.). These broadly applicable EPA regulations establish the basic framework that responsible parties such as the military follow to investigate, evaluate, and remediate hazardous substance problems at their facilities. Under the NCP, site managers conduct a remedial preliminary assessment (and, where appropriate, a site investigation) to determine whether a particular site should be given priority for long-term remedial response. The results of these evaluations are used to score the site under the EPA’s hazard ranking system (HRS) model. If the site scores above the HRS threshold, then the entire facility is placed on the NPL for possible remedial action.

Following that, site managers generally undertake a remedial investigation and feasibility study (RI/FS) to study the nature and extent of the contamination problem at the site and to develop alternative approaches for managing the site problem. In the course of preparing this feasibility study, RPMs along with the appropriate regulatory authority will establish a preliminary remediation objective for the site, and they will prepare a broad list of alternative ways in which the preliminary remediation objective at the site may be attained. This list is screened to eliminate clearly impractical alternatives. The remaining alternatives are

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

studied, compared, and evaluated against a set of nine criteria divided into three categories, described below.

NCP Threshold Criteria

The two threshold criteria in the National Contingency Plan are (1) to be protective of human health and the environment and (2) to comply with Applicable or Relevant and Appropriate Requirements (ARARs). In practice, the criterion “protective of human health” has usually if not always been embodied in quantitative risk assessment, specifically as described in the Risk Assessment Guidance for Superfund (RAGS). “Protective of human health” has been interpreted as having a calculated cancer risk between 10–6 and 10–4 or a Hazard Index < 1.0. As discussed earlier, meeting an absolute objective of risk reduction is frequently embodied in the more functional objective of preventing human exposure to site-related contaminants during the period that is subject to analysis. A purely administrative remedy (or perhaps a physical barrier) can conceptually meet this functional objective as well as the complete removal of contaminants. “Protection of the environment” is less clearly defined, and although the typical approach employed by EPA and responsible parties has also been risk assessment, the methods used are generally less quantitative and more variable, reflecting the greater complexity of the physical/biological system under consideration.

In contrast to the “protective” criterion, compliance with ARARs is more obviously concerned with absolute objectives, both in philosophy and in practice. This is perhaps most clearly reflected in the ARARs that are directly relevant to this committee’s charge. Drinking water Maximum Contaminant Levels (MCLs) and non-zero Maximum Contaminant Level Goals (MCLGs) are considered to be ARARs for groundwater remediation.8 This designation is independent of whether the particular groundwater is, in fact, currently used as a source of drinking water or is likely to be so used in the future, as long as it is capable of being used as a source of drinking water. Table 4-6 presents MCLs and MCLGs for the chlorinated solvents and drinking water equivalent levels (DWEL) and lifetime health advisory levels for the explosives of concern in this report. These values are commonly set as the objectives of source remediation.

Requiring groundwater to meet specific concentration targets independent of its uses is clearly not an attempt to protect human health, at least as far as toxic risk is concerned, since less stringent rules are set for actual public water sup-

8  

If MCLs or non-zero MCLGs are exceeded, action generally is warranted (EPA OSWER Directive # 9355.0-30, April 22, 1991). Where the cumulative carcinogenic site risk to an individual based on reasonable maximum exposure for both current and future land use is < 10–4, and the noncarcinogenic hazard quotient is < 1.0, action generally is not warranted unless there are adverse environmental impacts.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

TABLE 4-6 Toxicological and Regulatory Benchmarks for Source Zone Chemicals

Chemical

Oral RfD (mg/kg/d)

MRL (mg/kg/d)

Carcinogen Class

10–6 Conc. (µg/L)

MCLG (µg/L)

MCL (µg/L)

Chlorinated Solvents

Tetrachloroethene (perchloroethylene, PCE)

0.01

0.02 Acute

2A /—

0

5

Trichloroethene (TCE)

0.2 Acute

2A /—

0

5

cis-1,2-Dichloroethene

—/D

70

70

1,1-Dichloroethene (1,1-DCE)

0.05

—/ C

1,1,1-Trichloroethane (TCA)

