Characterizing lakebed seepage and geologic heterogeneity using resistivity imaging and temperature measurements

The contribution of groundwater-surface water exchange to lake budgets is poorly understood and depends in part on lakebed heterogeneities. These heterogeneities are difficult to characterize using traditional point sampling methods. The goal of this project was to use electrical resistivity to identify potential zones of groundwater discharge and recharge, providing focus for point measurements. Multiple resistivity surveys were conducted at Lake Lacawac, a small, glacially-formed lake in northeastern Pennsylvania. Two types of resistivity surveys were conducted. In a continuous resistivity profile, a multi-electrode cable was towed parallel to shore to look for spatial variability in resistivity around the lake. Two parallel to shore surveys were conducted, an inner and outer loop, to help characterize the lateral extent of sediment types. The results of these surveys suggested lithology changes both along the shoreline and with distance from shore. Follow-up resistivity data were collected using cables laid along the lake bottom perpendicular to the shoreline to look for finer scale zonation that affects seepage as a function of distance from shore. Follow-up seepage measurements showed that seepage rates are very low, which is consistent with the resistivity data from which we concluded that most of Lake Lacawac is lined with glacial clay and that the lake is essentially perched above the groundwater flow system, with just minor amounts of seepage in a few locations where fingers of sandy sediments extend a short distance from the shore. Discontinuities in these patches of transmissive sediments can result in reversals in the direction of seepage at nearby locations. We conclude that towed resistivity is useful as a rapid reconnaissance tool for mapping geologic heterogeneity. The results can be used to guide the more time-consuming but higher-resolution, lake bottom resistivity measurements, which in turn can guide the placement of seepage meters.