GROUNDWATER MIXING ALONG SOLUTION-ENHANCED FRACTURES DISCERNED BY ELEMENTAL AND STABLE ISOTOPE GEOCHEMISTRY

Karst aquifers are well known for their intricate stratigraphy and geologic structures, which make the groundwater characterization difficult as flowpaths and recharge sources are complex and difficult to evaluate. The North-Central Alabama region, an area with karst development and complex geologic structures, represents an excellent environment to evaluate similarities and differences between karst aquifers, identify recharge sources and characterize mixing processes along dissolution fractures, and to assess aquifer vulnerability due to groundwater withdrawals. Field parameters, elemental and stable isotope geochemistry of approximately ten closely spaced production wells of two karst aquifers, Bangor Limestone and Tuscumbia Fort Payne are employed in this study. Geochemical data illustrate two distinctive groundwater endmembers: higher major ion, conductivity, and Alkalinity concentration values of the groundwater in the Bangor Limestone compared with those of the groundwater in the Tuscumbia Fort Payne aquifer. Geochemical speciation modeling shows that groundwater in both aquifers is undersaturated with calcite but supersaturated with hematite and goethite. Sampling during the summer and fall of 2010 exhibit aquifer inter-flow mixing along solution fractures and confirm the distinctive groundwater geochemistry of the two aquifers. Static water level decreases over the summer (2 to 5.2 meters) leads to groundwater characterized by more reducing conditions (lower Eh values) and ¹⁸O and ²H enriched stable isotope signature during the fall (fall range - δ¹⁸O: -4.8±0.1 to -5.4±0.1‰ VSMOW n=9; δD: -25.4±1 to -27.4±1‰ VSMOW, n=9 and summer range - δ¹⁸O: -5.1±0.1 to -5.7±0.1‰ VSMOW, n=11; δD: -25.0±1 to -30.6±1‰ VSMOW, n=11) than the summer season. In addition, mixing between the groundwater members is more evident during the fall because of the seasonal change of water table and the significant groundwater pumping during the summer. Stable isotope data also show that recharge occurs locally. Thus, the combination of field parameters, elemental and stable isotope geochemistry can be successfully used to identify recharge sources, evaluate groundwater flow and transport pathways, and to improve understanding of how groundwater withdrawals impact aquifer vulnerability.