Environ. Sci. Technol. 1994, 28, 769-775
Degradation of Chlorinated Hydrocarbons and Groundwater Geochemistry: A Field Study Walt W. McNab, Jr., and 1. N. Narasimhan'
Earth Sciences Division, Lawrence Berkeley Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, Berkeley, California 94720 Published laboratory studies suggest that certain chlorinated hydrocarbons are subject to chemical degradation in groundwater through abiotic and biologically-mediated processes. However, relatively few field investigations have been conducted on the degradation of these compounds. We examine the issue of degradation with regard to several chlorinated aliphatics dissolved in groundwater at the Lawrence Livermore National Laboratory in California. The highly oxidizedstate of the localgroundwater, as indicated by geochemical observations, suggests that reductive degradation reactions of chlorinated ethenes and ethanes are not thermodynamically or microbially favorable. In contrast, it is known that certain chlorinated compounds, such as l,l,l-TCA, may degrade through redox-independent elimination or hydrolysis reactions. Indeed, statistical analyses and numerical modeling have provided evidence for the nonredox-driven transformation of l,l,l-TCAto 1,l-DCE and to possibly acetic acid at the site. Similar analyses conducted for PCE and TCE failed to indicate any evidence for reductive dehalogenation. Introduction Numerous laboratory studies reported in the literature in recent years suggest that volatile organic compounds (VOCs), including chlorinated alkanes and alkenes, may undergo chemical degradation through a number of mechanisms (1-8). Knowledge of such degradation processes is clearly important in predicting the fate of organic contaminants in groundwater systems. However, our ability to recognizeand quantify degradation processes in the field is greatly limited for a variety of reasons. Unlike in the laboratory, the chemical species of interest exist within a moving body of water, are subject to mechanical mixing and molecular diffusion, and are retarded by sorption to differing degrees. The subsurface environment is typically heterogeneous, both physically and chemically, so that patterns of solute transport and degradation may vary spatially. In addition, our ability to investigate the subsurface is constrained both physically and economically, so that the spatial and temporal resolution of monitoring data is often very coarse. Yet, despite these difficulties, an understanding of degradation processes on a site-specific basis is essential in the calculations of risk assessment and in the planning of remedial strategies. In this study, we present the results of a field study of the degradation of dissolved chlorinated solvents in groundwater at the Lawrence Livermore National Laboratory (LLNL) in Livermore, CA. The investigation involved (1)postulating which degradation reactions would be favorable at the site, given the contaminant inventory,
* To whom correspondence should be addressed at 364 Hearst Mining Building, University of California, Berkeley, Berkeley, CA 94720. 0013-936X/94/0928-0769$04.50/0
0 1994 American Chemical Soclety
the local groundwater geochemistry, and the results of previous published studies and (2) searching for patterns in the site-monitoring data indicative of degradation. As we discuss in this paper, the findings of our field investigation agree well with other published reports in the literature. Site History The LLNL site is located in the Livermore Valley of the California Coast Ranges, approximately 40 mi east of San Francisco (Figure 1). In the early 1980s, VOCs were detected in soil and groundwater in the site vicinity as a result of historical usage and disposal at the site since the 1940s. VOCs in groundwater occur in relatively low concentrations, underlying about 85% of the site or a total area of about 1.4 square mi. The vertical thickness of the VOCs plumes in groundwater varies from about 10to about 30 m, with VOCs seldom being found below a depth of 70 m. VOCs are relatively mobile in groundwater at LLNL and migrate at a rate of about half the velocity of the groundwater, which flows at a rate on the order of 20mlyear (9). Trichloroethene (TCE)and tetrachloroethene (PCE) are the predominant species and are currently present in concentrations of up to 4.8 and 1.1ppm, respectively (1992 data). High concentrations are localized, and total groundwater VOC concentrations exceed 1 ppm at only a few locations. Other VOCs identified at the site include 1,l-dichloroethene (l,l-DCE), cis- and trans-l,PDCE, l,l,l-trichloroethane (l,l,l-TCA),1,l-dichloroethane(1,lDCA), 1,2-DCA, Freon-113, carbon tetrachloride, and chloroform. Degradation Mechanisms of VOCs VOCs are known to degrade through a variety of processes, either abiotic or biologically-mediated. The preferred degradation pathway exhibited by a given compound