USE OF INTEGRATED SURFACE AND BOREHOLE GEOPHYSICS TO MAP BURIED EPIKARSTAL FEATURES 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. The dramatic subsurface topography underlying the site is of particular importance to the clean up and continued detoxification of the site and surrounding area. Subsurface lithology and structural features in the small valley immediately south of the Acme Superfund site were examined using Geoprobe^TM surveys, natural gamma, and EM conductivity well logs. Natural gamma logs were the most effective tool for defining subsurface lithology. Steel well casings on some wells and calibration problems with the EM sonde lead to poor quality conductivity logs. Bed thicknesses ranging from 30 cm up to 1 m were determined from the gamma logs using the full-width-at-half- maximum method. Depth to bedrock (approximately 6 to 7 m), and existence of cavities, were verified by Geoprobe^TM surveys. Results of the first phase of this project suggest hydraulic isolation of the surface and shallow groundwater (probably a perched aquifer) and the discovery of a network of karstic fractures and caverns in the deeper epikarstal bedrock. The epikarst appears to be saturated, forming a highly transmissive zone. Future work will include obtaining more well logs and examining results of recent Geoprobe^TM logs from the 2009 summer field camp.