Robert C. Borden

Intrinsic biodegradation of MTBE and BTEX in a gasoline‐contaminated aquifer

Three-dimensional field monitoring of a gasoline plume showed rapid decay of toluene and ethylbenzene during downgradient transport with slower decay of xylenes, benzene, and MTBE under mixed aerobic-denitrifying conditions. Decay was most rapid near the source but slower farther downgradient. Effective first-order decay coefficients varied from 0 to 0.0010 d−1 for MTBE, from 0.0006 to 0.0014 d−1 for benzene, from 0.0005 to 0.0063 d−1 for toluene, from 0.0008 to 0.0058 d−1for ethylbenzene, from 0.0012 to 0.0035 d−1 for m-, p-xylene, and from 0.0007 to 0.0017 d−1 for o-xylene. Laboratory microcosm studies confirmed MTBE biodegradation under aerobic conditions; however, the extent of biodegradation was limited.

Occurrence and Treatment of 1,4-Dioxane in Aqueous Environments

1,4-Dioxane is classified as a probable human carcinogen. It is used as a stabilizer for chlorinated solvents, particularly, 1,1,1-trichloroethane (TCA), and it is formed as a by-product during the manufacture of polyester and various polyethoxylated compounds. Improper disposal of industrial waste and accidental solvent spills have resulted in the contamination of groundwater with 1,4-dioxane. Volatilization and sorption are not significant attenuation mechanisms due to 1,4-dioxanes complete miscibility with water. At present, advanced oxidation processes (AOPs) are the only proven technology for 1,4-dioxane treatment. 1,4-Dioxane was believed to be very resistant to both abiotic and biologically mediated degradation due to its heterocyclic structure with two ether linkages. However, recent studies have shown that 1,4-dioxane can be biodegraded as a sole carbon and energy source, and that cost-effective biological treatment processes can be developed. Future work should be oriented towards the developme...

MTBE and aromatic hydrocarbons in North Carolina stormwater runoff

A total of 249 stormwater samples were collected from 46 different sampling locations in North Carolina over an approximate 1-year period and analyzed to identify land use types where fuel oxygenates and aromatic hydrocarbons may be present in higher concentrations and at greater frequency. Samples were analyzed by gas chromatography-mass spectrometry in ion selective mode to achieve a quantitation limit of 0.05 microg/l. m-,p-Xylene and toluene were detected in over half of all samples analyzed, followed by MTBE: o-xylene: 1,3,5-trimethylbenzene: ethylbenzene; and 1,2,4-trimethylbenzene. Benzene, DIPE, TAME and 1,2,3-trimethylbenzene were detected in < 10% of the samples analyzed. Median contaminant concentrations (when detected) varied from 0.07 microg/l for ethylbenzene to 0.11 microg/l for toluene. All of the locations with significantly higher contaminant concentrations were associated with direct runoff from a gas station or discharge of contaminated groundwater from a former leaking underground storage tank. For all of the aromatic hydrocarbons, the maximum observed contaminant concentrations were over an order of magnitude lower than current drinking water standards.

Hydrocarbon dissolution and transport: a comparison of equilibrium and kinetic models

Abstract The dissolution and transport of trapped nonaqueous phase liquid hydrocarbon (oil) was examined in a laboratory column study using aquifer material from a site contaminated with aviation gasoline. Two mathematical models were compared for simulating the dissolution of trapped oil. The first model assumes that mass transfer will be rapid relative to groundwater flow and that the concentrations in the aqueous and oil phases will be at equilibrium. The second model assumes that mass transfer between the aqueous and oil phases is best described by a linear resistance relationship. Both models assume that equilibrium concentrations in the oil and the aqueous phases can be estimated by equating activities in each phase. Numerical simulations using both models are generated and compared with column study results. Overall, the kinetic model appears to provide a somewhat better description of the dissolution process, although this model requires significantly greater computation times. A simple procedure is described to develop an order of magnitude estimate of the number of pore volumes of water required to flush individual hydrocarbons from the aquifer material.

