W. Sauck1, D. Werkema1, E. Atekwana2, and G. Aal2
1Dept. of Geosciences,Western Michigan Univ.
2Dept. of Geology, Univ. of Missouri, Rolla, MO
Shallow hydrocarbons (LNAPL), perched high in the unsaturated zone at a former refinery site in Kalamazoo, MI, were discovered and mapped using a dual coil electromagnetic (EM) induction system whose signal comes mainly from the 3–10 foot depth interval. The response of the LNAPL-impacted sediments in this environment was highly conductive, relative to the surrounding glacial drift. Additionally, several of the high-conductivity zones are associated with the ends of buried abandoned pipes (which are also readily mapped with the EM system). These conductive zones were investigated further with the Ground Penetrating Radar (GPR) and electrical resistivity methods. This area, at the former tank farm, consists of very heterogeneous outwash sands, gravels, and some till units.Water table was 20 to 25 feet below the surface. Multi-spacing electrical resistivity profiling was done along several transects over the anomalous conductive zones. Results showed a conductive zone centered between 3–6 foot depths. GPR was then used to profile across these conductive features. It showed that the anomalous zone was channel-form in cross-section. Additionally, attenuation of the radar signal strength by the anomalous zone corroborated the high conductivity of this zone. Finally, hand augering was done in the conductive zones, as well as in adjacent areas with normal background conductivity. Liquid hydrocarbons were encountered at 3–8 foot depths in all borings made in the anomalously conductive zones, while clean sands were found in areas of normal background conductivity. The base of the impacted zone was a thin, tight clay unit. Thus, the basal clay of the paleochannels was the impermeable unit that held the LNAPLs perched high in the vadose zone. Mapping of these shallow contaminated units has helped to understand the distribution of LNAPLs at the level of the water table.
This anomalous response, entirely contrary to what is normally expected for hydrocarbon contamination, has been seen elsewhere at old spill sites in Michigan. Hydrocarbons and sediments containing recently spilled liquid hydrocarbons normally show high resistivity. Other WMU research has shown that bacterial degradation in mature spills produces inorganic leachate and surfactants, which together allow electrically conductive paths to develop through the initially high-resistivity LNAPL-impacted layer.
AAPG Search and Discovery Article #90900©2001 AAPG Eastern Section Meeting, Kalamazoo, Michigan