Microorganisms present in deep Atlantic coastal plain sediments affect the geochemical evolution of groundwater and its chemical and isotopic composition, yet the factors controlling their origin, distribution, and diversity are poorly understood. The evolution of the groundwater chemistry, the fractionation of stable carbon isotopes, and the groundwater age are all indicators of the inorganic and microbial reactions occurring along a given flow path from groundwater recharge to groundwater discharge. In this study, tritium, 14C, and groundwater chemistry along three flow paths of the Middendoff aquifer in South Carolina were analyzed. The !4C ranged from 89 percent modern carbon (pmC) in the recharge zone to 9.9 pmC in the distal borehole; the/•3C remained relatively constant at ----22%•, suggesting microbial oxidation of organic carbon. Carbon isotope analyses of particulate organic carbon from core sediments and groundwater chemistry were used to model the carbon chemistry; the groundwater ages obtained from •4C ranged from modem to 11,500 years B.P. The highest frequencies of occurrence, numbers, and diversity of aerobic and anaerobic bacteria were found in boreholes near the recharge zone where the calculated ages were <1000 years B.P. The transport of microorganisms from the recharge zone may be responsible for this distribution as well as the electron acceptors necessary to support this diverse community of bacteria. The presence of both aerobic heterotrophs and anaerobic sulfate-and iron-reducing bacteria in the core sediments suggested the occurrence of anaerobic microsites throughout this otherwise aerobic aquifer. The highest in situ microbial respiration rate, as determined by modeling, was found along a flow path near the recharge area. It is likely that the electron acceptors necessary for supporting a diverse microbial community are depleted by the time the groundwater residence time in the Middendoff aquifer exceeds several hundred years. INTRODUCTION Abundant and diverse populations of microorganisms were found at depths of over 400 m in Atlantic coastal plain sediments at and near the U.S. Department of Energy's (DOE's) Savannah River Site (SRS) in South Carolina [Balkwill, 1989; Fredrickson .et al., 1989; Jones et al., 1989]. These microorganisms are capable of affecting both the chemical and isotopic composition of groundwater and its geochemical evolution [Chapelle et al., 1987, 1988; Murphy et al., 1989; Plumruer et al., 1990; Chapelle and Lovley, 1990a; McMahon et al., 1990; Lovley et al., 1990a; McMahon and ½hapelle, 1991]. The complex interaction between microorganisms and organic carbon in the subsurface is perhaps most evident when the inorganic and isotopic chemistry of groundwater are modeled along a flow path from groundwater recharge to groundwater discharge. Reaction-path modeling of the chemical evolution and the change in stable isotopes along a flow path requires an understanding of the role of aerobic and anaerobic microorganisms in the carbon cycle and the nature of particulat...
