Assesment of electrical resistivity method to map groundwater seepage zones in heterogeneous sediments at Mirror Lake, NH

Temporal and spatial variability makes locating zones of seepage difficult using traditional point measurements. The goal of this project was to employ 2D electrical resistivity, 3D electrical resistivity, and time-lapse resistivity to improve our understanding of how small-scale heterogeneity controls seepage. We collected underwater electrical resistivity data along the southwest shore of Mirror Lake, NH, as part of a multi-year assessment of the utility of geophysics for mapping groundwater seepage beneath lakes. We found that resistivity could predict out-seepage. A line collected perpendicular-to-shore along the lake bottom starting 27-m off shore and continuing 27-m on shore (1-m electrode spacing) showed the water table dipping away from the lake, the steep gradient indicative of high out-seepage in this area. Resistivity could also broadly delineate high-seepage zones. An 80-m line collected parallel to shore using 0.5-m electrode spacing was compared with measurements collected the previous year using 1-m electrode spacing. Both data sets show the transition from high-seepage glacial outwash to low-seepage glacial till, demonstrating reproducibility. However, even the finer 0.5-m electrode spacing was insufficient to resolve the heterogeneity well enough to predict seepage variability within each zone. In two sections along this 80-m line, one over glacial outwash, the other over till, we collected 14 parallel lines of resistivity, 13.5-m long and spaced 1-m apart to form a 13.5 x 13-m data grid. These lines were inverted using two methods: (1) individually using a 2-D inversion program and then interpolated to create a 3-D volume and (2) they were jointly inverted to create a 3-D volume. Examination of resistivity slices through these volumes highlights the heterogeneity of both these materials, suggesting groundwater flow takes indirect flow paths. However, only when there was a strong contrast in resistivities (the till section) could a possible groundwater flow path be identified. Time-lapse resistivity was used to determine the effect of the top layer of fine sediments. A 13.5-m long time-lapse resistivity survey was completed in the glacial till using 0.5-m electrode spacing showed that disturbing only a few millimeters of surficial sediments produced up to a 6% change in resistivity. This change was accompanied by changes in seepage, indicating that the fine layer of sediments is a major control on seepage patterns. This project showed that combining several electrical resistivity methods provides a better understanding of subsurface heterogeneity and aids in the placement of point measurements. However, in such heterogeneous material the goal of predicting seepage variation still remains difficult.