National Academies Press: OpenBook

Alternatives for Ground Water Cleanup (1994)

Chapter: 3 Performance of Conventional Pump-and-Treat Systems

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Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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3
Performance of Conventional Pump-and-Treat Systems

Between 1982 and 1992, 73 percent of the cleanup agreements at Superfund sites where ground water is contaminated specified the use of pump-and-treat technology (Kelly, 1994). At most of these sites, the cleanup goal is to restore the aquifer so that the water extracted from it will be suitable for drinking without further treatment. Yet, within the past few years, studies of pump-and-treat systems have indicated that drinking water standards may be essentially impossible to achieve in a reasonable time frame at certain sites (Keely, 1989; Mackay and Cherry, 1989; EPA, 1989a; Mercer et al., 1990; Doty and Travis, 1991; Travis and Doty, 1990). This chapter presents the Committee on Ground Water Cleanup Alternatives' assessment of how well existing pump-and-treat systems have performed and whether it is reasonable to expect that they can achieve drinking water standards.

The analysis presented in this chapter is based on a review of 77 sites where pump-and-treat systems have been studied and the committee members' own extensive experience with ground water cleanup. Appendix A shows the sites the committee evaluated and summarizes the performance of pump-and-treat systems at each site. At 69 of the 77 sites, the pump-and-treat systems have not yet reached cleanup goals, as indicated in Appendix A. However, the committee also found eight sites where pump-and-treat systems have apparently achieved cleanup goals.

Throughout this chapter are brief case studies of sites where goals have been reached and those where they have not. Although the chapter highlights many success stories, the committee wishes to emphasize that

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

TABLE 3-1 Continuum of Possible Results for Pump-and-Treat Systems

Result

Explanation

Unequivocal failure

Fails to contain subsurface sources of contamination and to clean up the plume of dissolved contaminants emanating from source areas

Prevention of additional exposure to contamination

Contains subsurface sources of contamination and prevents the plume of dissolved contaminants from increasing in size

Reduction of additional exposure and significant shrinkage of the area affected by the contamination

Contains subsurface sources of contamination and possibly reduces the amount of contaminant mass in source areas; cleans up part or all of the plume of dissolved contaminants to healthbased standards

Unequivocal success

Fully removes sources of contamination and cleans up the plume of dissolved contaminants to health-based standards

these successes are rare, as is evident in Appendix A. The committee also wishes to emphasize that whether a cleanup is labeled a success or a failure depends in part on the stringency of the cleanup goal. The success or failure of a cleanup should not be viewed as a simple ''yes'' or "no" but instead should be evaluated according to a continuum of possible results, from unequivocal failure to reduction in exposure to contaminants to unequivocal success, as shown in Table 3-1.

HOW PUMP-AND-TREAT SYSTEMS WORK

Conventional pump-and-treat systems are based on a theoretically very simple concept: contaminated ground water is extracted from the subsurface, and the extracted water is replaced with clean water. The clean water comes either from areas immediately adjacent to the contaminated zone or from water injected into the subsurface as part of the pump-and-treat process. (See Figure 1-1 in Chapter 1 for an example of a pump-and-treat system.)

Occasionally, the extracted water is discharged directly into a surface water body, such as a stream. Direct discharge is acceptable where the surface water standards allow higher contaminant levels than do the ground water standards and where the contaminant concentration in the extracted ground water is low enough that surface water standards will not be exceeded. More often, however, the extracted water requires treat-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

ment. The extracted water may be treated using any of a number of methods that have been well tested for removing contaminants from drinking water and industrial and municipal wastewater. For example, air strippers can remove volatile contaminants, granular activated carbon can remove dissolved organic contaminants, and biological systems can remove biodegradable contaminants. Once treated, the water may be discharged to a surface water body or reinjected underground. Reinjection can improve the system's efficiency and reduce cleanup time by speeding the flow of water and contaminants to the extraction wells.

Pump-and-treat systems can be designed for two very different goals: containment, to prevent the contaminant from spreading, and restoration, to remove the contaminant. In pump-and-treat systems designed for containment, the extraction rate is generally established as the minimum rate sufficient to prevent enlargement of the contaminated zone. In pump-and-treat systems designed for restoration, the pumping rate is generally established to be much larger than that required for containment so that clean water will flush through the contaminated zone at an expedited rate. Because of their reduced pumping requirements, pump-and-treat systems designed for containment are much less costly to operate than pump-and-treat systems designed for restoration. In all other fundamental ways, the two types of systems are identical. However, pump-and-treat systems designed for restoration face a much greater technical challenge than those designed for containment. Even when these systems extract contaminated water and replace it with clean water, undissolved contaminants may remain underground. The remaining contaminants will dissolve slowly over time, making complete restoration of the ground water impossible until all of the contaminants can be removed

PREVIOUS STUDIES OF PUMP-AND-TREAT SYSTEMS

Before 1989, the limitations of pump-and-treat systems were not fully appreciated. No large-scale studies of the effectiveness of pump-and-treat systems were available because most of the systems were so new that their long-term performance could not be assessed. In 1989, however, the Environmental Protection Agency (EPA) released a study of pump-and-treat systems that caused concern in the regulatory community and among businesses paying for the cleanups. After a detailed review of 19 sites where pump-and-treat systems were operating, the EPA determined that at none of these sites had the aquifers been restored to drinking water standards (EPA, 1989a,b,c). In 1991, the EPA reassessed the data from these 19 sites and reviewed 5 additional sites. The agency found

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

Components of a pump-and-treat system at a former coal gas plant: pumped water is filtered through the activated carbon units shown here. Courtesy of the Johns Hopkins University, Department of Geography and Environmental Engineering.

one site of the 24 where cleanup goals were apparently reached (EPA, 1992a,b). However, no follow-up monitoring was performed at the site to confirm the cleanup, and agency personnel have questioned the existing site data (Sutter and Glass, 1992).

After the EPA studies, others conducted their own investigations. Researchers at the Oak Ridge National Laboratory reexamined data from 12 of the EPA sites and evaluated 4 additional sites (Doty and Travis, 1991). Like the EPA researchers, the Oak Ridge investigators concluded that pump-and-treat systems had not restored the aquifers to drinking water standards at any of the sites. More recently, the American Petroleum Institute (API) released a study of 13 sites not included in the EPA or Oak Ridge studies (API, 1993). The API's results were more promising: the study identified five sites, all gasoline stations, where pump-and-treat systems have reached cleanup goals. In a fourth study, researchers representing the California Regional Water Quality Board reviewed the records of 37 pump-and-treat systems at semiconductor manufacturing sites in California's Santa Clara Valley (Bartow and Davenport, 1992). Like the API study, this study yielded results somewhat more promising than previous studies: the researchers found two sites where pump-and-treat systems have reduced concentrations to below health-based standards for all of the contaminants; they identified an

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

additional eight sites where the pump-and-treat systems have reached health-based cleanup levels for some but not all contaminants.

A common limitation of all the studies of pump-and-treat systems is that at most sites, systems have been operating for too short a time period to allow a final assessment of their effectiveness. The average starting year of all the systems in the EPA, Oak Ridge, API, and California Water Quality Control Board studies was 1985, which means these studies were based on only about five to seven years' worth of data, on average. A second problem with the studies is that for many of the early pump-and-treat systems, the designers did not fully appreciate the complexity of the subsurface and thus did not consider it in the system plans. For these systems, separating system success or failure from inadequate design is difficult. A third problem is that at many sites, surface sources of contamination such as heavily contaminated soils remain in place, raising questions about whether the inability to reach cleanup goals is due to continued leaching from these sources. As a consequence of these limitations, existing studies do not rule out the possibility that, given more time, optimal designs, and removal of surface sources of contamination, a larger number of pump-and-treat systems could reach cleanup goals.

Amidst the uncertainty raised by the recent studies, some analysts have suggested that pumping and treating may be a wasted effort (Travis and Doty, 1990). Such critics question whether, given the poor record in meeting health-based cleanup goals, pump-and-treat systems are worth operating. These critics emphasize the enormous cost of pumping and treating large volumes of ground water over long time periods. On the other hand, others view the technology more favorably, contending that pump-and-treat systems can significantly reduce the risks of exposure to ground water contamination by removing contaminant mass and by containing the plume to keep it from points of water use, even if they cannot return all of the aquifer to near-pristine conditions.

FEASIBILITY OF CLEANUP WITH PUMP-AND-TREAT SYSTEMS

The effectiveness of pump-and-treat systems depends strongly on hydrogeologic and contaminant properties. As the complexity of the hydrogeologic conditions and the contaminants increases, the likelihood that the pump-and-treat system will meet stringent cleanup goals decreases.

Table 3-2, developed by the committee, provides a framework for assessing the complexity of cleaning up contaminated ground water. In the table, the complexity of ground water cleanup increases with the complexity of contaminant chemistry, from left to right. The complexity

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

TABLE 3-2 Relative Ease of Cleaning Up of Contaminated Aquifers as a Function of Contaminant Chemistry and Hydrogeology

 

Contaminant Chemistry

Hydrogeology

Mobile, Dissolved (degrades/volatilizes)

Mobile, Dissolved

Strongly Sorbed, Dissolveda (degrades/volatilizes)

Strongly Sorbed, Dissolveda

Separate Phase LNAPL

Separate Phase DNAPL

Homogeneous, single layer

1b

1-2

2

2-3

2-3

3

Homogeneous, multiple layers

1

1-2

2

2-3

2-3

3

Heterogeneous, single layer

2

2

3

3

3

4

Heterogeneous, multiple layers

2

2

3

3

3

4

Fractured

3

3

3

3

4

4

a "Strongly sorbed" generally indicates contaminants for which the retardation coefficient is greater than 10. A retardation coefficient of 10 indicates that at any given time, 10 percent of the contaminant is dissolved in the water and 90 percent is sorbed to the aquifer solids.

b Relative ease of cleanup, where 1 is easiest and 4 is most difficult.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

of cleanup also increases with hydrogeologic complexity, from top to bottom. Conditions categorized as 1, shown in the upper left portion of the table, represent those that will be easiest to remediate. Conditions categorized as 4, shown in the lower right portion of the table, will pose the greatest technical challenge.

