VARIATION OF δ13C IN THE CARBONATE AQUIFERS OF THE CUMBERLAND PLATEAU IN SOUTHEAST KENTUCKY

Samples collected during 2010 and 2011 at Stream and NBC Caves, one input to and the primary output from the Redmond Creek karst aquifer, respectively, provide insight into the geochemical evolution of shallow groundwater along the Cumberland Plateau of southeast Kentucky. Additional samples from nearby seeps of oilfield brine offer comparative information on the geochemistry of deeper groundwater. In this abstract, we consider the variation of δ¹³C (VPDB) in these samples.

Sample TOC concentrations range between 1 and 3 mg/L. δ¹³C_(DOC) ranges between -26‰ and -29‰ with no spatial or temporal trends. These results correspond to regional measurements of soil δ¹³C and reveal the dominance of C3 vegetation in the mixed hardwood forest overlying much of the watershed.

The range of δ¹³C_(DIC) (-6.3‰ to -12.4‰) coincides with carbonic acid dissolution of limestone, which, by the stoichiometry of the equilibration reaction, half the DIC derives from bedrock (δ¹³C ~ +1.5‰, Ste. Genevieve Ls.), and half from soil-derived CO₂ (δ¹³C ~ -16‰). The data also weakly suggest that δ¹³C_(DIC) at NBC and Stream Caves is inversely proportional with discharge (R² = 0.52 and 0.77). In contrast, δ¹³C_(DIC) in the brine seeps average -5.3‰ and may reflect the input of additional bedrock DIC from a reaction with acidity from oxidized-H₂S.

We propose that SI_calcite that is inversely proportional with discharge (R² = 0.92, NBC Cave) principally governs δ¹³C_(DIC) variation. Accordingly, undersaturated waters in higher discharge have an excess of soil-derived pCO₂ and contribute to δ¹³C_(DIC) more depleted than in base flow. However, waters at Stream Cave are more enriched in ¹³C than at NBC Cave (-7.2‰ versus -8.8‰) despite having lower SI_calcite (-2.3 versus -0.3). At Stream Cave sulfur oxidation in overlying siliciclastics may contribute acidity and DIC with a higher than expected δ¹³C_(DIC). At NBC Cave, sulfate reduction in the brines underlying Redmond Creek may cause additional depletion of ¹³C.