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Ground water chemistry data collected over a six-year period show that the distribution of contaminants and redox processes in a shallow petroleum hydrocarbon-contaminated aquifer has changed rapidly over time. Shortly after a gasoline release occurred in 1990, high concentrations of benzene were present near the contaminant source area. In this contaminated zone, dissolved oxygen in ground water was depleted, and by 1994 Fe(III) reduction and sulfate reduction were the predominant terminal electron accepting processes. Significantly, dissolved methane was below measurable levels in 1994, indicating the absence of significant methanogenesis. By 1996, however, depletion of solid-phase Fe(III)-oxyhydroxides in aquifer sediments and depletion of dissolved sulfate in ground water resulted in the onset of methanogenesis. Between 1996 and 2000, water-chemistry data indicated that methanogenic metabolism became increasingly prevalent. Molecular analysis of 16S-rDNA extracted from sediments shows the presence of a mo re diverse methanogenic community inside as opposed to outside the plume core, and is consistent with water-chemistry data indicating a shift toward methanogenesis over time. This rapid evolution of redox processes reflects several factors including the large amounts of contaminants, relatively rapid ground water flow (~0.3 m/day [~1 foot/day]), and low concentrations of microbially reducible Fe(III) oxyhydroxides (~ 1 µmol/g) initially present in aquifer sediments. These results illustrate that, under certain hydrologic conditions, redox conditions in petroleum hydrocarbon-contaminated aquifers can change rapidly in time and space, and that the availability of solid-phase Fe(III)-oxyhydroxides affects this rate of change., Introduction Numerous observations suggest that plumes associated with petroleum hydrocarbon contamination of ground water systems tend to achieve a semisteady-state configuration over time (Baedecker et al. 1993; Rice et al. [...]

Author(s): Francis H. Chapelle (corresponding author) [1]; Kathleen O'Neill [2]; Paul M. Bradley [1]; Barbara A. Methé [2]; Stacy A. Ciufo [2]; LeRoy L. Knobel [3]; Derek R. Lovley (corresponding [...]

Microorganisms in surface water sediment can degrade methyl t-butyl ether (MTBE) to CO(sub 2) under denitrifying conditions. MTBE is a fuel additive that the EPA has classified as a possible carcinogen.

Methyl tert-butyl ether(MTBE) is a gasoline additive used for octane enhancement in the United States, which results in lower emissions of ozone and carbon monoxide precursor compounds, and it thought to have potentially dangerous health risks associated with it. Research of the amount of MTBE transported by groundwater flow shows that MTBE can quickly mineralize to carbon dioxide under artificial and natural oxic conditions.

The microorganisms collected from 11 stream and lake bed sediments in the United States. showed significant mineralization of methyl-tert-butyl ether (MTBE), indicating that aerobic bed sediment microbial processes may provide an environmental sink for MTBE in surface water systems and may help in the transience of MTBE in some surface waters.

This study examines the relationship of methanogens to the degradation of vinyl chloride.

The anaerobic oxidation of (1,2-14C)vinyl chloride (VC) and (1,2-14C)dichloroethene (DCE) to (super 14)CO(sub 2) was carried out. It has been observed that humic acid compounds can act as electron acceptors for the anaerobic oxidation of these organic contaminants. This finding suggest that the efficient mineralization of VC and DCE can be done by humic acids without accumulating reduced intermediates even under methanogenic conditions.

Intrinsic bioremediation appears to be an effective method in decontaminating highly-polluted aquifer systems. Results revealed that bed-sediment microorganisms indigenous to a creek have rapidly mineralized vinyl chloride (VC) and dichloroethene (DCE) under aerobic conditions. The rates of DCE and VC mineralization, however, depends on contaminant concentration.

Quantitative analysis of oxidation-reduction potentials (Eh) and levels of dissolved H2 was conducted in jet fuel-contaminated aquifers of Wurtsmith Air Force Base, MI. The Eh potentials of the contaminated aquifers were indicated by the presence of high H2 levels which is located in the center of the plume. The groundwater samples also exhibited high levels of oxygen, sulfate, methane and Fe(II) which act as potential electron acceptors for redox reactions.

Abundant electron donor availability enabled methanogenc bacteria (MB) to sequester a portion of the electron flow in a petroleum hydrocarbon-dominated aquifer, even in the presence of sufficient concentrations of sulfate to enable sulfate reduction. Limited electron donor availability, on the other hand, allowed sulfate-reducing bacteria (SRB) to outcompete MB for available substrate at a lower concentration of sulfate than when electron donor was abundant. Thus, fluctuations in electron donor availability influenced the competition between SRB and MB for control of electron flow.

Stable carbon isotopes in dissolved inorganic carbon (DIC) provide evidence of the zonation of 13C-depleted hydrocarbon biodegradation processes under the influence of hydrologically controlled electron-acceptor availability. A four-year study of biodegradation zonation in a sandy, noncarbonate shallow aquifer contaminated with jet-fuel petroleum hydrocarbons showed a range from -28 to +11.9% in (delta)13C values in DIC.

Fe(III) chelators such as EDTA, N-methyliminodiacetic acid, ethanol diglycine, humic acid, and phosphates induce benzene biodegradation of the aquifers contaminated with aromatic hydrocarbons. The chelated Fe(III) acts as an electron acceptor and is reduced during anaerobic benzene oxidation. The rate of degradation depends on the accessability of Fe(III) oxides to the aromatic hydrocarbon-degrading Fe(III) reducers.

