GEOPHYSICAL INVESTIGATION OF BEDROCK DEPTH AT THE WRR SUPERFUND SITE IN EAU CLAIRE, WI

Near-surface geophysical methods can provide valuable information for understanding the stratigraphy of sites with groundwater contamination. In this study, geophysical techniques were used to better characterize the soil-bedrock interface of a Superfund site in Eau Claire, Wisconsin. The study area was the Lowes Creek County Park that lies downgradient of the WRR Environmental Services facility outside of Eau Claire. Decades of manufacturing at this facility have resulted in several types of groundwater contamination, including dense non-aqueous phase liquids (DNAPLs). For remediation efforts to remove the DNAPLs to be effective, bedrock depth must be established. Limited borehole data available at the site indicate an unconfined sandy aquifer overlying sandstone bedrock, with a highly variable bedrock surface. Bedrock in some portions of the site is at a depth of 17 m below the ground surface, while other boreholes nearby show bedrock depths greater than 45 m. The borehole data are insufficient to adequately characterize the bedrock topography, but suggest that a complicated system of buried valleys may be present.

Four types of geophysical measurements were acquired to provide additional information on the bedrock topography. Geophysical data collection was concentrated in the areas where greater depths to bedrock were suspected, and data were acquired using seismic refraction, microgravity, resistivity, and electromagnetic induction techniques. Standard data processing techniques were used for all data sets, and the seismic data were analyzed using reciprocal delay-time methods. Analysis of the different types of geophysical data showed good agreement between the four techniques and with the borehole measurements of bedrock depth. While all techniques provided information on the site stratigraphy, seismic refraction and microgravity techniques had the greatest penetration depths and were most useful for estimating bedrock depth and understanding variations in bedrock topography. When these techniques were used to supplement borehole data, the bedrock structure could be better understood as two roughly parallel bedrock valleys running through the site. These valleys can then be targeted for drilling and DNAPL remediation.