Influence of Carbon and Microbial Community Priming on the Attenuation of Uranium in a Contaminated Floodplain Aquifer

The capacity for subsurface sediments to sequester radionuclide contaminants, such as uranium (U), and retain them after bioremediation efforts are completed is critical to the long‐term stewardship of re‐mediated sites. In U bioremediation strategies, carbon amendment stimulates bioreduction of U(VI) to U(IV), immobilizing it within the sediments. Sediments enriched in natural organic matter are naturally capable of sequestering significant U, but may serve as sources to the aquifer, contributing to plume persistence. Two types of organic‐rich sediments were compared to better understand U release mechanisms. Sediments that were artificially primed for U removal were retrieved from an area previously biostimulated while detrital‐rich sediments were collected from a location never subject to amendment. Batch incubations demonstrated that primed sediments rapidly removed uranium from the groundwater, whereas naturally reduced sediments released a sizeable portion of U before U(VI)‐reduction commenced. Column experiments confirmed that U release persisted for 65 pore volumes in naturally reduced sediments, demonstrating their sink‐source behavior. Acetate addition to primed sediments shifted the microbial community from sulfate‐reducing bacteria within Desulfobacteraceae to the iron‐reducing Geobacteraceae and Firmicutes, associated with efficient U(VI) removal and retention, respectively. In contrast, Geobacteraceae communities in naturally reduced sediments were replaced by sequences with similarity to Pseudomonas spp. during U release, while U(VI) removal only occurred with enrichment of Firmicutes. These investigations stress the importance of characterizing zones with heterogeneous carbon pools at U‐contaminated sites prior to the determination of a remedial strategy to identify areas, which may contribute to long‐term sourcing of the contaminants.     

Lessons learned from bacterial transport research at the South Oyster Site

This paper provides a review of bacterial transport experiments conducted by a multiinvestigator, multiinstitution, multidisciplinary team of researchers under the auspices of the U.S. Department of Energy (DOE). The experiments were conducted during the time period 1999-2001 at a field site near the town of Oyster, Virginia known as the South Oyster Site, and included four major experimental campaigns aimed at understanding and quantifying bacterial transport in the subsurface environment. Several key elements of the research are discussed here: (1) quantification of bacterial transport in physically, chemically, and biologically heterogeneous aquifers, (2) evaluation of the efficacy of conventional colloid filtration theory, (3) scale effects in bacterial transport, (4) development of new methods for microbial enumeration and screening for low adhesion strains, (5) application of novel hydrogeophysical techniques for aquifer characterization, and (6) experiences regarding management of a large field research effort. Lessons learned are summarized in each of these areas. The body of literature resulting from South Oyster Site research has been widely cited and continues to influence research into the controls exerted by aquifer heterogeneity on reactive transport (including microbial transport). It also served as a model (and provided valuable experience) for subsequent and ongoing highly-instrumented field research efforts conducted by DOE-sponsored investigators.     

Investigation and Remediation of a 1,2-Dichloroethane Spill Part II: Documentation of Natural Attenuation

A release of 1,2-dichloroethane. also known as ethylene dichloride (EDC), resulted in shallow subsurface freephase contamination of a Gulf Coast site in the southern United States. The site stratigraphy consists primarily of a low permeability, surficial peat. silt, and clay zone underlain by fractured clay; a confined 12 in deep sand ground water flow zone; a confined 21 m deep fine sand zone of limited ground water flow, followed by a deep aquitard. The Gumbo clay and sandy clay aquitard below the release area overlies and protects the 61 m deep Upper Chicot Aquifer, which is a confined regional aquifer. An ongoing recovery and hydraulic containment program from the primary impacted and laterally and vertically restricted shallow 40-foot sand zone has effectively recovered dense nonaqueous phase liquid (DNAPL) and contained dissolved phase EDC.
Natural attenuation of EDC was demonstrated through (1) a laboratory microcosm study substantiating the ability of the native microbial population in the deeper aquifer lo degrade EDC under anaerobic environmental conditions found at the site. (2) field investigations showing reductions in EDC concentrations over time in many of the wells on site, and (3) an evaluation of the ground water for EDC and its degradation products and oilier geo-chemical parameters such as dissolved oxygen, redox potential, and pH. Degradation products of EDC found in the field investigations included 2-chloroeihanol, ethanol. ethene, and ethane. Dissolved EDC concentrations in selected wells between the first recorded samples and the fourth quarter of 1997 ranged from greater than 4% to 99% reductions. First-order exponential decay half-lives ranged from 0.21 to 4.2 years for wells showing decreases in FDC concentrations over time. Elevated methane concentrations indicated carbon dioxide to be the major terminal electron acceptor.     

