VARIATIONS IN STORM HYDROCHEMISTRY AS AN INDICATOR OF THE MIXING BETWEEN DEEP AND SHALLOW KARST WATERS IN THE PENNSYLVANIA VALLEY AND RIDGE

Cedar Run Spring, a conduit-dominated spring in the Cumberland Valley of Pennsylvania, was monitored to understand variations in flow paths using variation in water chemistry during storm events. Data from continuous conductivity and water-level data loggers coupled with stormwater sampling described different responses to storm events depending on distribution of the rain event. Mg:Ca ratios can be used to track shallow conduit water (low ratio) and deeper matrix water (high ratio) that has dissolved more carbonate rock. Hydrochemical results of three storm events displayed three types of response. Antecedent conditions for the three storm events were moist, wet, and dry, respectively. In addition, storm intensity was high, moderate, and low, respectively. The hydrochemical response to the first storm event showed an initial increase in Mg:Ca, followed by a sharp decrease. The second storm event occurred less than a day after the first one, and chemical response showed little change during the storm. Apparently, the matrix water with higher Mg had already flushed out. The final storm event showed multiple peaks and drops in the Mg:Ca ratio because of a diffuse precipitation event. Samples collected on the rising limb displayed a dilution stage where younger water is mixing with older groundwater. The dilution stage is followed by enrichment in ion concentration on the falling limb. Following the falling limb response, there is another increase in ion concentration that is greater than pre-storm conditions. Increases in hydraulic head after storm events can promote the mixing of different source waters. Furthermore, the increase in overall conductivity on a distinct recovery limb may indicate the mixing between deeper matrix compartments and conduit water. Although the ion ratios varied during storms, indicating mixing, there was no distinct pattern for the rising and falling limb. Instead the source of water is dependent on the storm intensity and the antecedent conditions.