Influence of Contaminant Chemistry and Site Geology

As Table 3-2 shows, two types of contaminant characteristics can complicate ground water cleanup. The first characteristic is the tendency of the contaminant (organic or inorganic) to sorb to solid materials. As explained in detail in Chapter 2, chemical compounds dissolved in ground water interact with the solid media in the subsurface. As a result, at any given time, some of the chemical compound is dissolved in the ground water and some is attached to the solid media. Pump-and-treat systems can remove only dissolved contaminants. When a portion of the contaminant mass remains sorbed to solid media, it is possible that cleanup standards will not be met because the sorbed contaminants will desorb too slowly to be entirely removed but quickly enough to contaminate the clean ground water.

The second contaminant characteristic complicating cleanup is the tendency for certain organic contaminants to remain undissolved as a nonaqueous phase. As explained in Chapter 2, these phases may be organic chemicals present as light nonaqueous-phase liquids (LNAPLs) that tend to float on the water table, such as gasoline, or as dense non-aqueous-phase liquids (DNAPLs) that tend to sink, such as chlorinated solvents. Contaminants dissolve slowly from these nonaqueous-phase liquids (NAPLs) into the passing ground water. As a result, it is likely that cleanup standards will not be met when NAPLs are present.

As Table 3-2 shows, two types of contaminant characteristics can facilitate cleanup. As indicated in Chapter 2, certain chemicals degrade and/or volatilize. These processes may aid in cleanup when the processes occur naturally or when the remediation system takes advantage of them. For example, many LNAPLs are petroleum chemicals that degrade (when dissolved from the LNAPL) and/or volatilize. These processes, along with the fact that the LNAPLs usually rest above the water table, facilitate cleanup when a pump-and-treat system is combined with other technologies such as soil vapor extraction or bioventing (see Chapter 4).

In addition to contaminant characteristics, Table 3-2 shows three types of geologic characteristics that can complicate cleanup: multiple layers, heterogeneities, and fractured rock. When such geologic features

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

are present, different regions of the contaminated zone will flush at different rates, with high-permeability zones cleaned up first. Attainment of cleanup standards will often be determined by how quickly the lower permeability zones flush. Furthermore, as explained in Chapter 2, heterogeneous regions with low permeability and regions with fractured rock can store significant quantities of contaminants that the bulk of the ground water cannot reach. Chemical transfer from these regions is slow and is controlled by diffusion. Regardless of the amount of contaminated water extracted, as long as significant diffusion occurs out of the low-permeability zones, it is possible that ground water cleanup standards will not be met.

Geologic complexities and the presence of sorbed or nonaqueous-phase contaminants may affect the outcome of pumping and treating by causing progress toward cleanup to tail off above the cleanup goal. When the pump-and-treat system begins operation, contaminant concentrations may drop very rapidly, but after continued operation this rapid progress may cease at a level significantly above the cleanup goal. At such sites, there is no doubt that additional progress is still being made toward remediation, because the contaminant mass left in the aquifer is finite, and the pump-and-treat system continues to remove mass. In addition, the leveling effect often is observed only at some of the wells, near contaminant sources, while at other wells contaminant concentrations may continue decreasing. However, at wells where the concentration has leveled, continued progress toward reducing the concentration will be very slow, and the "final" stages of remediation may proceed for a very long time, as shown in Figure 3-1.

Geologic and chemical complexities may also affect the result of pumping and treating by causing regrowth of the contaminant plume when pumps are turned off, even after the cleanup goal has been reached. Plume regrowth may occur when nonaqueous-phase contaminants that were not extracted with the pump-and-treat system dissolve in the clean water. It may also occur when contaminants in zones of low permeability that were not flushed with the pump-and-treat system diffuse into the clean water.

Influence of the Quantity and Duration of Contamination

In addition to the geologic and chemical characteristics depicted in Table 3-2, two other factors are very important in determining the difficulty of cleanup: (1) the mass of contaminant released and (2) the length of time the contaminant remained in the subsurface before cleanup. The easiest sites to remediate are those at which only a small mass of chemi-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

FIGURE 3-1 The effect of tailing on cleanup time. The "theoretical removal" curve shows the number of aquifer volumes of ground water that must be pumped to remove the contamination, assuming all of it dissolves readily. The "removal with tailing" curve shows the number of aquifer volumes of ground water that must be pumped to remove the contamination when significant undissolved sources of contamination are present. Examples of such sources include contaminants present as pools of NAPLs and metals that have precipitated. SOURCE: Mercer et al., 1990.

cal has been released in a small area and pumping and treating begin soon after the release.

The length of time required for remediation generally increases with the amount of contaminant mass and the size of the source area. The size of the source influences cleanup time not only because larger quantities of contamination require more time to remove but also because the larger the source, the more difficult it is to identify and characterize the impact on ground water. The sites reviewed in Appendix A span a wide spectrum of source sizes and complexities. For example, at the Varian Associates site in Santa Clara, California, approximately 40 to 80 liters of 1,1,1-trichloroethane were lost in a one-time spill in 1984, whereas at the Aerojet site in Sacramento, California, potentially more than 4 million liters of chlorinated organic solvents were released at more than 100 source areas scattered about a 30-square-kilometer site starting in the early 1950s.

For contaminants that resist degradation, the length of time for remediation increases with the length of time between the contaminant release and the start of remediation, because some processes that control ground water migration, such as diffusion, are time dependent. On the

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

Cleanup of crude oil from a burst pipeline near Bemidji, Minnesota. The equivalent of 8,000 barrels of oil was removed directly, but 2,500 barrels remained in the subsurface, forming a plume of contamination in the ground water. Courtesy of Hans-Olaf Pfannkuch, University of Minnesota.

other hand, for contaminants that degrade to nontoxic products by chemical or biological processes, remediation may become easier as the time between the release and the start of remediation increases. Sites with ground water contamination range from coal gas generating facilities where releases occurred more than a century ago to service stations with ruptured underground storage tanks that are addressed within days of the release. Table 3-2 assumes that a "medium" amount of contaminant has resided in the subsurface for a "medium" length of time.

CATEGORIZING SITES FOR CLEANUP

Provided certain qualifications are kept in mind, the categories in Table 3-2 can provide a preliminary indication of the difficulty of clean-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

ing up a particular site. The table does not encompass all of the factors that make ground water cleanup a complex task. As indicated above, it does not consider chemical mass released and duration of contamination. In addition, it does not consider cleanup goals, which influence whether the cleanup is perceived as a success or failure. Nevertheless, the categories in Table 3-2 are useful as subjective criteria for evaluating sites.

Categorizing a site according to Table 3-2 requires information about the following site characteristics:

  1. site stratigraphy (i.e., a description of the geological layers),

  2. permeability of the layer(s),

  3. structural geology (especially information about fractures),

  4. types of chemicals in the subsurface,

  5. properties of chemicals in the subsurface, and

  6. estimates of the distribution of chemicals in the subsurface, including estimates of the potential or actual presence of LNAPLs or DNAPLs.

Determining the appropriate row in Table 3-2 requires an assessment of whether the site hydrogeology is homogeneous, heterogeneous, or fractured, which can be determined by analyzing the first three types of information (stratigraphy, permeability, and structural geology). The ideal hydrogeologic environment for pump-and-treat systems is one that is a single layer with a hydraulic conductivity greater than about 10-5 cm/s (Mercer et al., 1990). Determining the appropriate column in Table 3-2 requires a judgment about the likely contaminant behavior in the subsurface, which requires the above types of information about the contaminants (types, properties, and distribution), as well as information about the composition of the solid media comprising the aquifer. For example, the retardation coefficient, which depends on properties of the contaminant and the solid media, indicates whether or not the contaminant will sorb strongly. In general, pump-and-treat systems are best suited to recover mobile chemicals that have retardation coefficients less than 10, which in general means that at any given time at least 10 percent of the contaminant in the plume is dissolved in the ground water (see Chapter 2). Relatively volatile organic compounds, indicated in the first and third columns of Table 3-2, have high vapor pressures and Henry's Law constants greater than 10-3 atm-m3/mole (EPA, 1990). Degradation, also shown in the first and third columns, is both chemical and site specific. As explained in Chapter 2, a wide variety of compounds—from gasoline and other fuels to chlorinated solvents—are potentially biodegradable, but whether they will degrade in the field depends on site conditions (especially on the presence of electron acceptors and other compounds necessary to support microbial activity).

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

As an example of how this information can be used to categorize sites, a plume containing a contaminant with a retardation coefficient greater than 10 would belong in the fourth column under the contaminant chemistry heading of Table 3-2 because it would have a strong tendency to sorb. If the site geology is heterogeneous, then according to the table the site would be assigned a category of 3. The committee used this process to categorize the sites in Appendix A and to evaluate the feasibility of ground water cleanup under different types of site conditions.

Cleanup of Sites in Category 1

At sites with conditions in category 1 in Table 3-2, well-designed pump-and-treat systems generally should be able to restore the ground water to health-based standards in reasonable periods of time. These sites have uniform geologic characteristics and contaminants that are fully dissolved. Such ideal site conditions are rare. For example, of the 77 sites listed in Appendix A, only 2 are in category 1; the pump-and-treat system reached cleanup goals at one of these sites, a service station reviewed in the API study (API, 1993). At this site, the initial total concentration of the gasoline components benzene, toluene, ethylbenzene, and xylene (BTEX) was 1,021 parts per billion (ppb). After five years of pumping, the BTEX concentration was below the detection limit. This is a relatively small site and thus design and operation of the cleanup system were much simpler than at the more complex sites typical of the Super-fund program. The treatment system consisted of one extraction well operating at an estimated average rate of 95 liters per minute; it removed approximately 19 kg of contaminants in total. In addition to the smaller size of the site, another factor that may have aided the cleanup is the biodegradability of the contaminants. Numerous researchers have demonstrated that under the right conditions, BTEX is degradable even without human intervention (National Research Council, 1993). When this report was prepared, operators at this service station had applied to shut down the pump-and-treat system and commence post-remediation monitoring.

Cleanup of Sites in Category 2

Cleanup of sites in category 2 also is possible but is subject to greater uncertainties than cleanup of sites in category 1. Of the 77 sites in Appendix A, 12 sites are in category 2 and 2 sites are on the borderline between categories 2 and 3. Cleanup goals have been achieved at four of these sites. At these four sites, some or all of the contaminants were biodegradable compounds such as BTEX, ketones, and alcohols. Conse-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

quently, it is possible that some of the contamination was removed by biodegradation rather than by pumping and treating. It is also possible that at some sites, cleanup goals will be achieved in the future with continued pumping and treating.