or series of compounds is typically a reflection of local groundwater chemistry, microbiology, and the susceptibilityof the compound to hydrolysisor elimination. Redox Reactions. As a rule-of-thumb, highly halogenated VOCs, such as those found at LLNL, tend to degrade primarily through reductive reactions, while their less chlorinated counterparts tend to be subject more to oxidation (8). For the more halogenated compounds, susceptibility to reduction results from the relatively oxidized state of the carbon atoms which balances the negative charges associated with C1- or B r . The most familiar reduction mechanism for halogenated alkanes and alkenes is reductive dehalogenation (refs 1, 2,4,5, and others). This process involvesthe replacement of halogen atoms on the molecule by hydrogen atoms, thus consuming valence electrons from a suitable donor species and yielding a less-halogenated VOC. Vogel et al. (8)used standard potentials for a number of reductive dehalogenation half-reactions as a guide to redox constraints on Environ. Sci. Technol., Vol. 28. No. 5, 1994 780
Figure 1. LLNL area map. After Noyes (21).
the energetic favorability of the reactions. In general, such reactions occur at or below pE or Eh values low enough to exclude the presence of dissolved 02 and in some cases NOf. Furthermore, abundant evidence in the literature suggests that these reactions are biologically-mediatedby anaerobic microbes which primarily flourish under reducing conditions. Thus, the issue of reductive dehalogenation of each class of VOCs in LLNL groundwater (chlorinatedethenes, ethanes, and methanes) involvesthe assessment of redox conditions. There is some evidence that oxidative degradation mechanisms may be available to some chlorinated VOCs. These mechanisms include hydroxylation (the replacement of hydrogen atoms in halogenated alkanes with hydroxyl groups, yielding a halogenated alcohol) and epoxidation (replacement of the double carbon-carbon bond in halogenated alkenes with a single bond and the attachment of an oxygen atom to the molecule, thus producing an epoxide). These degradation mechanisms are less understood and are difficult to study due to the highly transient nature of the intermediate degradation products (8). To date, chlorinated alcohols and epoxides have not been detected in LLNL groundwater. We shall not consider these mechanisms further in the present study. Nonredox Reactions. Other VOC degradation mechanisms are known to occur which do not involve electron transfer. These include substitution reactions, such as the replacement of halogen atoms with OH- groups (hydrolysis), and elimination reactions such as dehydrohalogenation. Dehydrohalogenation involves the removal of a halogen atom and a hydrogen atom from a halogenated alkane and converting the carbon-carbon single bond into a double bond, thus producing a less chlorinated alkene. Nonredox reactions are typically abiotic in nature, and thus reaction rates obtained in the laboratory may be more easily extrapolated to the field than those of redox reactions, provided that pH and temperature effects are duly considered. Among the VOCs noted in LLNL groundwater, chlorinated ethenes appear to be highly resistant to hydrolysis 770
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and elimination reactions (10). However, such reactions have been observed in the laboratory for l,l,l-TCA, 1,lDCA, and 1,2-DCA
-
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with half-lives short enough that their effects may be measurable in the field. Reactions 1and 2 have been noted by a number of workers (see refs 6,7,11, and others), with the half-life of l,l,l-TCA generally ranging from 0.5 to 2.5 years. Other workers have observed the abiotic degradation of l,l,l-TCA but did not conclusivelyidentify the daughter product or products (10,12).Reactions 3 and 4 were observed by Jeffers et al. (IO) under alkaline conditions only, with generally long half-lives (61 and 72 years, respectively). Reactions 1-4 can be shown to be thermodynamically favorable in LLNL groundwater. However, given the long half-lives of 3 and 4 and the lack of experimental evidence for this reaction under near-neutral pH conditions, the most obvious candidate for nonredox-driven degradation in LLNL groundwater is l,l,l-TCA. Redox Conditions i n LLNL Groundwater
Sediments underlying the Livermore Valley consist primarily of consolidated marine deposits of Quaternary and Tertiary ages, overlain by unconsolidated flood plain deposits derived from these rocks. Characteristic iron oxide staining has been observed in Holocene alluvium and in the Upper Member of the Livermore Formation underlying the LLNL site (13),suggesting that relatively oxidizing conditions prevailed during deposition or subsequently thereafter. This is consistent with the lack of significant quantities of organic material in the sediments.
