Bacteria in deep coastal plain sediments of Maryland: A possible source of CO₂ to groundwater

Abstract

Nineteen cores of unconsolidated Coastal Plain sediments obtained from depths of 14 to 182 m below land surface near Waldorf, Maryland, were collected and examined for metabolically active bacteria. The age of the sediments cored range from Miocene to Early Cretaceous. Acridine orange direct counts of total (viable and nonviable) bacteria in core subsamples ranged from 10 8 to 10 4 bacteria/g of dry sediment. Direct counts of viable bacteria ranged from 10 6 to 10 3 bacteria/g of dry sediment. Three cores contained viable methanogenic bacteria, and seven cores contained viable sulfate‐reducing bacteria. The observed presence of bacteria in these sediments suggest that heterotrophic bacterial metabolism, with lignitic organic material as the primary substrate, is a plausible source of CO 2 to groundwater. However, the possibility that abiotic processes also produce CO 2 cannot be ruled out. Estimated rates of CO 2 production in the noncalcareous Magothy/Upper Patapsco and Lower Patapsco aquifers based on mass balance of dissolved inorganic carbon, groundwater flow rates, and flow path segment lengths are in the range 10 −3 to 10 −5 mmol L −1 yr −1 . Isotope balance calculations suggest that aquifer‐generated CO 2 is much heavier isotopically (∼—10 to + 5 per mil) than lignite (∼−24 per mil) present in these sediments. This may reflect isotopic fractionation during methanogenesis and possibly other bacterially mediated processes.