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.     

Creation of a Subsurface Permeable Treatment Zone for Aqueous Chromate Contamination Using In Situ Redox Manipulation

An in situ redox manipulation (ISRM) method for creating a permeable treatment zone in the subsurface has been developed at the laboratory bench and intermediate scales and deployed at the field scale for reduction/immobilization of chrornate contamination. At other sites, the same redox technology is currently being tested for dechlorination of TCE. The reduced zone is created by injected reagents that reduce iron naturally present in the aquifer sediments from Fe(III) to surface-bound and structural Fe(II) species. Standard ground water wells are used, allowing treatment of contaminants too deep below the ground surface for conventional treneh-and-fill technologies.
A proof-of-principle field experiment was conducted in September 1995 at a chromate (hexavalent chromium) contaminated ground water site on the Hartford Site in Washington. The test created a 15 m (˜50 feet) diameter cylindrical treatment zone. The three phases of the test consisted of (1) injection of 77, 000 L (20, 500 gallons) of buffered sodium dithionite solution in 17.1 hours, (2) reaction for 18.5 hours, and (3) withdrawal of 375, 000 L (99, 600 gallons) in 83 hours. The withdrawal phase recovered 87% to 90% of the reaction products. Analysis of post-experimental sediment cores indicated that 60% to 100% of the available reactive iron in the treated zone was reduced. The longevity of the reduced zone is estimated between seven and 12 years based on the post-experiment core samples. Three and half years after the field test, the treatment zone remains anoxic, and hexavalent chromium levels have been reduced from 0.060 mg/L to below detection limits (0.008 mg/L). Additionally, no significant permeability changes have been detected during any phase of the experiment.     

Magnetic properties and heavy-metal contents of contaminated seabed sediments of Penny's Bay, Hong Kong

Magnetic properties and heavy-metal concentrations of vibrocore samples were found to be potential indicators of shipping contamination in seabed sediments in Hong Kong Harbour. Geochemical results of 74 vibrocores located off Penny's Bay on Lantau Island revealed an enrichment of heavy metals in the upper 1-2 m of the cores within the eastern part of the study site. Whole-core magnetic susceptibility measurements also showed a greater concentration of magnetic particles in the surficial layer of these cores. A significant correlation exists between the magnetic susceptibility and the concentrations of Pb, Zn and Cu, as well as the Tomlinson pollution load index (PLI). The proximity of these cores to a major navigation fairway and an anchorage site suggests probable contamination of the surficial seabed sediments by shipping-related wastes. A study of the magnetic properties of one of the cores, VT60, revealed a difference in the magnetic properties between the contaminated and the uncontaminated sediments. Samples from the contaminated zone exhibited relatively stronger magnetic remanence and susceptibility. The two groups of samples also formed distinct trends on the hysteresis ratios plot. A level of unusually high magnetic susceptibility within the contaminated zone was attributable to the presence of strongly magnetized granules, which were probably refuse from shipping-related activities. Geochemical and magnetic results were also conducted on samples of different size-fractions from this core. The < 63 microns fraction was found to contain a relatively higher magnetic susceptibility and greater heavy-metal content.     

