In Situ Measurement of Electro-Osmotic Fluxes and Conductivity Using Single Wellbore Tracer Tests |
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| Authors: | Walt W McNab Jr Roberto Ruiz |
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| Institution: | Walt McNab, an environmental scientist at Lawrence Liver-more National Laboratory, holds an M.S. and a Ph.D. in mineral engineering (hydrogeology) and a B.A. in geology from the University of California, Berkeley. His research interests include ground water flow and transport simulation, ground water chemistry and geochemical modeling, statistical studies of ground water contaminant plumes, and remediation system design. He is currently at Exponent-Failure Analysis Associates, 1970 Broadway, Ste. 250, Oakland, CA 94612;(510) 208–2001;fax (510) 208–2039;;Roberto Ruiz is a senior mechanical engineering associate at Lawrence Livermore National Laboratory (7000 East Ave., Mail Stop 530, Livermore, CA 94550;). He has extensive experience in the design of chemical process systems and equipment as well as complex mechanical systems. Ruiz is a certified plant engineer and a member of the Association for Facilities Engineering. |
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| Abstract: | Electro-osmosis (EO), the movement of water through porous media in response to an electric field, offers a means for extracting contaminated ground water from fine-grained sediments, such as clays, that are not easily amenable to conventional pump-and-treat approaches. The EO-induced water flux is proportional to the voltage gradient in a manner analogous to the flux dependence on the hydraulic gradient under Darcy's law. The proportionality constant, the soil electro-osmotic conductivity or keo, is most easily measured in soil cores using bench-top tests, where flow is one-dimensional and interfering effects attributable to Darcy's law can be directly accounted for. In contrast, quantification of EO fluxes and keo in the field under deployment conditions can be difficult because electrodes are placed in ground water wells that may be screened across a heterogeneous mixture of lithologies. As a result, EO-induced water fluxes constitute an approximate radial flow system that is superimposed upon a Darcy flow regime through permeable pathways that may or may not be coupled with hydraulic head differences created by the EO-induced water fluxes. A single well comparative tracer test, which indirectly measures EO fluxes by comparing wellbore tracer dilution rates between background and EO-induced water fluxes, may provide a means for routinely quantifying the efficacy of EO systems in such settings. EO fluxes measured in field tests through this technique at a ground water contamination site were used to estimate a mean keo value through a semianalytic line source model of the electric field. The resulting estimate agrees well with values reported in the literature and with values obtained with bench-top tests conducted on a soil core collected in the test area. |
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