Oxygen-enhanced biodegradation of phenoxy acids in ground water at contaminated sites

The effects of adding oxygen to anaerobic aquifer materials on biodegradation of phenoxy acid herbicides were studied by laboratory experiments with aquifer material from two contaminated sites (a former agricultural machinery service and an old landfill). At both sites, the primary pollutants were phenoxy acids and related chlorophenols. It was found that addition of oxygen enhanced degradation of the six original phenoxy acids and six original chlorophenols. Inverse modeling on 14C 4-chloro-2-methylphenoxypropanoic acid (MCPP) degradation curves revealed that increasing the oxygen concentrations from <0.3 mg/L up to 7 to 8 mg/L shortened the lag phases (from approximately 150 d to 5 to 25 d) and increased first-order degradation rate constants by 1 order of magnitude (from approximately 5 x 10(-2) d(-1) to up to 30 x 10(-2) d(-1)). Additionally, the degree of MCPP mineralization was increased (30% to 50% mineralized at low oxygen concentrations and 50% to 70% mineralized at high oxygen concentrations, based on 14CO2 recovery). These positive effects on degradation were observed even at relatively low oxygen concentrations (2 mg/L). Furthermore, effects related to the addition of oxygen on the general geochemistry were studied. An oxygen consumption of 2.2 to 2.6 mg O2/g dw was observed due to oxidation of solid organic matter and, to some extent (0.5% to 11% of the total oxygen consumption), water-soluble compounds such as Fe2+, dissolved Mn, nonvolatile organic carbon, and NH4+. Overall, the results suggest that stimulated biodegradation by addition of oxygen might be a feasible remediation technology at herbicide-contaminated sites, although oxygen consumption by the sediment could limit the applicability.     

Fate of Point‐Source Pesticide Spill in Clayey Till after 15 Years

This study investigated the development of pesticide pollution two, three, and 17 years after spills of the herbicides dichlorprop, mecoprop (MCPP), MCPA, 2,4‐D (phenoxy acids), simazine, and terbutylazine (triazines) in a former orchard machinery service yard. The spills had occurred over several decades on a 23‐m thick, mainly anaerobic fractured clayey till aquitard. Angled monitoring wells were installed in the aquitard 3 years after the spills ceased in 1989. In 1993, monitoring revealed that high groundwater concentrations of dichlorprop (677 µg/L) and MCPP (139 µg/L) were accumulated as a zone of maximum pollution in anaerobic and largely immobile pore water at 5 to 6 m depth in the aquitard profile. In contrast, 2,4‐D was determined in only one water sample, and MCPA and simazine and terbutylazine were determined only in low concentrations (below10 µg/L), although these pesticides had been handled at the site in greater amounts than dichlorprop and MCPP according to detailed historic information obtained for the site. Repeated monitoring in the same wells after a further 14 years in 2007 revealed that no identifiable degradation of MCPP had occurred, while dichlorprop had degraded by 75% to 80% (estimated half‐life of approximately 5 years). Furthermore, degradation products related to the phenoxy acids had accumulated, especially 4‐CPP with a maximum concentration of 218 µg/L. In the same zone, MCPA and simazine had almost disappeared. As the pollution was mainly accumulated in largely immobile pore water of the aquitard clayey matrix, and the groundwater recharge was low (30 to 60 mm/year), only minor vertical displacement of the zone with maximum pollution zone had occurred during the 15 years of monitoring. However, concentrations of dichlorprop (0.01 to 0.02 µg/L), MCPP (0.1 to 0.2 µg/L), and 4‐CPP (0.6 to 0.7 µg/L) had spread along textural heterogeneities in the aquitard into the underlying sandy aquifer at 23‐m depth.     

