In Situ Measurement of Electro‐Osmotic Fluxes and Conductivity Using Single Wellbore Tracer Tests

Abstract

Electroosmosis (EO), the movement of water through porous media in response to an electric field, offers a means for extracting contaminated groundwater 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 electroosmotic conductivity or k eo , 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 k eo in the field under deployment conditions can be difficult because electrodes are placed in groundwater 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 well bore 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 groundwater contamination site were used to estimate a mean k eo value through a semi-analytic line source model of the electric field. The resulting estimate agrees well with values reported in the literature as well as values obtained by bench-top tests conducted on a soil core collected in the test area.

Electrolyte Management for Effective Long-Term Electro-Osmotic Transport in Low-Permeability Soils

“…Since hydraulic pumping in a heterogeneous matrix draws water primarily through the coarse-grained zones, circumventing finer-grained zones with higher contamination, mechanical pump-and-treat technology is not effective in the cleanup of clayey, fine-grained zones. We are therefore exploring the use of in-situ electro-osmotic pumping to flush contaminants from fine-grained sediments ( − ).…”

Field Measurements of Electro-osmotic Transport of Ground Water Contaminants in a Lithologically Heterogeneous Alluvial-Fan Setting

“…Because the source sediments at the LLNL demonstration site exhibit heterogeneous lithologies, electro-osmotic treatment of fine-grained sediments is being coupled directly with pump-and-treat extraction of ground water from coarse-grained sediments. This remediation strategy presents challenges in interpreting the effects of EO on contaminant transport because of the difficulty in separating the electro-osmotic and Darcian flux components within the same well field (McNab and Ruiz, 2001). This study presents a means for separating the two flux components based on the integration of lithologic data, initial contaminant distributions between lithologies, modeling, and statistical analyses of contaminant fluxes to the extraction wells.…”