Candidate: Douglas D. Werkema Jr.

Degree of: Doctor of Philosophy

Department: Geosciences

Title: Geolectrical Response of an Aged LNAPL Plume: Implications for Monitoring Natural Attenuation

Date: Friday, August 16, 2002, 4:00 p.m. - 6:00 p.m.
1122 Rood Hall

Committee:
Dr. Estella Atekwana, Chair
Dr. William Sauck
Dr. Daniel Cassidy
Dr. Anthony Endres

Abstract:
The geolectric stratigraphy at a site impacted with light non-aqueous phase liquid (LNAPL) is investigated for its link to natural biodergradation, its contribution to the conductive model for aged LNAPL contaminated sites, and the relationship to the natural hydrologic regime.

The highest conductivities (lowest resistivities) were observed in portion of soils where LNAPL was in residual and free phase. Corroborating evidence from bacteria enumeration from soil close to the VRP installations show orders-of magnitude increase in both heterotrophic and oil degrading microbes at the depths where the conductivity was at a maximum. The coincidence of peak microbial populations with zones of high conductivity suggests that the anomalous high conductivity in these zones may be due to increase in the pore fluid conductivity related to microbial degradation of LNAPL.

The conductivity response from VRPs within the free product plume, within the residual plume, and at a clean non-contaminated location is also compared. Select VRP depth sections and depth slices indicate that the conductivity is highest at the LNAPL free product locations, progressively lower in the LNAPL dissolved location and lowest in the clean (i.e. non-contaminated) location. 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 the contaminated locations. These results support the conductive model at LNAPL contaminated sites due to the effects of enhanced mineral dissolution of the aquifer materials resulting form biodegradation of the contaminant mass.

Further comparison of the temporal conductivity variation and water table fluctuations are also presented. The VRP results reveal that the natural hydrogeologic regime is suppressed in LNAPL contaminated areas as the fluctuating water table is not observed in these VRPs, but is evident in the non-contaminated VRP. Finally, the results demonstrate the potential for the sue of vertical resistivity probes in understanding the hydrogeologic dynamics at a LNAPL impacted site.

Overall, the findings from this study demonstrate the potential of geoelctrical imaging as a tool for assessing bio-attenuation of LNAPL impacted soils and focusing remediation efforts at discrete zones determined through VRP data analysis.

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