Alternatives for Ground Water Cleanup (1994) / Chapter Skim
Currently Skimming:
3 Performance of Conventional Pump-and-Treat Systems
3 Performance of Conventional Pump-and-Treat Systems
Pages 80-124
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 80... ...
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-andtreat systems have performed and whether it is reasonable to expect that they can achieve drinking water standards.
|
From page 81... ...
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.
|
From page 82... ...
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.
|
From page 83... ...
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 pumpand-treat systems have reached cleanup goals.
|
From page 84... ...
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.
|
From page 86... ...
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.
|
From page 87... ...
Regardless of the amount of contaminated water extracted, as long as significant diffusion occurs out of the lowpermeability zones, it is possible that ground water cleanup standards will not be met. Geologic complexities and the presence of sorbed or nonaqueousphase contaminants may affect the outcome of pumping and treating by causing progress toward cleanup to tail off above the cleanup goal.
|
From page 88... ...
_ ~ ALTERNATIVES FOR GROUND WATER CLEANUP 1.0 Cat o ~ 0.5 o c) :~ ._ o Thoeretical Removal \ Removal with Tailing o Water-filled Aquifer Volumes FIGURE 3-1 The effect of tailing on cleanup time.
|
From page 89... ...
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.
|
From page 90... ...
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.
|
From page 91... ...
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, 19931. At this site, the initial total concentration of the gasoline components benzene, toluene, ethylbenzene, and xylene (BTEX)
|
From page 92... ...
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.
|
From page 93... ...
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
|
From page 94... ...
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.
|
From page 95... ...
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 O ~ B X ~ Cp ~ '_ i'.
|
From page 96... ...
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.
|
From page 97... ...
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.
|
From page 98... ...
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.
|
From page 99... ...
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.
|
From page 100... ...
100 ALTERNATIVES FOR GROUND WATER CLEANUP Appropriate Uses for Pum~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.
|
From page 104... ...
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.
|
From page 105... ...
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: 2 x 108 liters 1 day 1 yr 380 liters/min 1440 min 365 days x 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.
|
From page 106... ...
2. 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.
|
From page 107... ...
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 Actionsfor 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~.
|
From page 108... ...
(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.
|
From page 109... ...
The effect of item 5leachate 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.
|
From page 110... ...
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.
|
From page 111... ...
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.
|
From page 112... ...
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.
|
From page 113... ...
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.
|
From page 115... ...
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 ricing lA7~t~r levels in injection wells that may signal clogging. ~ ~, ~c, ,, ~ ~ 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.
|
From page 116... ...
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
|
From page 117... ...
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.
|
From page 118... ...
· 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?
|
From page 119... ...
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.
|
From page 120... ...
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)
|
From page 121... ...
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 x 10-6 cm2/s, which results in a porous media molecular diffusion coefficient of 1 x 10~ cm2/s.
|
From page 122... ...
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 x 10-6 cm2/s.
|
From page 123... ...
Presentation before the National Research Council's Committee on Ground Water Cleanup Altematives, Washington, D.C., March 24, 1992. Smedes, H
|
From page 124... ...
1990. Can contaminated aquifers at Superfund sites be remediated?
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.