Abstract

It is often difficult to understand hydrologic processes in upland catchments where the subsurface architecture is unknown. To investigate such architecture, we are investigating a small watershed on a single lithology (sandstone) in Garner run, a first-order catchment within the Shavers Creek watershed in central Pennsylvania. Geochemical data acquired from a nest of wells in the stream bed suggest both shallow interflow and deeper groundwater contribute to stream flow.

Electrical Resistivity Imaging (ERI) is frequently applied to image subsurface structures with a detectable electrical resistivity contrast relative to the host medium, making it a suitable method for measuring and characterizing subsurface fluid flow paths and the controls on water–rock interactions. In June 2016, a 2D geoelectrical survey was performed across Garner Run using the Syscal pro-multi-electrode system. Data were acquired on a total of 240 electrodes equally spaced 2m apart, with Wenner and dipole-dipole array configurations combined to enhance resolution of geoelectrical structure. The estimated imaging depth based on a depth of investigation (DOI) metric was approximately 30m below the lowest point along the 2D-ERI line.

ERI reveal that the two sides of the catchment have strikingly different resistivity values, mandating a modification to the basic conceptual model of the site geological framework. Electrical resistivity values were obtained, ranging from <100 Ohm m to >10000 Ohm m. High resistivities below the northwest slope are consistent with highly resistant quartzite sandstone. Valley fill deposits are imaged below the northwest slope, supporting the conceptual model. However, the base of the valley fill deposits is characterized by a very strong resistivity contrast suggesting underlying electrically conductive bedrock, possibly representing shale rich sandstone or a clay layer. This strong electrical contact may explain the pronounced interflow inferred from stream geochemistry data as it is likely a contact that limits vertical transport of water. In summary, ERI provided valuable high-resolution information complementing historical information, bathymetric, geological, and lithostratigraphic data allowing the architecture of the critical zone of this catchment to be better understood.

Citation

Zarif, F., Slater, L.D., Brantley, S.L., Robinson, J., and Kessouri, P. (2016): Electrical Resistivity Imaging is Consistent with Shallow Interflow within the Garner Run subcatchment of the Susquehanna Shale Hills CZO. 2016 Fall Meeting, American Geophysical Union, San Francisco, CA, 12-16 Dec..

This Paper/Book acknowledges NSF CZO grant support.