3 /D

200

200

1,2-Dichloroethane (DCA)

0.2 Int.

2B / B2

0.4

0

5

Tetrachloromethane (carbon tetrachloride)

0.0007

0.007 Int.

2B / B2

0.3

0

5

Trichloromethane (chloroform)

0.01

2B / B2

0

80a

Dichloromethane (methylene chloride)

0.06

2B / B2

5.0

0

5

Other Hydrocarbons

Naphthalene

0.02

2B / C

Benzo(a)pyrene

2A / B2

0.005

0

0.2

Aroclor 1254 (PCB mixture)

0.00002

(0.00002)

—/ (B2)

(0. 1)

(0)

(0.5)

Aroclor 1260 (PCB mixture)

(0.00002)

—/ (B2)

(0.1)

(0)

(0.5)

Explosives

TNT

0.0005

—/ C

1.0

20b

2b

2,4-DNT

0.002

c

0.05

100b

HMX

—/—

4.0

2,000b

400b

RDX

—/—

0.3

100b

2b

NOTES: Dashes mean there is no information on a particular source. Parentheses are used where the table discusses a specific chemical, but where the given value was specified for a broader chemical class.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

Oral RfD = Oral Reference Dose: an EPA estimate of a dose at which chronic exposure is not expected to cause adverse effects.

MRL = Minimum Risk Level: an ATSDR estimate of a dose at which chronic exposure is not expected to cause adverse effects. Int. and Acute refer to intermittent and acute exposure, which are sometimes specified rather than chronic exposure. Carcinogen Class: A ranking of the weight of evidence that the substance is carcinogenic in humans. The first is the IARC (1 = known, 2A = probable, 2B = possible) classification, while the second is EPA’s (A = known, B1 = probable – human evidence, B2 = probable – animal studies,C = possible, D = not classifiable as to carcinogenicity) (EPA, 1986). EPA’s system has undergone several changes over the years, and no consistent c lassification by EPA appears to be available.

10–6 Conc. = Concentration of the chemical in water that would result in an excess cancer risk value of one-in-a-million from ingesting 2 liters of water a day.

MCLG = Maximum Contaminant Level Goal

MCL = Maximum Contaminant Level

aThis is the MCL for total trihalomethanes, of which chloroform is one.

bLast two columns for the explosives refer to DWEL (µg/L) and Lifetime Health Advisory (µg/L), respectively.

cNone recommended, potential human carcinogen (Group B2)

SOURCE: EPA (2002).

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

plies. That is, under the Safe Drinking Water Act, a public water supply (i.e., a utility with more than 15 service connections, or serving more than 25 customers) is not prohibited from exceeding MCLs in the water it supplies. Rather, it is simply required to notify the state and its customers when they are exceeded (40 CFR 141.31, 141.32).

Although MCLs have the advantage of being quantifiable (i.e., the concentrations of contaminants in water can readily be accurately measured), MCLs have major limitations with respect to both selection of remediation technologies and evaluation of benefits of source zone remediation efforts. As noted in EPA (2003b), the use of MCLs may inhibit source zone remediation attempts. This is because in the most prevalent hydrogeologic settings, minute amounts of DNAPL, sorbed mass, and dissolved mass in stagnant zones will remain after source zone remediation, such that desorption and reverse diffusion from the source zone into the dissolved phase plume region may maintain concentrations in some locations above MCLs for some time. Thus, even if removal of DNAPL is quite complete, attainment of MCLs throughout the source zone can almost never be expected immediately after source zone remediation. As a result, where attainment of MCLs is the absolute objective, the general conclusion might be that no technology is capable of meeting the objective and thus source zone remediation may not be attempted.