Table 3-2 indicates two broad types of sites in category 2: (1) sites with contaminants that are fully dissolved and therefore amenable to extraction but with geologic heterogeneities that interfere with contaminant extraction and (2) sites with homogeneous geologic conditions that facilitate ground water extraction but with contaminants that sorb to solid materials, interfering with their extraction. An example of a category 2 site is the service station described in Box 3-1. This site is similar to the category 1 service station described above, except that the site contains geologic heterogeneities. At this site, three years of pumping reduced the BTEX concentration below regulatory standards in monitoring wells. The company ceased pumping in May 1991; one-and-a-half years later, the last set of available monitoring data indicated that contaminant concentrations remained below regulatory levels.

In the cases where cleanup goals have apparently been achieved at category 2 sites, it is possible that some contamination may remain—either in low permeability zones that were not adequately flushed by the pump-and-treat system or sorbed to solid materials in the aquifer. It is uncertain whether such lingering contaminants will dissolve in the ground water in sufficient quantities that, at some future date, contaminant concentrations will again exceed regulatory levels. An example of such a situation is the tire manufacturing plant described in Box 3-2, which was contaminated with chlorinated solvents and BTEX. At this site, regulators have approved shutdown of the pump-and-treat system because cleanup goals have been achieved in all 25 extraction wells. However, monitoring well data show that a small contaminant ''hot spot'' remains. It is uncertain whether contaminants remaining in the soils in the hot spot may spread and create a risk at some future date.

Cleanup of Sites in Category 3

Complete cleanup of sites in category 3 is possible but is subject to significant uncertainties. Partial cleanup may be a more realistic scenario for many such sites. For example, 19 of the sites in Appendix A are in category 3, 2 sites are on the borderline between categories 2 and 3, and 8 sites are on the borderline between categories 3 and 4; cleanup goals have been achieved at only 3 of these 29 sites. All three sites were gas stations contaminated with BTEX, some of which may have biodegraded.

Four types of sites have category 3: (1) sites with fractured bedrock and dissolved or sorbing contaminants, (2) sites with sorbed contami-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

BOX 3-1 COMPLETE RESTORATION OF GROUND WATER CONTAMINATED WITH GASOLINE—SERVICE STATION, UNIDENTIFIED LOCATION

This example illustrates the ability of a pump-and-treat system to completely restore a site in category 2 according to Table 3-2 when the source of contamination is removed rapidly and the pump-and-treat system begins operating soon after the contaminant release. This site fits category 2 because the contaminants were primarily mobile and dissolved (although some LNAPLs were initially present) and the geology is heterogeneous. The contaminants—components of gasoline—are readily biodegradable (National Research Council, 1993), a property that may have aided the cleanup.

At this service station, gasoline spilled when the installation of a monitoring well resulted in the puncturing of a 38,000-liter (10,000-galion) underground fuel tank. The puncture caused the release of an undetermined amount of gasoline into the soil surrounding the tank. After the tank was emptied, an emergency response was initiated immediately, and 83,000 liters (22,000 gallons) of ground water mixed with gasoline were pumped from an excavated pit and hauled away for disposal. Within days, a monitoring well system was installed to delineate the contaminant plume and to provide information for designing the pump-and-treat system. The contaminated aquifer was composed of dense silty fine sand with zones of calcareous semiconsolidated sandstone. A single extraction well was put into operation approximately eight months after the spill.

Prior to pumping, the maximum concentrations of the chemical components of gasoline that had dissolved in the ground water were as follows: benzene, 3,600 ppb; toluene, 4,030 ppb; ethyl benzene, 730 ppb; xylene, 5,300 ppb; and methyl tertiary butyl ether, 6,700 ppb. After one year, the pump-and-treat system had reduced all of these concentrations in the extraction well to below regulatory standards. After three years, the system had reduced the concentrations below regulatory standards in all monitoring wells. In all but one well, contaminant concentrations were below levels detectable with standard analytical methods.

The company has ceased pumping at this site, and the concentrations in all the monitoring wells remain below cleanup levels. Mobile LNAPLs were never detected in any of the wells, probably because most of the LNAPL was removed during the emergency response. This case illustrates the potential for successful application of pump-and-treat systems under favorable conditions and the value of rapid action.

REFERENCE: API, 1993.

nants and heterogeneous geology, (3) sites with separate-phase LNAPLs and homogeneous or heterogeneous geology, and (4) sites with DNAPLs and homogeneous geology. At sites with fractured bedrock, removing contaminants from the fractures is difficult because of problems in circulating water through these regions. At sites with sorbed contaminants and heterogeneous geology, removal of the sorbed contaminants from zones of low permeability is extremely slow, controlled by the desorp-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

tion and diffusion rates. At sites with LNAPLs and DNAPLs, cleanup cannot be achieved unless the NAPLs are removed. Because ground water extraction is not generally efficient at cleaning up NAPLs, some other remedial action may be necessary. NAPLs that float on the water table may be partially removed by direct pumping. DNAPLs that have migrated below the water table may also be partially removed by direct pumping, but locating them is much more difficult than locating LNAPLs. NAPLs that are not removed or contained can cause regrowth of the contaminant plume, even after the pump-and-treat system has apparently reached cleanup goals. For example, at the Dayton, New Jersey, computer manufacturing facility described in Box 3-3, the pump-and-treat

BOX 3-2 RESTORATION OF GROUND WATER CONTAMINATED WITH SOLVENTS—SALINAS, CALIFORNIA

This case illustrates the ability of a pump-and-treat system to reduce the size of a large contaminant plume to health-based levels. However, it also illustrates that even where cleanup goals have apparently been achieved, some contamination may remain. This site fits category 2 in Table 3-2 because the contaminants were dissolved and the geology is heterogeneous, with multiple layers.

Firestone Tire and Rubber Company operated a tire manufacturing plant at this site, near Salinas, California, from 1963 until 1980. As part of the Resource Conservation and Recovery Act requirements for closing the storage area at the facility, Firestone was required to conduct environmental investigations, which began in 1983. The investigations revealed a range of contaminants in on- and off-site wells, the most significant of which were 1,1-dichloroethane (DCA), 1,1-dichloroethene (DCE), and 1,1,1-trichloroethane (TCA). Historical records indicated that TCA was the solvent most commonly used at the site; the other two compounds were produced by chemical and biological degradation of the TCA.

The investigations traced the sources of contamination to soil near the plant buildings. Firestone subsequently excavated 4,800 tonnes of soil from this area, disposed of it at a hazardous waste landfill, and backfilled the excavated area with clean material. Although TCA is a chlorinated solvent that may enter the subsurface as a DNAPL (see Box 3-3), in this case there was no evidence that significant quantities of the contaminant had migrated below the water table in DNAPL form. The potential DNAPL sources were excavated with the soil. The excavation of source areas and the absence of DNAPLs in the ground water helped facilitate cleanup.

The aquifer system beneath the site is made up of three interconnected zones: shallow (approximately 30 meters deep), intermediate (30 to 40 meters below ground surface), and deep (four subzones 60, 90, 120, and 150 meters deep). The zones are separated by clay and silt layers of varying thicknesses that are locally discontinuous. Only the deep water-bearing zones are used extensively.

The subsurface geology acted as a series of steps that forced contaminated ground water into the intermediate and deeper zones as it flowed away from the

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

system initially succeeded in reducing contaminant concentrations below regulatory standards, but the plume reemerged when the pump-and-treat system was turned off. Investigators attributed the regrowth of the plume to NAPLs that had not been removed or accounted for when the pump-and-treat system was designed.

Although full restoration is unlikely for many sites in category 3 except over extremely long time periods, cleanup of the majority of the plume is possible at these sites. For example, at the site described in Box 3-3, the pump-and-treat system eliminated the dissolved plume in six years, and the cleanup might have lasted if a containment system had been installed around the contaminant source areas before the pump-

site. The initial plumes were 900 meters long and 300 meters wide in the shallow aquifer. 120 meters long and 30 meters wide in the intermediate aquifer, and 2,000 meters long and 500 meters wide in the deep aquifer. However, although contaminants were present in the deep aquifer, risk assessments showed that the contaminant levels in this aquifer were below those requiring regulatory action: contaminant levels in the deep aquifer were below the maximum permissible levels allowed under the Safe Drinking Water Act, and the combined cancer risk from the contaminants was below 10-6. Therefore, the pump-and-treat system was designed to clean up only the shallow and intermediate aquifers and to prevent additional contamination from spreading to the deeper aquifers.

Ground water extraction began in 1986 in the shallow aquifer and in 1989 in the intermediate aquifer, with a total combined pumping rate between 2,100 and 2,800 liters per minute. By June 1992, health-based cleanup standards had been achieved in all 25 of the extraction wells. However, in a small "hot spot" between extraction wells, samples from monitoring wells showed levels of DCE above the drinking water standard of 6 µg/liter. Consultants at the site have hypothesized that the hot spot is located in a zone where, due to the placement of the extraction wells, there was no ground water flow while the pump-and-treat system was operating. Despite the presence of the hot spot, regulators allowed shutdown of the pump-and-treat system in November 1992 under the condition that monitoring would continue until June 1994. After pumping stopped, DCE concentrations increased at the not spot from 18 µg/liter to approximately 50 µg/liter but then returned to a level of approximately 20 µg/liter.

During the remediation at this site, the pump-and-treat system removed 97 kg of DCE, 72 kg of TCA, and 33 kg of DCA. This is a relatively small quantity, making cleanup at this site less difficult than at some other sites.

REFERENCES: International Technology Corporation, 1989; Smedes et al., 1993; R. Leonard Allen, International Technology Corporation, personal communication, 1994; Edwin Wing, International Technology Corporation, personal communication, 1994.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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and-treat system was shut down. At such sites, the part of the plume that does not contain nonaqueous phases or other significant contaminant sources in many cases can be cleaned up relatively rapidly, and the aquifer will remain clean as long as source areas are contained. Containment can be accomplished through physical barriers or by continued pumping around the source zone at a rate just sufficient to prevent contaminant migration.

Cleanup of Sites in Category 4

Cleanup of sites in category 4 to health-based standards is extremely unlikely, although in most cases containing the contamination and shrinking the contaminated area is possible at these sites. Sites in category 4 have either LNAPLs in fractured rock aquifers or DNAPLs in heteroge-

BOX 3-3 THE REEMERGENCE OF A CONTAMINANT PLUME AFTER CLEANUP—DAYTON, NEW JERSEY

This case illustrates how contaminant plumes can reemerge after an apparently successful cleanup when sources of contamination are left in place. This site is in category 3 according to Table 3-2 because it contains DNAPLs and because the geology is homogeneous.