Accumulation of Pesticides in Anaerobic Clayey Till‐Controls and Implications for Groundwater

Pesticide residuals after point‐source pesticide spills in clay‐rich aquitards may potentially affect underlying groundwater for many decades due to slow release of accumulated pollution in the clayey matrix material of the aquitard. In this study, we evaluated factors behind different degrees of accumulation of phenoxy acids (MCPP, dichlorprop, 2,4‐D MCPA) and triazines (simazine and terbutylazine) observed in an old pesticide pollution described by Jørgensen et al. (2016a, this issue). By using leaching experiments, it was shown that a zone of maximum concentrations of MCPP and dichlorprop (mg/L) observed by Jørgensen et al. (2016a, this issue) represented accumulated potentially mobile pollution in anaerobic, however largely immobile pore water of the clayey matrix in the upper few meters of the unoxidized aquitard zone. By using sorption experiments, we determined 9 to16 times higher mobility by diffusion and flow for the phenoxy acids (R = 1 to 2) than for the triazines (R = 9 to 16) in the clayey matrix material of the aquitard. This indicated that more rapid and greater accumulation could occur for the phenoxy acids in the clayey matrix than for the triazines. In contrast, the relative mobility of the phenoxy acids and triazines was much closer in sand‐filled fractures and thin sand layers/lamina in the clay, suggesting that the migration of the same compounds along these textural preferential flow paths into the underlying aquifer was less different. Despite that a greater mass had originally been spilled of 2,4‐D and MCPA having similar mobility as the accumulated MCPP and dichlorprop, these compounds were not accumulated in the zone of maximum concentrations. It is suggested that the controls, which initially allowed for the observed separate accumulation of MCPP and dichlorprop as a zone of maximum pollution, were due to the combination of high persistence and high mobility for these specific pesticides in the clayey till matrix material of the aquitard. The investigation showed that over time the initial high concentrations of the accumulated phenoxy acids (MCPP, dichlorprop) transformed into high concentrations of related mobile degradation products (e.g., 4‐CPP and 2‐CPP), which extended the total time of groundwater pollution beyond the disappearance of the original phenoxyacids.     

Natural Gradient Tracer Test to Evaluate Natural Attenuation of MTBE Under Anaerobic Conditions

A natural gradient tracer test using perdeuterated MTBE was conducted in an anaerobic aquifer to determine the relative importance of dispersion and degradation in reducing MTBE concentrations in ground water. Preliminary ground water chemistry and hydraulic conductivity data were used to place the tracer within an existing dissolved MTBE plume at Port Hueneme, California. Following one year of transport, the tracer plume was characterized in detail.
Longitudinal dispersion was identified as the dominant mechanism for lowering the perdeuterated MTBE concentrations. The method of moments was used to determine the longitudinal and lateral dispersion coefficients (0.85 m²/day and 0.08 m²/day, respectively). A mass-balance analysis, carried out after one year of transport, accounted for 110% of the injected mass and indicated that no significant mass loss occurred. The plume structure created by zones of higher and lower hydraulic conductivity at the site was complex, consisting of several localized areas of high tracer concentration in a lower concentration plume. This is important because the aquifer has generally been characterized as exhibiting fairly minor heterogeneity. In addition, the tracer plume followed a curved flowpath that deviated from the more macroscopic direction of ground water flow inferred from local ground water elevation measurements and the behavior of the existing plume. Understanding the mass balance, plume structure, curvature of the tracer plume, and consequently natural attenuation behavior required the detailed sampling approach employed in this study. These data imply that a detailed understanding of site hydrogeology and an extensive sampling network may be critical for the correct interpretation of monitored natural attenuation of MTBE.     

Fate of MTBE Relative to Benzene in a Gasoline-Contaminated Aquifer (1993–98)

Methyl tert -butyl ether (MTBE) and benzene have been measured since 1993 in a shallow, sandy aquifer contaminated by a mid-1980s release of gasoline containing fuel oxygenates. In wells downgradient of the release area, MTBK was detected before benzene, reflecting a chromatographic-like separation of these compounds in the direction of ground water flow. Higher concentrations of MTBE and benzene were measured in the deeper sampling ports of multilevel sampling wells located near the release area, and also up to 10 feet (3 m) below the water table surface in nested wells located farther from the release area. This distribution of higher concentrations at depth is caused by recharge events that deflect originally horizontal ground water flowlines. In the laboratory, microcosms containing aquifer material incubated with uniformly labeled ¹⁴C-MTBE under aerobic and anaerobic. Fe(III)-reducing conditions indicated a low but measurable biodegradation potential (<3%¹⁴C-MTBW as ¹⁴CO₂) after a seven-month incubation period, Tert -butyl alcohol (TBA), a proposed microbial-MTBE transformation intermediate, was detected in MTBE-contaminated wells, but TBA was also measured in unsaturated release area sediments. This suggests that TBA may have been present in the original fuel spilled and does not necessarily reflect microbial degradation of MTBE. Combined, these data suggest that milligram per liter to microgram per liter decreases in MTBE concentrations relative to benzene are caused by the natural attenuation processes of dilution and dispersion with less-contaminated ground water in the direction of flow rather than biodegradation at this point source gasoline release site.     

