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Geoelectrical Evidence of Bicontinuum Transport in Ground Water

K. Singha
Dept. of Geosciences, Penn State University, 311 Deike Building, University Park, PA 16802

F.D. Day-Lewis
Branch of Geophysics, U.S. Geological Survey, 11 Sherman Place, Storrs, CT 06268

J.W. Lane, Jr.
Branch of Geophysics, U.S. Geological Survey, 11 Sherman Place, Storrs, CT 06268

Abstract

however, observed transport behavior appears inconsistent with the standard advective-dispersive model; instead, concentration histories show long tailing behavior, non-Gaussian breakthrough, and/or rebound after pumping for mass removal has ceased. These phenomena have prompted the consideration of dual-domain, rate-limited mass transfer (RLMT) as a controlling process. Determination of parameters describing mass-transfer between mobile and immobile domains - or even verifying the occurrence of RLMT - is problematic because geochemical data-collection methods preferentially sample the mobile component of the pore space. We present direct evidence of RLMT at the field scale during an aquifer storage and recovery experiment. We observe a hysteretic relation between measurements of pore-fluid conductivity (from borehole fluid samples) and bulk earth conductivity (from borehole electrical-resistivity). This hysteresis contradicts advective-dispersive transport and the standard petrophysical model relating pore-fluid and bulk conductivity, but can be explained by bicontinuum transport models that include first-order RLMT. Using a simple model, we demonstrate that geoelectrical methods can be used to bound estimates of mass transfer-rates and immobile porosity that are otherwise difficult to estimate in situ. These findings suggest that RLMT is one of the fundamental processes controlling solute transport and the efficiency of aquifer remediation, and suggest that similar analyses in other geologic settings may help evaluate the prevalence of RLMT.


Final copy as submitted to the American Geophysical Union (AGU) for publication as: Singha, K., Day-Lewis, F.D., Lane, J.W., Jr., 2007, Geoelectrical evidence of bicontinuum transport in ground water, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract H12C-02.

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