The ubiquitous presence of dissolved 02 (up to 8 mg/L) in groundwater samples provides additional evidence for oxidizing conditions. It is well-recognized that attempts to classify redox conditions in groundwater using dissolved oxygen concentrations or with direct & measurements are unreliable (14-17). A more practical approach is to examine individual redox indicator species present in the system. In LLNL groundwater, the speciation of manganese and chromium provides additional evidence for oxidizing conditions. Manganese. The presence of manganese in the solid phase can be useful for bracketing possible groundwater pE values (18). Ubiquitous black specks noted in LLNL sediments during the installation of groundwater monitoring wells have been found to consist largely of MnO2 (13). However, concentrations of Mn2+in the vast majority of monitoring wells sampled for manganese (132 out of 162) were below the detection limit of approximately 1 X 10-7 mol/L at the time of measurement. Given the mean pH of 7.6 in LLNL groundwater and the half-reaction for the equilibration of MnO2 (birnessite or pyrolusite) with aqueous Mn2+ Mn2+f 2H20
4H+ + 2e- + Mn02(s)
100
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it may be shown that the redox couple-specific pE is on the order of at least 9.0-9.5 (or an E h of 500-600 mv), implying fairly oxidizing conditions. Chromium. Aqueous chromium may exist in more than one oxidation state and thus may also be used to assess redox conditions in groundwater. Two important chromium oxidation states in groundwater are trivalent chromium, Cr3+, and hexavalent chromium, Cr6+. Analyses performed on samples from various LLNL monitoring wells have included tests for total chromium and Cr6+.In an evaluation of data from 39 monitoring wells, it was found that in over 80% of the samples the ratio of Cr6+ to total Cr exceeded 0.5, with Cr6+apparently the exclusive form of chromium in over 50% of the wells. Although historical releases of chromate (Cr6+) wastes to LLNL groundwater have occurred, detections of Cr6+noted in deep wells below the contaminated zones and in wells upgradient from the facility indicate the existence of natural sources of Cr6+(13). Thus, Cr6+is the dominant natural form of chromium in LLNL groundwater. Equilibrium between Cr(OH)2+ (trivalent chromium) and Cr0d2-(hexavalent chromium) at pH 7.6 implies a couplespecific pE on the order of 7.5-8.0 (& of 450-500 mV) (18). For Cr6+to dominate, higher pE values are implied. Thus, the speciation of dissolved chromium is consistent with that of manganese in suggesting relatively oxidizing conditions in groundwater at the site. Implications for ReductiveDehalogenation. Given the oxidizing nature of LLNL groundwater, it appears that reductive dehalogenation reactions of the VOCs present are unlikely to occur. Most of the reductive dehalogenations can be shown outright to be thermodynamically unfavorable. Moreover, the anaerobic microbe populations which mediate these reactions are not likely to flourish in such an environment. In fact, the respective end member compounds of the chloroethene and chloromethane reductive dehalogenation chains, vinyl chloride and methyl chloride, have never been detected in several thousand individual groundwater samples collected at the
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Time (Elapsed Days) Figure 2. Simulated temporal variation of the l,I-DCE:l,l,l-TCA concentratlon ratio (mol/L) with distance: Degradation scenario (top) and nondegradatlon scenario (bottom).