In 1977, routine monitoring at a municipal drinking water supply well near Dayton, New Jersey, revealed contamination by chlorinated solvents, primarily TCA and perchloroethylene (PCE). Investigators from the New Jersey Department of Environmental Protection (NJDEP) traced the source of contamination to a nearby IBM plant that had manufactured ink ribbons for printers and punch cards for computer data. They determined that tanks used to store TCA and PCE had leaked, eventually contaminating the ground water.

IBM removed the storage tanks that were the suspected source of contamination and began cleaning up the ground water with a pump-and-treat system in 1978. The company installed 14 extraction wells and 9 injection wells and evaluated the performance of the system at more than 100 monitoring wells. In 1980, IBM and the NJDEP reached an agreement that the company would continue pumping and treating until it could demonstrate that further treatment would not significantly reduce contaminant concentrations.

By 1984, the pump-and-treat system had nearly eliminated the contaminant plume. Levels of TCA and PCE were undetectable at most monitoring wells near the drinking water supply well. Near where the chemical storage tanks had been located, concentrations of both PCE and TCA were well below 100 µg/liter at all but one well, down from maximum concentrations of 12,000 µg/liter for TCA and 8,000 µg/liter for PCE before treatment. Consultants for both IBM and South Brunswick Township, which operated the drinking water well, agreed that further pumping and treating would not yield substantial reductions in contaminant levels. The NJDEP agreed that IBM could cease treatment in 1984, with contin

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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neous or fractured rock aquifers. Removal of NAPLs from fractured rock and heterogeneous regions poses the most extreme of technical challenges because of the difficulty of circulating water through these regions and the difficulty of dissolving NAPLs. Of the 34 sites in category 4 and the 8 sites on the borderline between categories 3 and 4 in Appendix A, none have been fully cleaned up.

Although full restoration is not possible with existing technology for sites in category 4, varying degrees of cleanup may be possible at some of these sites. One example of such a site is the semiconductor manufacturing facility described in Box 3-4, where the pump-and-treat system eliminated the plume of dissolved contaminants. The contamination remaining at this site is confined to a source zone within a bentonite-slurry wall; the areal extent of this source zone is small compared to the original area of the dissolved plume. In effect, the dissolved plume at this site fit

ued monitoring to ensure that contaminant concentrations did not rise above 100 µg/liter off site.

After the pump-and-treat system was shut down, monitoring wells indicated gradual increases in contaminant concentrations and a reemergence of the contaminant plume. IBM's consultants warned the NJDEP in 1987 that within a year, contaminant concentrations would exceed the 100-µg/liter action level. By 1990, contaminant concentrations in some monitoring wells were higher than they had been before cleanup began in 1978. The NJDEP agreed that IBM should restart a modified version of the original pump-and-treat system. The new system operates fewer wells than the originate the goal is to prevent off-site contaminant migration and, as the plume shrinks, ultimately reduce pumping to a rate just sufficient to contain contamination within the source area. Pumping will likely continue at this site indefinitely.

Researchers at the site concluded that the contaminant plume reemerged because of the presence of NAPLs in the soil and ground water near the contaminant source area. Although the researchers never observed the NAPLs directly, the very high concentrations of TCA and PCE present in the ground water before cleanup, as well as the site history and the known characteristics of TCA and PCE, indicate that undissolved sources of these chemicals are trapped in the subsurface.

Although this case shows that contaminant concentrations can rebound if contaminant sources are not removed or contained, ironically it also demonstrates that pump-and-treat technology can work for cleaning up dissolved contamination. The pump-and-treat system at this site nearly eliminated the contaminant plume, and the cleanup would have been a long-term success if a containment system had been installed around the contaminant source area.

REFERENCES: Robertson, 1992; Stipp, 1991; EPA, 1989b.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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category 2 (and therefore cleanup was possible), while the source zone fits category 4 (and therefore contamination remains). Another example of such a site is the abandoned quarry described in Box 3-5. Here, the pump-and-treat system has successfully isolated the contaminant source zones (in fractured rock) and has prevented further discharge of dissolved contaminants to the Schuylkill River, which interconnects with the aquifer.

Despite such relative successes, it is important to realize that at some sites hydrogeologic conditions may prevent isolation of contaminant source areas. For example, at the computer manufacturing facility described in Box 3-6, an extensive pump-and-treat system consisting of 14 wells has been unable to contain contaminant source areas. At this site the pump-and-treat system has effectively stabilized the plume of dis-

BOX 3-4 CLEANUP OF DISSOLVED CONTAMINANTS AND ISOLATION OF THE CONTAMINANT SOURCE—SAN JOSE, CALIFORNIA

This example illustrates the ability of pump-and-treat systems to meet cleanup standards in part of the contaminated zone if the contaminant source is isolated and prevented from continuously regenerating the dissolved plume. This site fits category 4 according to Table 3-2 because it contains DNAPLs in a heterogeneous geologic setting.

In 1981, Fairchild Semiconductor Corporation discovered a rupture in a waste solvent tank at its San Jose, California, manufacturing facility. The company initiated a ground water investigation, which identified contamination on site. Off site sampling then revealed that one large municipal water supply well and five private wells were contaminated.

The company began plume delineation efforts immediately, ultimately installing 124 monitoring wells. The plume was located in a complex hydrogeologic setting composed of stratified alluvial sand and gravel deposits. Silt and day layers separated the sands and gravels into separate water-bearing zones (the shallowest called aquifer A the next B, and so on). Investigations established that the plume was 1,700 meters long and 300 meters wide, the majority of which was off site, downgradient of the area in which it was generated by dissolution of contaminants from the solvent tank. The primary contaminant was TCA, with lesser concentrations of DCE and Freon 113. The waste solvent tank rupture is itself almost conclusive evidence that DNAPL chemicals were released to the subsurface. In addition, the very high TCA concentrations—above the aqueous solubility in samples from the A aquifer and at 93 percent of solubility in the B aquifer—further support the conclusion that DNAPL had penetrated into the subsurface.

The pump-and-treat system was initially designed for full aquifer restoration. The company installed 11 extraction wells to contain both the source area and

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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solved contaminants, but because uncontained contaminant sources remain, the plume has not decreased in size.

When planning an approach for remediating a site, it is extremely important to recognize from the outset the presence of complexities such as those indicated in category 4. Failure to account for these complex conditions can result in the establishment of unrealistic cleanup goals. For example, at the waste lagoon site described in Box 3-7, the government planned that the site would be cleaned up to drinking water standards within five years. However, after four years of pumping it became clear that drinking water standards could not be attained within the foreseeable future because the aquifer contained DNAPLs trapped in fractured bedrock.

the dissolved plume. The system attained full hydraulic containment in 1982. In 1985, the company constructed a bentonite slurry wall around the entire plant to isolate the source area. Subsequently, contaminant concentrations in the off-site plume (outside the slurry wall) dropped to below drinking water standards. In 1991, the company shut down the pumping system for the off-site plume.

Analyses of monitoring data indicate that the pump-and-treat system removed approximately the amount of dissolved mass that was estimated to be within the plume initially (1,400 kg) but that approximately 12 times the initially contaminated volume of water was removed during this time. These findings suggest that (1) the extraction system was not perfectly efficient, resulting in extraction of uncontaminated ground water from outside the plume boundaries, and/or (2) the contaminated area needed to be flushed several times to remove the mass that was present within the plume, perhaps because of contaminant sorption to aquifer soils or because of diffusion into clay lenses. However, the fact that the pump-and-treat system removed an amount of contaminant mass approximately equal to that present in the dissolved phase initially suggests that very little of the contaminant mass was strongly sorbed.

This case illustrates that under some circumstances, pump-and-treat systems may clean up the plume of dissolved contaminants. It appears that cleanup was possible in this case primarily because the company isolated the source area from contact with the flowing ground water and secondarily because sorption of the contaminants was slight or negligible. The company is attempting to clean up the source area with a combination of excavation and vapor extraction. Because it is unlikely that the slurry wall is uniformly impermeable, pumping from within the slurry wall (to maintain an inward hydraulic gradient) must continue until the source area is remediated to ensure that no contaminants will escape and regenerate the plume.

REFERENCES: Harman et al., 1993; EPA 1989b.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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BOX 3-5 CONTAINMENT OF DNAPLS IN FRACTURED ROCK—KING OF PRUSSIA, PENNSYLVANIA

This example illustrates the use of a pump-and-treat system to contain contaminated ground water at a site where restoration was deemed infeasible because of the presence of DNAPLs in a fractured bedrock aquifer, conditions in category 4 according to Table 3-2.

This site, known as Tyson's Dump, is an abandoned sandstone quarry located 25 km northwest of central Philadelphia. The site is located just south of the Schuylkill River, which is a major river that is approximately 600 meters wide in the vicinity of the site, flowing from west to east. The site was used for the disposal of various septic and chemical wastes from 1969 to 1973. Large quantities of 1,2,3-trichloropropane, a DNAPL with a density of 1.4 g/cm3, were disposed of in the quarry. Prior to site remediation, this chemical was found in water samples collected at downstream local water supply intakes.

The Tyson's Dump site is underlain by a thin veneer of colluvium, fill, and floodplain deposits that overlie sandstone siltstone members of the Stockton Formation. The beds in the Stockton Formation dip to the north-northwest, under the Schuylkill River, at approximately 12 degrees. Ground water flow is primarily along bedding plane fractures and partings and high-angle joints, as is illustrated in Figure 3-2. Shallow bedrock ground water flow is toward the Schuylkill River both from the south side of the river in the vicinity of the Tyson's Dump site and from the north side of the river.

A zone of ground water contamination, characterized primarily by 1,2,3-trichloropropane, extends from the quarry northward to the river and under the river to the north bank of the river. The primary mode of transport for the 1,2,3-trichloropropane was transport as a DNAPL phase downdip along bedding plane fractures from the quarry to under the river. Dissolution of the DNAPL has created an extensive dissolved plume. The extent and depth of the DNAPL under the river has led all parties involved with the site to conclude that the DNAPL cannot be effectively recovered and that the DNAPL will continue to act as a source of dissolved contaminants in ground water. As a result, the cleanup remedy for the site is a containment system that prevents further discharge of dissolved contaminants to the Schuylkill River. The initial ground water containment system began operation in 1988. Operation of this initial system has apparently significantly reduced the amount of contaminants discharging to the Schuylkill River, as 1,2,3-trichloropropane is not currently detected at the water supply intakes at a detection limit of 0.5 µg/liter.

