CARBON DIOXIDE DYNAMICS WITHIN THE CRITICAL ZONE OF A KARST LANDSCAPE: SAVOY EXPERIMENTAL WATERSHED, ARKANSAS

Within a highly heterogenous subsurface flow type, karst, the critical zone evolves largely through the congruent weathering of calcite driven by dissolved concentrations of carbon dioxide. Flow within this particular type of carbonate setting results in hydraulic conductivities that vary over many orders of magnitude. Through solutional enlargement, dissolved carbon dioxide develops preferred flow paths that focus flow and transport dissolved species and gases. Though gas transport within the subsurface is typically assumed to be diffusive, modeling results suggest that advective transport driven by both water and buoyant gas flow are likely to produce strong perturbations to vertical depth profiles of CO2. These dynamics likely have important impacts on the depth distribution of calcite weathering

To examine whether the predicted advective effects are observed in the field, we are conducting measurements at the Savoy Experimental Watershed (SEW), a University of Arkansas collaborative research site for the study of water science and animal waste management in mantled karst terrain. CO2 measuring units composed of a Sensair K-30 CO2 gas analyzer with a range up to 100,000 ppm and Arduino based datalogger have been deployed in two vadose zone wells of varying depth and one soil depth well. Loggers are housed in a weather proof casing and powered by a 12 V lead acid battery attached to a solar power charging station. The CO2 sensors are coated with a waterproof breathable membrane enabling direct measurement of dissolved CO2. CO2 data collected from these loggers are compared to data collected from a primary spring that drains the catchment as well as comprehensive weather data, including soil moisture content, collected every five minutes within the same basin. Through multiple deployed loggers, we are able to examine CO2 concentration dynamics over time and space within the catchment.