ABSTRACTConcentrations of HCO3and 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). HCO3concentrations in this area range from 150‐200 mg/1 and δ13C values of dissolved HCO3are approximately –13.0 per mil. Concentrations of HCO3decrease 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, HCO3concentrations 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‐3in this downgradient water shows that dissolution of isotopically heavy carbonate shell material cannot by itself explain the observed δ13C trend. Rather, the Na:HCO‐3ratio 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) CO2is implied. Possible sources of aquifer‐generated CO2include bacterially‐mediated fermentation of lignitic aquifer materia
展开▼