Observations on the biota of an estuarine beach at Hardys Bay (NSW, Australia) following a spillage of aldrin

An industrial accident resulted in a spillage of aldrin onto an estuarine beach at Hardys Bay, New South Wales. Samples of water and sediment were taken 3 weeks later to measure the level of contamination resulting from the spill. The abundances of some intertidal biota were also determined at the site of the spillage and in other uncontaminated areas.

Three weeks after the spill, samples of water were not contaminated by aldrin but samples of sediment from the surface in the vicinity of the drain contained aldrin. In comparison to uncontaminated locations the abundances of crustacea were greatly reduced in the vicinity of the drain but abundances of polychaetes were not significantly different. The field data on the relative susceptibility of the two groups of animals agrees well with laboratory derived LC50 data for crustacea and polychaetes.

The study suggests that degree of contamination of water and sediment may not always be an accurate reflection of acute impact on biota, and that different components of the biota can show different reactions to the presence of a toxic chemical.     

The relationship of uranium isotopes to oxidation/reduction in the Edwards carbonate aquifer of Texas

The concentration of dissolved uranium and²³⁴U/²³⁸U alpha activity ratio (“A.R.”) were determined in water samples from 23 locations in the Edwards carbonate aquifer of south central Texas by isotope dilution methods and alpha spectrometry. This aquifer consists of two parts, an updip oxidized portion and a downdip reduced portion. At some places the boundary is associated with faulting and at others it is not. The boundary between the two portions of the aquifer can be located by an abrupt change in chemical properties of the water such as a large increase in concentration of Cl^?, SO₄^2? and total dissolved solids, the presence of H₂S and a decrease in Eh in moving from updip to downdip. Compared with the oxidized samples, the uranium concentration is much lower and the A.R. higher in the reduced samples so that the uranium from each portion falls in a distinct field. The oxidized aquifer samples show very little variation in the measured uranium parameters even though, in some cases, there is evidence that the water has flowed through some tens of kilometers of aquifer. Samples collected near the boundary at those places not associated with faulting yield dissolved uranium values which fall in neither field and which, for the most part, cannot result from mixing of the oxidized and reduced waters. These samples probably result from changes in location of the oxidation-reduction boundary.     

Applying the HPT‐GWS for Hydrostratigraphy,Water Quality and Aquifer Recharge Investigations

The primary objective of this study was to evaluate use of the hydraulic profiling tool‐groundwater sampler (HPT‐GWS) log data as an indicator of water quality (level of dissolved ionic species) in an alluvial aquifer. The HPT‐GWS probe is designed for direct push advancement into unconsolidated formations. The system provides both injection pressure logs and electrical conductivity (EC) logs, and groundwater may be sampled at multiple depths as the probe is advanced (profiling). The combination of these three capabilities in one probe has not previously been available. During field work it was observed that when HPT corrected pressure (P_c) indicates a consistent aquifer unit then bulk formation EC can be used as an indicator of water quality. A high correlation coefficient (R ² = 0.93) was observed between groundwater specific conductance and bulk formation EC in the sands and gravels of the alluvial aquifer studied. These results indicate that groundwater specific conductance is exerting a controlling influence on the bulk formation EC of the coarse‐grained unit at this site, and probably many similar sites, consistent with Archie's Law. This simple relationship enables the use of the EC and P_c logs, with targeted water samples and a minimum of core samples, to rapidly assess groundwater quality over extended areas at high vertical resolution. This method was used to identify both a brine impacted zone at the base of the aquifer investigated and a groundwater recharge lens developing below storm water holding ponds in the upper portion of the same aquifer. Sample results for trace level, naturally occurring elements (As, Ba, U) further demonstrate the use of this system to sample for low level groundwater contamination.     

