WERKEMA JR., DOUGLAS D., Western Michigan University, Dept. of Geosciences, Kalamazoo, MI

ABSTRACT: Geoelectrical Response During Microbial LNAPL Degradation: Implications for Monitoring Bioremediation

The geoelectrical response of Light Non-Aqueous Phase Liquid (LNAPL) contamination in shallow unconsolidated sediment has been based on the model of high apparent resistivity. The validity of this model has been questioned by the results of recent investigations and suggests anomalously low apparent resistivity coincident with zones impacted by LNAPL. To account for this discrepancy, it is proposed that the occurrence of anomalously low apparent resistivities in and below free product and residual product LNAPL plumes is the result of elevated total dissolved solids (TDS) resulting from sediment grain leaching due to acids produced as a by-product of LNAPL microbial degradation.

This hypothesis is tested via both field and laboratory investigations. In situ vertical resistivity probes (VRPs) are installed throughout the known plume, emanating from the former Crystal Refinery in Carson City, Michigan, and off the plume. Monthly monitoring includes apparent resistivity measurements, sediment core analysis for bacterial population count, hydrogeochemical analysis and sedimentology. Results show anomalously low apparent resistivities coincident with maximum bacterial populations. The laboratory component designed as an analog to, and verification of, the field conditions is just beginning; however, preliminary results support the above hypothesis and validate the field data.

Overall, this project develops a greater understanding of the vertical apparent resistivity distribution at an LNAPL impacted site, as well as provides insights into the natural dynamic hydrogeologic processes as observed by VRPs. The information gained from this study should allow an improved basis for the geoelectrical monitoring of LNAPL biodegradation.

This abstract is on pages 1875-1876.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid