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

In carbonate settings, the critical zone evolves largely through the congruent weathering of calcite driven by dissolved carbon dioxide. This calcite dissolution process generates one of the most highly heterogeneous types of subsurface flow media, karst, where hydraulic conductivities can vary over many orders of magnitude. Preferred flow paths, which develop by solutional enlargement, focus flow and transport of both 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. High resolution in situ time series data of dissolved CO2 were obtained at SEW from a well, a soil sensor, and the primary spring that drains the catchment, enabling us to examine CO2 concentration dynamics over time and space within the catchment. At each site, an Arduino based logger, using a Sensair K-30 CO2 sensor with a range up to 100,000 ppm, is housed in weather proof housing and powered by a 12 V lead acid battery. The CO2 sensors are coated with a waterproof breathable membrane enabling direct measurement of dissolved CO2. This configuration enables inexpensive, high frequency (hourly), and low power monitoring of dissolved CO2. Comprehensive weather data, including soil moisture content, is collected in five-minute intervals at a central weather station.