THE IMPACT OF VENTILATION ON DISSOLUTION RATES WITHIN KARST CONDUITS

Within carbonate aquifers, the development of karst conduits is primarily driven by the dissolution of calcite by carbonic acid. In the shallow subsurface, concentrations of CO2 often rise well above atmospheric concentrations due to root respiration and decay of other organic matter. Therefore, in most karst settings, the bulk of the acidity that drives dissolution is derived from subsurface sources of carbon dioxide. However, the development of solutionally enlarged fractures and conduits can enable the circulation of atmospheric gases, largely due to thermally produced buoyant flows. When sufficient ventilation occurs, CO2 concentrations in the subsurface decrease, reducing the capability of water to dissolve carbonate minerals. Here we examine time series data of dissolved CO2 measured at Copperhead and Langle Springs, located at the Savoy Experimental Watershed near Fayetteville, Arkansas, and within Blowing Springs Cave, in Bella Vista, Arkansas. All sites display seasonal patterns with higher CO2 concentrations in the summer. This results in part from higher rates of respiration during warmer periods. However, two of the three sites also display sudden decreases and oscillations in dissolved CO2 that are most easily explained by ventilation. In the case of Blowing Springs Cave, time series data of cave airflow velocity demonstrate that cave airflow direction is a first order control on both CO2 concentrations and dissolution rates within the cave stream. In contrast, variations in dissolved load during storm events play a relatively minor role in determining long term dissolution rates. Comparison of CO2 dynamics at Copperhead and Langle Springs, which are part of a karst underflow-overflow system, suggest that ventilation may play a stronger role once karst systems become mature.