Geophysical Investigation Of Vadose Zone Conductivity Anomalies At A Former Refinery Site, Kalamazoo, Mi

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Electromagnetic (EM-31), magnetic, resistivity, and ground penetrating radar (GPR) methods were used to examine part of a 60 acre former refinery site in Kalamazoo, MI. The study area is underlain by about 7 meters of highly variable glacial drift, which in turn overlies an outwash sand unit. Water table occurs about 6-8 meters depth. Three large above ground storage tanks (ASTs) formerly occupied the study area and were removed by the US EPA in 1997. Magnetic and EM-31 surveys revealed eight remnants of buried pipe ranging from 20 m to more than 100 m in length. Emanating from six of these abandoned pipe segments are extensive conductivity anomalies, with values ranging from 16 mS/m to 30 mS/m vs. background values of 8 mS/m to 15 mS/m. GPR and dipole-dipole resistivity profiles were measured over anomalously conductive zones to provide lateral and vertical resolution. Three hypotheses are proposed to explain the shallow high conductivity zones: 1) past leakage of brine, 2) fortuitous coincidence of the pipes with high-clay zones, or 3) past leakage of hydrocarbons which have since undergone biodegradation, resulting in the release of inorganic ions enhancing the pore fluid conductivity. Shallow hand augured soil borings into these conductive zones encountered hydrocarbons perched on thin clay lenses, or trapped in low permeability sandy-silt layers. Soil borings in background, non-contaminated areas showed only clean sand and silt. Because the conductivity of the contaminated zones is higher than for similar uncontaminated areas, the conductive model for mature hydrocarbon spills at the water table can be extended upward to include vadose zone occurrences of Light Non-Aqueous Phase Liquids (LNAPL). Thus, this conductive model can be invoked to successfully map subhorizontal vadose zone migration paths taken by past hydrocarbon spills in this environment. The findings from this study have significant implications for the detection and monitoring of vadose zone LNAPL contamination. Introduction and Site History The study site is a former petroleum refinery located on the east side of the City of Kalamazoo (Figure 1) encompassing more than 60 acres of land. Refining activities ceased in the 1980’s, but the storage tanks were used until the mid-1990’s, when excessive leakage finally mandated the closure of the site. Most of the refining machinery and facilities were concentrated in the SW corner of the property, SW of the map of Figure 1. The rest of the approximately 60-acre property was used for bulk storage tanks and ancillary loading, pumping, and transfer facilities. Above ground pipelines and apparently some below ground pipelines connected the various parts of the refinery and storage facilities. In 1997, the US EPA removed all above ground structures except for a garage and two office structures. Since then, the Michigan Dept. of Environmental Quality (MDEQ) has been leading a program (via subcontractors) to clean the soils and groundwater, which were impacted by petroleum and refined product spills, as well as by sludge dumping. At this time, fairly early in the remediation process, efforts have been concentrated in capturing or intercepting the free product before it moves into the Davis Creek environment. The site is slated for brownfield development and the part of the site is