Alternatives for Ground Water Cleanup (1994) / Chapter Skim
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5 Characterizing Sites for Ground Water Cleanup
5 Characterizing Sites for Ground Water Cleanup
Pages 193-212
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From page 193... ...
The lack of proper characterization has often occurred even after huge sums of money have been spent and considerable time has elapsed in characterizing the site. Proper site characterization is an essential component of designing a ground water cleanup system, but at the same time perfection in site characterization is unattainable.
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From page 194... ...
Site characterization studies need not be designed to develop as complete a picture as possible of the subsurface environment. Highly detailed characterization studies may be appropriate at research sites, but they are not needed at most sites.
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From page 195... ...
However, efforts should be made to identify subsurface geologic environments where DNAPL accumulations may be present. Given the purpose of site characterization as expressed in these documents and the limited number of currently available remedial alternatives for contaminated ground water, it follows that site characterization studies can focus on collecting the data required to choose among a limited number of alternatives.
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From page 196... ...
uration on fracture surfaces remains as a long-term source of dissolved contaminants. An example of a site with a mineral phase acting as a long-term source of ground water contamination is the United Chrome site in Oregon (listed in Appendix A)
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From page 197... ...
Natural attenuation may be acceptable for some zones where it can be shown that natural biological, chemical, and physical processes will lower contaminant concentrations to cleanup goals before the contaminants reach receptors; it may also be acceptable for ground water that discharges into surface water bodies where it can be demonstrated that the ground water will not be used and the resulting concentrations in the surface water body will be below applicable standards. Once the goal of remediation is established, site characterization studies should be designed to collect the data required to design a technically effective and cost-effective remedial action to obtain the desired end result (whether containment, active restoration, or natural attenuation)
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From page 198... ...
\t x., / Pumping Well for Source Containment Restored Aquifer ~ _ _ , Pumping Well for Source Containment 1 Natural Attenuation Ground Water Flow i5S ~ / Restored Aquifer jig /~ ___________ ~ FIGURE 5-2 Options for managing sites with contaminated ground water. In the option shown at the top, a pumping well is installed to prevent enlargement of the plume of dissolved contaminants but not to clean up the plume.
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From page 199... ...
In simplest terms the required data include · information to define or estimate the horizontal and vertical extent of ground water contamination, · information to estimate the locations of contaminant source areas, · information to describe the hydrogeologic setting, and · information to estimate the site's restoration potential.
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From page 200... ...
· Description of regional and she geology Stratigraphy, including thickness, lateral extent, continuity of units, and preferential pathways · Depth to ground water · Hydraulic gradients · Hydraulic conductivity, storage coefficient, effective porosity · Temporal variability in water levels · Ground water recharge and discharge · Ground water/surface water interactions FIGURE 5-3 Categories of data required to select and design a ground water remedial action (adapted from EPA, 1993b)
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From page 201... ...
Unfortunately, the relatively low contaminant concentrations typically found in ground water are difficult to measure with these nondisruptive methods. Similarly, minimally intrusive techniques, such as soil vapor analyzers, can be useful for indicating the location of plumes near the water table, but they are not useful for indicating deep areas of contamination.
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From page 202... ...
What constitutes adequate vertical definition of a ground water problem is a frequently asked question in site characterization studies. At many sites, especially those with DNAPLs in fractured rock, contamination can extend to depths of 300 meters and more.
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From page 203... ...
. DNAPL sources are difficult to identify because they are denser than water, can migrate below the water table (often to great depths)
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From page 204... ...
The 1 percent rule is frequently used in site characterization studies, and, given the available characterization technologies, it is the best means for estimating the extent of DNAPLs in the subsurface. At many sites where DNAPL presence is suspected, investigators are reluctant to define the vertical extent of contamination because they fear remobilizing the DNAPLs and inducing new downward migration.
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From page 205... ...
At the United Chrome site listed in Appendix A, the conclusion that a mineral phase was controlling the cleanup rate was a hypothesis that was developed and tested after data from a pump-andtreat system indicated that concentrations were not changing with time. From a theoretical perspective, describing the processes controlling min
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From page 206... ...
Restoration Potential Traditionally, site characterization studies have been conducted with
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From page 207... ...
Thus, an explicit step in the site characterization process should be to estimate the restoration potential of each subarea of the contaminated site. Table 3-2, in Chapter 3, shows a method for estimating restoration potential developed by the committee.
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From page 208... ...
Because of the complexity of the subsurface environment and the long time frames required for ground water cleanup, site characterization must not be viewed as a onetime event or even a several-time event that ends when the final remedial investigation report is submitted. Hydrogeologic and water quality data will be collected throughout the life of a ground water cleanup system, and these data should be reviewed regularly to determine if the cleanup system is meeting its objectives and, if not, whether adjustments are needed.
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From page 209... ...
;; :. ` ~ Insbllation of Containment System to Prevent Expansion of Dissolved Plume Pilot i Stud ~ Interim Actlon Hydraulic ,ainment l ~ _W Installation of Containment System for Source Areas .~ FIGURE 5-4 Iterative stages of site characterization (adapted from EPA, 1993b)
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From page 210... ...
Remediation efforts have also suffered from the inability of site characterization to accurately predict early on the restoration potential of the site and to quantify and locate subsurface sources of contamination. Research to improve the speed of site characterization and its ability to locate sources of contamination would greatly improve the ability to carry out effective ground water cleanups.
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From page 211... ...
· What measures-technical and institutional-are needed to ensure that knowledge about the site gained during the course of remediation will be considered in reassessing cleanup objectives and the design of the treatment system during long-term cleanup projects? CONCLUSIONS Based on a review of the processes for characterizing contaminated sites in order to design ground water cleanup systems, the committee reached the following conclusions: · The goal of site characterization studies should be the collection of sufficient data to select and design a remedial action.
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From page 212... ...
· Improved methods are needed for assessing the scope and distribution of contamination. Exishng site characterization techniques car~not always accurately locate subsurface sources of contamination and are often costly and slow.
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