Viable bacteria are present (105-107 cells per gram sediment) in the clastic water-table and Hawthorn aquifers that overlie and recharge the Floridan aquifer near Hilton Head Island, South Carolina. Incubation of cored sediments shows that indigenous bacteria are capable of producing carbon dioxide (CO2) as they metabolize sedimentary organic material. Estimated in situ CO2 production rates, based on dissolved inorganic carbon (DIC) changes and ground-water flow rates, are in the range of 10-2 mmol . L-1, yr-1.

Two independent lines of evidence, (1) isotope balance calculations on DIC along ground-water flow paths and (2) direct measurement of the isotopic composition of CO2produced by incubation of Hawthorn sediment, suggest that bacterially produced CO2 in the Hawthorn aquifer is much heavier isotopically (δ13C ∼ -10‰ to -15‰) than associated sedimentary organic material (δ13C ∼ -21‰). The relatively heavy δ13C values of DIC in water from the Floridan aquifer, which receives much of its recharge from the overlying Hawthorn, reflect this source of isotopically heavy CO2.

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