USGS Groundwater Information: Branch of Geophysics
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Carole D. Johnson, U.S. Geological Survey, Storrs, Conn.
John W. Lane, Jr., U.S. Geological Survey, Storrs, Conn.
John H. Williams, U.S. Geological Survey, Troy, N.Y.
F.P. Haeni, U.S. Geological Survey, Storrs, Conn.
An integrated suite of geophysical methods was used to characterize the hydrogeology of a fractured-rock aquifer to identify contamination or pathways for contaminant migration near a former landfill at the University of Connecticut, Storrs, Connecticut. Surface-geophysical methods were used to identify the dominant direction of fracture orientation and to locate potential leachate plumes. Two shallow, electrically conductive anomalies near surface-water discharge areas north and south of the landfill were interpreted as leachate plumes. Two other sheet-like electromagnetic (EM) anomalies were identified and targeted for drilling and borehole-geophysical investigation. These methods were used to determine the location, orientation, and lateral continuity of fractures and to quantify the hydraulic properties of the transmissive fractures.
One borehole was located to intersect an anomaly observed at a depth of about 18 meters. The EM-conductivity log measured a high electrical conductivity anomaly at a depth of 21 meters, which coincides with a layer observed to contain sulfide mineralization. The lack of high conductivity fluids in the borehole supports the interpretation that this anomaly is caused by a lithologic change rather than by fractures that contain conductive leachate.
The second borehole was positioned to intersect a conductive feature at a depth of about 18 meters. A fracture with similar strike and dip was observed at a depth of about 22 meters in optical- and acoustic-televiewer images, in borehole-radar surveys, and was determined to be hydraulically conductive during heat-pulse flowmeter tests. This feature was also characterized by a high-conductivity spike in the EM log. Although the specific conductance of the fluid in this depth zone was high, it could not account for the spike in the EM log. The anomaly is interpreted to be caused by a combination of conductive fluids in the fracture and of conductive minerals in the rock. Water-quality samples from a discretely isolated zone near a depth of 22 meters indicated the presence of landfill leachate. This investigation illustrates the effectiveness of the use of combined geophysical methods for identification `and evaluation of electrically conductive contaminant plumes.
Final copy as submitted to Symposium on the Application of Geophysics to Engineering and Environmental Problemsfor publication as: Johnson, C.D., Lane, J.W., Jr., Williams, J.H., and Haeni, F.P., 2001, Application of geophysical methods to delineate contamination in fractured rock at the University of Connecticut landfill, Storrs, Connecticut: in Symposium on the Application of Geophysics to Engineering and Environmental Problems, Denver, Colorado, March 4-7, 2001, Proceedings: Wheat Ridge, Colo., Environmental and Engineering Geophysical Society, CD-ROM.
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