Geochemical and microbiological factors that control long-term performance of subsurface permeable reactive barriers were evaluated at the Elizabeth City, North Carolina, and the Denver Federal Center, Colorado, sites. These ground water treatment systems use zero-valent iron filings (Peerless Metal Powders Inc.) to intercept and remediate chlorinated hydrocarbon compounds at the Denver Federal Center (funnel-and-gate system) and overlapping plumes of hexavalent chromium and chlorinated hydrocarbons at Elizabeth City (continuous wall system). Zero-valent iron at both sites is a long-term sink for carbon, sulfur, calcium, silicon, nitrogen, and magnesium. After about four years of operation, the average rates of inorganic carbon (IC) and sulfur (S) accumulation are 0.09 and 0.02 kg/m2/year, respectively, at Elizabeth City where upgradient waters contain <400 mg/L of total dissolved solids (TDS). At the Denver Federal Center site, upgradient ground water contains 1000 to 1200 mg/L TDS and rates of IC and S accumulation are as high as 2.16 and 0.80 kg/m2/year, respectively. At both sites, consistent patterns of spatially variable mineral precipitation and microbial activity are observed. Mineral precipitates and microbial biomass accumulate the fastest near the upgradient aquifer-Fe0 interface. Maximum net reductions in porosity due to the accumulation of sulfur and inorganic carbon precipitates range from 0.032 at Elizabeth City to 0.062 at the Denver Federal Center (gate 2) after about four years. Although pore space has been lost due the accumulation of authigenic components, neither site shows evidence of pervasive pore clogging after four years of operation.

A series of laboratory tests were conducted to evaluate whether denitrification would be a suitable alternative for biorestoration of an aquifer contaminated with JP-4 jet fuel. Microcosms were prepared from both uncontaminated and contaminated aquifer material from the site, in an anaerobic glovebox, amended with nitrate, nutrients, and aromatic hydrocarbons, and incubated under a nitrogen atmosphere at 12 °C. With uncontaminated core material, there was no observable lag period prior to removal of toluene whereas 30 days was required before biodegradation commenced for xylenes, ethylbenzene, and 1,2,4-trimethylbenzene. An identical test with contaminated aquifer material exhibited not only much longer lag periods but decreased rates of biodegradation; benzene, ethylbenzene, and o-xylene were not significantly degraded within the 6-month time period even though active denitrification occurred at this time. First-order biodegradation rate constants ranged from 0.016 to 0.38 day™1 for uncontaminated core material and from 0.022 to 0.067 day™1 for contaminated core material. Tests with individual compounds in uncontaminated core indicated that benzene and m-xylene inhibited the basal rate of denitrification. These data demonstrate that several aromatic compounds can be degraded under denitrifying conditions, but rates of biodegradation may be lower in material contaminated with JP-4 jet fuel.

The effect of the addition of common fermentation products on the dehalogenation of tetrachloroethene was studied in methanogenic slurries made with aquifer solids. Lactate, propionate, crotonate, butyrate, and ethanol stimulated dehalogenation activity, while acetate, methanol, and isopropanol did not. Although the ecological and public health risks associated with tetrachloroethene (PCE) contamination may be the most severe when spills affect groundwater, little is known about the environmental conditions necessary to initiate and sustain dehalogenation activity in contaminated aquifers. This study was done with core material collected from a site impacted by both aviation gasoline and chloroethenes at a Coast Guard Air Station at Traverse City, Mich. Collection of core material, making of microcosms, sampling, and chloroethene analysis were done as described previously (11). The microcosms differed in that 10 g of aquifer material and 20-ml serum bottles were used. The final concentrations of donors added were as follows: acetate, 4 mM; lactate, 3 mM; propionate, 3 mM; butyrate, 2 mM; crotonate, 2 mM; methanol, 4 mM; ethanol, 2 mM; and isopropanol, 3 mM. The PCE addition was 1 ,ul of a methanol stock solution (0.751 g of PCE per 10-ml total volume in methanol) to give a PCE initial concentration of approximately 30 ,uM. A small amount of headspace was left in these bottles for ease of handling. All bottles were well shaken after all amendments had been made. Four fatty acids and three alcohols were tested for the ability to act as the source for reducing equivalents for PCE dehalogenation. The production of trichloroethene (TCE) and total dichloroethenes in microcosms, using these compounds as electron donors, is shown in Fig. 1. Samples from microcosms amended with lactate or ethanol had TCE present at the first sampling point at 6 days (Fig. 1). Butyrate, crotonate, and propionate also supported dehalogenation, although the lag period was longer, while acetate, methanol, and isopropanol did not support dehalogenation above that observed in the unamended control during this time period (Fig. 1). The measurements from the early time points were quite variable because of differences in the onset of dehalogenation. However, at the last two time points, significant differences (Student's t test with ao = 0.05) in the total amount of dehalogenation products present were observed between microcosms amended with lactate, ethanol, propionate, crotonate, or butyrate and those which were unamended or were amended with acetate,'methanol, or isopropanol. Several researchers have demonstrated the stimulation of reductive dehalogenation of chloroethenes by addition of * Corresponding author.

The Solvent Extraction Residual Biotreatment (SERB) technology was evaluated at a former dry cleaner site in Jacksonville, FL, where an area of tetrachloroethylene (PCE) contamination was identified. The SERB technology is a treatmenttrain approach for complete site restoration, which combines an active in situ dense nonaqueous-phase liquid (DNAPL) removal technology, cosolvent extraction, with a passive enhanced in situ bioremediation technology, reductive dechlorination. During the in situ cosolvent extraction test, approximately 34 kL of 95% ethanol/5% water (v:v) was flushed through the contaminated zone, which removed approximately 60% of the estimated PCE mass. Approximately 2.72 kL of ethanol was left in the subsurface, which provided electron donorfor enhancement of biological processes in the source zone and downgradient areas. Quarterly groundwater monitoring for over 3 yr showed decreasing concentrations of PCE in the source zone from initial values of 4-350 microM to less than 150 microM during the last sampling event. Initially there was little to no daughter product formation in the source zone, but after 3 yr, measured concentrations were 242 microM for cis-dichloroethylene (cis-DCE), 13 microM for vinyl chloride, and 0.43 microM for ethene. In conjunction with the production of dissolved methane and hydrogen and the removal of sulfate, these measurements indicate that in situ biotransformations were enhanced in areas exposed to the residual ethanol. First-order rate constants calculated from concentration data for individual wells ranged from -0.63 to -2.14 yr(-1) for PCE removal and from 0.88 to 2.39 yr(-1) for cis-DCE formation. First-order rate constants based on the change in total mass estimated from contour plots of the groundwater concentration data were 0.75 yr(-1) for cis-DCE, -0.50 yr(-1) for PCE, and -0.33 yr(-1) for ethanol. Although these attenuation rate constants include additional processes, such as sorption, dispersion, and advection, they provide an indication of the overall system dynamics. Evaluation of the groundwater data from the former dry cleaner site showed that cosolvent flushing systems can be designed and utilized to aid in the enhancement of biodegradation processes at DNAPL sites.

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