Distribution of terminal electron-accepting processes in an aquifer having multiple contaminant sources

Abstract

Concentrations of electron acceptors, electron donors, and H 2 in groundwater were measured to determine the distribution of terminal electron-accepting processes (TEAPs) in an alluvial aquifer having multiple contaminant sources. Upgradient contaminant sources included two separate hydrocarbon point sources, one of which contained the fuel oxygenate methyl tertbutyl ether (MTBE). Infiltrating river water was a source of dissolved NO 3, SO 4 and organic carbon (DOC) to the downgradient part of the aquifer. Groundwater downgradient from the MTBE source had larger concentrations of electron acceptors (dissolved O 2 and SO 4) and smaller concentrations of TEAP end products (dissolved inorganic C, Fe 2+ and CH 4) than groundwater downgradient from the other hydrocarbon source, suggesting that MTBE was not as suitable for supporting TEAPs as the other hydrocarbons. Measurements of dissolved H 2 indicated that SO 4 reduction predominated in the aquifer during a period of high water levels in the aquifer and river. The predominant TEAP shifted to Fe 3+ reduction in upgradient areas after water levels receded but remained SO 4 reducing downgradient near the river. This distribution of TEAPs is the opposite of what is commonly observed in aquifers having a single contaminant point source and probably reflects the input of DOC and SO 4 to the aquifer from the river. Results of this study indicate that the distribution of TEAPs in aquifers having multiple contaminant sources depends on the composition and location of the contaminants and on the availability of electron acceptors.