3D TIME-LAPSE GEOELECTRICAL MONITORING OF MOISTURE CONTENT IN AN EXPERIMENTAL WASTE ROCK PILE: VALIDATION USING HYDROGEOLOGICAL DATA

Abstract

The hydrogeological behavior of heterogeneous and unsaturated media can be challenging to assess, especially where classical hydrogeological instrumentation cannot be directly installed such as in the core of waste rock piles. In this paper, the authors present the results of several 3D Electrical Resistivity Tomography surveys carried out in 2017 for time-lapse monitoring of water infiltration events in an experimental waste rock pile. This pile was built according to a recently proposed waste rock disposal method at the Lac Tio mine (RTFT, Québec, CA) that aims at diverting water flow from potentially reactive waste rock, thus limiting metal leaching and contamination of the effluent. The pile has been instrumented with soil moisture sensors and lysimeters to monitor water content over time and collect percolating water. In addition, 192 buried electrodes have been used to carry hourly measurements with an optimized protocol of 1000 configurations uploaded on a Terrameter LS (ABEM) to monitor internal flow of water sprinkled on the top of the pile with a water truck. Time-lapse 3D ERT data were inverted to yield the 3D model of soil electrical resistivity over time before, during and after induced infiltration events in the pile. While resistivity results show consistent variations associated with increased moisture content, conversion of resistivity into volumetric water content is not straightforward. This challenge is related, in part, to changes in the distribution of water resistivity over time in the pile, which in turn strongly affects resistivity within the waste rock. Laboratory column measurements have been conducted to assess the relationship between global (waste rock) resistivity, water resistivity and moisture content for samples from the pile. The images of water content obtained with ERT are then validated with hydrogeological measurements and modeling of the pile. This assessment indicates that geoelectrical monitoring is an efficient tool to monitor water moisture in a complex media.