Assessment of electrical resistivity method to map groundwater seepage zones in heterogeneous sediments

Underwater electrical‐resistivity data were collected 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 locate shoreline sections where water is seeping out of the lake. A resistivity line 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 gradient indicative of lake discharge in this area. Resistivity could also broadly delineate high‐seepage zones. An 80‐m line run parallel to shore using a 0.5‐m electrode spacing was compared with measurements collected the previous year using 1‐m electrode spacing. Both data sets showed 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. For example, over a 12.5‐m stretch where seepage varied from 1–38 cm/day, resistivity varied horizontally from 700–3900 ohm‐m and vertically in the top 2‐m from 900–4000 ohm‐m without apparent correlation with seepage. 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 spaced 1 m apart to form a 13.5 × 13 m data grid. These lines were inverted individually using a 2‐D inversion program and then interpolated to create a 3‐D volume. Examination of resistivity slices through this volume highlights the heterogeneity of both these materials, suggesting groundwater flow takes sinuous flow paths. In such heterogeneous materials the goal of predicting the precise location of high‐seepage points remains elusive.