SPECIAL REPORT
Contaminated Groundwater: The National Research Council's Study of Groundwater Cleanup Alternatives
I
n recent debates in Congress about reauthorizing the Superfund statute and the Resource Conservation and Recovery Act (RCRA), a key point of contention has been the degree to which contaminated groundwater should he cleaned up. Different interest groups disagree strongly on such issues as "How clean is clean?" and whether industrial sites should be cleaned up to the same standards as sites nearer to residential areas. But often overlooked in this debate is a fundamental question: Can existing technology restore contaminated groundwater to health-based standards? In the past several years, numerous scientific publications have raised serious concerns about the ability of technology to restore contaminated groundwater (1-5).When Congress enacted RCRA and Superfund and later amended these laws, policy makers assumed that technology could reverse the contami~~~
J A C Q U E L I N E A. MACDONALO Notional Research Council Washington, DC20418
M I C H A E L C, KAVANAUGH ENVIRON Corporation
Emerwille. CA 94608 362 A Environ. Sci. Technol., Vol. 28. NO.8, 1994
nation problems of the past. As a result, the cleanup goal at most RCRA and Superfund sites is to restore contaminated groundwater to drinking-water standards. Achieving these standards was the groundwater cleanup goal selected for 270 of approximately 300 Superfund cleanup decisions issued between October 1987 and September 1991 (61.
According to a new National Research Council (NRC) study puhlished this month, the answer to whether contaminated groundwater can he restored to drinking-water standards is not a simple "yes" or "no." Groundwater restoration may be possible for sites with relatively simple contamination scenarios, and at more complex sites cleanup of part of the contaminated area to health-based standards may be possible. But for the most complex sites, which includes the majority of Superfund sites, existing technologies may he unable to restore significant areas to health-based standards. The three-year NRC study, Alternatives for Ground Water Cleanup, assessed the degree to which contaminated groundwater can be restored. The study was carried out by the 19-member Committee o n Ground Water Cleanup Alternatives, chaired by Michael C. Kavanaugh, which included experts in hydrogeology, environmental engineering, chemistry, epidemiology, risk assessment, and environmental
0013-936X/94/0927-362A$04.50/0 0 I F 4 American Chemical Society
policy. The membership reflected not only the range of technical expertise required to assess the potential for groundwater cleanup but also the range of interest groups involved in this issue, from environmental organizations to industry. A major focus of the study was a review of "pump-and-treat" systems, the prevailing method used to restore groundwater in the United States. Pump-and-treat systems are employed at approximately threequarters of Superfund sites where groundwater is contaminated and at most sites governed by RCRA and state laws. These systems pump the contaminated water to the surface through a series of extraction wells, treat it at the surface to remove the contaminants, and then either reinject the water underground or dispose of it off site. In theory, these
systems clean the groundwater by flushing enough clean water through the subsurface to dissolve and remove all of the contaminants. Pump-and-treat performance In evaluating the performance of pump-and-treat systems, the committee searched for reports in technical journals, government and industry trade group publications, and technical conference proceedings. Although thousands of pumpand-treat systems are operating in the United States (the exact number is unknown), comprehensive studies of the performance of individual pump-and-treat systems ape rare. In all, the committee identified 77 sites where sufficient data were available to evaluate the performance of the system. Most of these sites were reviewed in studies by
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the American Petroleum Institute, the Department of Energy's Oak Ridge National Laboratory, EPA, and the San Francisco Bay Regional Water Quality Control Board (7-201. The committee analyzed the data as reported in these studies, evaluating the quantity of water flushed through the site with the pumpand-treat system, the number of years in operation, the hydrogeologic conditions, the system's effectiveness in containing the contamin a t i o n , a n d the system's effectiveness in reaching cleanup goals. In considering the data from these 77 sites, three factors are important to keep in mind. First, the time period during which pump-and-treat systems have been operating at these sites is relativelv short. On average, the available'data covered only about five to seven years. It is possible that continued operation of these systems will yield more successes. Second, even at sites that were reportedly cleaned up, it is possible that contamination remains but has not yet been detected by monitoring wells. Third, at some sites, cleanup with pump-and-treat systems might be achieved more rapidly if the extraction rates were higher. The committee compared the extraction rate to the size of the contaminated area at 24 sites to determine the number of pore volumes of contaminated water extracted each year, which indicates the number of times the pump-andtreat svstem flushes the site. At 13 of the 54 sites, the extraction rate is less than 1 pore volume per year. Attainment of cleanuD criteria at most sites under the most favorable circumstances can be expected to take decades, with extraction rates of less than one pore volume per year. On the basis of the data from the 77 sites, the committee concluded that cleaning u p groundwater to health-based standards using conventional pump-and-treat systems may be feasible at a limited number of sites having relatively simple characteristics. The sites that are simplest to clean up are those where the contaminants are present in dissolved form, the geology is relatively homogeneous, and the contamination is recent. However, the committee determined that portions of most sites will remain contaminated above health-based levels even after long-term operation of a conventional pump-and-treat system.
