An integrated surface-geophysical investigation of the University of Connecticut landfill, Storrs, Connecticut: 2000

Abstract

A surface-geophysical investigation to characterize the hydrogeology and contaminant distribution of the former landfill area at the University of Connecticut in Storrs, Connecticut, was conducted in 2000 to supplement the preliminary hydrogeologic assessment of the contamination of soil, surface water, and ground water at the site. A geophysical-toolbox approach was used to characterize the hydrogeology and contaminant distribution of the former landfill. Two-dimensional direct-current resistivity, inductive terrain-conductivity, and seismic-refraction surface-geophysical data were collected and interpreted in an iterative manner with exploratory drilling, borehole geophysics, and hydraulic testing. In this investigation, a geophysical-toolbox approach was used to 1) further define previously identified conductive anomalies and leachate plumes; 2) identify additional leachate plumes, possible fracture zones, and (or) conductive lithologic layers in the bedrock; and 3) delineate bedrock-surface topography in the drainage valleys north and south of the landfill. Resistivity and terrain-conductivity surveys were used to further delineate previously identified geophysical anomalies to the north and southwest of the landfill. A conductive anomaly identified in the terrain-conductivity survey to the north of the landfill in 2000 had a similar location and magnitude as an anomaly identified in terrain-conductivity surveys conducted in 1998 and 1999. Collectively, these surveys indicated that the magnitude of the conductive anomaly decreased with depth and with distance from the landfill. These anomalies indicated landfill leachate in the overburden and shallow bedrock. Results of previous surface-geophysical investigations southwest of the landfill indicated a shallow conductive anomaly in the overburden that extended into the fractured-bedrock aquifer. This conductive anomaly had a sheet-like geometry that had a north-south strike, dipped to the west, and terminated abruptly about 450 feet southwest of the landfill. The sheet-like conductive anomaly was interpreted as a fractured, conductive lithologic feature filled with conductive fluids. To further delineate this anomaly, two two-dimensional resistivity profiles were collected west of the sheet-like conductive anomaly to assess the possibility that the sheet-like conductive anomaly continued to the west in its down-dip direction. Each of the north-south oriented resistivity profiles showed bullet-shaped rather than linear-shaped anomalies, with a relatively smaller magnitude of conductivity than the sheet-like conductive anomaly to the east. If these bullet-like features are spatially connected, they may represent a linear, or pipe-like, conductive anomaly in the bedrock with a trend of N290°E and a plunge of 12°. Additional surveys were conducted to assess the apparent southern termination of the sheet-like conductive feature. Terrain-conductivity surveys indicated the sheet-like feature was not continuous to the south. A two-dimensional resistivity line and a coincident terrain-conductivity profile indicated the presence of a steep, eastward dipping, low magnitude, electrically conductive anomaly on the eastern end of the profile. Although the sheet-like conductive anomaly apparently did not continue to the south, the survey conducted in 2000 identified an isolated, weak conductive anomaly south of the previously identified anomaly. Inductive terrain-conductivity surveys performed north of the sheet-like conductive anomaly and west of the landfill indicated the anomaly did not extend to the north into the area of the former chemical-waste disposal pits. No conductive plumes or conductive features were observed in the subsurface bedrock west of the landfill. A conductive anomaly was identified in the southern section of the new terrain-conductivity grid. The magnitude and distribution of the apparent conductivity of this anomaly was identified as a nearly vertical sheet-like conductive feature. The anomaly extended north-south for 150 feet, and the depth and (or) magnitude of the conductive anomaly decreased towards the south. Based on the location, orientation, and dip, this feature was interpreted as a separate anomaly rather than as a continuation of the sheet-like, conductive bedrock feature that was previously identified southwest of the landfill. No other new conductive anomalies were identified south or west of the landfill. Seismic-refraction surveys were used to delineate the depth to the water table and the depth and topography of the bedrock surface. The seismic-refraction surveys at the northern and southern ends of the landfill confirm the presence of shallow bedrock at 25 feet below land surface. Seismic-refraction surveys conducted southwest of the landfill in a minor topographic valley indicate the bedrock is about 10 feet below land surface.