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TEMPORAL AND SPATIAL VARIABILITY OF HIGH RESOLUTION IN SITU VERTICAL APPARENT RESISTIVITY MEASUREMENTS AT A LNAPL IMPACTED SITE

D. Dale Werkema Jr.1, Estella A. Atekwana2, Anthony Endres3, and William A. Sauck1
1Dept. of Geosciences
Western Michigan University
Kalamazoo, MI 49008
2Dept. of Geology and Geophysics
University of Missouri-Rolla
Rolla, MO. 65409
3Environmental Geophysics Facility
Department of Earth Sciences
University of Waterloo
Waterloo, Ontario N2L 3G1

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Abstract

Ten high resolution vertical resistivity probes (VRPs) have been installed within plume and off plume locations at a LNAPL contaminated site adjacent to a former oil refinery in Carson City, MI. The VRPs are sealed PVC wells with stainless steel screws, serving as electrodes, spaced every 2 inches (5.08 cm) with depth. VRP measurements of apparent resistivity and monitoring well measurements of the water table were made monthly during a full calendar year. The apparent resistivity response from VRPs within the free product plume, within the residual plume, and at a clean non-contaminated location is compared. Further comparison of the temporal resistivity variation and water table fluctuations are also presented. The VRP results reveal that the natural hydrogeologic regime is not present in LNAPL contaminated areas as the fluctuating water table is not observed in these VRPs, but is evident in the non-contaminated VRP. Select VRP depth sections and depth slices indicate that the apparent resistivity is lowest at the LNAPL free product locations, progressively higher in the LNAPL dissolved location and highest in the clean (i.e. non-contaminated) location. These VRP sections also respond in like form to precipitation events, however at the contaminated locations the magnitude of change is greatly suppressed. A simple analysis using Archie's Law reveals that a large pore water saturation and a large pore water conductivity enhancement is necessary to produce the VRP field results from contaminated locations. We believe these results support our conductive model at LNAPL contaminated sites due to the effects of enhanced mineral dissolution of the aquifer materials resulting from biodegradation of the contaminant mass. Finally, the results demonstrate the potential for the use of vertical resistivity probes in understanding vadose zone processes and hydrogeologic dynamics at a LNAPL impacted site.

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