GEOPHYSICAL IDENTIFICATION OF SUBSURFACE CAVITIES AND FRACTURES NEAR A SUPERFUND SITE SOUTH OF ROCKFORD, ILLINOIS

The ACME Superfund site is one of many Superfund sites in Northern Illinois. This 20 acre (8.1 ha) site was contaminated by various volatile organic compounds (VOC’s) and heavy metals during the 1960-1980s. These contaminants have seeped into the Wise Lake dolomite (Galena Group) and contaminated local ground water supplies. Hydrogeological information is especially crucial immediately south of the ACME site, where up to 10 m of unconsolidated sediments overlie the fractured and karstic Wise Lake Formation in a small valley adjacent to residential wells. In order to help formulate a conceptual hydrogeological model, subsurface lithology and structural features are examined using a variety of surface and subsurface geophysical techniques, including resistivity, frequency-domain electromagnetics, seismic refraction and ground-penetrating radar (GPR), as well as natural gamma and electrical conductivity logs. The methodology involves examining karst features and their geophysical response over an adjacent upland to east, and in a quarry to the north, then applying this information to interpret geophysical surveys in the valley south of the ACME site. Ground-penetrating radar, 1D and 2D resistivity surveys proved effective at mapping the bedrock surface beneath heterogeneous sediments. GPR sections, in particular, show a prominent reflection at 100-135 ns (6-7 m depth), corresponding to the bedrock surface. This reflection is visible on both 100 MHz and 50 MHz sections. In the subsurface natural gamma logs were most effective for defining lithology. Conventional natural gamma logs provide great detail on the nature of unconsolidated deposits overlying the bedrock. Depth to bedrock and the existence of shallow cavities (2-3 m deep in areas of shallow bedrock) were confirmed from Geoprobe^TM direct-push surveys.