A Study of Long-Term MTBE Attenuation in the Borden Aquifer, Ontario, Canada

In 1988 and 1989, a natural gradient tracer test was performed in the shallow, aerobic and aquifer at Canadian Forces Base (CFB) Borden. A mixture of ground water containing dissolved oxygenated gasoline was injected below the water table along with chloride (Cl-) as a conservative tracer. The migration of BTEX, MTBE, and Cl was monitored in detail for 16 moths. The mass of BTEX compounds in the plume diminished significantly with time due to intrinsic aerobic biodegradation, while MTBE showed only a small decrease in mass over the 16-month period. In 1995/96, a comprehensive ground water sampling program was undertaken to define the mass of MTBE still present in the aquifer. Since the plume had migrated into an unmonitored section of the Borden Aquifer, numerical modeling and geostatistical methods were applied to define an optimal sampling grid and to improve the level of confidence in the results. A drive point profiling system was used to obtain ground water samples. Numerical modeling with no consideration of degradation pedicted maximum concentrations in excess of 3000 μg/L; field sampling found maximum concentrations of less than 200 μg/L. A mass balance for the remaining MTBE mass in the aquifer eight years after injection showed that only 3% of the original mass remained. Sorption, volatilization, a biotic degradation, and plant uptake are not considered significant attenuation processes for the field conditions. Therefore, we suggest that biodegradation may have played a major role in the attenuation of MTBE within the Borden Aquifer.     

Long-Term Ground Water Quality Impacts from the Use of Hexazinone for the Commercial Production of Lowbush Blueberries

Lowbush blueberries, native to eastern Canada and Maine, are an important economic crop in these areas. Herbicides containing the active ingredient hexazinone are commonly applied to blueberry fields, and there is a high frequency of detection of relatively low concentrations of hexazinone in domestic wells located close to areas of lowbush blueberry production. The objective of this study was to determine the long-term impacts from hexazinone-based herbicide use on ground water quality in the immediate growing areas.
Physical and chemical hydrogeologic data were collected for an outwash sand and gravel aquifer in southwestern New Brunswick, Canada. The majority of the land overlying the aquifer is devoted to lowbush blueberry production. Twenty-one nested monitoring wells were sampled for hexazinone and hexazinone metabolites over a four-year period. Hexazinone was consistently detected at values of 1 to 8 parts per billion (ppb) in all but two of these wells, one that is upgradient of herbicide applications, and one that is downgradient with anoxic conditions. Hexazinone metabolites B and A1 were also detected in all but two of the 21 wells at values ranging from 0.5 to 2.5 ppb. The hexazinone and metabolite data suggest both aerobic and anaerobic degradation of hexazinone. Complete degradation of hexazinone appears to occur only in the one downgradient well exhibiting anoxic ground water conditions. Concentrations of hexazinone and its metabolites in the ground water were essentially constant over the four-year period.     

