The distribution of microbially mediated terminal electron-accepting processes (TEAPs) was investigated in four hydrologically diverse groundwater systems by considering patterns of electron acceptor (nitrate, sulfate) consumption, intermediate product (hydrogen (H2)) concentrations, and final product (ferrous iron, sulfide, and methane) production. In each hydrologic system a determination of predominant TEAPs could be arrived at, but the level of confidence appropriate for each determination differed. In a portion of the lacustrine aquifer of the San Joaquin Valley, for example, all three indicators (sulfate concentrations decreasing, H2 concentrations in the 1-2 nmol range, and sulfide concentrations increasing along flow paths identified sulfate reduction as the predominant TEAP, leading to a high degree of confidence in the determination. In portions of the Floridan aquifer and a petroleum hydrocarbon-contaminated aquifer, sulfate reduction and methanogenesis are indicated by production of sulfide and methane, and hydrogen concentrations in the 1-4 nmol and 5-14 nmol range, respectively. However, because electron acceptor consumption could not be documented in these systems, less confidence is warranted in the TEAP determination. In the Black Creek aquifer, no pattern of sulfate consumption and sulfide production were observed, but H2 concentrations indicated sulfate reduction as the predominant TEAP. In this case, where just a single line of evidence is available, the least confidence in the TEAP diagnosis is justified. Because this methodology is based on measurable water chemistry parameters and upon the physiology of microbial electron transfer processes, it provides a better description of predominant redox processes in groundwater systems than more traditional Eh-based methods. IntroductionEvaluating oxidation-reduction processes is fundamental to understanding the hydrochemistry of groundwater systems. Redox reactions affect the speciation and mobility of dissolved constituents, especially metals and organic compounds, that are important from a water quality and health perspective. In spite of this importance, methods for evaluating redox conditions in anaerobic groundwater systems remain problematic. The early expectation that platinum electrode measurements [Sato, 1960] or measurement of redox couples could be used quantitatively to define an equilibrium redox potential (Eh) of groundwater has not been realized. This reflects the fact that the basic assumption of thermodynamic equilibrium is not appropriate for most hydrologic systems [Thorstenson, 1984;Lindberg and RunnelIs, 1984].The introduction of a kinetic, as apposed to an equilibrium, framework for describing microbially mediated terminal electron-accepting processes (TEAPs) in groundwater systems Paper number 94WR02525. 0043-1397/95/94 WR-025 25 $ 05.00 native way to describe redox processes in groundwater systems. At the most basic level, microbially mediated redox processes proceed sequentially so that electron donors and acceptors are con...

Measurements of dissolved hydrogen and other biologically active solutes in groundwater from a shallow petroleum hydrocarbon-contaminated aquifer indicate that the distribution of microbial terminal electron-accepting processes (TEAPs), such as methanogenesis, sulfate reduction, and ferric iron (Fe (!!I)) reduction, is highly dynamic in both time and space. Delivery of sulfate to methanogenic zones by infiltrating recharge or lateral transport can result in a TEAP shift from methanogenesis to sulfate reduction. Conversely, lack of .recharge and consumption of available sulfate can result in a shift from sulfate reduction to methanogenesis. Temporal shifts between sulfate and Fe (III) reduction were also observed. Time lags associated with TEAP shifts ranged from less than 10 days to about 35 months. The relation between TEAP and biodegradation rates of a variety of organi• compounds indicate that biodegradation rates of petroleum hydrocarbons probably vary temporally and spatially in a contaminated aquifer.

The purpose of this investigation was to determine whether tree-core analysis could be used to delineate shallow groundwater contamination by chlorinated ethenes. Analysis of tree cores from bald cypress [Taxodium distichum (L.) Rich], tupelo (Nyssa aquatica L.), sweet gum (Liquidambar stryaciflua L.), oak (Quercus spp.), sycamore (Platanus occidentalis L.), and loblolly pine (Pinus taeda L.) growing over shallow groundwater contaminated with cis-1,2-dichloroethene (cDCE) and trichloroethene (TCE) showed that those compounds also were present in the trees. The cores were collected and analyzed by headspace gas chromatography. Bald cypress, tupelo, and loblolly pine contained the highest concentrations of TCE, with lesser amounts in nearby oak and sweet gum. The concentrations of cDCE and TCE in various trees appeared to reflect the configuration of the chlorinated-solvent groundwater contamination plume. Bald cypress cores collected along 18.6-m vertical transects of the same trunks showed that TCE concentrations decline by 30-70% with trunk height. The ability of the tested trees to take up cDCE and TCE make tree coring a potentially cost-effective and simple approach to optimizing well placement at this site.

Several practical considerations for measuring concentrations of dissolved molecular hydrogen (H 2 ) in groundwater, including (1) sampling methods, (2) pumping methods, and (3) effects of well casing materials were evaluated. Three different sampling methodologies (a downhole sampler, a gas-stripping method, and a diffusion sampler) were compared. The downhole sampler and gas-stripping methods gave similar results when applied to the same wells. The diffusion sampler, on the other hand, appeared to overestimate H 2 concentrations relative to the downhole sampler. Of these methods, the gas-stripping method is better suited to field conditions because it is faster (∼30 min for a single analysis as opposed to 2 h for the downhole sampler or 8 h for the diffusion sampler), the analysis is easier (less sample manipulation is required), and the data computations are more straightforward (H 2 concentrations need not be corrected for water sample volume). Measurement of H 2 using the gas-stripping method can be affected by different pumping equipment. Peristaltic, piston, and bladder pumps all gave similar results when applied to water produced from the same well. It was observed, however, that peristaltic-pumped water (which draws water under a negative pressure) enhanced the gasstripping process and equilibrated slightly faster than either piston or bladder pumps (which push water under a positive pressure). A direct current (dc) electrically driven submersible pump was observed to produce H 2 and was not suitable for measuring H 2 in groundwater. Measurements from two field sites indicate that iron or steel well casings produce H 2 , which masks H 2 concentrations in groundwater. PVC-cased wells or wells cased with other materials that do not produce H 2 are necessary for measuring H 2 concentrations in groundwater. ES970085C X Abstract published in Advance ACS Abstracts, August 15, 1997.