ELECTRICAL RESISTIVITY MEASUREMENTS OF GROUNDWATER STORAGE CAPACITY IN GLACIAL SOILS

In protected wilderness areas, subsurface hydrological properties can be difficult to assess due to restrictions placed on invasive practices such as the installation of groundwater wells. Here, we examine the use of electrical resistivity tomography (ERT) to assess the storage capacity of near surface (unconfined) glacial soils. Past research at the study site, located in the Medicine Bow Mountains of Wyoming, quantified basin inputs and outputs of water as precipitation and streamflow; however, a major unknown in the water balance is storage, particularly the capacity of glacial soils to store groundwater. The study area is a glacial lake in a cirque basin with a watershed area of 24.7 ha. The watershed consists of an overburden of till and quartzite boulders deposited during Quaternary glaciations overlying Medicine Bow quartzite bedrock. ERT data were collected using a dipole-dipole array configured with 2-m electrode spacing along a 46 m long east-west trending profile. The inverted ERT data suggest three subsurface units separated by sharp resistivity gradients. These include (1) a western surficial layer that thickens eastward from 1.5 to 4 m with resistivity values ranging from ~2700 to 6000 ohm-m; (2) an underlaying subsurface layer with resistivity values ranging from ~50 to 1500 ohm-m; and (3) an eastern unit with resistivity greater than 13000 ohm-m that extends from the ground surface to the depth of ERT penetration (approximately 7 m) that is separated from units 1 and 2 by a sharp vertical resistivity boundary. Surface observations of soil, the presence of a shallow (0.25 m deep) stream channel, and well-established vegetation above unit 1 suggests that this unit represents an unconfined glacial soil aquifer. The contact between units 1 and 2 is interpreted as the boundary between unsaturated and saturated soil. Unit 3 is interpreted to represent crystalline bedrock, based on the high resistivity values, coupled with minimal vegetation, and surface exposure of large quartzite boulder fields. The ERT data suggest that up to 7 m of glacial soil is available for groundwater storage.