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Characterization of a former uranium mill site using frequency-domain electromagnetic and electrical resistivity surveys

T.R. Brosten (tbrosten@usgs.gov)
U.S. Geological Survey, 11 Sherman Place Unit 5015, Storrs, CT 06269, United States

F.D. Day-Lewis (daylewis@usgs.gov)
U.S. Geological Survey, 11 Sherman Place Unit 5015, Storrs, CT 06269, United States

G.P. Curtis (gpcurtis@usgs.gov)
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States

J.W. Lane, Jr. (jwlane@usgs.gov)
U.S. Geological Survey, 11 Sherman Place Unit 5015, Storrs, CT 06269, United States

Abstract

Frequency-domain electromagnetic induction (FDEM) surveys were conducted at a former uranium mill site near Naturita, Colorado, to rapidly characterize site lithology and estimate variations in site hydrogeologic properties. The FDEM surveys were performed by towing a multi-frequency EM instrument behind an all-terrain vehicle permitting rapid high-density data collection over the 1.5 km x 0.5 km study site. Analysis of FDEM apparent conductivity maps was used to determine the placement of several electrical resistivity tomography (ERT) transects across areas of interest and adjacent to monitoring wells. Direct correlation between FDEM apparent electrical conductivity data and hydraulic conductivity measurements, recorded at discrete borehole intervals from slug tests, is poor due to the high electrical conductivity in this environment. FDEM apparent conductivity is an effective measurement over an integrated volume on the order of several cubic meters resulting in poor correlations with data recorded at discrete depth intervals. To fully capitalize on the information content of the FDEM data, we inverted the raw apparent electrical conductivity data using a code based on a one-dimensional (1D) forward model and two-dimensional (2D) regularization. This approach produces a three-dimensional (3D) volume of subsurface electrical conductivity. Correlation between measured hydraulic conductivity and inverted EM model conductivities, at depths of the well screens, appears promising and is the subject of ongoing research. The ERT data were also inverted to produce 2D cross sections. Where possible, ERT and FDEM conductivity models were correlated with water depths, lithologic logs, and hydraulic conductivity measurements from nearby monitoring wells. In general, ERT and FDEM results compare favorably, with the former providing information about deeper structures and the latter providing higher density spatial information.


Final copy as submitted to American Geophysical Union (AGU) for publication as: Brosten, T.R., Day-Lewis, F.D., Curtis, G.P., and Lane, J.W., Jr., 2009, Characterization of a former uranium mill site using frequency-domain electromagnetic and electrical resistivity surveys [abs.]: EOS Transactions, American Geophysical Union, Spring Meeting Supplement, Joint Assembly, 24-27 May 2009, Toronto, Canada.

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