MARINE RESISTIVITY AS A TOOL FOR CHARACTERIZING SEEPAGE ZONE AT LAKE LACAWAC, PA

The groundwater-surface water transition zones of lakes and streams are dynamic and difficult to characterize. The spatial variability of seepage zones makes them difficult to locate using traditional point measurements such as seepage meters. The goal of this project is to use marine resistivity to identify potential seepage zones, providing focus for future studies. Multiple resistivity surveys were conducted at Lake Lacawac, a small, glacially formed lake located within the Lacawac Sanctuary in Wayne County, northeastern Pennsylvania. One target for these surveys was the resistivity contrast between groundwater and surface water. Another target was resistivity contrasts created by geologic heterogeneities that can control groundwater discharge into the lake. Two types of surveys were conducted using an 8 channel SuperSting resistivity system. In a continuous resistivity profile, an 11-electrode cable was towed behind a boat powered by a trolling motor. This cable was used with electrode separations of 2 and 4 meters, giving a depth of penetration of approximately 4 and 8 meters respectively. This streamer was towed close to shore to look for spatial variability in the resistivity of benthic sediments at different locations around the lake. A second 28-electrode array was laid on the lake bottom perpendicular to shore. This array had a fixed spacing of 1 meter, giving a depth of penetration of approximately 5 meters. These surveys targeted changes in resistivity versus distance from shore. Results of these resistivity surveys were inverted using EarthImager to generate 2D models of the underlying sediment and fluid. These models suggested several locations where groundwater could be discharging into the lake. Temperature sensors placed on the lake bottom and buried in benthic sediment were used to provide ground truth about interpreted areas of seepage. Temperature data from at least one of these locations is consistent with groundwater discharge. Future work will examine more of these potential seepage zones and will also include surveys repeated over time to look for temporal variations in the groundwater-surface water interface.