Magnetic signature of hydrocarbon-contaminated soils and sediments at the former oil field H?nigsen, Germany

Magnetic properties of hydrocarbon (HC) containing soils and sediments from two sites (Site A and B) of the former oil-field H?nigsen were analyzed in order to determine whether magnetic methods can be employed to delineate HC contamination of soils and sediments. Magnetic parameters such as magnetic susceptibility and induced isothermal remanent magnetizations, as well as soil and sediment properties such as pH, iron content and water content, HC content and most probable number counts of iron-metabolizing microorganisms were determined. The magnetic concentration-dependent parameters for HC contaminated samples were 25 times higher in soils from Site A than in sediment samples from Site B. However, at Site B the magnetic susceptibility was still four times higher in comparison to lithologically similar non-contaminated sediment samples from a third Site C. Newly formed magnetite containing mainly single domain particles was responsible for the magnetic enhancement, whereas superparamagnetic grains represented only a minor component. Site A had an acidic pH compared to neutral pH at Site B, and a higher crystalline and bioavailable total iron content. Nevertheless, Site B samples contained significant numbers of both iron(II)-oxidizing and iron(III)-reducing microorganisms indicating that microbial iron cycling might have taken place at this site and potentially played a role for iron mineral transformation, including magnetite (trans)formation. The content of total non-polar hydrocarbons (TNPH) at Site A was one order of magnitude higher than at Site B. Only at Site A magnetic susceptibility correlated well with TNPH. Our results demonstrate that HC contaminated samples had an enhanced magnetite content compared to non-contaminated soils and sediments. Therefore, magnetic methods may provide a quick and cost-effective way to assess HC contamination in soils and sediments. However, more field sites and laboratory investigations are needed to reveal the complex nature of the processes involved.     

Ground-Water Quality at a Creosote Waste Site

An abandoned creosote facility in Conroe, Texas, has become a field site for the National Center for Ground Water Research (NCGWR) at Rice University. Ground-water contamination in the shallow aquifer beneath the site was characterized by sampling soils and water quality at 14 monitoring wells and 35 boreholes. Results from six sampling trips over two years for inorganic and organic chemical concentrations in the ground water show wells around the site were contaminated to levels above 800 μg/l for naphthalene, 400 μg/1 for methyl naphthalene, and 150 μg/1 for dibenzofuran. Conservative constituents, traced by chloride concentrations up to 75 mg/l, have migrated 300 ft (90 m) downgradient of the site. Organic contaminants have been adsorbed and microbially degraded in their migration from the waste source as evidenced by their attenuated concentrations. Detailed field pump tests have been performed to evaluate hydraulic conductivity at several of the shallow wells. The U.S. Geological Survey (USGS) Solute Transport Model (Konikow and Bredehoeft, 1978) has been used to predict chloride plume patterns and evaluate parameters which govern transport processes at the Conroe waste site.     

Application of Electromagnetic Logging to Contamination Investigations in Glacial Sand-and-Gravel Aquifers

Electromagnetic (EM) logging provides an efficient method for high-resolution, vertical delineation of electrically conductive contamination in glacial sand-and-gravel aquifers. LM. gamma, and lithologic logs and specific conductance data from sand-and-gravel aquifers at five sites in the northeastern United States were analyzed to define the relation of KM conductivity to aquifer lithology and water quality. Municipal waste disposal, septic waste discharge, or highway deicing salt application at these sites has caused contaminant plumes in which the dissolved solids concentration and specific conductance of ground water exceed background levels by as much as 10 to 20 limes.
The major hydrogeologic factors that affected KM log response at the five sites were the dissolved solids concentration of the ground water and the silt and clay content in the aquifer. KM conductivity of sand and gravel with uncontaminated water ranged from less than 5 to about 10 millisiemens per meter (mS/m); that of silt and clay zones ranged from about 15 to 45 mS/m: and that of the more highly contaminated zones in sand and gravel ranged from about 10 to more than 80 mS/m. Specific conductance of water samples from screened intervals in sand and gravel at selected monitoring well installations was significantly correlated with KM conductivity.
CM logging can be used in glacial sand-and-gravel aquifer investigations to (1) determine optimum depths for the placement of monitoring well screens: (2) provide a nearly continuous vertical profile of specific conductance to complement depth-specific water quality samples; and (3) identify temporal changes in water quality through sequential logging. Detailed lithologic or gamma logs, preferably both, need to be collected along with the F.M logs to define zones in which elevated EM conductivity is caused by the presence of sill and clay beds rather than contamination.     

