This study investigates the effectiveness of direct current electrical resistivity as a tool for assessing ground water/surface water interactions within streams. This research has shown that patterns of ground water discharge can be mapped at the meter scale, which is important for understanding stream water quality and ecosystem function. Underwater electrical resistivity surveys along a 107-m stream section within the Burd Run Watershed in South Central Pennsylvania identified three resistivity layers: a resistive (100 to 400 Omega m) surface layer corresponding to the streambed sediments, a conductive (20 to 100 Omega m) middle layer corresponding to residual clay sediments, and a resistive (100 to 450 Omega m) bottom layer corresponding to the carbonate bedrock. Tile probing to determine the depth to the bedrock and resistivity test box analysis of augered sediment samples confirmed these interpretations of the resistivity data. Ground water seeps occurred where the resistivity data showed that the residual clays were thinnest and bedrock was closest to the streambed. Plotting the difference in resistivity between two surveys, one conducted during low-stage and the other during high-stage stream conditions, showed changes in the conductivity of the pore fluids saturating the sediments. Under high-stream stage conditions, the top layer showed increased resistivity values for sections with surface water infiltration but showed nearly constant resistivity in sections with ground water seeps. This was expressed as difference values less than 50 Omega m in the area of the seeps and greater than 50 Omega m change for the streambed sediments saturated by surface water. Thus, electrical resistivity aided in characterizing ground water discharge zones by detecting variations in subsurface resistivity under high- and low-stream stage conditions as well as mapping subsurface heterogeneities that promote these exchanges.

Characterizing groundwater interaction with streams is essential for understanding contaminant transport. We are investigating the use of multielectrode resistivity to improve the detection of seepage points, and the mapping of gaining and losing stream reaches. Our field area is the Burd Run watershed, Shippensburg, PA. Burd Run is of interest as a geophysical test case because the stream is dilute where it flows from the metasedimentary ridge of South Mountain across a colluvium wedge. Fluid conductivity increases from 35 µS/cm to 440 µS/cm in three abrupt increments as the stream flows across the steeply dipping carbonate units of the Great Valley before discharging into Mill Spring Creek. The increases can be attributed to the introduction of carbonate groundwater, both from municipal discharge and from vertical seeps within the streambed. Continuous dipole-dipole surveys were conducted over a 107 m reach using a 28-electrode cable with a 1-m electrode spacing deployed on the streambed. The survey resolved the conductivity contrast of the streambed sediments over the carbonate bedrock and detected a vertical zone of higher electrical conductivity that correlates with observed streambed seeps.

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