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...

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The chemical quality of ground water in the upper glacial (water-table) aquifer in Nassau and Suffolk Counties was examined in relation to 10 types of land use to evaluate the effect of human activities on ground water. The upper glacial aquifer, which consists of unconsolidated Pleistocene deposits composed primarily of sand and gravel, overlies thick Cretaceous deposits that form the bulk of the island's aquifer system. The aquifers are the sole source of drinking water for more than 2.6 million people in the two-county area. Contamination has restricted the use of the upper glacial aquifer for public-water supply in developed areas; thus, its use is restricted except in rural areas of Suffolk County. Median concentrations of total dissolved solids and detection frequencies of tetrachloroethylene, trichloroethylene, and 1,1,1-trichloroethane were higher in sewered areas than in unsewered areas. This indicates that the effects of sewering in the highly developed areas of Nassau and Suffolk Counties may be obscured by other factors, such as (1) concentration of commerce and industry in sewered areas, which are sources of leaks and spills of these contaminants, and (2) potential leakage from sewers. Also, some contaminants that were present in areas before sewer installation may persist within the aquifer system. Public Water Supply, provided lists of contaminated public-supply wells and analytical data on organic compounds in samples collected during local studies in Nassau County. James Mulligan and Richard Liebe of the Nassau County Department of Public Works, Water Supply Unit, provided analytical data on organic compounds in samples collected from the county's observation-well network. Arthur Kunz and Edith Tannenbaum of the Long Island Regional Planning Board, and Peter French of Resources Planning Associates, Inc., provided land-use data and help in data interpretation. Carole Swick and Roy Fedelem of the Long Island Regional Planning Board provided population-density statistics. Funding was provided by the U.S. Geological Survey's Toxic Waste-GroundWater Contamination Program.