Carbon speciation and composition of natural microbial communities in polluted and pristine sediments of the Eastern Mediterranean Sea

Sediment samples collected from polluted (Augusta Bay) and pristine regions of the Eastern Mediterranean Sea (South Ionian Sea, Thracian Sea) were analyzed for black carbon, aliphatic hydrocarbons and phospholipid ester-linked fatty acids (PLFA). The aim of the study was to investigate the anthropogenic and biogenic inputs into the Eastern Mediterranean Sea and to evaluate the effects of refractory organic matter (e.g. black carbon) and the level of hydrocarbon contamination on benthic microbial community composition. Black carbon, generally considered to be chemically and biologically inert, comprised a significant but highly variable fraction of the sedimentary carbon pool in the analyzed sediments with a ratio to total organic carbon ranging from 0.02 to 0.66. Principal component analysis of the chemical characteristics of the sediments (organic carbon content, black carbon, bioavailable organic carbon, chlorophyll a, phaeopigments, aliphatic hydrocarbons) revealed clustering of samples along a gradient from the most productive and contaminated region of Augusta Bay to the carbon-poor and pristine sediments of the Thracian Sea. PLFA analysis revealed that gram-negative bacteria and microeukaryotes were most abundant in Augusta Bay and in the most impacted station of the Thracian Sea. The high levels of branched and odd-chain fatty acids recorded for these stations is probably linked to the elevated amounts of hydrocarbons at these stations; e.g. microbial communities may have developed the ability to degrade either naturally occurring aliphatic hydrocarbons or hydrocarbons derived from oil contamination.     

Spatial variability of in situ microbial activity: biotracer tests

Biotracer tests have been proposed as a means by which to characterize the in situ biodegradation potential for field-scale systems. In this study, field experiments were conducted at two sites to evaluate the utility of the biotracer method for characterizing the spatial variability of microbial activity. The first site is a mixed waste-contaminated surficial aquifer in Utah, and the second site is a chlorinated solvent-contaminated regional aquifer in Tucson, Arizona. Mass recovery of the biotracer decreased approximately linearly with increasing residence time for the Tucson site. Similar behavior was observed at the Utah site, except in the region adjacent to the injection zone, where percent recoveries were much lower than those predicted using a correlation determined using data collected downgradient of the injection zone. First-order biodegradation rate coefficients obtained from model calibration of the tracer data varied between 0.2 and 0.5/day for the Tucson site. For the Utah site, the values varied between 0.1 and 0.6/day downgradient of the injection wells, and between 0.7 and 2.6/day near the injection wells. Considering the large range over which biodegradation rate coefficients can vary, the rate coefficient exhibited relatively minimal spatial variability (factor of 2.5) for the Tucson site. Conversely, the spatial variability of the rate coefficient was an order of magnitude greater for the Utah site. These differences in variability are consistent with conditions associated with the respective sites. For example, the greater microbial activity observed in the vicinity of the injection wells for the Utah site is consistent with the biomass distribution determined from analysis of core samples, which shows larger bacterial cell densities for the region near the injection wells. These results illustrate the utility of biotracer tests for in situ characterization of microbial activity (e.g., biodegradation potential), including evaluation of potential spatial variability.     

