DETECTION OF CONTAMINANT PLUMES BY BOREHOLE GEOPHYSICAL LOGGING

Two borehole geophysical methods - electromagnetic induction and natural gamna radiation logs - were used to vertically delineate landfill leachate plumes in a glacial aquifer. Geophysical logs of monitoring wells near two landfills in a glacial aquifer in west-central Vermont show that borehole geophysical methods can aid in interpretation of geologic logs and placement of monitoring well screens to sample landfill leachate plumes. Zones of high electrical conductance were delineated from the electromagnetic log in wells near two landfills. Some of these zones were found to correlate with silt and clay units on the basis of drilling and gamma logs. Monitoring wells were screened specifically in zones of high electrical conductivity that did not correlate to a silt or clay unit. Zones of high electrical conductivity that did not correlate to a silt or clay unit were caused by the presence of ground water with a high specific conductance, generally from 1000 to 2370 muS/cm (microsiemens per centimeter at 25 degrees Celsius). Ambient ground water in the study area has a specific conductance of approximately 200 to 400 muS/cm. Landfill leachate plumes were found to be approximately 5 to 20 feet thick and to be near the water table surface.