Mapping Road Salt Discharge from Groundwater Using Electrical Resistivity, Mirror Lake, New Hampshire

We are using electrical resistivity surveys, seepage meter measurements, and drivepoint sampling to characterize the movement and contribution of chloride enriched groundwater into Mirror Lake, New Hampshire. The source of the chloride is road salt from nearby Interstate Highway 93, and steady increases in chloride in the lake have been observed since the highway's construction from 1969 to 1971. The relative contribution of surface water and groundwater to lake chloride is not well-quantified. Electrical resistivity surveys assess the spatial extent of a zone of contaminated groundwater located in the lake sediments near the northeast inlet. Two types of surveys have been conducted: floating cable and a cable placed directly on the lake bottom. The surveys identified a plume beneath the lake of low resistivity (<100 ohm m). Surrounding lake sediment resistivity is closer to 1000 ohm m. The plume is 10 to 20 meters wide and extends down into the substrate at least 4 to 5 meters. Shallow pore water samples from the plume had chloride concentrations of 9 mg/L while lake water samples had a chloride concentration of 2.5 mg/L. Seepage meter measurements and water sampling guided by the electrical resistivity map constrain the contribution of both groundwater and surface water to the lake chemistry. Seepage into and out of lakes is often difficult to quantify, but the combination of geophysics and hydrologic measurements can improve our understanding of the hydrologic processes affecting lake water chemistry.