SUBSURFACE CHARACTERIZATION OF A SUPERFUND SITE, NEW HAVEN, MISSOURI, USING GEOPHYSICAL TECHNIQUES

The fall 2003 Electrical and Electromagnetic Methods class at the University of Missouri-Rolla conducted a geophysical survey at the Operable Unit 1 Superfund site, New Haven, Missouri. Contamination at the site consists of a shallow (~15-50 ft) tetrachloroethene (PCE) plume that migrates towards the Missouri River. The objective of this geophysical study was to aid the United States Geological Survey in the characterization of the subsurface at the site, as the bedrock topography is important in controlling the distribution of dense non-aqueous phase liquids (DNAPL) such as PCE in the subsurface. Electrical resistivity (ER) and ground penetrating radar (GPR) surveys were conducted to determine the general character of bedrock at the field site. Students were responsible for survey setup, data acquisition, processing, and interpretation. Two basic resistivity layers were observed: a shallow layer of lower resistivity and a deeper, higher resistivity layer. The deeper is interpreted to be dolomite bedrock, while the shallower layer is alluvial overburden consisting of silty sand and clay layers with interbedded sand lenses. The presence of the conductive alluvium limited the depth of penetration of the GPR signal to less than 5 m. The GPR data showed the location of near surface reflectors and buried utilities, but due to the limited depth penetration, was unable to image the bedrock surface. Depth to bedrock varied along the resistivity profiles from 6 meters below ground level to more than 20 meters closer to the Missouri River. The ER profiles indicated abrupt drops greater than 15 meters in the bedrock surface consistent with the outline of a possible paleo cliff face of the Missouri River valley. Rise in the base level of the Missouri River and subsequent flooding has since resulted in the deposition of alluvium over the bedrock surface burying the paleo valley walls of the river. The bedrock surface controls the vertical distribution of the DNAPL. However, the ER profiles show discontinuous silt/clay layers with sand lenses that may impact contaminant distribution and migration. Further geophysical investigation is needed as a complex bedrock structure is suggested to lie beneath the site. This project provided useful hands-on training for the students while yielding information relevant to the USGS for the remediation design for the site.