REFERENCE: EPA 1992b.

Appropriate Uses for Pump-and-Treat Systems

In summary, the committee found that there is a spectrum of possible uses for pump-and-treat systems, depending on site conditions. At relatively simple sites, pump-and-treat systems may be able to restore the ground water to health-based standards. At more complex sites,

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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FIGURE 3-2

Hydrogeology and contaminant migration at the Tyson's Dump site (see Box 3-5).

SOURCE: EPA, 1992b.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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BOX 3-6 CONTAMINANT STABILIZATION WITH A PUMP-AND-TREAT SYSTEM—SAN JOSE, CALIFORNIA

This example illustrates a situation in which a pump-and-treat system prevented further contaminant migration but was unable to shrink the plume of dissolved contaminants. Because of its heterogeneous geology and the presence of DNAPLs, this site matches category 4 in Table 3-2. The primary reasons that plume shrinkage was not achieved appear to be the very high ground water flow rates at this site coupled with the fact that the system was not effective in isolating contaminant source areas.

IBM Corporation discovered ground water contamination at this facility during audits at all of its manufacturing plants after discovering a contamination problem at a different site. The sources of contamination appear to be what would previously have been considered minor spillage of chemicals during routine filling of underground storage tanks. During site investigations IBM removed all tanks and tested all tanks and pipes for leakage. The company found no tank leaks but did identify one leaking pipe.

Site investigations began in 1978 immediately after discovery of the problem. The primary contaminants of concern are TCA DCE, and Freon 113. The subsurface is composed of a complex interlayered assemblage of sand and gravel units and less permeable units of silts and days. The sand and gravel aquifers are not completely isolated from one another because there are discontinuities in the less permeable layers that separate them vertically. The contaminant plume is on the order of 4,000 meters long and 460 meters wide. The plume volume prior to the commencement of pumping and treating was estimated to be 2 million m3, containing a total dissolved mass of contaminants on the order of 130 kg. The pump-and-treat system began operating in 1983. Between 1983 and 1986, 12 extraction wells were put in operation; 8 of the wells were designed to prevent further migration of contaminants (to hydraulically isolate the source). Two additional wells were put in operation in 1990, one to aid in source containment and the other to aid in dissolved plume control and removal. Based on the monitoring data, the system does not yet appear to have hydraulically isolated the source area, possibly in part because of the very high natural ground water flow rates (3 meters per day) and in part because of extreme heterogeneity of the subsurface.

Although to date approximately 18 times the estimated initial contaminated water volume has been extracted, there has been little change in the areal extent of the plume. Peak concentrations in the plume have been reduced somewhat, but not dramatically. The mass of contaminants removed is almost four times what site investigators initially estimated was present in dissolved form. This latter fact suggests that significant contaminant mass continues to be released to the ground water, this mass may have sorbed to the aquifer media and may be present in subsurface DNAPL sources. It does not appear that operation of the existing pump-and-treat system will result in restoration of the aquifer to the cleanup criteria in the foreseeable future. The inability of the pump-and-treat system to shrink the plume is likely due to the very high ground water flow rates, which act to spread the plume, and to geologic heterogeneity, which has prevented isolation of the contaminant source areas. It is conceivable that a different configuration of wells and pumping schedules could have improved performance, but it is not evident how the information required to design an improved system could be gained with reasonable effort in such a complex hydrogeologic environment.

REFERENCES: Harman et al., 1993; EPA 1989b.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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BOX 3-7 PUMPING AND TREATING A DNAPL SITEVILLE MERCIER, PROVINCE OF QUEBEC

This example illustrates how not realizing the full complexity of a site can result in unrealistic cleanup goals. Because of the presence of DNAPL in fractured rock, this site fits category 4 in Table 3-2.

The lagoons at this site, in the Municipality of Ville Mercier, Province of Quebec, were used for industrial waste disposal between 1968 and 1972. Before 1972, the lagoons had been mined, leaving sand and gravel pits overlaying a glacial till and fractured bedrock aquifer. Ground water contamination was recognized as early as 1971. A variety of chemicals have subsequently been identified in the subsurface, but the most prevalent is DCA which accounts for approximately 42 percent by weight of the organic compounds present.

In 1984 the government of Quebec established a system of three wells to pump contaminated ground water and built a water treatment plant about 300 meters south of the former lagoons to treat the pumped water. The government intended that this pump-and-treat system would remove essentially all the contamination from overburden and bedrock and restore these aquifers to drinking water use within five years.

By about 1988, after four years of pumping, it became obvious that ground water pumping would not restore the shallow and deeper aquifers within any practical period of time. During approximately four years of pumping and removal of 6 billion liters of ground water, the initially very high concentrations of DCA were reduced by dilution as less contaminated and uncontaminated ground water was drawn toward the wells. However, health-based cleanup goals were not reached, and many experts have agreed that they will not be reached in the foreseeable future because of the presence of DNAPLs (Mercier Remediation Panel, 1993; Martel, 1988; Jackson et al., 1991).

According to Martel (1988), DNAPL from the lagoons penetrated downward through the ground water in the sand and gravel formation. Some of the DNAPL pooled on top of a low-permeability basal till. In some locations, the till formed a barrier and prevented the movement of DNAPL through the fractured porous bedrock. However, because the till is sloped and rests erratically on bedrock, the DNAPL continued to move down the slope and eventually penetrated into the fractures in the bedrock. Likewise, Pakdel et al. (1990) determined that the widespread occurrence of volatile hydrocarbons in ground water at the site is generally the result of lateral movement of LNAPL and downward penetration of DNAPL from the lagoons. Along its downward path, the DNAPL left behind residual ganglia in pores and fractures. Martel estimated the lifetime of this residual at several decades to centuries (Martel, 1988).

Without removing the subsurface DNAPL sources, which can persist for decades to centuries, ground water contamination cannot be eliminated from this site. Although excavation can be used to remove some of the DNAPL from the soils above the water table, this is not true for the saturated zone below 5 to 10 meters. Unfortunately, there are currently no remedial methods available for completely removing DNAPL sources below the water table. Because of the residual DNAPLs at this site, Martel concluded that ''ground water withdrawal from [the Ville Mercier] aquifer is not a suitable solution.'' Martel further noted that the pump-and-treat system "is actually a confinement measure preventing the propagation of contaminants rather than a restoration measure."

REFERENCES: Jackson et al., 1991; Martel, 1988; Mercier Remediation Panel, 1993; Pakdel et al., 1990.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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pump-and-treat systems may clean up the dissolved portion of the contaminant plume, while either a pump-and-treat system or other method contains the remaining contamination. At the most complex sites, cleanup to health-based standards may be impossible, and contaminant containment may be the only feasible option.

It is important to recognize that contaminated sites may consist of areas with varying degrees of complexity. For example, a site may contain a contaminant source area that should be categorized as 4 according to Table 3-2 and is therefore extremely difficult or impossible to clean up. However, the same site may have a dissolved plume in which cleanup is possible. Similarly, a contaminated site may have areas with relatively homogeneous geology, in which cleanup is possible, and areas where the geology is more complex. Thus, in choosing remediation alternatives for a given site, it may be necessary to view the site as having several different components, each of which should be approached with a different remediation scheme.

It is also important to recognize that at sites where pump-and-treat systems have achieved cleanup goals, the long-term effectiveness of the cleanup may still be uncertain, and the site may require continued monitoring. Although the ground water may test clean at the wells, it is possible that secondary sources of contamination remain in undiscovered non-aqueous phases or zones of low permeability. Continued monitoring at these sites is necessary to establish that the cleanups are permanent.

CLEANUP TIMES FOR PUMP-AND-TREAT SYSTEMS

Under Superfund and other ground water cleanup laws, the goal is not only to return ground water to a usable condition, but also to do so in a reasonable time frame. Consequently, an important consideration in evaluating the effectiveness of pump-and treat systems is not only whether they can work, but also how long cleanup will take. Time is one factor that regulatory agencies may consider in determining whether cleanup is technically feasible. For example, a recent EPA guidance document specifying factors for determining whether ground water cleanup is technically impracticable states that "very long restoration timeframes (e.g., longer than 100 years) may be indicative of hydrogeologic or contaminant-related constraints to remediation" (EPA, 1993).

Like performance capability, cleanup time varies widely with site conditions. Theoretical cleanup times range from years to centuries or more, depending on contaminant and geologic characteristics. Furthermore, the scientific community has not agreed on the best methods for estimating cleanup times under complex geologic and chemical condi-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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tions. The models and equations used for time estimates are subject to continuing scientific investigation and controversy. A long time is required for cleanup at most sites, as is demonstrated by the large number of pump-and-treat systems currently in operation and the very few systems that have attained cleanup goals.

Predicting the time required to clean up contaminated ground water would be relatively straightforward if the volume of water that required extraction was equal to the volume of contaminated ground water. For example, if one assumes as representative conditions a contaminated site that is 4 hectares (10 acres), with an aquifer that is 17 meters thick and that has a porosity of 30 percent, then a total of 200 million liters of water are contaminated, and a simple computation shows the cleanup time:

Volume of contaminated water

With a pumping rate of 380 liters per minute, the time required to pump the equivalent of the volume of contaminated water is one year:

Unfortunately, this will not be the cleanup time in a real aquifer system. The volume of water that must be extracted will be generally much larger than the volume of contaminated ground water, for many of the same reasons that the performance of pump-and-treat systems varies.

Processes That Affect Cleanup Time

Five primary processes in the subsurface explain why the volume of water that must be extracted to clean up an aquifer is greater than the volume of contaminated ground water:

  1. Mixing of clean ground water and contaminated ground water: All pump-and-treat systems will cause some mixing of clean and contaminated ground water during extraction. Mixing increases the volume of water that needs to be extracted. In practice, most systems are not designed to minimize mixing; as a result, significant mixing of clean and contaminated ground water often occurs.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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Excavation of a contaminant source area at a former coal gas plant. Courtesy of the Johns Hopkins University, Department of Geography and Environmental Engineering.

