DISTRIBUTION OF TERMINAL ELECTRON-ACCEPTING PROCESSES IN AN AQUIFER HAVING MULTIPLE CONTAMINANT SOURCES

Concentrations of electron accepters, electron donors, and Hz in groun dwater were measured to determine the distribution of terminal electro n-accepting processes (TEAPs) in an alluvial aquifer having multiple c ontaminant sources. Upgradient contaminant sources included two separa te hydrocarbon point sources, one of which contained the fuel oxygenat e methyl tertbutyl ether (MTBE). Infiltrating river water was a source of dissolved NO3, SO4 and organic carbon (DOC) to the downgradient pa rt of the aquifer. Groundwater downgradient from the MTBE source had l arger concentrations of electron accepters (dissolved O-2 and SO4) and smaller concentrations of TEAR end products (dissolved inorganic C, F e2+ and CH4) 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 tha t SO4 reduction predominated in the aquifer during a period of high wa ter levels in the aquifer and river. The predominant TEAP shifted to F e3+ reduction in upgradient areas after water levels receded but remai ned SO4 reducing downgradient near the liver. This distribution of TEA Ps is the opposite of what is commonly observed in aquifers having a s ingle contaminant point source and probably reflects the input of DOC and SO4 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 a nd on the availability of electron accepters. (C) 1997 Elsevier Scienc e Ltd.