Arsenic Geochemistry and Hydrostratigraphy in Midwestern U.S. Glacial Deposits

Arsenic concentrations exceeding the U.S. EPA's 10 μg/L standard are common in glacial aquifers in the midwestern United States. Previous studies have indicated that arsenic occurs naturally in these aquifers in association with metal-(hydr)oxides and is released to groundwater under reducing conditions generated by microbial oxidation of organic matter. Despite this delineation of the arsenic source and mechanism of arsenic mobilization, identification of arsenic-impacted aquifers is hindered by the heterogeneous and discontinuous nature of glacial sediments. In much of the Midwest, the hydrostratigraphy of glacial deposits is not sufficiently characterized to predict where elevated arsenic concentrations are likely to occur. This case study from southeast Wisconsin presents a detailed characterization of local stratigraphy, hydrostratigraphy, and geochemistry of the Pleistocene glacial deposits and underlying Silurian dolomite. Analyses of a single core, water chemistry data, and well construction reports enabled identification of two aquifers separated by an organic-rich aquitard. The upper, unconfined aquifer provides potable water, whereas arsenic generally exceeds 10 μg/L in the deeper aquifer. Although coring and detailed hydrostratigraphic characterization are often considered impractical, our results demonstrate that a single core improved interpretation of the complex lithology and hydrostratigraphy. This detailed characterization of hydrostratigraphy facilitated development of well construction guidelines and lays the ground work for further studies of the complex interactions among aquifer sediments, hydrogeology, water chemistry, and microbiology that lead to elevated arsenic in groundwater.     

Geochemical effects on metals following permanganate oxidation of DNAPLs

The application of in situ chemical oxidation for dense, nonaqueous phase liquid (DNAPL) remediation requires delivery of substantial levels of oxidant chemicals into the subsurface to degrade target DNAPLs and to satisfy natural oxidant demand. This practice can raise questions regarding changes in subsurface conditions, yet information regarding potential effects, especially at the field scale, has been lacking. This paper describes an evaluation of the effects on metals associated with in situ chemical oxidation using potassium permanganate at Launch Complex 34 (LC34), Cape Canaveral Air Station, Florida. At LC34, high concentrations of permanganate (1 to 2 wt%) were injected into the subsurface as part of a demonstration of DNAPL remediation technologies. In a companion experimental effort at the Colorado School of Mines, field samples were characterized and laboratory batch and mini-column studies were completed to assess effects of permanganate oxidation on metals in the subsurface one year after completion of the field demonstration. Results indicated there was potential for long-term immobilization of a portion of introduced manganese and no treatment-induced loss in subsurface permeability due to deposition of manganese oxides particles, which are a product of the oxidation reactions. Permanganate treatment did cause elevated manganese, chromium, and nickel concentrations in site ground water within the treated region. Some of these metals effects can be attenuated during downgradient flow through uncontaminated and untreated aquifer sediments.     

Arsenic Cycling in Hydrocarbon Plumes: Secondary Effects of Natural Attenuation

Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe‐hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. At a crude‐oil‐contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented. We collected groundwater samples at the site annually from 2009 to 2013 to examine if As is released to groundwater and, if so, to document relationships between As and Fe inside and outside of the dissolved hydrocarbon plume. Arsenic concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained less than 5 µg/L As. Combined with previous data from the Bemidji site, our results suggest that (1) naturally occurring As is associated with Fe‐hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe‐hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. We develop a conceptual model of secondary As release due to natural attenuation of hydrocarbons that can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.     

