Mapping High‐ <scp>TDS</scp> Groundwater Near Impoundments Using Ground and Waterborne Towed Electromagnetics

Long-term monitoring at landfills and impoundments containing coal combustion products (CCPs) or other industrial wastes is essential for detecting possible leachate releases to groundwater and mapping contamination plumes. This study evaluates a novel, non-invasive geophysical approach-towed time-domain electromagnetic (TEM) surveys-for non-invasive and rapid assessment of groundwater quality near landfills and impoundments that have the potential to release plumes with higher total dissolved solids (TDS) than groundwater. CCPs are one such example where releases can have relatively high sulfate, sodium, and/or calcium concentrations resulting in high TDS and, therefore, high electrical conductivity. This makes electromagnetic (EM) methods suitable for their detection and monitoring. Recent advancements in TEM technology enable efficient subsurface imaging over extensive areas using antennas towed by vehicles on land or boats on water bodies. TEM surveys provide valuable information about overburden thickness, geological structures, lithology, and pore-fluid TDS. We conducted integrated ground-based and waterborne TEM surveys at a CCP complex adjacent to a river in the eastern United States. Despite challenging site conditions, including railroad tracks, high-voltage power lines, and power-generation infrastructure, high-quality TEM data were collected. Over 20 line-km of data were acquired and inverted using laterally constrained two-dimensional (2D) and spatially constrained three-dimensional (3D) inversions. Results successfully delineated geological boundaries and identified conductive anomalies consistent with elevated TDS indicative of potential leachate plumes. Geophysical interpretations agreed well with water-quality data from nearby monitoring wells. This work highlights the effectiveness of integrated ground-based and floating TEM surveys for high-resolution characterization around CCP impoundments.