site. The end member of the chloroethane chain, chloroethane, has been detected only twice in over 2000 samples. Analysis of Field Data
We start with the hypotheses that the degradation of l,l,l-TCA through nonredox mechanisms may be favorable at the site, while the reductive dehalogenationsof the chlorinated VOCs are not. The next logical question is how to best use the data to confirm or refute these hypotheses. One obvious approach is to integrate under the inferred concentration contours for a given compound at different times, yielding an estimate for the change in mass in solution over time. However, the 300+ monitoring wells at the site cover an area of approximately 1.5 square mi, implying a spatial density on the order of only 1well/ h. Thus, the error associated with integrating under inferred concentration contour intervals may be very large. In addition, monitoring wells are generally sampled on a quarterly basis. Therefore, the spatial and temporal data resolution available at the LLNL does not readily permit accurate estimates of mass changes over time. A more suitable method for searching for evidence of degradation Environ. Sci. Technol., Vol. 28, No. 5, 1994 771
0
500
lo00
Flgure 3. l,l,l-TCA plume In southeastern sectlon of LLNL slte (Arroyo Seco area). After Thorpe et al. ( 73).
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Figure 4. 1,l-DCE plume In southeastern sectlon of LLNL slte (Arroyo Seco area). After Thorpe et al. ( 73).
is to examine trends in the concentration ratios of daughter- and parent compound members of degradation chains. Modeling. In a dynamic groundwater system, two basic mechanisms are responsible for changes in the concentration ratios of any two compounds over time. One mechanism is retardation due to sorption. If two chemically-stable compounds are initially present at the same location and extent within an aquifer, the more mobile species will obviously migrate further downgradient with respect to the less mobile specieswith time. Consequently, the concentration ratio of the more mobile species to the less mobile species should tend to decrease with time in monitoring wells located near the center of the less mobile plume. Chemical degradation is the other mechanism by 772
Environ. Scl. Technol., Vol. 28, No. 5, 1994
which concentration ratios may change. If two chemicals of equivalent mobility, one stable and one subject to degradation, are initially present at the same location, the ratio of the stable chemical to the degrading chemical will clearly increase over time at all locations. This ratio increase will be more pronounced if the degrading compound produces the nondegrading species as a daughter product. For l,l,l-TCAand l,l-DCE, both of the abovescenarios are applicable. l,l,l-TCA is more hydrophobic, and thus presumably less mobile, than 1,l-DCE;however, 1,l-DCE may also be a dehydrohalogenation daughter product of l,l,l-TCA. Because of the superimposed effect of retardation on the l,l-DCE:l,l,l-TCA concentration ratio, interpretation of the data in terms of degradation is a
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Flguro 5. Measured temporal variation of the 1,l-DCE: 1,1,1-TCA concentration ratio (top) and that of TCE:PCE (bottom) with distance in Arroyo Seco area wells (mol/L).
more complex task. In order to qualitatively evaluate the expected variation in the l,l-DCE:l,l,l-TCA ratio influenced by both degradation and sorption, a solute transport model was used to compare two possible scenarios: chemical degradation effects combined with retardation and retardation alone, Both scenarios were based upon hydrologic data collected from the LLNL facility. These data included an estimated mean Darcy velocity of 1.66 X m/day, a mean porosity of 0.3 (resulting in a pore velocity of 5.53 X m/day), and an apparent mean longitudinal dispersivity of 80 m (9). 1,l-DCEwas assumed to have a retardation factor of 1.0 (Le., negligible adsorption) as a control. The retardation factor of l,l,l-TCA was assumed to be 2.0, based upon the difference between the published water solubility values of the two chemicals as well as the organic carbon partition coefficients and the octanol-water partition coefficients (19). These values are in general agreement with Tompson (9), who states that retardation coefficients for VOCs in groundwater at the LLNL facility generally range between 1.2 and 2.1. For simplicity, it was assumed that transport is onedimensional (i.e., transverse dispersion is neglected). This
