Variations in lake seepage were studied along a 130 m shoreline of Mirror Lake NH. Seepage was downward from the lake to groundwater; rates measured from 28 seepage meters varied from 0 to -282 cm/d. Causes of this variation were investigated using electrical resistivity surveys and lakebed sediment characterization. Two-dimensional (2D) resistivity surveys showed a transition in lakebed sediments from outwash to till that correlated with high- and low-seepage zones, respectively. However, the 2D survey was not able to predict smaller scale variations within these facies. In the outwash, fast seepage was associated with permeability variations in a thin (2 cm) layer of sediments at the top of the lakebed. In the till, where seepage was slower than that in the outwash, a three-dimensional resistivity survey mapped a point of high seepage associated with heterogeneity (lower resistivity and likely higher permeability). Points of focused flow across the sediment-water interface are difficult to detect and can transmit a large percentage of total exchange. Using a series of electrical resistivity geophysical methods in combination with hydrologic data to locate heterogeneities that affect seepage rates can help guide seepage meter placement. Improving our understanding of the causes and types of heterogeneity in lake seepage will provide better data for lake budgets and prediction of mass transfer of solutes or contaminants between lakes and groundwater.

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 0.5m 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 x 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.

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