CARBON AND NUTRIENT CYCLING IN SOUTH-CENTRAL INDIANA KARST: PRELIMINARY RESULTS

Karst terrains possess unique hydrologic and geochemical characteristics with unique carbon and nutrient cycling dynamics. Dissolution of carbonate rocks liberates dissolved inorganic carbon (DIC) from calcite and soil gas while epikarst soils release dissolved organic carbon (DOC) as delayed biological capture from the atmosphere. These processes mobilize carbon from the atmosphere and lithosphere into karst aquifers at rates influenced by nitrogen, phosphorous, and sulfur cycles, among other factors.

As part of a long-term study, we are monitoring concentrations of DIC and nutrients in the classic karst landscape of the Mitchell Plateau in south-central Indiana. In this region, well-developed epikarst facilitates the mixing of meteoric, surface, and groundwater in extensive subsurface drainage systems. Direct infiltration links agricultural practices on the surface with the water quality below. Our samples come from sites representing inflow, throughflow, and outflow in the Bluespring and Lost River karst basins. The sites include: Bluespring Caverns, an extensive cave system that drains roughly 25 km² through overlying epikarst; Flood Creek, a perennial sinking stream; Wesley Chapel Gulf, a karst window where Lost River surfaces; and Orangeville Rise, where a portion of the waters from Lost River arise.

Our analyses have detected multiple spikes of nitrogen and phosphorous as high as 3.98 mg/L and 1.41 mg/L respectively, that may correlate with land use along the sampled flow path. As spring of 2019 was unusually wet in south-central Indiana, anomalous amounts of nutrient-rich runoff may have entered subsurface drainage from farm fields. Measurements of alkalinity show site specific distributions likely related to water contributions and dissolution processes. Flood Creek has a maximum measured alkalinity of 107.2 mg/L CaCO₃ while Orangeville Rise has a maximum measured alkalinity of 195.0 mg/L CaCO₃. A positive correlation is found at each site between sulfate concentrations, ranging from below detectable limits of 10 mg/L at Flood Creek to 35 mg/L at Orangeville Rise, and alkalinity, suggesting sulfur plays a role in DIC production in the Mitchell Plateau. As our monitoring continues into 2020, we expect our interpretations to become more robust as they are corroborated or contradicted by new data.