simplification is appropriate since the purpose of the modeling exercise is strictly comparative. An integral finite difference transport code capable of modeling sequential decay chains was utilized to perform the simulations (20). A hypothetical, one-dimensional section of an aquifer was discretized into 40 vol elements of identical size, each 25 m in length, resulting in a total length of 1000 m. An explicit time step of 50 days was selected for both scenarios. The simulation was run for a total of 9100 days (approximately 25 years), requiring 182 time steps. Presumably, VOCs were introduced into LLNL groundwater as a result of human activities for a number of years before the active sources were curtailed (13). To mimic this effect, for the first scenario, it was assumed that l,l,lTCA was the exclusive contaminant initially present at the inflow boundary at a constant concentration of 1ppm for the first 10years. During the entire 25-year simulation, l,l,l-TCA degraded into 1,l-DCE at a spatially and temporally constant rate corresponding to a half-life of 2.0 years. For the second scenario, l,l,l-TCA and 1,lDCE were both assumed to be present initially as contaminants, each with a constant concentration of 1 ppm at the inflow boundary for the first 10 years. Degradation mechanisms were not considered for the second scenario. The simulated ratios of the concentrations of 1,l-DCE: l,l,l-TCA (on a mol/L basis) for both scenarios are shown on Figure 2 for several points along the length of the column. The distinct observable difference between the two modeling cases is the change in the 1,l-DCE:l,l,lTCA concentration ratio over time at individual locations. For the reactive case, an increase in the ratio is predicted for at each observation point; for the nonreactive case, the ratio declines. Thus, according to the model, degradation effects should overwhelm retardation effects on the ratio given the set of parameters used. The most well-defined coexisting plumes of l,l,l-TCA and 1,l-DCE are located in the southwestern portion of the lab along the Arroyo Seco (Figures 3 and 4, respectively). The two plumes appear to be somewhat associated, although the 1,l-DCEplume is larger and extends further downgradient. A number of representative wells were selected for analysis of the behavior of the two chemicals based on their spatial distribution along the apparent Envlron. Scl. Technol., Vol. 28, No. 5, 1884 778
>iO.O
Slope Flgum 7. DLsMbutlon of temporal trends In the 1.1DCE:l.l.l-TCA
and the TCEFCE comentratkm ratios (rnl/L).
longitudinal axis of the l,l,l-TCA plume. The actual trends in the temporal evolution of the 1,l-DCEl,l,lTCA concentration ratio for the wells identified in these figures are shown on Figure 5. An increase in this ratio in the data is clearly evident. This is qualitatively consistent with the degradation scenario and entirely inconsistent with the nonreactive scenario. To investigate the effect of the hydrolysis of l,l,l-TCA to acetic acid on the l,l-DCEl,l,l-TCA concentration ratio, the degradation scenario was modeled for a second time with acetic acidasanadditionaldaughter product,using adegradation half-life of 2.0 and 10.0 years for the production of acetic acid and l,l-DCE, respectively. Only a slight change in the ratio evolution resulted. Thus, although the ratio methoddoes indeedstrongly suggest degradation of 1,lJTCA, it provides little assistance in identifying either 1,lDCE or acetic acid as the primary degradation product. For comparative purposes, a similar data analysis was performed on PCE and TCE concentrations measured in wellsinthesamearea,withTCEsuspectedofbeingslightly more mobile than PCE due to differences in hydrophobicity. The concentration ratios of TCEPCE over time, also shown on Figure 5, do not indicate any increasing trend. Thus, the data do not exhibit evidence for the reductive dehalogenation of PCE to TCE using the ratio analysis. Variations in Mass. Semiquantitative evidence for the degradation of 1.1,l-TCA was also be obtained by examining apparentchangesintotalmassofthecompound over time in the Arroyo Seco plume. Although this calculation was not feasible over the entire LLNL site, the relatively well-defined plume in the Arroyo Seco area allowed such a calculation on a local level. Relative total masses of both dissolved 1,lJ-TCA and 1.1-DCE along the Arroyo Seco were calculated hy integrating the line segments connecting the wells identified on Figures 3 and 4. The integration was performed TI4
Envton. Scl. Tscmol.. Vd. 28.
