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

Abstract

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.

A paleotemperature record derived from dissolved noble gases in groundwater of the Aquia Aquifer (Maryland, USA)

“…In the most distant Aquia well (MD4), ␦ 13 C rises to a value of Ϫ8‰. These observations are in agreement with published data (Chapelle and Knobel, 1985).…”

“…The most distant well (MD4) is dominated by Na ϩ and HCO 3 Ϫ ions. This evolution of the Aquia water is in agreement with the patterns described by Chapelle and Drummond (1983) and Chapelle and Knobel (1985).…”

“…Such an explanation could be unusually strong chemical and isotopic exchange processes along the flow paths leading to the low-14 C region that are not resolved by the models typically used for converting 14 C activities into ages. The chemical and isotopic evolution of Aquia water including the processes influencing the carbon system in the Aquia aquifer have been discussed and modeled by Chapelle (1983), Chapelle and Drummond (1983), Chapelle and Knobel (1985), Plummer et al (1994) and Appelo (1994). Their results can be summarized as follows:…”

“…In region III (more than 80 km from the outcrop) HCO 3 Ϫ and Na ϩ increase rapidly, whereas Ca 2ϩ and Mg 2ϩ remain low. This is explained by dissolution of magnesium-calcite in the presence of biogenic CO 2 and concurrent cation exchange (Chapelle and Knobel, 1985), or by proton exchange in the course of freshening of the aquifer (Appelo, 1994).…”

“…It has been shown based on mass balance considerations (Chapelle, 1983) and isotopic data (Chapelle and Knobel, 1985) that calcite cementation is of secondary origin, i.e., due to calcite precipitation from the groundwater. Chapelle and Knobel (1985) reported ␦ 13 C values ranging from Ϫ10.4 to Ϫ2.2‰ for the calcite cement, and from Ϫ0.6 to ϩ 1.9‰ for the shell material.…”