Stable Carbon Isotopes of HCO 3 in the Aquia Aquifer, Maryland: Evidence for an Isotopically Heavy Source of CO 2

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Abstract
Concentrations of HCO 3 and δ 13 C values of dissolved inorganic carbon change along the hydrologic gradient of the Aquia aquifer. In the outcrop area, meteoric recharge rapidly dissolves carbonate shell material (δ 13 C ∼ 0.0 per mil) in the presence of soil‐gas CO 2 (δ 13 C∼–26 per mil). HCO 3 concentrations in this area range from 150‐200 mg/1 and δ 13 C values of dissolved HCO 3 are approximately –13.0 per mil. Concentrations of HCO 3 decrease gradually (to about 130 mg/1) and δ 13 C values become slightly heavier (to about –11.4 per mil) as water flows away from the outcrop area. These trends reflect progressive dissolution of metastable carbonate shell material and subsequent precipitation of secondary calcite cement. At about 40 miles downgradient in the flow system, HCO 3 concentrations increase sharply (to about 400 mg/1), and δ 13 C values become significantly heavier (δ 13 C ∼–6.2 per mil). The observed 1:1 molar ratio of Na + to HCO ‐ 3 in this downgradient water shows that dissolution of isotopically heavy carbonate shell material cannot by itself explain the observed δ 13 C trend. Rather, the Na:HCO ‐ 3 ratio demonstrates that shell material dissolution must take place in the presence of CO 2 . Because the aquifer in this area is closed to both soil‐gas and atmospheric CC>2, an aquifer‐generated source of isotopically heavy (δ 13 C ∼–5.3 per mil) CO 2 is implied. Possible sources of aquifer‐generated CO 2 include bacterially‐mediated fermentation of lignitic aquifer materials and bacterially‐mediated methanogenesis.

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