  1. Geologic heterogeneities: Geologic heterogeneities, such as at sites in categories 2 through 4 in Table 3-2, can increase cleanup times, just as they increase the difficulty of reaching health-based cleanup goals. The cleanup time is often determined by how fast the lower-permeability zones flush. In addition, in regions where there is almost no ground water flow, the cleanup time may be determined by the rate of contaminant diffusion from the low-permeability zones, which is an extremely slow process.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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  1. Nonaqueous phases: NAPLs, such as at sites in categories 3 and 4 in Table 3-2, can slow cleanup, just as they interfere with reaching health-based cleanup goals. At sites with NAPLs, the cleanup time for pump-and-treat systems will be a function of how quickly these liquids dissolve.

  2. Sorbed contaminants: Sorbed contaminants, as at sites in categories 2 and 3, can slow cleanup. At sites with sorbed contaminants, the cleanup time will depend on how quickly the contaminants desorb, which depends on contaminant solubility and the organic carbon content of the soils.

  3. Leachate from remaining contaminant sources: When the original source of contamination or contaminated soils near the original source remain, it is obvious that a pump-and-treat system will be unable to attain cleanup standards. Nevertheless, at many sites listed in Appendix A, it is likely that chemicals are still migrating to the water table beneath the original source areas, prolonging the cleanup.

In contrast to these five processes, which all work to increase the volume of water that must be extracted to attain a cleanup standard, there are processes that work to decrease the volume of water that must be extracted. These processes are biological, chemical, and physical phenomena that cause the chemicals to degrade or volatilize (see Chapter 2). For many of the most common ground water contaminants, these processes are not very important. However, for some important ground water contaminants, such as gasoline-derived contaminants and chlorinated phenoxy pesticides, degradation processes are a dominant factor in determining the time required to reach cleanup goals.

Methods for Estimating Cleanup Times

A convenient way to view the problem of estimating cleanup time is to consider the number of pore volumes that must be pumped from the contaminated zone to attain the cleanup goals. One pore volume equals the amount of water stored in the contaminated portion of the aquifer. The number of pore volumes required for cleanup (in other words, the number of times the contaminated region must be flushed) will be a function of the cleanup standard, the initial contaminant concentrations, and the five processes listed above. The 1988 EPA document Guidance on Remedial Actions for Contaminated Ground Water at Superfund Sites describes two approaches for estimating ground water cleanup times that are implicitly based on the number of pore volumes: the "batch flushing model" and the "continuous flushing model" (EPA, 1988). These two ap-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
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proaches consider only the processes described in items 1 and 4 listed above (i.e, mixing and sorption).

The two approaches described in the EPA guidance document address the situation in which clean water is circulated through a region that initially contains contaminated ground water. The approaches assume simple advective displacement of the contaminated water, thus neglecting dispersive transport. The approaches further assume that the contaminant concentration in the influent water is always zero as the water enters the contaminated region but that it adjusts instantly to a concentration in equilibrium with the remaining sorbed contaminant mass after the water enters the contaminated region.

In the two EPA approaches, the assumed mechanism of contaminant removal is the same: clean water enters the contaminated region; contaminant mass is transferred from the soils to the water; water is removed and with it the dissolved contaminants; and the process is repeated. Thus, the two approaches are based on the same physical assumptions. Because both approaches assume instantaneous equilibrium between the sorbed and dissolved phases, if properly formulated they should give the same results except for numerical error. In this sense, the distinction between the "batch flush" and "continuous flush" methods described in the EPA's guidance document is misleading.

The EPA batch flush model represents an explicit finite-difference approximation of the solution to the governing differential equation. This approximation is relatively difficult to use and is subject to numerical errors. The exact solution to the governing differential equation, which is relatively simple to apply, is the following expression for the number of pore volumes, PV, required to reach the cleanup concentration, Cwt, in ground water:

where R is the retardation factor and Cwo is the initial contaminant concentration in the ground water. The derivation of this equation and the finite-difference approximation is described by Zheng et al. (1992).

The batch flush model is a useful approach for estimating cleanup times in a simple aquifer system with chemicals for which interaction with the solid matrix can be represented by linear sorption. For example, this approach was used for estimating cleanup times at the Lone Pine, New Jersey, Superfund site, which has a relatively simple aquifer system and dissolved chlorinated solvents (Zheng et al., 1991). The selected remedy at this site had an estimated cleanup time of 165 years.

In general, the batch flush model will underestimate cleanup time

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

because it does not account for the processes described above in items 2, 3, and 5 (i.e., heterogeneities, NAPLs, and leachate from the original source of contamination). In addition, if the interaction between the dissolved chemical and the chemical attached to the solid media is not represented by linear sorption, as is the case for most inorganic compounds, the batch flush model will tend to underestimate cleanup time. For example, Stephanatos et al. (1991) contend that the use of linear sorption models may introduce errors that underestimate cleanup times. They recommend estimating sorption effects with site-specific leaching tests or the EPA's Organic Leachate Model.1 Stephanatos et al. found that for an iron-arsenic compound at the Whitmoyer Laboratories Superfund site, the effect of sorption as estimated from soil leaching tests is nonlinear. A linear sorption model predicted a cleanup time of 160 years, but the nonlinear model estimated a time of 50,000 years.

Detailed, computer-based models that include all the major processes affecting contaminant flow are available for estimating cleanup times (see, for example, Zheng et al., 1992; National Research Council, 1990; EPA, 1985). However, given budget and time constraints typical for hazardous waste investigations, the site-specific data necessary to run such models are rarely collected. Even in research settings, collecting all the necessary data is difficult. As a result, these types of models have been used to estimate cleanup times at only a limited number of sites. In addition, even when these models have been used, they have most often been used only to describe processes represented by the batch flush model and have overlooked the other important influences on cleanup time.

The following examples illustrate the effects of items 2 and 3—heterogeneities and NAPLs—on aquifer cleanup times. These examples, although simple, demonstrate processes that significantly increase ground water cleanup times at many, if not most, sites. The effect of item 5—leachate from contaminant source areas—is not included in the following discussion because it is axiomatic that cleanup goals will not be achieved if significant quantities of contaminants continually enter the aquifer from surface source areas.

Example 1: The Effect of Heterogeneities on Cleanup Time

The effect of geologic heterogeneities on ground water cleanup times can be illustrated by considering an aquifer comprised of sands and clay lenses and contaminated with trichloroethene. If the aquifer has been contaminated for a long time, it is reasonable to assume that contaminant concentrations in the ground water prior to cleanup will be the same in the sand portions of the aquifer and the clay lenses. Concentrations in

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

FIGURE 3-3 Changes in average relative trichloroethene concentration in clay lenses of varying thicknesses as a function of time. This figure shows that the time required for contaminants to diffuse out of clay lenses can be considerably long and can substantially prolong the cleanup effort.

the sand portions of the aquifer will decrease rapidly after the start of cleanup because advection, which is rapid, will be the dominant process controlling contaminant migration. However, concentrations in the clay lenses will decrease slowly because the dominant process controlling contaminant migration out of the lenses is molecular diffusion, which is very slow.

Figure 3-3 shows the estimated time required to reduce the average concentration of trichloroethene in clay lenses with thicknesses of 0.3, 0.6, and 1.2 meters to various average relative concentration levels for this hypothetical example.2 In a clay lens with a thickness of 0.6 meters, approximately 6 years will be required to reduce the average concentration to 50 percent of the initial concentration, and approximately 25 years will be required to reduce the average concentration to 10 percent of the initial concentration. The time required to reduce the concentration to 10 percent of the initial concentration will be 0.25 years at a thickness of 6 cm, 6 years at a thickness of 30 cm, and 100 years at a thickness of 1.2 meters. This example demonstrates that clay lenses can provide a long-term source of contamination to permeable portions of an aquifer and thus can significantly increase the cleanup time.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

Example 2: The Effect of Residual Napls on Cleanup Time

The effect of residual NAPLs on cleanup times can be illustrated by considering an aquifer composed of fine-grained sands with a residual trichloroethene content of 30 liters/m3 (a volumetric NAPL content of about 3 percent). The concentration of trichloroethene in the ground water in this aquifer volume will be a function of the size of the residual NAPL globules and the kinetics of globule dissolution. Using theoretical calculations, it is possible to show that dissolving all of the trichloroethene in the volume will require 122 years.3 However, the theoretical calculations oversimplify the actual dissolution process. The concentration of trichloroethene in the ground water will change as the residual trichloroethene globules dissolve. As the globules dissolve, their size and surface area will decrease, and the dissolution rate will be lower. As a result, the actual time required to completely dissolve residual NAPLs in this example may be significantly longer than 122 years.

Example 3: The Effect of Dnapl Pools on Cleanup Time

Dissolution of DNAPL from a pool that has migrated to the base of an aquifer will occur at a much slower rate than dissolution of residual NAPL globules because significant dissolution occurs only at the top of the pool. As an example, consider a 10-meter-long trichloroethene pool in a sandy aquifer. Figure 3-4 shows the time required to reduce the thickness of the DNAPL pool by 1 cm at ground water velocities in the range of 0.01 to 1 meter per day for transverse dispersivities of 0.1, 0.01, and 0.001 meters.4 (The transverse dispersivity, a property of the aquifer medium, describes the amount of dispersion occurring in a direction perpendicular to the ground water flow direction.) These ranges of ground water velocities and dispersivities are representative of those found in most aquifers. At a ground water velocity of 0.03 meters per day, about 4 years will be required to remove a centimeter from the surface of the DNAPL pool with a transverse dispersivity of 0.1 meter, about 10 years with a dispersivity of 0.01 meter, and about 30 years with a dispersivity of 0.001 meter. Actual dissolution rates are likely to be slower than those shown on Figure 3-4 because of site-specific limitations to trichloroethene dissolution that are not considered in the calculations. These limitations may include microscale irregularities on the DNAPL pool and/or the presence of microlayers such as films or microorganisms at the interface between the DNAPL and the water.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

FIGURE 3-4 Calculated time to remove 1 cm from a DNAPL pool as a function of ground water velocity for various transverse dispersivities (a). (Transverse dispersivity describes the tendency for contaminants to diffuse in a direction perpendicular to the direction of ground water flow.) This figure shows that dissolution of DNAPL pools is an extremely slow process.