No. 5, 1994
by averaging the concentrations on an annual basis to minimize scatter, linearly weighting the concentrations based upon the distances between the wells, and summing up the areas of the resulting trapezoids. A corresponding graph of the variation of the relative totalmasses of 1,lJTCA and 1,l-DCE shows a pronounced decline in 1,l.lTCA concentrations compared to 1,l-DCE (Figure 6). It is unlikely that this relationship between the total masses of l,l,l-TCA and 1,l-DCE is due topreferential transport of 1.1,l-TCA as it is likely to be less mobile than 1,l-DCE. The decline in theconcentrationof l,l,l-TCA is equivalent toaneffectivehalf-lifeontheorder of2yean3,whichagrees very well with published values. However, the degree to which other mechanisms such as transverse dispersion influence the observed decline is difficult to establish. 1,l-DCE does not exhibit any evidence of decline and may exhibit aslight increase. Given the presumed greater mobility of l,l-DCE, it would be expected that some loss due to transverse dispersion should be observable over the 5-year period. It is conceivable that the generation of 1.1-DCE from the degradation of l,l,l-TCA may compensate to a degree for such a loss. However, because of mobility differences between the two compounds,and the pwsibility of acetic acid asan additional daughter product, it is not possible to establish a direct mass-balance relationship between the observed relative masses of l,l,lTCA and 1,l-DCE. Site-Wide Statistical Patterns. To gain further insight into the behavior of 1,lJ-TCA and l,l-DCE, temporal trends in the concentration ratio were studied in 64 individual wells located across the entire LLNL facility. Wells were selected for analysis when at least three different data points (Le., sampling dates) were available for which l,l,l-TCA and 1,l-DCE were both detected above 1ppb. Temporal trends in the ratio were evaluated byusinglin~regressiontoesti~te theaverage slope of the logarithm of the l,l-DCEl,l,l-TCA concen-
tration ratio. Positive values in this trend indicate increases in the ratio with time in a given well; negative values indicate a decrease. A histogram of the distribution of these slopes is shown on Figure 7, The distribution is noticeablyskewed to the right, implying that the 1,l-DCE l,l,l-TCAratio is increasing in more wells than decreasing. The same analysis for TCE and PCE applied to 23 wells in the Arroyo Seco area is depicted on the same histogram. Unlike the case for 1,l-DCE and l,l,l-TCA, this trend distribution does not appear to be skewed significantly, suggesting that average TCE:PCE concentration ratio in the area is largely stable with time. A final possible indicator of the degradation of l,l,lTCA to 1,l-DCE is the association of the two chemicals. Out of approximately 5600 individual samples evaluated for l,l,l-TCA and 1,l-DCE across the entire LLNL facility, less than 3% of the samples contained detectable l,l,lTCA concentrations but no detectable 1,l-DCE concentrations. This very strong association among such a large number of samples indeed suggests 1,l-DCE as a degradation product of l,l,l-TCA,given the reported detection of this reaction elsewhere in the literature (6, 7). In contrast, out of some 400 individual groundwater samples collected from the Arroyo Seco area, which were included as part of our evaluation, approximately 18% contained detectable concentrations of PCE but no detectadle concentrations of TCE, despite the higher concentrations of these chemicals and the more extensive plumes in comparison to TCA and DCE. Summary
The observed behavior of the various VOCs detected in LLNL groundwater agrees well with published laboratory studies concerning the transformation of the individual compounds present at the site. Geochemical evidence indicates that LLNL groundwater is very oxidizing and, thus, largely precludes reductive dehalogenationreactions for chlorinated ethenes and ethanes. Indeed, there is no apparent evidence for significantreductive dehalogenation of PCE to TCE in the areas of the LLNL site which were studied. A review of the literature indicatesthat hydrolysis rates for most of the chlorinated ethenes are extremely slow and thus will have no significant impact on the concentrations of these chemicals in LLNL groundwater. However, l,l,l-TCA does apparently degrade abiotically at a measurable rate under environmental conditions. A comparison of the spatial and temporal trends in concentration between l,l,l-TCA and 1,l-DCE suggests that l,l,l-TCA may be degrading into other compounds in LLNL groundwater. Possible daughter products include 1,l-DCE itself or acetic acid. An approximate half-life for this reaction on the order of 2 years is suggested from one analysis of the data. This value is consistent with reports in the literature of 0.5-2.5 years. Oxidized degradation products of VOCs such as alcohols and epoxides have not been detected in LLNL groundwater samples. It is possible that these materials rapidly mineralize to COz and hence escape detection. Acknowledgments
This study was supported by the Environmental Restoration Division (ERD) of the Lawrence Livermore
National Laboratory, under the auspices of the U. S. Department of Energy under Contract W-7405-Eng-48, through the Earth Sciences Division of the Lawrence Berkeley Laboratory. We wish to thank Dr. John Ziagos of the ERD and Eric Nichols and Zafer Demir of Weiss Associates,Inc., for their assistance and helpful comments on this paper. We also wish to thank the three anonymous reviewers for their constructive criticisms.