Furthermore, depending on where they are measured, MCLs can constitute a confusing absolute objective. Consider a case in which 99.9 percent of the DNAPL is removed, and some wells in the source zone show concentrations below MCLs. Nonetheless, a sample taken from a well far downgradient of the remaining DNAPL may continue to be above MCLs for a considerable period, given the time required for the effects of the remedy to be felt away from the source. In fact, downgradient concentrations may initially be virtually unchanged even though both the mass flux from the site and the time required for the site to return to precontamination conditions have been dramatically reduced. The position of the monitoring well with respect to any remaining contamination is also a factor. In this case, a well that is directly downgradient of the remaining mass may not show a large change in concentration compared to a well located directly downgradient of a portion of the source that was effectively treated. Furthermore, contaminant concentrations in groundwater from downgradient wells are highly dependent upon specific well design (screen interval) and location, making it difficult to interpret the significance of measured concentrations. Thus, the relationship between monitoring well concentrations and source remediation is complex, which should be kept in mind when selecting MCLs as metrics.

NCP Balancing Criteria

The five balancing criteria in the NCP, designed to guide the selection of the most appropriate among several remedies that could meet the threshold criteria,

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

are (3) long term effectiveness and permanence, (4) reduction of mobility, toxicity, or volume, (5) short term effectiveness, (6) implementability, and (7) cost. These criteria, in the aggregate, address multiple types of objectives. Cost is likely an absolute objective and is clearly independent of either of the two “threshold” criteria. In contrast, effectiveness primarily addresses a judgment of functionality, presumably relative to the threshold criteria. Implementability appears to address both functional and procedural objectives.

NCP Modifying Criteria

The modifying criteria help to clarify cases where more than one alternative is judged suitable according to the threshold and balancing criteria. These are (8) state acceptance and (9) community acceptance. That EPA has placed them in the least valued of its categories clearly indicates that these objectives are not, from an agency point of view, absolute objectives.

Applying the above-described criteria, EPA attempts to select the remedial alternative that utilizes “permanent solutions and treatment” to the “maximum extent practicable” and that is “cost-effective,” in the sense that the costs it entails are in proportion to the treatment effectiveness. In making this decision, EPA has considerable discretion and flexibility. In consultation with state officials, EPA next issues, for public comment, a proposed plan that sets forth the agency’s recommended remedial alternative. After public review and comment, EPA will make a final remedy selection, which it will document in a formal ROD.

Once the ROD has been published, EPA (or one or more responsible parties, sometimes including the Department of Defense) goes about designing, constructing, and implementing the selected remedy. If at the close of that cleanup phase contamination left in place is at levels above those allowing for unrestricted land use, then long-term monitoring and sometimes institutional controls are required. Such monitoring and controls must be in place until the site no longer poses an unacceptable risk to the environment or human health. At some sites, in fact, they may be required indefinitely. The effectiveness of long-term monitoring and institutional controls over long periods is discussed elsewhere (EPA, 1998, 1999; NRC, 1999b, 2000a,b, 2003).

The RCRA Corrective Action Program

Although the Superfund regulatory system described above is the source of most of the regulatory requirements that affect the cleanup of military facilities, Superfund is supplemented in that respect by other governmental requirements. In 1984, Congress enacted a comprehensive set of amendments to the Resource Conservation and Recovery Act (RCRA). Among other things, those amendments made it clear that owners and operators of treatment, storage, and disposal

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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facilities (TSDFs) (including military facilities of this type) must investigate and, as necessary, clean up past as well as present releases of hazardous wastes from their properties (hence, RCRA Corrective Action Program).

This program is administered by EPA and by those states that have been authorized by EPA to administer their own state hazardous waste programs. Although RCRA does not expressly require source treatment, in the early 1990s, EPA proposed corrective action regulations that adopted essentially the CERCLA remedy selection factors for RCRA sites (these regulations have not yet been finalized). EPA’s One Cleanup Program initiative also reinforces its view that one set of rules should apply to all similar sites regardless of the statutory program. Under RCRA, state-administered hazardous waste programs are required to be “equivalent” to the federal hazardous waste program (Section 6926(b)). Nonetheless, such programs contain numerous details, and they include considerable variations in their corrective action components as well as in their other features. In view of this substantive diversity, this report does not attempt to summarize the corrective action requirements imposed by particular states.