Anaerobic biodegradation of alkylbenzenes and trichloroethylene in aquifer sediment down gradient of a sanitary landfill

Abstract The objective of this investigation was to evaluate the anaerobic biodegradability of benzene, toluene, ethylbenzene, ortho-, meta- and para-xylene (BTEX) and trichloroethylene (TCE) in aquifer sediment down gradient of an unlined landfill. The major organic contaminants identified in the shallow unconfined aquifer are cis -dichloroethylene ( c -DCE) and toluene. The biodegradative potential of the contaminated aquifer was measured in three sets of microcosms constructed using anaerobic aquifer sediment from three boreholes down gradient of the landfill. The degradability of BTEX and TCE was examined under ambient and amended conditions. TCE was degraded in microcosms with aquifer material from all three boreholes. Toluene biodegradation was inconsistent, exhibiting biodegradation with no lag in one set of microcosms but more limited biodegradation in two additional sets of microcosms. TCE exhibited an inhibitory effect on toluene degradation at one location. The addition of calcium carbonate stimulated TCE biodegradation which was not further stimulated by nutrient addition. TCE was converted to ethylene, a harmless byproduct, in all tests. Benzene, ethylbenzene and xylene isomers were recalcitrant in both ambient and amendment experiments. Biodegradation occurred under methanogenic conditions as methane was produced in all experiments. Bromoethane sulfonic acid (BES), a methanogenic inhibitor, inhibited methane and ethylene production and TCE biodegradation. The results indicate the potential for intrinsic bioremediation of TCE and toluene down gradient of the Wilders Grove, North Carolina, landfill. The low concentrations of TCE in monitoring wells was consistent with its biodegradation in laboratory microcosms.

Mineralization of 1,4-dioxane in the presence of a structural analog.

A mixed culture with the ability to aerobically biodegrade 1,4-dioxane in the presence of tetrahydrofuran (THF) was enriched from a 1,4-dioxane contaminated aquifer. This consortium contained 3–4 morphologically different types of colonies and was grown in mineral salts media. Biodegradation of 1,4- dioxane began when THF concentrations in batch experiments became relatively low. No biodegradation of 1,4-dioxane was observed in the absence of THF and the measured cell yield was similar during degradation of 1,4-dioxane with THF or with THF alone. However, when the consortium was grown in the presence of 14C-1,4-dioxane plus THF, 2.1% of the radiolabeled 1,4-dioxane was present in the particulate fraction. The majority of the 14C (78.1%) was recovered as 14CO2, while 5.8% remained in the liquid fraction. This activity is interesting since the non-growth substrate is mineralized, yet only minimally assimilated into biomass. Using THF as the growth substrate, 1,3-dioxane, methyl t-butyl ether, ethyl t-butyl ether and t-amyl methyl ether.

Dissolution and biorestoration of nonaqueous phase hydrocarbons: Model development and laboratory evaluation

A one-dimensional numerical model is presented for simulating the enhanced biorestoration of aquifer material contaminated with residual nonaqueous phase liquid (NAPL) hydrocarbon. The model simulates the simultaneous dissolution, transport, and biodegradation of individual hydrocarbons under oxygen-limiting conditions. Partitioning between the NAPL and aqueous phase is modeled as a linear first-order process where the NAPL is composed of two fractions, a fast oil fraction with a high mass transfer rate and a slow oil fraction with a much lower mass transfer rate. Microorganisms are assumed to be present attached to the soil grains and are assumed to be immobile. Biodegradation of the problem compounds is simulated as a multistep process where the parent compounds are first biotransformed to oxygenated intermediates. These intermediates may then be mineralized to CO2 and H2O. Comparison of model simulations with experimental results from soil columns containing residual hydrocarbon and an active microbial population indicate that the model is capable of simulating the extent of hydrocarbon biodegradation using reasonable model parameters. Model simulations indicate that if benzene, toluene, and xylene are assumed to be completely mineralized, the model will greatly underestimate the extent of biotransformation. The model overestimated the extent of hydrocarbon biotransformation at low substrate concentrations.

