CALL FOR PROPOSALS:

ORGANIZERS

  • Harvey Thorleifson, Chair
    Minnesota Geological Survey
  • Carrie Jennings, Vice Chair
    Minnesota Geological Survey
  • David Bush, Technical Program Chair
    University of West Georgia
  • Jim Miller, Field Trip Chair
    University of Minnesota Duluth
  • Curtis M. Hudak, Sponsorship Chair
    Foth Infrastructure & Environment, LLC

 

Paper No. 10
Presentation Time: 4:15 PM

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


FLOREA, Lee J., Department of Geological Sciences, Ball State University, 2000 W. University Ave, Muncie, IN 47306, WYNN, Jonathan G., Department of Geology, University of South Florida, 4202 E. Fowler Avenue, SCA 528, Tampa, FL 33620, LAWHON, Nicholas, Department of Geography and Geology, Western Kentucky University, 1906 College Heights Blvd, Bowling Green, KY 42101-1066, STINSON, Chasity L., Department of Biology, Texas State University, 601 University Drive, San Marcos, TX 78666 and WALDEN, William, 882 Old Sawmill Road, Monticello, KY 42633, mr_chaos@hotmail.com

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 CO213C ~ -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.



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