Two “environmental indicators,” developed by EPA, suggest that the absolute objectives of RCRA are to eliminate the most immediate public health and environmental risks. The two indicators, “Current Human Exposures under Control” and “Migration of Contaminated Groundwater under Control,” measure whether people are currently being exposed to unacceptable levels of environmental contamination, and whether existing groundwater contamination is growing and/or affecting nearby surface waterbodies. The functional objectives that are frequently called for in site-specific agreements between owners and operators of TSDFs and regulatory authorities are typically defined in terms of concentrations of particular contaminants as measured at the boundaries of given units of real property.

Human Health and Environmental Risk Assessment Framework

CERCLA and the NCP define a regulatory process for characterizing the level of hazard presented by site contaminants and identifying the degree of cleanup required. This regulatory process has historically been focused on the metric of risk (NRC, 1999b). During the site investigation process, information is collected to identify the sources of contamination, the extent of contamination, and the environmental characteristics and conditions contributing to exposure and potential risk. This information is used to conduct human health and ecological risk assessments during the RI/FS, following the guidance provided by the EPA known as Risk Assessment Guidance for Superfund (RAGS) (EPA, 1989, 1991a). Other risk-based methodologies, including the ASTM Risk-Based Corrective Action approach (ASTM, 1998), which is similar to the RAGS process, may also be used.

Risk assessment applied to environmental cleanup of hazardous waste sites is the process of determining the level of risk posed by chemicals at the site to

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
×

(primarily local) human and ecological receptors. The risk assessment process integrates information on the physical conditions at the site, the nature and extent of contamination, the toxicological and physical–chemical characteristics of the contaminants, the current and future land use conditions, and the dose–response relationship between projected exposure levels and potential toxic effects (see Table 4-6 for toxicological data on DNAPL constituents and chemical explosives). The end result of human health and ecological risk assessment is a numerical value of potential additional risk to the hypothetical receptor from the contaminant source. The calculated risk values are compared to an acceptable target risk level or to a range of acceptable risk defined by the NCP or by state regulations. If the risk estimate is greater than the acceptable target risk level, target cleanup level objectives are identified for the site using the assumptions developed in the risk assessment related to potential levels of exposure.

The overall purpose of risk assessment is to address the absolute objective of protecting human health and the environment. The risk assessment will determine if site-specific risk is above acceptable limits and the extent to which site risk needs to be reduced to meet the absolute objective. The risk assessment will also provide information that will support development of functional objectives, such as identifying which chemicals and exposure pathways contribute most to elevated risk. It will also help define metrics of success related to remediation. For example, the risk assessment may determine that levels of chlorinated solvents in groundwater would result in unacceptable risk if the groundwater is used as a source of potable water. If the ability to reduce the groundwater contaminant concentrations were limited by a lack of available technologies, the metric of risk reduction might be met by supplying an alternate source of potable water to residents.

RAGS and the other risk-based methods commonly used to evaluate site risk and establish cleanup levels provide a standardized, systematic approach for estimating site risk. The standardized approach allows for relatively easy implementation of the methods at a large number of sites and allows sites to be prioritized for cleanup action. These methodologies and their strengths and weaknesses for different applications have been described and evaluated in other NRC reports (e.g., NRC, 1983, 1999b). Since all contaminated military facilities conduct their site investigations and cleanups under either RCRA or CERCLA (NRC, 2003), it is likely that at identified Army sites where source remediation is an option, a risk assessment has already been conducted or will be conducted in the future.

Distinctions between Human Health and Ecological Risk Assessment

The methods typically used in human health risk assessment are highly prescribed by RAGS and similar risk-based methodologies. RAGS requires that risk estimates for humans be protective of individuals and be based on the maximum exposure that is reasonably likely to occur. This risk estimate tends to be

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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conservative, that is, it is more likely to overestimate than underestimate true risk. Site-specific information concerning exposure can be used in calculating the risk estimate when the information is well documented. More typically, however, default exposure assumptions identified by the standardized risk-based methodologies and toxicity criteria developed by EPA based on laboratory animal testing data are used. Because of the uncertainly inherent in using animal data to predict toxicity in humans, the toxicity criteria recommended for use by EPA have incorporated modifying factors that result in far lower allowed chemical intake in humans. For practical reasons, the use of these standardized assumptions for exposure and toxicity in evaluating human health risk is encouraged by the regulators. The outcomes of this approach are relatively limited flexibility in accounting for site-specific conditions and risk estimates that represent higher-than-average exposure conditions. Box 4-4 further discusses the role of uncertainty in risk assessment calculations, and how uncertainty can be more quantitatively assessed in lieu of using the default assumptions discussed above.