A study was conducted to determine whether the rates of microbial hydrocarbon degradation in low-temperature groundwater systems are more depressed than degradation in more temperate aquifers. Microbial metabolism in sediments from two different aquifers in Adak, AK, showed low rates for aquifers in temerate systems, suggesting that the assumption is incorrect.

The effect of the environment on microbial mineralization of trinitrotoluene (TNT) in contaminated soil is investigated. The results showed that the addition of cellobiose and syringate had an inhibiting effect on the mineralization process. Microorganisms were capable of metabolizing TNT but they preferred other substances when possible. There were indications that aerobic and anaerobic conditions can enhance TNT degradation.

Introduction The role of organic acids in promoting mineral dissolution in the subsurface has received considerable attention, particularly with respect to oilfield waters (Crossey et al., 1986; Kharaka et al., [...]

Raising the bioavailability of Fe(III) on addition of suitable ligands increases its potential as a substitute to oxidants for the bioremediation of petroleum-affected aquifers. The addition of organic ligands to Fe(III) increases its bioavailability and the ligand presence facilitates the comparison of degeneration rates of aromatic hydrocarbons in anoxic aquifer sediments to those in oxic sediments.

Reduction-oxidation reactions in anoxic groundwaters was investigated using concentrations of dissolved H2. The latter was used to determine if Fe(III) reduction, sulfate reduction or methanogenesis was the terminal electron accepting process in both pristine and contaminated aquifers. Although the study could not obtain the predominating terminal electron accepting processes in contaminated aquifers, it revealed a similarity between the H2 concentrations of the two aquifers and those measured in aquatic sediments.

Atrazine mineralization potential under aerobic conditions in alluvial-aquifer sediments is dependent on the structure of the atrazine compound, according to a recent study conducted in an Ohio flood plain. Less than .1% of an atrazine compound with a ring structure was mineralized in 23 days. In contrast, ethyl-2-carbon atrazine mineralization, under aerobic conditions, was observed in shallow sediments and was sometimes detectable in deep sediments. Thus, atrazine compounds, which are often used in herbicides, may pass through to the water table if the structures are ringed.

Tests of the hypothesis that the development and localization of high-iron ground water in an Atlantic Coastal Plain is controlled by the activity and interaction of Fe(III)- and sulfate reducing bacteria are discussed. Sediments recovered from three continuous core drilling operations were subjected to assay analysis. It was found that microbial competition favors the zone of Fe(III) reduction over sulfate reducing bacteria in the aquifer.

The possibility that microorganisms are catalyzing the ongoing reduction of Fe(III) in the sediments of deep (20-250 m) aquifers was investigated. Acetate-oxidizing, Fe(III)-reducing bacteria were recovered from deep subsurface sediments, but only from sediments in which it appeared that Fe(III) reduction was the terminal electron-accepting process for oxidation of organic matter. The Fe(III)-reducing microorganisms were capable of reducing ferric oxides present in deep subsurface sediments. Although Fe(III) reduction in subsurface sediments is frequently regarded as an abiological reaction, the enzymatic reduction of Fe(III) by microorganisms reported here is the first mechanism of any kind actually shown to have the potential to couple the oxidation of organic matter to carbon dioxide with the reduction of Fe(III) under the environmental conditions typically found in deep aquifers. We propose that microbially catalyzed Fe(III) reduction is responsible for such late postdepositional phenomena as the formation of variegated red beds and the release of high concentrations of dissolved iron into anaerobic ground waters.

Dissolved organic carbon (DOC) is the smallest amount of organic carbon present in aquifer systems and is typically dwarfed by amounts of particulate organic carbon (POC) and adsorbed organic carbon (AOC). Research conducted over the last half century, however, has shown that these dissolved, particulate, and adsorbed compartments interact dynamically with each other. That suggests the hypothesis that the bioavailability of DOC in groundwater may indicate the bioavailability of the associated POC and AOC compartments as well. If that proves to be the case, it would greatly simplify the process of evaluating the bioavailability of total organic carbon present in groundwater systems. That hypothesis was examined by (1) comparing DOC bioavailability between two aquifers receiving modern atmospheric recharge, but with the recharge passing through POC/AOC sources of substantially different geologic ages, and (2) measuring POC/AOC bioavailability in sediments in from two aquifers before and after injection with bioavailable DOC consisting of dissolved sugars and emulsified vegetable oil. The results of both comparisons are consistent with the hypothesis that DOC bioavailability in groundwater reflects the bioavailability of the associated POC and AOC compartments and vice versa. Thus, DOC bioavailability may be a useful indicator of an aquifer's potential to drive reduction/oxidation processes that affect the chemical quality of groundwater. [ABSTRACT FROM AUTHOR]

Researchers from the U.S. Geological Survey demonstrated that oxidative acetogenesis, followed by oxidative acetogenesis may be effective in the microbial anaerobic degradation of vinyl chloride to CH4.

Vinyl chloride is a highly toxic and carcinogenic compound most often found in aquifer systems. Biodegradation of vinyl chloride by indigenous microorganisms has been proposed for contaminated, anaerobic aquifer systems. The approach requires iron(III)-reducing conditions to promote anaerobic oxidation of the compound. This process of rapid microbial degradation is also known as anaerobic mineralization.

James E. Landmeyer [a,b] Francis H. Chapelle [a] Paul M. Bradley [a] Abstract Stable hydrogen-isotope values of ground water (δ[H.sup.2]) and dissolved hydrogen concentrations ([H.sub.2(aq)]) were quantified in a petroleum-hydrocarbon [...]