Electrical Imaging of Saline Tracer Migration for the Investigation of Unsaturated Zone Transport Mechanisms

Abstract

Abstract. Abstract: Better understanding of field-scale unsaturated zone transport mechanisms is required if the fate of contaminants released at the surface is to be predicted accurately. Interpretation of results from direct tracer sampling in terms of operative hydraulic processes is often limited by the poor spatial coverage and the invasive nature of such techniques. Cross-borehole electrical imaging during progress of saline tracer migration is proposed to assist investigation of field-scale solute transport in the unsaturated zone. Electrical imaging provides non-destructive, high density and spatially continuous sampling of saline tracer transport injected over an area of the ground surface between two boreholes. The value of electrical imaging was tested at a field site on an interfluve of the UK Chalk aquifer. Improved understanding of active transport mechanisms in the unsaturated zone of the UK Chalk is required to predict its vulnerability to surface pollutants. In a tracer experiment in May 1996, a conductive saline tracer was infiltrated over 18 m2 at an average rate of 47 mm day-1 for 56 hours. Cross-borehole images obtained during and after infiltration show a large, homogenous, resistivity reduction in the top 3 m, no change between 3 m and 6 m depth, and smaller, inhomogeneous, resistivity reductions below 6 m depth. The resistivity has reduced at down to 15 m depth less than 2 days after tracer infiltration began. Hydrological interpretation of a sequence of electrical images obtained prior to, during, and up to three months after tracer injection suggests: (1) rapid tracer entry into the soil zone and upper 2 m of weathered Chalk, (2) intergranular transport of the bulk of the tracer, (3) a significant fissure flow component transporting tracer to at least 15 m depth in 31 hours, and (4) vertical changes in transport mechanisms possibly caused by interception of fissures by marl layers. The results of this experiment suggest that electrical imaging can assist the description of unsaturated zone hydraulic mechanisms through visual identification of spatial and temporal variations in transport processes.

Estimation of unsaturated hydraulic parameters in sandstone using electrical resistivity tomography under a water injection test

“…The dependence of electrical resistivity variations on changes in soil water content through empirical or semiempirical relationships (e.g., Archie, 1942) or established in-situ relationships (e.g., Farzamian et al, 2015) is the key mechanism that permits the use of time-lapse ERT to monitor water movement in time-lapse mode. Several studies have been conducted to monitor salt tracer tests or water infiltration through the unsaturated zone using ground surface ERT (e.g., Barker and Moore, 1998;Park, 1998;Hayley et al, 2009) and crosshole ERT (e.g., Slater et al, 1997;Daily et al, 1992;Binley et al, 2002aBinley et al, , 2002bDeiana et al, 2007). Also, ground surface ERT (e.g., Robert et al, 2012;Cassiani et al, 2006) and crosshole ERT (e.g., Binley et al, 1996;Slater et al, 2000Slater et al, , 2002Singha and Gorelick, 2005) are extensively used in saturated zone study.…”

An experiment of non‐invasive characterization of the vadose zone via water injection and cross‐hole time‐lapse geophysical monitoring

“…Binley, Henry‐Poulter and Shaw ; Slater et al . , , ; Kemna et al . ; Singha and Gorelick ; Vanderborght et al .…”

“…This fact has led to the increasing use of non‐invasive, geophysical methods, particularly ground‐penetrating radar (GPR) and electrical resistivity tomography (ERT), often in a cross‐borehole configuration, to investigate the vadose zone (Slater et al . ; Binley et al . , ; Alumbaugh et al .…”

Charged Body Potential Monitoring of an Electrolyte Plume Emanating from a Dripping Source

“…Ref., [25,26] To avoid density effects, this KCl solution was only moderately conductive compared to some other experiments in the literature where highly conductive tracers were used (e.g.…”