The ecological risk assessment process is far less prescribed in the published risk-based methodologies than the human health risk evaluation process for several reasons. In the ecological risk assessment process, ecological risk does not exist unless receptors and habitat are currently present at a site or are likely to be present in the future. Highly developed industrial sites are less likely to sustain ecological receptors and habitat. Unlike human health risk assessment, where risk to only one species is evaluated, ecological risk assessment must consider all ecological receptors present or potentially present in all environmental media potentially impacted. This evaluation requires a site-specific survey to determine what types of receptors (plants, animals, invertebrates, etc.) are present in each medium (soil, surface water, sediment, etc.). Finally, ecological risk assessment evaluates risk to the population of each species present, being concerned with risk to individual members of the population only when the receptor is classified as a threatened or endangered species by state or federal regulations. The available risk-based methodologies present a general framework for ecological risk evaluation (EPA, 1991b, 1992c, 1994, 1997b), but the type of evaluation conducted for a specific site is typically negotiated with the regulatory agency having responsibility for the site.

Exposure Pathways at Army Facilities

Explosives and DNAPL contamination at Army facilities can represent very long-term sources of contamination for soil, groundwater, and surface water. If the explosives or DNAPL contamination is present in relatively shallow soil (4–6 meters below ground surface), direct contact with contaminated soil (ingestion, dermal contact) could occur through or as a result of excavation activities that might bring contaminated subsurface soil to the surface. Army facility occupants and offsite occupants may indirectly contact contaminants in shallow soil

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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BOX 4-4
Evaluation of Variability and Uncertainty in Risk Assessment

The inherent variability in exposure variables or population response and the lack of knowledge about specific parameters used in estimating risk can both affect the outcome of a risk assessment and the degree of confidence associated with the results. Evaluation of these sources of uncertainty is necessary to allow risk assessment results to be viewed in the appropriate context. “Variability” refers to the true heterogeneity or diversity that occurs within a population or a sample. Examples of factors that have associated variability include contaminant concentration in a medium (air, water, soil, etc.), differences in exposure frequencies or duration, or, in the case of ecological risk assessments, inter- and intraspecies variability in dose–response relationships. EPA risk assessment guidance (EPA, 1989) states that risk management decisions at Superfund sites will generally be based on an individual that has a reasonable maximum exposure (RME). The intent of the RME is to estimate a conservative exposure case (i.e., well above the average case) that is still within the range of possible exposures based on both quantitative information and professional judgment. In addition, EPA recommends conducting a central tendency exposure estimate (CTE), which is a measure of the mean or median exposure. The difference between the CTE and RME gives an initial impression of the degree of variability in exposure and risk between individuals in an exposed population (EPA, 2001a).

If a risk assessment has been conducted using a point estimate approach, a range of point estimates can be developed to represent variability in exposures. To calculate RME risk estimates using this approach, EPA has developed recommended default exposure values to use as inputs to the risk equations (EPA, 1992a, 1996a, 1997b, 2001b). A CTE risk estimate is calculated using central estimates for each of the exposure variables, which are available from EPA guidance and other sources. For both RME and CTE risk estimates, site-specific data are used if they are available. The point estimate approach to risk assessment does not determine where the CTE or RME risk estimates lie within the risk distribution, and the likelihood that an estimated risk will be sustained cannot be determined. This leads to uncertainty as to what level of remedial action is justified or necessary.