Aquifer vulnerability to pesticide migration through till aquitards

This study investigates the influence of key factors-mainly recharge rate and degradation half-life--on downward migration of the widely used pesticide mecoprop (MCPP) through a typical clayey till aquitard. The study uses the numerical model FRAC3Dvs, which is a three-dimensional discrete fracture/matrix diffusion (DFMD) numerical transport model. The model was calibrated with laboratory and field data from a site near Havdrup, Denmark, but the overall findings are expected to be relevant to many other sites in similar settings. Fracture flow and MCPP transport parameters for the model were obtained through calibration using well-characterized laboratory experiments with large (0.5 m diameter by 0.5 m high) undisturbed columns of the fractured till and a field experiment. A second level of upscaling and sensitivity analysis was then carried out using data on hydraulic head, fracture spacing, and water budget from the field site. The simulations of downward migration of MCPP show that MCPP concentration and mass flux into the underlying aquifer, and hence the aquifer vulnerability to this pesticide compound, is mainly dependent on the degradation rate of the pesticide, the overall aquitard water budget, and the ground water recharge rate into the aquifer. The influence of flow rate, matrix diffusion, and degradation rate are intertwined. This results in one to four orders of magnitude higher MCPP flux into the aquifer from aquifer recharge rates of 20 and 120 mm/yr, respectively, for no degradation and MCPP half-life of 0.5 yr. From a sensitivity analysis it was found that the range of MCPP flux into the aquifer varied less than one order of magnitude due to (1) changing fracture spacing from 1 to 10 m, or (2) preferential flow along inclined thin sand layers, which represent common conditions for the current and other settings of clayey till in Denmark and other glaciated areas in Europe and North America. The results indicate that for aquifers overlain by fractured clayey tills, the vulnerability to contamination with pesticides (pesticide flux into the aquifer) and other widespread agricultural contaminants is going to vary strongly in the watershed as a function of the distribution of aquitard water budget (flow rate) and aquitard redox environment (controlling contaminant degradation rates), even if the thickness of the till is relatively constant. DFMD modeling of cause-effect relationships within such systems has great potential to support decisions in planning, regulation, and contaminant remediation.     

Analysis of recharge-induced geochemical change in a contaminated aquifer

Recharge events that deliver electron acceptors such as O2, NO3, SO4, and Fe3+ to anaerobic, contaminated aquifers are likely important for natural attenuation processes. However, the specific influence of recharge on (bio)geochemical processes in ground water systems is not well understood. The impact of a moderate-sized recharge event on ground water chemistry was evaluated at a shallow, sandy aquifer contaminated with waste fuels and chlorinated solvents. Multivariate statistical analyses coupled with three-dimensional visualization were used to analyze ground water chemistry data (including redox indicators, major ions, and physical parameters) to reveal associations between chemical parameters and to infer processes within the ground water plume. Factor analysis indicated that dominant chemical associations and their interpreted processes (anaerobic and aerobic microbial processes, mineral precipitation/dissolution, and temperature effects) did not change significantly after the spring recharge event of 2000. However, the relative importance of each of these processes within the plume changed. After the recharge event, the overall importance of aerobic processes increased from the fourth to the second most important factor, representing the variability within the data set. The anaerobic signatures became more complex, suggesting that zones with multiple terminal electron-accepting processes (TEAPs) likely occur in the same water mass. Three-dimensional visualization of well clusters showed that water samples with similar chemical associations occurred in distinct water masses within the aquifer. Water mass distinctions were not based on dominant TEAPs, suggesting that the recharge effects on TEAPs occurred primarily at the interface between infiltrating recharge water and the aquifer.     

Monitored Natural Attenuation of Manufactured Gas Plant Tar Mono- and Polycyclic Aromatic Hydrocarbons in Ground Water: A 14-Year Field Study

Site 24 was the subject of a 14-year (5110-day) study of a ground water plume created by the disposal of manufactured gas plant (MGP) tar into a shallow sandy aquifer approximately 25 years prior to the study. The ground water plume in 1988 extended from a well-defined source area to a distance of approximately 400 m down gradient. A system of monitoring wells was installed along six transects that ran perpendicular to the longitudinal axis of the plume centerline. The MGP tar source was removed from the site in 1991 and a 14-year ground water monitored natural attenuation (MNA) study commenced. The program measured the dissolved mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs) periodically over time, which decreased significantly over the 14-year period. Naphthalene decreased to less than 99% of the original dissolved mass, with mass degradation rates of 0.30 per year (half-life 2.3 years). Bulk attenuation rate constants for plume centerline concentrations over time ranged from 0.33 ± 0.09 per year (half-life 2.3 ± 0.8 years) for toluene and 0.45 ± 0.06 per year (half-life 1.6 ± 0.2 years) for naphthalene. The hydrogeologic setting at Site 24, having a sandy aquifer, shallow water table, clay confining layer, and aerobic conditions, was ideal for demonstrating MNA. However, these results demonstrate that MNA is a viable remedial strategy for ground water at sites impacted by MAHs and PAHs after the original source is removed, stabilized, or contained.     

