Stable Carbon Isotopes of HCO3 in the Aquia Aquifer, Maryland: Evidence for an Isotopically Heavy Source of CO2

Concentrations of HCO₃ and δ¹³C 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 (δ¹³C ∼ 0.0 per mil) in the presence of soil-gas CO₂ (δ¹³C∼–26 per mil). HCO₃ concentrations in this area range from 150-200 mg/1 and δ¹³C values of dissolved HCO₃ are approximately –13.0 per mil. Concentrations of HCO₃ decrease gradually (to about 130 mg/1) and δ¹³C 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, HCO₃ concentrations increase sharply (to about 400 mg/1), and δ¹³C values become significantly heavier (δ¹³C ∼–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 δ¹³C trend. Rather, the Na:HCO^-₃ ratio demonstrates that shell material dissolution must take place in the presence of CO₂. Because the aquifer in this area is closed to both soil-gas and atmospheric CC>2, an aquifer-generated source of isotopically heavy (δ¹³C ∼–5.3 per mil) CO₂ is implied. Possible sources of aquifer-generated CO₂ include bacterially-mediated fermentation of lignitic aquifer materials and bacterially-mediated methanogenesis.