Changes In Geoelectrical Properties Accompanying Microbial Degradation Of Lnapl

Abstract

Field geophysical studies have identified anomalously high conductivities within and below the free product zone in soils at sites with contamination by light, nonaqueous phase liquids (LNAPL). Laboratory experiments and simple numerical modeling studies were conducted to test the hypothesis that these anomalously high conductivities result from products of LNAPL biodegradation. These experiments consisted of 20-L glass vessel reactors with 18 L of inoculated sand and 6 L of pore water. Diesel fuel was added to the top of the sand. These experiments simulated a smear zone with a top layer of LNAPL. Duplicate reactors of the following types were maintained for 120 days; nutrients added (at levels observed in the field), no nutrients added, killed (autoclaved) control with nutrients. The killed control showed no signs of diesel fuel biodegradation. The biologically active reactors showed evidence of diesel fuel biodegradation (e.g., reduced dissolved oxygen, increasing numbers of oil-degrading microbes). Diesel fuel biodegradation was accompanied by increases in the concentrations of volatile organic acids, calcium ion, pore water conductivity, total dissolved solids, surfactant production, and diesel fuel emulsification. These results are complemented by the numerical modeling results, which also showed potential enhancements in aqueous phase conductivity of 3700 μS/cm when complete mineralization is assumed. However, the more realistic equilibrium models predict enhancements in the 500-1500 μS/cm range. From this study, two important observations are made that have significant ramifications on the measured geoelectrical properties at aged LNAPL sites. First, microbial degradation of LNAPLs produces a variety of acids that enhance chemical weathering of the aquifer materials, resulting in high TDS content and thereby increasing the conductivities of the pore waters. Second, emulsification of the LNAPL by surfactant production has the potential to change the wetting phase from LNAPL wetted to water wetted, providing electrically conductive paths within LNAPL saturated zones. Both of these observations are consistent with our field investigations where we have reported conductivity values 2-5 times background values from contaminated zones. Finally, our field studies have also shown that the LNAPL saturated zone is conductive and not resistive. Thus, the above laboratory experiment and numerical modeling results demonstrate that LNAPL biodegradation readily explains the temporal changes in conductivity observed in geophysical investigations of impacted aquifers. * Now at the University of Missouri-Rolla, Dept. of Geology and Geophysics