STORM RESPONSE IN URBAN SPRINGS

Karst aquifers respond more quickly to precipitation events than other types of aquifers. However, continuous monitoring and geochemical analysis can provide the necessary information to better understand aquifer recharge pathways and sources during precipitation events. The Chester Valley area, in Chester County Pennsylvania, is underlain by carbonate units in an east-northeast to west-southwest trend bounded on the north and south by crystalline igneous and metamorphic units. The springs selected for this study are in commercial or residential land use in an urban setting. All springs were from the Elbrook Formation, which is a fractured dolomite unit. Geochemical analysis of halide concentrations, chlorides and bromide, in the water samples from the commercial spring site shows contamination by road salt. High chloride concentrations (150 mg/L -280 mg/L) were measured in the spring, summer, and fall seasons; therefore, it is likely that the trend will continue during the winter as well. Storm response was rapid, on the order of one to three hours. This is in contrast to storm response measured in past studies in a rural setting showing 1-2 days response time from the start of the precipitation event to increase in water stage and conductivity at the monitored sites. The water stage response is also on the order of 10-30 centimeters higher in the rural than urban setting. The maximum water stage responses at the urban springs are less than 10 centimeters increase from baseflow conditions. Urban springs all show a decrease in conductivity, which further indicates a rapid storm response. The fast response and lower water stage at these urban springs are due to smaller drainage/recharge areas for the springs compared to those in rural setting.