Carbon dioxide and dissolution rate dynamics within a karst underflow-overflow system, Savoy Experimental Watershed, Arkansas, USA

摘要

Within numerical models of karst development, geochemical and hydrological boundary conditions are typically assumed to be constant. However, rates of calcite dissolution in natural karst systems can vary substantially in time. In particular, variation in carbon dioxide (CO2) concentrations has been shown to produce time variability in dissolution rates in karst streams, but controls on CO2 variation within karst systems are relatively poorly quantified. Here we analyze hourly in-situ measurements of dissolved CO2, discharge, and specific conductance at a pair of karst underflow-overflow springs and examine potential drivers of variability in CO2 and dissolution rates. The springs display strong seasonal variability in CO2 and saturation state as well as moderate variation during storm events. Though both springs have elevated CO2 concentrations in the summer season, the overflow spring experiences a substantial decrease in dissolved CO2 below a critical discharge threshold. We hypothesize that this decrease results from ventilation within the overflow portion of the system as segments of the flow path transition from full pipe to open channel flow. The overflow spring experiences substantially lower average dissolution rates than the underflow spring, despite larger discharge and chemical variability during high flow events at the overflow spring. Though open systems are frequently presumed to have higher dissolution rates, because of their ability to replenish CO2 that is consumed in the dissolution process, these data suggest that dissolution rates within the closed portion of the system may in some cases be higher due to the inability of closed flow paths to ventilate excess CO2 to the atmosphere. Such conditions are likely to occur in conduits where CO2 concentrations are elevated above atmospheric levels and average flow through times are short compared to the time scale over which water reaches equilibrium with respect to calcite.