APPLICATION OF UNDERWATER RESISTIVITY IN THE ASSESSMENT OF GROUNDWATER-SURFACE WATER INTERACTION, BURD RUN WATERSHED, SHIPPENSBURG, PA

We are testing a new application of electrical resistivity to characterize groundwater-surface water interaction within the Burd Run watershed near Shippensburg, PA. Burd Run flows from South Mountain over a colluvium wedge into the steeply dipping carbonate units of the Great Valley before discharging into Mill Spring Creek. From South Mountain to Mill Spring Creek there are three significant increases in the surface water conductivity. These increases are attributed to the introduction of carbonate groundwater in the form of spring discharge and seeps within the streambed. Fluid conductivity along the 5-km reach of Burd Run we studied increases in a series of jumps from 35 µS/cm to 440 µS/cm. Periods of low flow provided insight into key reaches on which to focus resistivity studies. We chose a reach of the stream with known springs to construct a 107m continuous dipole-dipole survey. We used a 28 electrode cable with a 1-m electrode separation, with the electrodes placed directly on the streambed. The resulting resistivity section revealed conductive vertical features beneath known springs as well as the distribution of sandy-clay-loam, clay, and resistive carbonate bedrock. The complexity of the geologic controls on hydrologic flow in this karst terrain made the initial testing of this method ideal. Ultimately, stream-bottom resistivity sounding can aid in the placement and distribution of point measurement equipment, such as seepage meters and piezometers, for monitoring groundwater-surface water interaction on a watershed scale.