If a risk assessment has been conducted using probabilistic techniques, parameter distributions are used as inputs to the risk equations rather than single values. These distributions characterize the interindividual variability inherent in each of the exposure assumptions, and they are used with mathematical processes such as Monte Carlo simulation to estimate risk. The simulation output is a distribution of risks that would occur in the population, which provides a better understanding of where the CTE and RME risks occur in the distribution. A technique known as one-dimensional Monte Carlo analysis can be used to estimate the probability of occurrence associated with a particular risk level of concern (e.g., cancer risk of 10–6) (EPA, 2001a).

Uncertainty is also inherent in every human health and ecological risk assessment because one’s knowledge of actual exposure conditions and receptor

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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response to chemical exposure is imprecise. The degree of uncertainty depends to a large extent on the amount and adequacy of the available facility-specific data. Typically, the most significant areas of uncertainty associated with receptor exposure include exposure pathway identification, exposure assumptions, assumptions of steady-state conditions, environmental chemical characterization, and modeling procedures. The toxicity values used in risk assessment must also be viewed in light of uncertainties and gaps in toxicological data. Information concerning the effect of a chemical on humans is often limited. Toxicity data are often based on data derived from high-dose studies using a specially bred homogeneous animal population. These data are extrapolated for use in predicting risk to a heterogeneous human population that is more likely to experience a low-level, long-term exposure (EPA, 2001a).

Ideally, the uncertainty associated with each parameter used in the risk assessment would be carried through the evaluation process in order to characterize the uncertainty associated with the final risk estimates. However, since actual exposure conditions cannot be fully described, a variety of modeling strategies are available to evaluate uncertainty. If a risk assessment has been conducted using a point estimate approach, parameter uncertainty is usually addressed in a qualitative manner for most variables (EPA, 2001a). For example, the uncertainty section of a point estimate risk assessment document might note that soil sampling conducted may not be representative of overall contaminant concentrations and, as a result, the risk estimate may over- or underestimate actual risk. Uncertainty in the environmental concentration term is addressed quantitatively to a limited extent in a point estimate approach by using the 95 percent upper confidence limit (UCL) for the arithmetic mean concentration in the risk estimate, which accounts for uncertainty associated with environmental sampling and site characterization (EPA, 1992b, 1997c, 2001a). The 95 percent UCL is combined in the same risk calculation with various central tendency and high-end point estimates for other exposure factors.

If a risk estimate is conducted using probabilistic methods, the uncertainty associated with the best estimate of the exposure or risk distribution can be quantitatively estimated using a two-dimensional Monte Carlo analysis. This analysis can provide a quantitative measure of the confidence in the fraction of the population with a risk exceeding a particular level. Additionally, the output from this analysis can provide a quantitative measure of the confidence in the risk estimate for a particular fraction of the population (EPA, 2001a).

Compared to a point estimate risk assessment, a probabilistic risk assessment based on the same state of knowledge can provide a more complete characterization of variability in risk and a quantitative evaluation of uncertainty. In deciding whether a probabilistic assessment of risk should be performed, the key question is whether this type of analysis (vs. a point estimate assessment) is likely to provide information that will help in risk assessment decision making. To assist site managers in deciding what type of risk assessment is best suited to their site, decision-making tools such as a tiered approach developed by EPA based on “scientific management decision points” are available to help identify the complexity of analysis that may be needed (EPA, 2001a).

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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through inhalation of DNAPL contaminants that have volatilized and migrated to the ground surface.

If groundwater contaminated with explosives or DNAPLs is used as a potable water source at an Army facility, exposure to facility occupants can occur through direct contact with the water during ingestion and via dermal contact. Indirect contact through inhalation of DNAPL volatile organic compounds that become airborne during water use or through migration to the ground surface or into occupied structures is also a possibility. The same type of exposure can occur offsite if contaminant migration has occurred or could occur in the future, and if groundwater is used by offsite residents as a source of potable water.

Ecological receptors are most likely to contact contaminants from explosives or DNAPL after the contaminants have migrated through groundwater and have discharged to surface water. In these cases, the threat may be somewhat less given the dilution of the contaminant that is likely to occur once it is discharged to the surface water and given the rapid volatilization of many DNAPL contaminants to air. Ecological receptors are not likely to contact explosive or DNAPL contaminants in soil below the top meter unless excavation activities bring contaminated soil to the surface.

