Hysteresis In The Electrical Conductmty-Saturation Relationship Of A Sand

There is a growing interest in the use of electrical measurements for the detection,<br>mapping and monitoring of zones of non-aqueous phase liquid (NAPL) contamination and<br>for the measurement of soil moisture content in the unsaturated zone (for example,<br>electrical resistivity tomography during air sparging). The interpretation of these<br>measurements depends on an understanding of the conductivity-saturation relationship.<br>Previous laboratory experiments have shown that this relationship is hysteretic. This<br>paper discusses our laboratory investigation of the effect of fluid saturation and saturation<br>history on the electrical conductivity of partially saturated sand.<br>Our ongoing experiments involve 2- and 4-electrode measurements of the complex<br>electrical conductivity of a sand sample during drainage-imbibition cycles with water-air<br>and water-NAPL systems. This paper presents the results of a drainage-imbibition<br>experiment using air and water as the pore fluids. Capillary pressure measurements were<br>made concurrently with the electrical measurements. Hysteretic behaviour was observed<br>in both the conductivity-saturation and the capillary pressure-saturation relationships. In<br>particular, the conductivity at a given saturation was found to be significantly lower during<br>imbibition than during drainage over most of the saturation range. The results of these<br>studies suggest that the saturation history should be considered when interpreting<br>electrical measurements from the unsaturated zone and areas of NAPL contamination.