Literature Cited (1) Bouwer, E. J.; Rittman, B. E.; McCarty, P. L. Environ. Sci. Technol. 1981,15, 596-559. (2) Bouwer, E. J.; McCarty, P. L. Appl. Environ. Microbiol. 1983,45, 1295-1299. (3) Barker, J. F.; Tessman, J. S.; Plotz, P. E.; Reinhard, M. J . Contam. Hydrol. 1986,l , 171-189. (4) Barrio-Lage, G.; Parsons, F. Z.; Nassar, R. S.; Lorenzo, P. A. Environ. Sci. Technol. 1986,20, 96-99. (5) Wilson,B.H.; Smith,G. B.;Rees,J. F. Environ. Sci. Technol. 1986,20,997-1002. (6) Vogel, T. M.; McCarty, P. L. Environ. Sci. Technol. 1987, 21,1208-1213. (7) Vogel, T. M.; McCarty, P. L. J . Contam. Hydrol. 1987,1, 229-308. (8) Vogel, T. M.; Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1987,21, 722-736. (9) Tompson, A. F. B. Flow and Transport Within the Saturated Zone Beneath Lawrence Livermore National Laboratory: Modeling Considerations for Heterogeneous Media; Environmental Restoration Division, Lawrence LivermoreNational Laboratory: Berkeley,CA, 1990,UCID21828. (10) Jeffers, P. M.; Ward, L. M.; Woytowitch, L. M.; Wolfe, N. L. Environ. Sci. Technol. 1989, 23, 965-969. (11) Pearson, C. R.;McConnell, G. R o c . R. SOC.London, B 1975, 189,305-332. (12) Dilling, W. L.; Tefertiller, N. B.; Kallos, G. J. Environ. Sci. Technol. 1975,9,833-838. (13) Thorpe, R. K., Isherwood, W. F., Dresen, M. D., WebsterScholten, C. P., Eds. CERCLA Remedial Investigations Report for the LLNL Livermore Site; Environmental Restoration Division, Lawrence Livermore National Laboratory: Berkeley, CA, 1990; UCAR-10299; Vol. 2, Section 4. (14) Nicholson, R. V.; Cherry, J. A.; Reardon, E. J. J. Hydrol. 1983,63,131-176. (15) Lindberg, R. D.; Runnells, D. D. Science 1984, 225, 925927. (16) Drever, J. I. The Geochemistry of Natural Waters, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1988; p 288. (17) Barcelona, M. J.; Holm, T. R.; Schock, M. R.; George, G. K. Water Resour. Res. 1989,25,991-1003. (18) Hem, J. D. Geochim. Cosmochim. Acta 1977,41, 527-538. (19) U.S. Environmental Protection Agency. Basics of Pumpand- Treat Ground- Water Remediation Technology;U. s. Environmental Protection Agency. Robert S. Kerr Environmental Research Laboratory: Ada, OK, 1990; EPA-600/ 8-901003. (20) McNab,W. W.; Narasimhan,T. N. WaterResour.Res. 1993, 29,2737-2746. (21) Noyes, C. D. M.S. Thesis, University of California, Davis, 1991. Received for review May 7,1993. Revised manuscript received October 28, 1993. Accepted January 24, 1994.'
* Abstract published in Advance ACS Abstracts, March 1,1994.
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