RELATION OF STORM CHEMOGRAPHS TO STORM HYDROGRAPHS: INTERPRETATIONS FROM THE KARST SPRINGS AT FORT CAMPBELL, KENTUCKY/TENNESSEE

The response of springs to storm events constitutes a useful probe of the internal structure of karst aquifers. Specifically, information can be gleaned by comparing the chemical response (chemograph) with the discharge response (hydrograph). Bulk chemical response was determined by continuous conductivity measurements for six springs located on or near the Fort Campbell Army Base in western Kentucky and Tennessee. The springs drain shallow ground water basins in Mississippian limestones on the western Highland Rim. The chemographs lagged the hydrographs for some springs and the recession period was often longer for the chemographs than for the hydrographs. Detailed chemographs for two springs were determined by chemical analysis of samples collected over short time intervals during a storm event. The calcite saturation index (SI_c) and carbon dioxide partial pressure (P_CO2) were calculated for each sample using MINTEQ. In both springs, the CO₂ pressure increased during the chemograph recession suggesting that the input of diffuse infiltration from the soil zone is important during this period. The addition of the high P_CO2 water at this time causes a corresponding decrease in the SI_c. The result of this correspondence is that the lowest SI_c observed during the storm event occurred during the chemograph recession and lagged behind the hydrograph recession. Hence discharge of the most aggressive spring water lagged the period of rapid chemical change. It appears that the full chemograph, specifically the changes in CO₂ pressure, reveals detail on the mixing of water in the conduit system with water from fracture storage or dispersed infiltration.