Atmosphere/Landscape CO2 Interactions in Mammoth Cave National Park: Hydrochemistry of Cascade River, Great Onyx Cave

High-resolution analysis of water/CO2/carbonate rock geochemistry was conducted at the Great Onyx Groundwater Basin in Mammoth Cave National Park. The purpose of this work has been to undertake hydrochemical characterization of the karst groundwater in Cascade River, the largest cave stream in the upper level of Great Onyx Cave, which is below a relatively pristine karst landscape. Hydrochemical monitoring of Cascade River at Biz Falls using field, laboratory, and high-resolution electronic data-sonde measurements has provided a detailed picture of the stream’s behavior with respect to the transport and influence of carbon dioxide in the surface and groundwater system as well as quantifying water/rock interactions. Comparison of cool and warm season hydrochemical dynamics shows waters of Cascade River are influenced only by atmospheric CO2 in winter while the warming of the seasons adds additional CO2 from a surface source, presumably soil respiration influenced by degradation of soil organic material and root respiration. The waters of Cascade River stay undersaturated with respect to the limestone bedrock so that the aquifer framework is continuously dissolving, though more rapidly in winter. In all seasons, storms have a rapid impact on the groundwater showing a relatively direct hydrologic connection from the surface through a significant conduit that most likely stays water filled. These results advance the understanding of hydrology and geochemistry of the Great Onyx Groundwater Basin, representing an important first step towards long-term study