Determining Technical Impracticability Based on Cleanup Time

When regulators are deciding whether to consider ground water cleanup technically impracticable because the predicted cleanup time is long, they should evaluate cleanup time and the number of pore volumes required to attain cleanup, rather than cleanup time alone. Cleanup time is a function of both the number of pore volumes pumped per year and the number of pore volumes required for cleanup. The former can be controlled by design of the cleanup system, while the latter is a function of the ground water system. When cleanup time alone is the criterion for determining technical infeasibility, there may be an incentive to design pump-and-treat systems that minimize the number of pore volumes extracted per year. (There are legitimate technical reasons for pumping at low rates. For example, some system designs may minimize pore volumes extracted per year in order to maximize mass recovery when diffusion processes dominate.) No good criterion has yet been developed for deciding what number of pore volumes would constitute technical infeasibility; the number will probably be site specific. In addition, when considering what cleanup time constitutes infeasibility, it is important to

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

realize that continued pumping over long time periods will remove additional contaminant mass and will therefore likely reduce risk, even though cleanup goals may not be achieved.

The committee reviewed data for the 24 sites described in the EPA study of pump-and-treat systems (EPA, 1992a,b) to obtain a rough estimate of the number of pore volumes per year that were being extracted at the sites in the study. These estimates, which are based on the extraction rates, plume areas, and plume thicknesses reported in the study, are listed in Table 3-3. At 13 of the 24 sites, the estimated extraction rate is less than one pore volume per year. Attainment of cleanup criteria at most sites under the most favorable of circumstances can be expected to take decades with extraction rates of less than one pore volume per year. The sites in the EPA study where the cleanup criteria were met in the dissolved part of plume, Fairchild San Jose (see Box 3-4) and IBM Dayton (see Box 3-3), are sites where extraction rates exceed one pore volume per year. At the two sites with the highest extraction rates, Ponders Corner and Site A, the cleanup criteria have not yet been attained, even though the extraction systems at both sites have operated for a number of years. The lack of attainment of cleanup standards at both sites can be explained by continued leaching of chemicals from source materials above the water table; in addition, it is possible that NAPLs may be present in the source areas at both sites. Some of the sites with very small extraction rates in terms of pore volumes per year are those where the pump-and-treat systems are designed only for containment and not for restoration.

Currently, data indicating the number of pore volumes pumped are not reported at most sites. Nevertheless, the time required for a pump-and-treat system to extract one pore volume of ground water from the contaminated zone is a fundamental system parameter that should be documented for all pump-and-treat systems. Assessments of ground water cleanup time should include estimates of the number of pore volumes that must be extracted to attain cleanup goals. The models described above are generally the most appropriate means for making these calculations, keeping in mind that specifying appropriate parameters for some of the important contaminant transport processes may be difficult, and the uncertainty in specifying the appropriate parameters may result in underestimated cleanup times.

IMPROVING SYSTEM PERFORMANCE THROUGH PROCESS MONITORING

This chapter has documented that how well a pump-and-treat system will perform and how long it will take are uncertain at the outset of cleanup and vary widely with site conditions. As a consequence, the

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

TABLE 3-3 Pore Volumes Pumped per Year at 24 Sites

 

 

Plume Dimension

Site Name

Extraction Rate (liters/min)

Area (ha)

Thickness (meters)

Pore Volumes (per year)

Years to Pump 20 Pore Volumes

Year Pumping Started

Amphenol

980

3.6

30

1.55

13

1987

Black & Decker

38

4.5

12

0.12

164

1988

Des Moines

4,900

53

15

1.08

19

1987

DuPont Mobile

680

15

9

0.85

24

1985

Emerson Electric

110

1.2

15

1.08

19

1984

Fairchild San Jose

15,000

30

55

1.59

13

1982

General Mills

1,500

45

15

0.38

52

1985

GenRad

150

4.0

6

1.08

19

1987

Gilson Road

1,100

6.5

34

0.92

22

1981

Harris

1,200

24

27

0.31

65

1984

IBM Dayton

3,800

24

24

1.12

18

1978

IBM San Jose

23,000

310

76

0.17

118

1982

Lathrop

2,300

310

58

0.02

907

1982

Mid-South Wood Products

160

6.0

52

0.09

228

1985

Nichols Engineering

250

0.8

30

1.75

11

1988

Olin

24,000

97

24

1.74

12

1974

Ponders Corner

7,800

9.3

24

5.84

3

1984

Savannah River

2,100

420

46

0.02

1045

1985

Site A

140

0.3

6

14.59

1

1988

Tyson's Dump

450

26

110

0.03

735

1988

Utah Power and Light

760

3.6

53

0.68

29

1985

Verona Well Field

9,100

51

37

0.86

23

1984

Ville Mercier

2,800

3,100

24

0.01

3015

1983

Western Processing

830

5.7

20

1.30

15

1988

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

operation of pump-and-treat systems should be viewed as a long-term project, in which the system's design is modified in response to improved understanding of the site. In effect, operation of the pump-and-treat system should become part of a continued site characterization process. Continued monitoring of the system is essential to determine the performance of the original design and to predict and subsequently assess the improvement in performance that might result from modifications made during operation.

What To Monitor

The parameters to be monitored and the necessary frequency of monitoring vary from one situation to the next, but generally the minimum requirements for both are established by the lead regulatory agency. Typically, the following types of monitoring data are necessary to track performance of the cleanup system:

  • water levels or piezometric heads at numerous sampling points throughout and around the contaminated zone to allow estimation of water flow directions and the portion of the aquifer that the extraction system is controlling;

  • contaminant concentrations in ground water at numerous sampling points throughout and around the contaminated zone to allow estimation of the areal and vertical extent of contamination and the remaining dissolved contaminant mass;

  • contaminant concentrations in the extracted ground water to assess progress toward the cleanup goal and to estimate the cumulative mass of contaminants removed from the aquifer;

  • contaminant concentrations in the treatment system effluent to assess performance of the treatment system and compliance with discharge requirements;

  • flow rates from the extraction wells and through the treatment system to confirm that the system is operating to specifications; and

  • other operational parameters, such as line pressures, that indicate proper operation or incipient failure of pumps and filters or rising water levels in injection wells that may signal clogging.

Given, as described throughout this report, that contaminated sites often have lingering subsurface sources of contamination, it would be advantageous to monitor the decrease or change in distribution of contaminant mass within source zones. Unfortunately, the tools currently available for source monitoring have not proven to meet the need or, in some cases, have been realized as potentially worsening the contamination problem. There is a great need for reliable, accurate techniques for

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

source mass monitoring, and several promising techniques are now under research and development.

Modifying Existing Pump-and-Treat Systems

Monitoring data may be used to assess performance of a pump-and-treat system by determining progress toward six general end points, which will vary with the site:

  1. elimination of contaminant migration beyond the extraction system,

  2. decrease in the size of the contaminated area,

  3. decrease in the contaminant concentration in the extracted ground water,

  4. decrease in the contaminant concentration remaining within the aquifer,

  5. increase in the cumulative mass of contaminants extracted from the subsurface, and

  6. minimization of the volume of water extracted for containment (and therefore the costs of treatment and disposal).

If progress toward any of these objectives does not meet expectations, modification of the system's design or operation should be considered (or may be required). This iterative process will lead to a remedial program that changes over time, with the twin purposes of meeting remedial objectives and minimizing costs.

For example, if the initial design does not meet the first goal, plume capture, the system may be modified by installing additional wells and/or by increasing the pumping rates in existing wells. The optimal modification should be predictable given the additional insight gained from head or water level data collected during initial system operation. Subsequent monitoring should show whether the modifications were adequate or whether the system will require further tuning.

If progress toward goals 2 through 5, which indicate successful contaminant removal, is considerably slower than expected, then one or more assumptions used in the original system design are incorrect. Such disappointing progress may result because of the unanticipated discovery of the types of geologic and chemical complications discussed earlier in this chapter and shown in Table 3-2. Monitoring may provide insight about the most important of these complications. For example, researchers conducting field tests at the Rocky Mountain Arsenal cleanup site learned from multilevel monitoring that variations in hydraulic conductivity within the aquifer were an important cause of the unanticipated tailing of contaminant concentrations above cleanup goals after pumping and treating (Mackay and Thorbjarnarson, 1990). The consultants work-

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

Drilling rig used to install wells at a contaminated site. Courtesy of Rice University, Department of Environmental Science and Engineering.

ing at this site had assumed, in essence, that the site belonged in category 2 according to Table 3-2 when in fact it was more appropriately categorized as 3 due to heterogeneities.

Where monitoring reveals especially complex conditions (as in categories 3 and 4 in Table 3-2) and the tailing of contaminant concentrations at an asymptote, there may be a need to adjust the system design and to reevaluate the remedial objective and projected cleanup time. In such cases, as discussed earlier in this chapter, the most realistic remedial objective might be plume capture. Continued monitoring will be necessary

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

to confirm that the plume is completely contained and to determine whether changes in extraction rates and/or locations would conserve money and/or water.

In the ideal case that progress in cleaning up the dissolved plume is initially relatively rapid or is enabled by source isolation, particularly for sites in category 1 or 2 in Table 3-2, subsequent monitoring is likely to reveal that portions of the aquifer have been cleaned adequately as remediation proceeds. In such cases, one or more of the extraction wells may no longer be removing significant amounts of contaminants. As monitoring confirms this situation, some of the wells may be shut down, or the extraction rate in some or all of the wells may be adjusted (some decreased and perhaps some increased, depending on the interactions between the wells). The goal of these adjustments is to minimize remaining costs while continuing the progress toward achieving cleanup goals.

If the remediation has apparently reached the original cleanup goals or a relaxed cleanup criterion has been agreed upon, active remediation may cease. However, as illustrated graphically by the Dayton, New Jersey, case discussed in Box 3-3, monitoring should continue well beyond the time of active remediation. The goal is to ensure that residual contamination is not sufficient to cause the reestablishment of a contaminant plume. If monitoring reveals significant contamination, then new alternatives for remediation or isolation of the contamination will have to be evaluated.

RESEARCH NEEDS FOR IMPROVING THE PERFORMANCE OF PUMP-AND TREAT SYSTEMS

Much of the research needed to design better pump-and-treat systems exists (although this research has not all been transferred to practitioners in the field). Current research focuses on new technologies to couple with pump-and-treat systems and is discussed in Chapter 4. The key problems requiring further research to improve the performance of conventional pump-and-treat systems are all related to developing better methods for site characterization. Especially important is research to address the following questions:

  • How can NAPLs, especially DNAPLs, be better characterized in the subsurface?

  • How can partitioning of chemicals between the aqueous phase and NAPL and sorbed phases be more accurately quantified?

  • How can this information be used to more accurately estimate cleanup times?