The physical extent of the contamination and the timeframe required for its reduction to levels that represent an acceptable risk affect several elements of exposure assessment and subsequent risk characterization:

  • The higher the concentration of the contaminant in the environmental medium, the higher the potential intensity of the exposure.

  • The more widespread the source of contamination, the larger the potential population of receptors that may contact the contaminant and/or the higher the potential exposure frequency.

  • The longer it takes to remove contamination from the environment, the longer the potential exposure duration.

In many cases, these factors require that the overall objective of protecting human health and the environment be met through a combination of treatment and long-term site management actions.

Time-Scale Considerations for Risk Assessment

The risk-based methods typically used at contaminated sites evaluate carcinogenic and noncancer risk to a hypothetical individual over the course of the person’s lifetime. These methods do not factor the lifetime of a source of contamination into risk estimates. They do not typically evaluate the size of the population potentially at risk, nor do they consider risk beyond the lifetime of an individual (i.e., they do not consider cumulative risk to the entire population exposed for the lifetime of the source of contamination). These shortcomings are

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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serious, given that source zone cleanup may take decades to complete for technical and financial reasons, and some level of contamination is likely to be left in place9 for an extended period of time.

Some types of chemical sources that represent particularly long-term problems in groundwater (e.g., chlorinated solvents) are known or are presumed to be highly toxic to humans. Only very low concentrations of these contaminants would be allowed in the groundwater if it were a source of drinking water. The timeframe required to achieve these low-level concentrations, either through natural site recovery or various remedial alternatives, may be so long as to be inconsistent with the timeframes implicit in risk-based methodologies. This can severely limit the ability of the risk assessor to differentiate what may be significant public health impacts, if they occur some time in the future.

The RAGS model, for example, is a static examination of risk for a fixed population, assuming constant conditions for 30 years (40 years for a family farm) under Reasonable Maximum Exposure conditions. More conservative variants of this model may address full lifetime exposure. There are also more realistic models that address changes in contaminant concentration over time, as well as residential mobility, aging, and other demographic factors influencing exposure (e.g., Price et al., 1996; Wilson et al., 2001), but these also fail to address timeframes of contamination that may span centuries.

Accordingly, existing risk metrics may be unable to demonstrate benefits from source remediation efforts, if the primary effect of those efforts is to reduce the time over which the source contributes to elevated contaminant concentrations in groundwater. If, hypothetically, a remedy were to have the effect on contaminant concentrations after an interval of several decades, it would not be detectable with current risk metrics. For the population that will reside in the area in the future, however, the risks from use of the groundwater have been substantially reduced. That is, in the absence of a remedy that will be effective within 30 years, existing analytical frameworks obscure important distinctions between remedies that are effective in 100 vs. 500 years.

Techniques are available for longer-term types of risk evaluation; these techniques and associated models have been used for many years to evaluate risk associated with Department of Energy legacy waste sites where very long-lived radionuclides will be present in the environment for thousands of years (Yu et al., 1993; EPA, 1996b). Unfortunately, with chemical contaminants, estimation of population risk over the lifetime of the contaminant source is not typically conducted because there is no regulatory requirement to conduct such an evaluation, nor is there a currently prescribed regulatory context for considering the results of

9  

“Contamination left in place,” as used in this report, is consistent with the interagency definition as hazardous substances, pollutants, or contaminants remaining at the site above levels that allow for unlimited use and unrestricted exposure (Air Force/Army/Navy/EPA, 1999).

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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such an evaluation. [It should be noted, however, that even though tools for evaluating long-lived contaminants are available, they are considered imperfect in predicting long-term risk because they rely on unverifiable assumptions about the future behavior of people and institutions (NRC, 2000a).] Existing risk assessment frameworks are badly in need of explicit reconsideration to better reflect the physical realities at sites if the best attainable remedies for these sites are to be selected.

