Logsdon River is a major, base-level stream within the Turnhole Bend Drainage basin of the Mammoth Cave System. The Logsdon River system has provided a unique opportunity to examine the geochemical evolution of a stream flowing through a major karst conduit that can be traversed for 10 km. This study examines CO2 inputs at the upstream portion of the river, which provide major control for the river’s hydrochemistry. Samples were collected from the upstream portion of Logsdon River at what is referred to as the S-188 sump and also nearby at Crowbar Dome over the course of 44 weeks from May 2012 through April 2013. The concentrations of CO2 for samples were calculated from field and laboratory analysis. The CO2 concentrations were examined during the study period to assess potential sources of CO2 input to the karst system in the context of seasonal variation. Seasonal fluctuations were found to be greatest in the near surface sample site, Crowbar Dome. Attenuation of seasonal variation of CO2 pressures in the upstream Logsdon River S-188 Sump suggests both surface inputs plus additional inputs of CO2 entering the system, perhaps from the decay of organic material in the saturated passages upstream beyond the accessible portion of the Logsdon River S-188 Sump. This in-cave source of CO2 has some control on hydrochemistry, and thus waterrock interaction and speleogenesis of the karst landscapes in south-central Kentucky