Hot Methanol Extraction for the Analysis of Volatile Organic Chemicals in Subsurface Core Samples from Dover Air Force Base, Delaware

The evaluation of contaminant concentrations in ground water and soil is an essential aspect of most hazardous waste remedial investigations. This paper describes methods applied toward obtaining, preserving, and analyzing subsurface samples for the determination of VOC concentrations in the saturated region of an unconfined coastal plain aquifer at Dover Air Force Base (DAFB). Delaware. The described protocol involved headspace-free subsampling of cores, field preservation of subsamples in methanol, and overnight extraction of the VOCs at elevated temperature (70°C). Methanol-extracted compounds were subsequently transferred to hexane and analyzed by gas chromatography. The method was found to achieve quantitative extraction from the aquifer sands in a single step, although extraction from fine-grained and more strongly sorbing aquitard samples required multiple methanol extractions to achieve comparable recovery. An extensive set of DAFB results is presented as an indication of how these methods can be applied toward characterizing field-scale contamination with a high degree of resolution and accuracy. These data have enabled a quantitative estimate of the contaminant distribution at this site and offer valuable insight into processes of contaminant migration into the underlying aquitard. In these regards, the methods used are believed to have provided much more accurate results than could have been obtained using more commonly applied techniques.     

Contamination of an Unconfined Sand Aquifer by Waste Pulp Liquor: A Case Study

A zone of contaminated ground water has been identified in an unconfined sand aquifer adjacent to a pit into which spent pulp liquor was intermittently discharged from 1970 to 1979. A network of multilevel sampling, bundle-type piezometers was installed. Up to seven depthspecific sampling points were incorporated into each piezometer providing a cost-effective means for three-dimensional mapping of hydraulic head and water quality in the unconfined sand aquifer. Ground-water samples retrieved from this network showed an area of contamination 900 m long, 400 m wide, and more than 25 m deep. This plume is dispersed about the ground-water flow lines passing beneath the waste disposal pit, and it terminates at a vigorous ground-water discharge area located 800 m from the pit. The contaminated ground water is characterized by elevated concentrations of sodium (3,000 mg/1), chloride (590 mg/1), alkalinity (2,700 mg/1), total organic carbon (2,000 mg/1), chemical oxygen demand (10,800 mg/1), biological oxygen demand (2,000 mg/1), tannin and lignin (780 mg/1), and lower sulphate (1 mg/1) compared to background ground waters in the area. The apparent rate of sodium migration is more than 50 m/yr and is close to the average linear ground-water velocity. Removal of some organic matter by biological transformation has produced the increased alkalinity in the contaminated ground water and somewhat reduced pH. Tannin and lignin are relatively inert compared to other organic compounds found in the waste liquor. Extremely low sulphate levels occurring in the highly contaminated ground waters indicate the existence of conditions favorable for microbially-mediated sulphate reduction.     

In Situ Abiotic Detoxification and Immobilization of Hexavalent Chromium

Detailed site characterization data from the former electroplating shop at the U.S. Coast Guard Air Support Center, Elizabeth City, North Carolina, suggested that the elevated Cr(VI) in the capillary fringe area had contaminated the ground water at the site. Most of the mobile Cr(VI) is present in the capillary fringe zone of the aquifer under an oxidizing environment. Current literature suggests that the reduction of Cr(VI) to Cr(III) through in situ redox manipulation in the presence of a reductant is an innovative technique for remediating chromate-contaminated sediments and ground water. The objective of this study was to evaluate the effectiveness of sodium dithionite in creating a reductive environment to remediate Cr(VI) present in soil. Sodium dithionite, a strong reductant, was injected into a small area of the vadose zone where elevated Cr(VI) was identified. Several striking changes observed in the target zone during the post-injection monitoring periods include a significant decrease in Eh(SHE), as much as ∼700 mV, absence of dissolved oxygen for 48 weeks, and the increase of Fe(II) concentrations. Results indicated that the in situ remedial treatment of Cr(VI) in the capillary fringe area was effective and consequently the concentration of Cr(VI) in ground water dropped below the MCLG level. This research demonstrated the effectiveness of in situ abiotic remediation by reducing Cr(VI) concentrations, mobility, and toxicity in soils and ground water within a short period of time. Therefore, sodium dithionite would be a feasible and cost-effective option for a full-scale remedial approach for the contaminated site at the U.S. Coast Guard Facility.