T135 Dynamic Gradients in Karst Aquifers

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room M101AB

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

Paper number: T135-11
Program: Technical Sessions
Time: 4:15 PM – 4:30 PM

Authors

Abstract

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 SIcalcite 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 SIcalcite (-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.