The committee developed a system for evaluating the difficulty of cleaning up groundwater based on a scale of 1through 4 (Table l ) , where sites rated 1are easiest to remediate and those rated 4 are the most difficult to remediate. Based on an as sessment ofthe 77 sites according tc this rating system, the committet determined t h a t groundwater cleanup is likely to be extremely complex at a very large numer of sites. For example, only 2 of the 77 sites received a rating of 1, and 34 received a rating of 4. While the 77 sites were not drawn from a statistical sample of the total universe of contaminated sites, the committei nevertheless believes that they a r i representative of conditions founc at many contaminated sites. At 8 of the 77 sites the committei reviewed, cleanup goals were re portedly achieved using conven tional pump-and-treat systems after one to several years of operation. A typical example of a site where cleanup goals were apparently achieved is a service station where a 10,000-gal underground fuel tank ruptured. At this site, emergency workers pumped out 22,000 gal of groundwater mixed with gasoline from an excavated pit around the tank, and a pump-and-treat system consisting of one extraction well began operating within eight mouths. After 3 years, the pump-and-treat system reduced contaminant concentrations from parts-per-million levels to below regulatory standards in all of the monitoring wells. The committee attributed the success of cleanup in part to the relatively simple characteristics of the site and the rapid initial response. Unlike most sites, the source of contamination at this site was quickly controlled a n d a large fraction removed. The plume of contaminated groundwater was therefore relatively small. In addition, the pumpand-treat system removed dissolved contaminants before they diffused deep into the solid materials in the aquifer. Finally, many of the ingredients of gasoline biodegrade rapidly, reducing the quantity of contaminants requiring removal by the pump-and-treat system. Much more common in the committee’s review were sites where pump-and-treat systems have reduced the size of the contaminated zone but even after several years of operation have been unable to restore the full site to health-based standards. An example of this type of site is a computer manufacturing
tive ease ot cleaning up contarnmatea grounawater , , , I Jifficulty of cieanup is influenced by the hydrogeologic conditions and contaminant chemistry at a site. The NRC report classified the relative easi of cleanup as a function of these two conditions on a scale of 1 to 4, where is the easiest and 4 the most difficult.
HydmgwlDgy
Homogeneous, single layer Homogeneous. multiple layers Heterogeneous, sinale - laver , Heterogeneous, multiple layers Fractured
Mobile, dissolved (degrades! Mobile, voiatilizes) dissolved
1
Contaminant chemistry Strongly sorbed, dissolved Strongly Separate (degrades/ sorbed, phase volatilizes) dissolved LNAPL-
Separal phase ONAPL
1-2
2
2-3
2-3
3
3
3
3
4
4
1
2
2 3
The 1-4 scale used m this table shwld not be viewed as obiective and fixed, but as a subiecl flexible method for evaluatimg sites Otherlanom that influence ease of cleanup. such as the t contaminant mass at a site and the length o f l i r n e ~ i n ~steWBS released. are not shown ~nthis la Source: Reference 1 ‘Light nonaqueous phase liqud ’Dense nonaqueaus.phase lquid
facility where a waste solvent tank ruptured and spilled the solvents 1.1,l-trichloroethane and Freon 113. By the time the rupture was discovered, the contaminant plume was relatively large-1,700 m long and 300 m wide, the majority of it off site. The site owners installed a pump-and-treat system consisting of 11extraction wells, initially with the goal of restoring the entire site to health-based standards. After nine years of pumping, cleanup goals had been achieved in the off-site portion of the contaminant plume, and the company was able to shut down off-site pumping wells. However, efforts to reach cleanup goals continue on site, 12 years after the pump-and-treat system was installed. Investigators have determined that the contaminant characteristics and geologic conditions at this site are extremely complex. Some of the solvents appear to have migrated deep into the subsurface as dense nonaqueous-phase liquids (DNAPLsl, and the geologic setting is highly heterogeneous. These factors will make it extremely difficult, perhaps impossible in the foreseeable future, to reach cleanup goals in the area nearest to the original source of contamination. In effect, this site consists of two distinct zones: one consisting primarily of dissolved contaminants, where cleanup is feasible, and one consisting of sources of contamination that
