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Broadband geoelectrical signatures of water-ethanol solutions in Ottawa Sand

R. Henderson (rdh98001@engr.uconn.edu)
U.S. Environmental Protection Agency, Office of Research and Development, Las Vegas, NV, United States

D.D. Werkema (werkema.d@epa.gov)
U.S. Environmental Protection Agency, Office of Research and Development, Las Vegas, NV, United States

R. Horton (rhorton@usgs.gov)
U.S. Geological Survey, Crustal Imaging and Characterization Team, Denver, CO, United States

J.W. Lane, Jr. (jwlane@usgs.gov)
U.S. Geological Survey Office of Groundwater, Branch of Geophysics, Storrs, CT, United States

Abstract

Ethanol has fast become the most widely used and distributed biofuel since its introduction as a fuel oxygenate to replace MTBE in gasoline and the emergence of "Flex Fuel" vehicles. Where ethanol is included in gasoline as a fuel oxygenate, subsurface releases have resulted in increased solubility and the transport of harmful BTEX compounds. In the case of "neat ethanol" (pure or slightly denatured) spills, large quantities of ethanol may initially reduce microbial populations, exacerbate pre-existing subsurface contamination in soil and groundwater, and potentially form explosive conditions through methanogenesis. There is a growing need for rapid assessment of subsurface releases to allow for quick remedial action following such releases. Surface and borehole geophysical measurements could provide rapid results to assess the extent of a release; however, little work has been done to understand the signature of ethanol in the subsurface. Here, we measure the broadband geoelectrical signature of various ethanol-water mixtures in a matrix of Ottawa sand to determine select geoelectrical parameters which may be applied in field scale studies. In the lower frequency range (Hz to kHz), resistivity and induced polarization parameters were measured and compared to the well known Cole-Cole model. At high frequencies (MHz to GHz), the dielectric constant of several ethanol-water solutions was measured. We use the empirical complex refractive index model (CRIM) to compare measured and predicted values of the dielectric constant. The low frequency electrical resistivity ranged from approximately 570 ohm-m (water in sand) to approximately 2300 ohm-m (pure ethanol in sand). The dielectric constant ranged from approximately 17.8 (water in sand) to 9.2 (pure ethanol in sand). Work is ongoing to determine induced polarization parameters. These initial results, however, suggest that geoelectrical field measurements would be useful to delineate an ethanol release in the environment soon after a spill under suitable site conditions. Based on the propensity of ethanol for biodegradation, however, more work is needed to assess the temporal evolution of an ethanol spill in geologically complex environments.


Final copy as submitted to American Geophysical Union (AGU) for publication as: Henderson, R.D., Werkema, D.D., Jr., Horton, R.J., and Lane, J.W., Jr., 2009, Broadband geoelectrical signatures of water-ethanol solutions in Ottawa Sand [abs.]: EOS Transactions, American Geophysical Union, v. 90, no. 52, Fall Meeting Supplement, abstract NS23A-1133.

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