Fractionation of stable isotopes in perchlorate and nitrate during in situ biodegradation in a sandy aquifer

Environmental context. Perchlorate (ClO4–) and nitrate (NO3–) are common co-contaminants in groundwater, with both natural and anthropogenic sources. Each of these compounds is biodegradable, so in situ enhanced bioremediation is one alternative for treating them in groundwater. Because bacteria typically fractionate isotopes during biodegradation, stable isotope analysis is increasingly used to distinguish this process from transport or mixing-related decreases in contaminant concentrations. However, for this technique to be useful in the field to monitor bioremediation progress, isotope fractionation must be quantified under relevant environmental conditions. In the present study, we quantify the apparent in situ fractionation effects for stable isotopes in ClO4– (Cl and O) and NO3– (N and O) resulting from biodegradation in an aquifer. Abstract. An in situ experiment was performed in a shallow alluvial aquifer in Maryland to quantify the fractionation of stable isotopes in perchlorate (Cl and O) and nitrate (N and O) during biodegradation. An emulsified soybean oil substrate that was previously injected into this aquifer provided the electron donor necessary for biological perchlorate reduction and denitrification. During the field experiment, groundwater extracted from an upgradient well was pumped into an injection well located within the in situ oil barrier, and then groundwater samples were withdrawn for the next 30 h. After correction for dilution (using Br– as a conservative tracer of the injectate), perchlorate concentrations decreased by 78% and nitrate concentrations decreased by 82% during the initial 8.6 h after the injection. The observed ratio of fractionation effects of O and Cl isotopes in perchlorate (ϵ18O/ϵ37Cl) was 2.6, which is similar to that observed in the laboratory using pure cultures (2.5). Denitrification by indigenous bacteria fractionated O and N isotopes in nitrate at a ratio of ~0.8 (ϵ18O/ϵ15N), which is within the range of values reported previously for denitrification. However, the magnitudes of the individual apparent in situ isotope fractionation effects for perchlorate and nitrate were appreciably smaller than those reported in homogeneous closed systems (0.2 to 0.6 times), even after adjustment for dilution. These results indicate that (1) isotope fractionation factor ratios (ϵ18O/ϵ37Cl, ϵ18O/ϵ15N) derived from homogeneous laboratory systems (e.g. pure culture studies) can be used qualitatively to confirm the occurrence of in situ biodegradation of both perchlorate and nitrate, but (2) the magnitudes of the individual apparent ϵ values cannot be used quantitatively to estimate the in situ extent of biodegradation of either anion.

Anaerobic Biodegradation of Alkylbenzenes in Laboratory Microcosms Representing Ambient Conditions

Abstract A microcosm study was performed to document the anaerobic biodegradation of benzene, toluene, ethylbenzene, m- xylene, and/or o-xylene in petroleum-contaminated aquifer sediment from sites in Michigan (MI) and North Carolina (NC) and relate the results to previous field investigations of intrinsic bioremediation. Laboratory microcosms, designed to simulate ambient conditions, were constructed under anaerobic conditions with sediment and groundwater from source, mid-plume, and end-plume locations at each site. The general patterns of biodegradation and electron acceptor utilization in the microcosms were consistent with field data. At the MI site, methane was produced after a moderate lag period, followed by toluene degradation in all sets of microcosms. At the NC site, biodegradation of the target compounds was not evident in the source area microcosms. In the mid-plume microcosms, toluene and o-xylene biodegraded first, followed by m-xylene and benzene, a pattern consistent with contaminant deca...

Evaluation of Slow Release Substrates for Anaerobic Bioremediation

ABSTRACT A variety of food-grade organic substrates were evaluated to identify materials that could be used to support long-term anaerobic bioremediation processes in the subsurface. In this work, the rate and extent of biogas production was used as an indicator of the potential for substrate fermentation to H2 and acetate, the primary electron donors used in reductive dechlorination. The rate and extent of biogas (primarily CO2+ CH4) evolution varied widely between the different substrates. For many of the substrates, biogas generation declined to very low levels within 100 days of substrate addition. However, a few substrates including several vegetable oils and sucrose esters of fatty acid (SEFAs) did support biogas production for extended time periods. Column studies demonstrated that both soybean oil and a SEFA could support sulfate reduction, methanogenesis and reductive dechlorination of perchloroethene (PCE) to cis-dichloroethene (cis-DCE) for over 14 months. The slower degradation rate of the SEFAs could be used to control substrate degradation rate in the subsurface, increasing substrate lifetime and reducing the required reinjection frequency.