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

CONCLUSIONS

Based on a review of the case studies listed in Appendix A and the experience of committee members, the committee reached the following conclusions about the performance of pump-and-treat systems:

  • At some sites with simple geology and dissolved contaminants, pump-and-treat systems appear to be capable of cleaning up ground water to health-based standards in a relatively short time. Such simple sites are the exception rather than the rule. Nevertheless, the committee found examples of sites where pump-and-treat systems achieved health-based cleanup goals for one or more contaminants. At such sites, it is important to recognize that continued monitoring is necessary to verify the long-term effectiveness of the cleanup. Contaminants may remain attached to solid materials or stored in nonaqueous phases in the subsurface even when ground water from monitoring wells meets regulatory standards.

  • At many of the sites where pump-and-treat systems have attained cleanup goals, the contaminants of concern are readily biodegradable . Pump-and-treat systems have achieved health-based goals at sites contaminated with gasoline, sites where contaminants are fully dissolved, and sites with both dissolved and LNAPL plumes (where the source has been removed) in shallow aquifers. The success of pump-and-treat systems at these sites may in part be due to biodegradation processes that convert contaminants to nontoxic products.

  • The chemical nature of contaminants can prevent pump-and-treat systems from restoring aquifers to health-based standards in a relatively short time. Pump-and-treat systems cannot restore aquifers except over very long time periods (hundreds or thousands of years) where NAPL contaminants remain unless the NAPLs are contained or removed. For contaminants that strongly sorb to solid materials in the subsurface, cleanup times using pump-and-treat systems may also be very long.

  • The geologic conditions of the site can prevent pump-and-treat systems from restoring aquifers to health-based standards in a relatively short time. Clay lenses and other heterogeneities, fractured bedrock, and zones of low hydraulic conductivity can trap contaminants and prevent the large-scale water circulation necessary for effective flushing of the subsurface.

  • At sites where complete aquifer restoration to health-based standards is impossible or impractical due to the chemical nature of the contaminants or geologic complexity, pump-and-treat systems can prevent the contamination from spreading and can clean up or shrink the

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

dissolved portion of the contaminant plume. Pump-and-treat systems can prevent contaminant migration by establishing a hydraulic barrier around the site. They can shrink the contaminant plume by pumping out contaminated water, which is then replaced at the outer edges of the plume with clean water. These accomplishments reduce the risk posed by the contamination by minimizing the area affected by the contamination.

  • Cleanup times for pump-and-treat systems vary widely depending on site conditions and pumping rates. For small sites with simple geology and dissolved contaminants, cleanup times may be relatively short, on the order of years. The presence of geologic heterogeneities, nonaqueous-phase contaminants, sorbed contaminants, and contaminant sources above the water table can extend cleanup times by anywhere from a few years to thousands of years and can make predicting the time highly uncertain. Because cleanup time also depends on the pumping rate (which system operators can control), evaluations of cleanup time should consider the number of pore volumes of ground water that must be extracted to achieve cleanup, in addition to the estimated cleanup time.

  • The operation of pump-and-treat systems should be viewed as a long-term project, in which the system's design is modified in response to improved understanding of the site. Because of the complexity of the contaminated subsurface, the performance of a pump-and-treat system will always be uncertain until the system is tested by beginning the cleanup. Monitoring provides the information necessary to optimize the system's performance and ultimately determine whether it will be able to reach cleanup goals.

NOTES

1.  

See 51 Fed. Reg. 21,653, June 13, 1986; 51 Fed. Reg. 27,062, July 29, 1986; 51 Fed. Reg. 41,088, November 13, 1986.

2.  

To prepare Figure 3-3, the committee assumed that the areal extent of the clay lenses is such that contaminant flow out of lenses is essentially one-dimensional and can be described by Fick's law:

where .Jx is the contaminant flux from the clay lens, De is the porous media molecular diffusion coefficient, and C is the concentration of the contaminant in the clay lens. (The porous media diffusion coefficient is a function of the water molecular diffusion coefficient for the contaminant corrected for the porosity and the tortuosity of the day lens.) The committee further assumed that the relative concentration of the trichloroethene in the clay lens is one (unit concentration) as a result of the long period of contamination, while the trichloroethene concentration in the sands is zero because the pump-and-treat system has

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

   

removed the contaminant from the water stored in the sand portion of the aquifer. Using these initial and boundary conditions, the committee followed a solution method for Fick's law similar to that described by Carslaw and Jaeger (1959) for heat flow from a solid bounded by parallel planes. The committee assumed that the porosity of the clay lens is 0.35, the retardation factor for trichloroethene in the clay lens is 2, and the water molecular diffusion coefficient for trichloroethene is 8.3 × 10-6 cm2/s, which results in a porous media molecular diffusion coefficient of 1 × 10-6 cm2/s.

3.  

For this example, the committee based its computations on a 1 m3 volume of aquifer and used the following assumptions: (1) a porosity of 0.3, (2) a ground water flow rate of 0.03 meters per day (typical of conditions in fine-grained sands), (3) a dissolved trichloroethene concentration equal to 10 percent of the aqueous solubility of this compound (a value consistent with field observations but less than that indicated by theoretical calculations) (Hunt et al., 1988), and (4) a random distribution of the trichloroethene globules. The density of trichloroethene is 1.47 g/cm3, and its water solubility is about 1,100 mg/liter. With these assumptions and trichloroethene properties, the following calculations show the time required to dissolve the trichloroethene globules:

Total contaminant mass

=

30 liter/m3 × 1 m3 × 1.47 g/cm3 × (100 cm/m)3 × 10-3 m3/liter

 

=

44,100 g

Concentration of dissolved trichloroetherne

=

10% × 1,100 mg/liter

 

=

110 rag/liter

Mass flux through 1-m2 area

=

0.03 meter/day × 1 m2 × 110 mg/liter × 10-3 g/mg × 103 liter/m3 × 0.3

 

=

0.99 g/day

Time required to dissolve residual trichloroethene

=

44,100 g/(0.99 g/day)

 

=

122 years

4.  

The rate at which the trichloroethene dissolves from the pool was estimated from the following equation, which is based on Hunt et al. (1988, 1989):

where

T is the thickness of DNAPL removed per unit time; Vx is the ground water velocity; Y is the aquifer thickness; Cs is the water solubility of the DNAPL; L is the length of the DNAPL

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

   

pool in the direction of ground water flow; erfc is the complementary error function; ρ is the DNAPL density; n is porosity; αt is the transverse dispersivity (which describes dispersion normal to the ground water flow direction); and De is the porous media molecular diffusion coefficient. For a given DNAPL pool geometry, the rate of dissolution in most aquifers is a function primarily of the ground water velocity and the transverse dispersion length. For this example, the committee assumed that the aquifer has a porosity of 0.35, a thickness of 5 meters, and a bulk molecular diffusion coefficient of 1 × 10-6 cm2/s.

REFERENCES

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Carslaw, H. S., and J. C. Jaeger. 1959. Conduction of Heat in Solids, 2nd ed. Oxford:Clarendon Press.


Doty, C. B., and C. C. Travis. 1991. The Effectiveness of Groundwater Pumping as a Restoration Technology. Knoxville: University of Tennessee, Waste Management Research and Education Institute.


EPA (Environmental Protection Agency). 1985. Modeling Remedial Actions at Uncontrolled Hazardous Waste Sites. EPA/540/2-85/001. Cincinnati, Ohio: EPA, Risk Reduction Engineering Laboratory.

EPA. 1988. Guidance on Remedial Actions for Contaminated Ground Water at Superfund Sites. EPA/540/G-88/003. Directive 9283.1-2. Washington, D.C.: EPA, Office of Solid Waste and Emergency Response.

EPA. 1989a. Evaluation of Ground-Water Extraction Remedies, Volume 1: Summary Report. EPA/540/2-89/054a. Washington, D.C.: EPA.

EPA. 1989b. Evaluation of Ground-Water Extraction Remedies, Volume 2: Case Studies. EPA/540/2-89/054b. Washington, D.C.: EPA.

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EPA. 1990. Handbook on In Situ Treatment of Hazardous Waste-Contaminated Soils. EPA/540/2-90/002. Cincinnati, Ohio: EPA, Risk Reduction Engineering Laboratory.

EPA. 1992a. Evaluation of Ground-Water Extraction Remedies: Phase II, Volume 1—Summary Report. Publication 9355.4-05. Washington, D.C.: EPA, Office of Emergency and Remedial Response.

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EPA. 1993. Guidance for Evaluating the Technical Impracticability of Ground-Water Restoration. Directive 9234.2-25. Washington, D.C.: EPA, Office of Solid Waste and Emergency, Response.


Harman, J., D. Mackay, and J. Cherry. 1993. Final Report to U.S. Air Force. Waterloo, Ontario, Canada: Waterloo Centre for Ground Water Research.

Hunt, J. R., N. Sitar, and K. S. Udell. 1988. Nonaqueous phase liquid transport and cleanup 1: analysis of mechanisms. Water Resources Res. 24(8):1247-1258.

Hunt, J. R., N. Sitar, and K. S. Udell. 1989. Correction to nonaqueous phase liquid transport and cleanup 1: analysis of mechanisms . Water Resource Res. 25(6):1450.

Suggested Citation:"3 Performance of Conventional Pump-and-Treat Systems." National Research Council. 1994. Alternatives for Ground Water Cleanup. Washington, DC: The National Academies Press. doi: 10.17226/2311.
×

International Technology Corporation. 1989. Remedial Investigation: Former Firestone Facility. Salinas, Calif.: John Steinbeck Library.


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There may be nearly 300,000 waste sites in the United States where ground water and soil are contaminated. Yet recent studies question whether existing technologies can restore contaminated ground water to drinking water standards, which is the goal for most sites and the result expected by the public.

How can the nation balance public health, technological realities, and cost when addressing ground water cleanup? This new volume offers specific conclusions, outlines research needs, and recommends policies that are technologically sound while still protecting health and the environment.

Authored by the top experts from industry and academia, this volume:

  • Examines how the physical, chemical, and biological characteristics of the subsurface environment, as well as the properties of contaminants, complicate the cleanup task.
  • Reviews the limitations of widely used conventional pump-and-treat cleanup systems, including detailed case studies.
  • Evaluates a range of innovative cleanup technologies and the barriers to their full implementation.
  • Presents specific recommendations for policies and practices in evaluating contamination sites, in choosing remediation technologies, and in setting appropriate cleanup goals.
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