The combination of a complex hydrogeologic framework, narrow land masses surrounded by saltwater, and extensive ground-water public-supply pumpage has produced localized areas of elevated chloride concentrations in ground water. This project uses advanced surface and borehole geophysical methods to determine the extent of saltwater intrusion and road-salt plumes at selected sites in eastern Suffolk County, Long Island, NY. Two-dimensional resistivity sections have been integrated with borehole geophysical logs and water-quality data to produce cross-sections of road-salt plumes. Borehole geophysical logs are collected twice a year to determine variations in chloride concentration and thicknesses of intrusive saltwater wedges or plumes. Temporal variations in EM conductivity logs have indicated movement of conductive ground water. Ongoing research includes comparison of other surface geophysical techniques, such as Induced Polarization and frequency domain electromagnetic surveys, and development of a chloride-concentration relation with measured EM log response.
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“USGS demonstrating borehole geophysical method for detection of chloride plumes”