CONCLUSIONS AND RECOMMENDATIONS

As mentioned in the opening of this chapter, clear definitions of absolute and functional objectives and metrics for success are not evident in most of the reports (both Army and non-Army) reviewed by the committee. This has made it difficult to determine the “success” of projects under any consistent definition. Reports of early (pre-2000) projects seldom contained sufficient rationale for how and why certain technologies were selected. More recent projects discuss objectives such as concentration reduction in the dissolved phase plume or reduction of source mass, but there is seldom evidence to suggest that the technology selected would meet those specific objectives. Indeed, within the Army several source remediation technologies have been piloted and then selected for scaling up in the absence of having specific cleanup objectives prior to the pilot projects. As an illustration of this, in situ chemical oxidation might have been attempted for a small portion of the source zone during a pilot study and found to achieve a certain percentage of mass removal. The committee observed that this would subsequently lead to full-scale implementation of the technology (1) without considering whether mass removal would meet the objectives of full-scale cleanup (which may be, for example, protection of human health) or (2) in the absence of any objectives for full-scale cleanup. Thus, in many cases observed by the committee, the decision to proceed with larger-scale remediation was not based on a demonstrated ability to achieve cleanup objectives. Rather, if the pilot test showed significant concentration reductions or mass removal, it was simply assumed that a larger-scale project would bring more widespread reductions.

The following recommendations regarding objectives for source remediation are made.

Remedial objectives should be laid out before deciding to attempt source remediation and selecting a particular technology. The committee observed that remedies are often implemented in the absence of clearly stated objectives, which are necessary to ensure that all stakeholders understand the basis of subsequent remediation decisions. Failure to state objectives in advance virtually guarantees stakeholder dissatisfaction and can lead to expensive and fruitless “mission creep” as alternative technologies are applied. This step is as important as accurately characterizing source zones at the site.

Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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A clear distinction between functional and absolute objectives is needed to evaluate options. If a given objective is merely a means by which an absolute objective is to be obtained (i.e., it is a functional objective), this should be made clear to all stakeholders. This is particularly important when there are alternative methods under consideration to achieve the absolute objectives, and when it is known or is likely that different stakeholders have a different willingness to substitute objectives for one another.

Each objective should result in a metric; that is, a quantity that can be measured at the particular site in order to evaluate achievement of the objective. Objectives that lack metrics should be further specified in terms of subsidiary functional objectives that do have metrics. Furthermore, although decisions depend upon both technical and nontechnical factors, once a decision has been made, the focus should be on the technical metric to determine if remediation is successful.

Objectives should strive to encompass the long time frames characteristic of many site cleanups that involve DNAPLs. In some existing frameworks, timeframes are very short (rarely longer than 30 years) relative to the persistence of DNAPL (up to centuries), such that alternative actions with significant differences in terms of the speed with which a site can be remedied cannot be distinguished. Within life cycle cost analysis, the chosen timeframe and discount rate can significantly affect cost estimations for different remedies. Decision tools with a more realistic temporal outlook have been developed in other areas of environmental science (e.g., storage and disposal of radioactive materials). Their application to DNAPL problems needs to be considered by the Army and by the site restoration community as a whole.

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Suggested Citation:"4 Objectives for Source Remediation." National Research Council. 2005. Contaminants in the Subsurface: Source Zone Assessment and Remediation. Washington, DC: The National Academies Press. doi: 10.17226/11146.
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Next: 5 Source Remediation Technology Options »
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At hundreds of thousands of commercial, industrial, and military sites across the country, subsurface materials including groundwater are contaminated with chemical waste. The last decade has seen growing interest in using aggressive source remediation technologies to remove contaminants from the subsurface, but there is limited understanding of (1) the effectiveness of these technologies and (2) the overall effect of mass removal on groundwater quality. This report reviews the suite of technologies available for source remediation and their ability to reach a variety of cleanup goals, from meeting regulatory standards for groundwater to reducing costs. The report proposes elements of a protocol for accomplishing source remediation that should enable project managers to decide whether and how to pursue source remediation at their sites.

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