Attenuation of a Mixed Chromium and Chlorinated Etjeme Ground Wate Plume in Estuarine Influenced Glaciated Sediments

The natural attenuation behavior of a ground water contaminant plume containing chromium and chlorinated ethenes in glaciated sediments was assessed using traditional and nontraditional methods. The mixed waste is transported through and attenuated within an estuarine influenced ground water aquifer of spatially varying redox character and organic carbon content. Contaminant fate and speciation were assessed as a function of geochemical conditions. Total, speciation-based, and sequential chemical extraction analyses were performed to determine contaminant partitioning and the redox capacity of the aquifer. Chromium speciation and partitioning were correlated with the reductive capacity and redox conditions of the aquifer sediments spatially distributed within the aquifer. Reductive dechlorination and partitioning of chlorinated ethenes were correlated with the organic carbon content and redox conditions of the aquifer sediments. The data showed that sharp redox gradients existed within the aquifer. Active reduction and retardation of both chromium and chlorinated ethenes was exhibited. The aqueous hexavalent chromium concentrations decreased to near nondetect levels in the vicinity of the receptor, whereas degradation products of higher-order chlorinated ethenes increased as a fraction of the total chlorinated ethene concentrations along the length of the plume. The potential for competition for reducing power under specific cases within the aquifer was suggested by the data, highlighting the need to include contaminant interactions in natural attenuation assessments.     

Influence of water flux and redox conditions on chlorobenzene concentrations in a contaminated streambed

Significant natural attenuation may occur on the passage of groundwater plumes through streambed sediments because of the transition from anaerobic to aerobic conditions and an increased microbial activity. Varying directions and magnitudes of water flow in the streambed may enhance or inhibit the supply of oxygen to the streambed and thus influence the redox zoning. In a field study at a small stream in the industrial area of Bitterfeld‐Wolfen, we observed the variability of hydraulic gradients, streambed temperatures, redox conditions and monochlorobenzene (MCB) concentrations in the streambed over the course of 5 months. During the observation period, the hydrologic conditions changed from losing to gaining. Accordingly, the temperature‐derived water fluxes changed from recharge to discharge. Redox conditions were highly variable between ? 170 and 368 mV in the shallow streambed at a depth of 0·1 m below the streambed surface. Deeper in the streambed, at depths of 0·3 m and 0·5 m, the redox conditions were more stable between ? 198 and ? 81 mV and comparable to those typically found in the aquifer. MCB concentrations in the streambed at 0·3 and 0·5 m depth increased with increasing upward water flux. The MCB concentrations in the shallow streambed at 0·1 m depth appeared to be independent of the hydrologic conditions suggesting that degradation of MCB may have occured. Copyright © 2010 John Wiley & Sons, Ltd.     

Estimating Aquifer Sensitivity to Nitrate Contamination Using Geochemical Information

Methods for predicting aquifer sensitivity to contamination typically ignore geochemical factors that affect the occurrence of contaminants such as nitrate. Use of geochemical information offers a simple and accurate method for estimating aquifer sensitivity to nitrate contamination. We developed a classification method in which nitrate-sensitive aquifers have dissolved oxygen concentrations > 1.0 mg/L, Eh values >250 mV, and either reduced iron concentrations < 0.1 mg/L or total iron concentrations < 0.7 mg/L. We tested the method in four Minnesota aquifer systems having different geochemical and hydrologic conditions. A surficial sand aquifer in central Minnesota exhibited geochemical zonation, with a rapid shift from aerobic to anaerobic conditions 5 m below the water table. A fractured bedrock aquifer in east-central Minnesota remained aerobic to depths of 50 m, except in areas where anaerobic ground water discharged upward from an underlying aquifer. A bedrock aquifer in southeast Minnesota exhibited aerobic conditions when overlain by surficial deposits lacking shale, whereas anaerobic conditions occurred under deposits that contained shale. Surficial sand aquifers in northwest Minnesota contained high concentrations of sulfate and were anaerobic throughout their extent. Nitrate-nitrogen was detected at concentrations exceeding 1 mg/L in 135 of 149 samples classified as sensitive. Nitrate was not detected in any of the 109 samples classified as not sensitive. We observed differences between our estimates of sensitivity and existing sensitivity maps, which are based on methods that do not consider aquifer geochemistry. Because dissolved oxygen, reduced iron, and Eh are readily measured in the field, use of geochemistry provides a quick and accurate way of assessing aquifer sensitivity to nitrate contamination.     

