CO2 STORM HYSTERESIS IN KARST SPRINGS

Studying how the chemistry of karst spring water reacts to periods of increased discharge can provide insights into how these systems store and transmit water. Changes in the partial pressure of carbon dioxide (P_CO2) in karst spring water during storm events, in combination with cation analysis, is helpful in discriminating soil water, epikarst water, point recharge storm event water, and matrix water during storm events. Comparing arrival times of these source waters can be used to infer recharge and flow patterns within karst systems.

Direct, prolonged, high frequency field P_CO2 data is difficult to acquire and CO₂ degassing from automatically-collected storm samples inhibits meaningful P_CO2 measurements. To assess CO₂ in storm samples, P_CO2 values are calculated through solution modeling in PHREEQc using alkalinity measurements, temperature, ion, and pH data. Storm events were captured at Springhouse Spring and Smullton Sinks, located in Centre County, PA, under both dry and wet antecedent conditions. Under dry antecedent conditions, Springhouse Spring exhibited stable Mg/Ca and P_CO2 values throughout the storm response. This indicates that unmixed recharge in small- to medium-sized storm events did not reach the spring. This is in contrast to other events at the spring during wet periods which exhibit hysteresis in Mg/Ca ratios. Clockwise Mg/Ca hysteresis was observed in storm samples taken from Smullton Sinks, over the course of a rain event, with an initial rise in Mg/Ca, suggesting matrix flushing (piston flow). The same storm event exhibited counterclockwise P_CO2 hysteresis. P_CO2 data support the piston flow interpretation with low P_CO2 matrix water arriving at the spring first, followed by high P_CO2 soil/epikarst water.