Field and Laboratory Studies of the Anomalous Elect rical Conductivity Zones Associated with LNAPL Contamination in MI
D. Werkema1, W. Sauck1, and E. Atekwana2
1Dept. of Geosciences, W. Michigan Univ.
2Dept. of Geology, Univ. of Missouri, Rolla, MO
High conductivity, rather than the expected low conductivity, has been measured geophysically in the LNAPL free- and residual-product zones at various contaminated sites in Michigan. The contaminants ranged from crude oil to JP-4 and gasoline. The dominant geologic material at these sites was sand, with appreciable amounts of carbonate. The effect has been seen at two former refinery sites, and at a former US Air Force Base (Wurtsmith).Water samples from the uppermost parts of the aquifer beneath these LNAPL plumes displayed electrical conductivities 3 to 5 times greater than background. Based on these results and earlier USGS hydrogeochemical studies at a pipeline spill in Bemidji, MN, we developed a working model which involved bacterial degradation of LNAPL, production of organic acids, carbonic acid, and surfactants. The acids were buffered by the calcite and dolomite grains, producing an inorganic leachate rich in Ca, Mg, and bicarbonates. Annual vertical cycling of the water table, in the presence of emulsifying surfactants, tended to break up the initially continuous LNAPL layer, allowing conductive leachate paths through the LNAPL to form.
The zones have been investigated in detail by permanent vertical arrays of electrodes -Vertical Resistivity Probes (VRP), by multispaced horizontal resistivity profiling at the surface, and by horizontal ground penetrating radar (GPR) profiling. The VRPs were measured repeatedly over a period of several years to better understand seasonal changes and individual infiltration events. Surface resistivity techniques could only map the shallower low-resistivity anomalies. The GPR could clearly delineate some of the leachate plumes because of the clearly attenuated GPR signal below the leachate plume.
Preliminary laboratory experiments in 18-liter reactor vessels showed rapid development (4–6 weeks) of large bacterial populations and the production of conductive leachate and surfactants. Larger semi-static column experiments have helped clarify a number of the processes and phenomena. These columns had the capability for resistivity measurements via a fixed vertical array of electrodes, as well as liquid sampling from any level with a hypodermic needle through a rubber septum along the sidewall of the column.
AAPG Search and Discovery Article #90900©2001 AAPG Eastern Section Meeting, Kalamazoo, Michigan