Detection of Contaminant Plumes by Borehole Geophysical Logging

Two borehole geophysical methods—electromagnetic induction and natural gamma radiation logs—were used to vertically delineate landfill leachate plumes in a glacial aquifer. Geophysical logs of monitoring wells near two land-fills in a glacial aquifer in west-central Vermont show that borehole geophysical methods can aid in interpretation of geologic logs and placement of monitoring well screens to sample landfill leachate plumes.
Zones of high electrical conductance were delineated from the electromagnetic log in wells near two landfills. Some of these zones were found to correlate with silt and clay units on the basis of drilling and gamma logs. Monitoring wells were screened specifically in zones of high electrical conductivity that did not correlate to a silt or clay unit. Zones of high electrical conductivity that did not correlate to a silt or clay unit were caused by the presence of ground water with a high specific conductance, generally from 1000 to 2370 μS/cm (microsiemens per centimeter at 25 degrees Celsius). Ambient ground water in the study area has a specific conductance of approximately 200 to 400 μS/cm. Landfill leachate plumes were found to be approximately 5 to 20 feet thick and to be near the water table surface.     

Monitored Natural Attenuation of Contaminants in the Subsurface: Processes

Among the alternatives considered for the remediation of soil and ground water at hazardous wastes sites are the use of natural processes to reduce or remove the contaminants of concern, Under favorable conditions, the use of natural attenuation can result in significant cost savings and compensate for uncertainties encountered in complex subsurface settings. In order to demonstrate that natural processes are effective in reaching established goals it is necessary to determine that transformation processes are taking place at a rate which is protective of human health and the environment, and that these processes will continue for an acceptable period of time.
While chemical transformation, dispersion, dilution, sorption, and volatilization are discussed, aerobic and anaerobic degradation comprise the major processes for the reduction of contaminant mass in the subsurface. In discussing the mechanisms of natural attenuation, chlorinated aliphatics and petroleum hydrocarbons are used as examples because of their significant impact on subsurface contamination and the effect of their physiochemical properties on attenuation processes.     

Vadose zone-attenuated artificial recharge for input to a ground water model

Accurate representation of artificial recharge is requisite to calibration of a ground water model of an unconfined aquifer for a semiarid or arid site with a vadose zone that imparts significant attenuation of liquid transmission and substantial anthropogenic liquid discharges. Under such circumstances, artificial recharge occurs in response to liquid disposal to the vadose zone in areas that are small relative to the ground water model domain. Natural recharge, in contrast, is spatially variable and occurs over the entire upper boundary of a typical unconfined ground water model. An improved technique for partitioning artificial recharge from simulated total recharge for inclusion in a ground water model is presented. The improved technique is applied using data from the semiarid Hanford Site. From 1944 until the late 1980s, when Hanford's mission was the production of nuclear materials, the quantities of liquid discharged from production facilities to the ground vastly exceeded natural recharge. Nearly all hydraulic head data available for use in calibrating a ground water model at this site were collected during this period or later, when the aquifer was under the diminishing influence of the massive water disposals. The vadose zone is typically 80 to 90 m thick at the Central Plateau where most production facilities were located at this semiarid site, and its attenuation of liquid transmission to the aquifer can be significant. The new technique is shown to improve the representation of artificial recharge and thereby contribute to improvement in the calibration of a site-wide ground water model.     

