Variation in Sediment and Geochemistry as Evidence of Changes in Flowpaths to a Karst Spring in Southeastern Pennsylvania

Abstract

The purpose of this study was to determine whether the pathways through which groundwater travels in a karst aquifer change between seasons and during storm events. A specific goal was to determine how these pathways affect the water chemistry and sediment in the discharge from a karst spring. From July 2002 through December 2003, storm and continuous monitoring data were collected from Nolte Spring, a former community water supply in southeastern Pennsylvania. Storm samples were collected using an automatic sampler. Additional samples were collected by hand during site visits performed on a bi-weekly to monthly basis. All samples were analyzed for suspended sediment and ions. Datalogging sensors were installed in the spring for the entire study period to record stage, conductivity and temperature. A conceptual model for the study spring was devised to describe storm response and seasonal changes in terms of recharge and groundwater flow pathways. The stage and conductivity at baseflow varied during the study period, as characterized by two end­members. The drought end-member was characterized by low stage and high conductivity. Frequent snowmelt and rain led to high stage and low conductivity during the flood end-member. The relatively high conductivity observed at times during the study period is indicative of slower groundwater flow paths to the spring. Storm responses were determined by different pathways based on recharge type, which was classified as point or diffuse recharge, based on the mean storm intensity. Point recharge events are caused by high intensity storms (> or equal to 1.5 cm/day) and the storm response was an initial increase in conductivity followed by a decrease. Diffuse recharge events are caused by low intensity storms(< or equal to 1.5 cm rain/day) and snowmelt, and the response was a decrease in conductivity. Geochemistry and sediment mineralogy were also affected by the flow pathways. During the drought, the P-CO2 was the lowest of the monitoring period, indicating outgassing from dewatered conduits. The Sic was slightly above saturation, and calcite was observed in the sediment samples, indicating possible calcite precipitation. In the growing season of 2003, P-CO2 was the highest of the monitoring period, due to increased recharge and plant activity. Calcite was also observed in the sediment collected during this time. Various calcite morphologies suggest dislodging of calcite particles from the wall rock and transport of precipitated calcite from another area of the karst aquifer or the soil-bedrock interface as possible reasons for the existence of calcite in the spring. During Winter and early Spring 2003 the suspended sediment mineralogy was mostly siliciclastics such as clays and silt. Sediment concentrations in Nolte Spring were lower than expected, possibly due to the absence of stagnant conduits in the karst aquifer where sediment would accumulate and be flushed during storms. The karst system feeding Nolte Spring has characteristics of both conduit and matrix/fracture flow. Specifically, calcite undersaturation points to conduit flow, whereas stable temperature and low coefficient of variation of hardness (CV) points to matrix/fracture flow. The results of this study also reveal the importance of long-term continuous monitoring, which shows the changeable nature of the karst aquifer in terms of storm water pathway and spring response.