A PRACTICAL APPROACH TO SITE CHARACTERIZATION OF A CONTAMINATED FRACTURED BEDROCK AQUIFER USING GEOPHYSICAL TECHNIQUES

A subsurface gasoline release in the area of West Hill in Killington, Vermont resulted in the contamination of 32 domestic supply wells. A drinking water supply well adjacent to the release was acting as a conduit for vertical contaminant migration into the deep fractured bedrock aquifer. A total of 8.5 million gallons of contaminated ground water have been removed from the supply well for treatment through a low-profile air stripper. A well sampling program conducted since 1993 at 70 supply wells has demonstrated a general north – south trending contaminant migration pattern. A borehole geophysical survey conducted in May 1994, which included single point resistance, fluid temperature, spontaneous potential, and heat-pulse flow meter measurements identified significant fractures between 87-187 and 335-395 feet below ground surface (bgs) and indicated a general downward migration of groundwater to a depth of approximately 340-400 feet bgs at a rate up to 0.15 gpm. Although gasoline-compound concentrations in the well have decreased substantially since remedial efforts began, they remain significantly above Vermont drinking-water standards. A supplemental geophysical investigation was performed in August 1999 to investigate and improve our understanding of the hydrogeology at the site and determine if alternative remedial actions exist. The results of a fracture trace analysis and electrical resistivity survey confirmed the general north-south orientation of fractures and the intersection of at least two fractures in the vicinity of the contaminated supply well. Results of discrete interval sampling indicate that contaminated ground water is entering the supply well throughout the length of the well under both pumping and non-pumping conditions, and that contaminant concentrations in samples collected from the fracture zone between 335-369 feet bgs were two or more times the level found at all other fracture locations. Results of the investigation have assisted in determining replacement supply well locations, making susceptibility determinations and establishing practical monitoring schedules.