may be impossible to remove. The committee’s analysis of 77 sites indicated that at many sites, project managers will confront problems like those at the computer manufacturing facility. At least 34 of the 77 sites the committee reviewed contain nonaqueous-phase liquid (NAPL) contaminants in heterogeneous geologic environments. Similarly, EPA estimates that 60% of Superfund sites have a medium to high likelihood of containing dense nonaqueous-phase liquid contaminants (11). Under such conditions, health-based cleanup goals are unlikely to be achieved in the foreseeable future for significant portions of the contaminated area. Innovative technologies In addition to analyzing conventional pump-and-treat systems, the committee evaluated innovative groundwater and soil cleanup technologies currently under testing and development. Many of the innovations the committee reviewed may be able to decrease the costs and/or increase the efficiency of groundwater cleanup. However, the committee determined that no existing technology, conventional or innovative, can overcome all of the difficulties associated with groundwater cleanup. In its review, the committee divided innovative technologies into two categories: “enhanced pumpand-treat systems” and “alternative
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technologies” (Table 2). Enhanced pump-and-treat systems circulate fluids, either water or air, through the subsurface. For example, s o i l vapor extraction a n d a i r sparging systems circulate air to remove volatile contaminants. In s i t u bioremediation systems circulate air andlor water a m e n d e d with substancethat promote contaminant biodegre dation. A l t h o u g h these method. have advantages, such as increasing the amount o f contaminant removed per unit volume of fluid circulated or treating contaminants in place in the subsurface, the requirement that they circulate f l u i d s underground means that a l l w i l l have some of the key limitations o f conventional pump-and-treat systems. For example, enhanced pump-andtreat systems will have difficulty removing contaminants f r o m heterogeneous geologic regions because circulating fluids through these regions i s problematic. Likewise, they will have d i f f i c u l t y removing NAPL contaminants because p r o m o t i n g contact between the NAPLs a n d the treatment fluids i s difficult. T h e alternative technologies d o n o t i n v o l v e pumping f l u i d s . By eliminating the need t o pump fluids and treat them above ground, these technologies can reduce the costs o f site management. H o w e v e r , t h e l o n g - t e r m effectiveness o f these methods has n o t b e e n demonstrated. For example, the in s i t u reactive barriers listed in Table 2 are in t h e early research a n d testing stages; i t i s n o t k n o w n whether a l l of the engineering challenges o f using t h e m c a n b e o v e r c o m e a n d whether they w i l l be effective for prolonged use. In addition, these methods are slow. In s i t u reactive barriers, for example, can treat contaminants o n l y at the rate o f natural groundwater flow, w h i c h generally ranges f r o m 0.03 m t o 0.3 m ( - 0.1 t o 1ft) per day. Another alternative method, physical containment, does n o t result in permanent site cleanup. Although none of the innovative technologies the committee evaluated can restore groundwater at a l l sites to health-based standards, many of these technologies can decrease the costs and increase the efficiency of groundwater c l e a n u p and therefore should he used more widely. However, barriers exist t’ using m a n y o f these technologies The major barrier i s management c the risk o f failure, w h i c h i s inheren in using less-tested technologies..i the difficult-to-characterize subsur366 A Environ. Sci. Technol.. Vol. 28, No. 8, 1994
iative groundwater cleanup technologies ‘OlOgY
Lkrcrlptlon
mstrated pumpand-heat technologiess apor Flushes air through soil above :tion the water table to extract volatile contaminants I Pumps materials through the nediation- subsurface to stimulate arowth of caroons organisms that biooegraire contam
