Geophysical Investigation of Vadose Zone Conductivity Anomalies at a Hydrocarbon Contaminated Site: Implications for the Assessment of Intrinsic Bioremediation

Abstract

Electromagnetic (EM-31), dc resistivity, and ground penetrating radar (GPR) methods were used to investigate part of a former refinery site in Kalamazoo, Michigan, USA. The study area is underlain by about [Formula: see text] of heterogeneous glacial drift deposits overlying a sandy outwash unit. Three large above ground storage tanks (ASTs) formerly occupied the study area and were removed by the U.S. Environmental Protection Agency (USEPA) in 1997. EM-31 surveys revealed eight remnants of buried pipe ranging from [Formula: see text] to more than [Formula: see text] in length. Emanating from some of these abandoned pipe segments are extensive near surface conductivity anomalies, with values ranging from [Formula: see text] to greater than [Formula: see text] vs. background values of less than [Formula: see text]. GPR and dipole-dipole resistivity surveys performed along selected profiles with anomalous conductive zones revealed a channel form within the shallow subsurface. Shallow hand augured soil borings into these conductive zones encountered hydrocarbons perched on thin clay lenses, or trapped in low permeability sandy-silt layers, while borings in background conductivity areas showed only clean sand and silt. We also observed that areas underlain by clays in association with perched hydrocarbons exhibited a higher conductivity (greater than [Formula: see text] and up to [Formula: see text]) when compared to areas underlain by uncontaminated clays. Based on these observations and results of our prior work in similar settings, we infer that the conductivity anomalies may be due to the occurrence of intrinsic biodegradation of the hydrocarbons. Thus, the previous conductive model originally proposed by our research group for ‘aged’ hydrocarbon spills at the water table can be extended upward to include vadose zone occurrences of biodegradation of these contaminants. The findings from this study have significant implications for the use of geophysical techniques in the assessment of vadose zone biodegradation.