APPLICATION OF ELECTROMAGNETIC LOGGING TO CONTAMINATION INVESTIGATIONS IN GLACIAL SAND-AND-GRAVEL AQUIFERS

Electromagnetic (EM) logging provides an efficient method for high-res olution, vertical delineation of electrically conductive contamination in glacial sand-and-gravel aquifers. EM, gamma, and lithologic logs a nd specific conductance data from sand-and-gravel aquifers at five sit es in the northeastern United States were analyzed to define the relat ion of EM conductivity to aquifer lithology and water quality. Municip al waste disposal, septic waste discharge, or highway deicing salt app lication at these sites has caused contaminant plumes in which the dis solved solids concentration and specific conductance of ground water e xceed background levels by as much as 10 to 20 times. The major hydrog eologic factors that affected EM log response at the five sites were t he 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 ranged 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 gr avel ranged from about 10 to more than 80 mS/m. Specific conductance o f water samples from screened intervals in sand and gravel at selected monitoring well installations was significantly correlated with EM co nductivity. EM logging can be used in glacial sand-and-gravel aquifer investigations to (1) determine optimum depths for the placement of mo nitoring well screens; (2) provide a nearly continuous vertical profil e of specific conductance to complement depth-specific water quality s amples; and (3) identify temporal changes in water quality through seq uential logging. Detailed lithologic or gamma logs, preferably both, n eed to be collected along with the EM logs to define zones in which el evated EM conductivity is caused by the presence of silt and clay beds rather than contamination.