USGS Groundwater Information: Branch of Geophysics
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APPLICATION OF ELECTROMAGNETIC LOGGING TO CONTAMINATION INVESTIGATIONS IN GLACIAL SAND-AND-GRAVEL AQUIFERS
John H. Williams, U.S. Geological
Survey, Albany, NY
Wayne W. Lapham, US Geological Survey, Reston, VA
Thomas H. Barringer, US Geological Survey, West Trenton, NJ
Electromagnetic (EM) logging provides an efficient method for high-resolution, vertical delineation of electrically conductive contamination in glacial sand-and-gravel aquifers. EM, gamma, and lithologic logs and specific conductance data from sand-and-gravel aquifers at five sites in the northeastern United States were analyzed to define the relation of EM conductivity to aquifer lithology and water quality. Municipal waster disposal, septic water discharge, or highway deicing salt application at these sites has caused contaminant plumes in which the dissolved solids concentration and specific conductance of ground water exceed background levels by as much as 10 to 20 times.
The major hydrogeologic factors that affected EM log response at the five sites were the dissolved solids concentration of the ground water and the silt and clay content in the aquifer. EM conductivity of sand and gravel with uncontaminated water range from less than 5 to about 10 millisiemens per meter (mS/m); that of silt and clay zones ranged from about 15 to 45 Ms/m; and that of the more highly contaminated zones in sand and gravel ranged from about 10 to more than 80 Ms/m. Specific conductance of water samples from screened intervals in sand and gravel at selected monitoring well installations was significantly correlated with EM conductivity.
EM logging can be used in glacial sand-and-gravel aquifer investigations to (1) determine optimum depths for the placement of monitoring well screens; (2) provide a nearly continuous vertical profile of specific conductance to complement depth-specific water quality samples; and (3) identify temporal changes in water quality through sequential logging. Detailed lithologic or gamma logs, preferably both, need to be collected along with the EM logs to define zones in which the elevated EM conductivity is caused by the presence of silt and clay beds rather than contamination.
Final copy as submitted to Ground Water Monitoring and Remediation Review for publication as: Williams, J. H., Lapham, W. W., and Barringer, T. H. , R. W., 1993, Application of electromagnetic logging to contamination investigations in glacial sand-and-gravel aquifers: Ground Water Monitoring and Remediation Review, vol. 13, no. 3, p.129-138.
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