The Geochemistry of Boron in a Landfill Monitoring Program

Ground water monitoring data collected during the past eight years at a permitted municipal solid waste (MSW) disposal facility located in the midwestern United States indicated fluctuations in typical leachate indicator parameter concentrations. Apparent trends in the data inferred leachate outbreak, generating suspicion as to the integrity of the landfill liner. Eight ground water monitoring wells were installed in three distinct geologic units at the landfill facility, including glacial drift, silurian dolomite, and a post-glacial peat fen, which is downgradient from the landfill. Piezometer nests were used to define ground water gradients at the site. Using boron as an indicator, the occurrence of analytes of concern in the downgradient monitoring wells were shown to be indicative of the natural geochemistry of site ground water. This work emphasizes the importance of understanding site hydrogeology during the interpretation of ground water quality data.     

Evidence for Enhanced Mineral Dissolution in Organic Acid-Rich Shallow Ground Water

Total concentrations of formate, acetate, and isobutyrate varied from less than 5 to greater than 9,000 μmol/l over distances of < 3 m in ground water from a shallow hydrocarbon contaminated aquifer. Laboratory incubations of aquifer material indicate that organic acid concentrations were dependent on the amount of hydrocarbon loading in the sediment and the relative rates of microbial organic acid production and consumption. In heavily contaminated sediments, production greatly exceeded consumption and organic acid concentrations increased. In lightly contaminated sediments rates were essentially equal and organic acid concentrations remained low. Concentrations of dissolved calcium, magnesium, and iron generally were one to two orders of magnitude higher in organic acid-rich ground water than in ground water having low organic acid concentrations. Carbonate and Fe(III)-oxyhydroxide minerals were the likely sources of these elements. Similarly, concentrations of dissolved silica, derived from quartz and k-feldspar, were higher in organic acid-rich ground water than in other waters. The positive relation (r = 0.60, p < .05, n = 16) between concentrations of silica and organic acids suggests that the microbially mediated buildup of organic acids in ground water enhanced quartz/k-feldspar dissolution in the aquifer, although it was not the only factor influencing their dissolution. A model that included organic acid microequivalents normalized by cation microequivalents significantly strengthened the correlation (r = 0.79, p < .001, n = 16) between dissolved silica and organic acid concentrations, indicating that competition between silica and cations for complexation sites on organic acids also influenced quartz/k-feldspar dissolution. Physical evidence for enhanced mineral dissolution in organic acid-rich waters included scanning electron microscopy images of highly corroded quartz and k-feldspar grains from portions of the aquifer containing organic acid-rich ground water. Microporosity generated in hydrocarbon contaminated sediments may adversely affect remediation efforts that depend on the efficient injection of electron acceptors into an aquifer or on the recovery of solutes from an aquifer.     

A Practical Approach to Evaluating Natural Attenuation of Contaminants in Ground Water

The extent of natural attenuation is an important consideration in determining the most appropriate corrective action at sites where ground water quality has been impacted by releases of petroleum hydrocarbons or other chemicals. The objective of this study was to develop a practical approach that would evaluate natural attenuation based on easily obtained field data and field tested indicators of natural attenuation. The primary indicators that can he used to evaluate natural attenuation include plume characteristics and dissolved oxygen levels in ground water. Case studies of actual field sites show that plumes migrate more slowly than expected, reach a steady state, and decrease in extent and concentration when natural attenuation is occurring. Background dissolved oxygen levels greater than 1 to 2 mg/L and an inverse correlation between dissolved oxygen and contaminant levels have been identified through laboratory and field studies as key indicators of aerobic biodegradation. an important attenuation mechanism. Secondary indicators such as geochemical data, and more intensive methods such as contaminant mass balances, laboratory microcosm studies, and detailed ground water modeling can demonstrate natural attenuation as well. The recommended approach for evaluating natural attenuation is to design site assessment activities so that required data such as dissolved oxygen levels and historical plume flow path concentrations are obtained. With the necessary data, the primary indicators should be applied to evaluate natural attenuation. II the initial evaluation suggests that natural attenuation is a viable corrective action alternative, then a monitoring plan should be implemented to verify the extent of natural attenuation.