Core Ideas 3D time‐lapse ERT is used to monitor water infiltration for mining environmental issues. Geoelectrical images provide information where no hydrogeological data is available. Water resistivity must be taken into account to understand bulk resistivity variations. Electrical resistivity of water is used as a tracer to reconstruct water infiltration. Infiltration model integrating both hydrogeological and geophysical data is proposed. Open‐pit mines often generate large quantities of waste rocks that are usually stored in waste rock piles (WRPs). When the waste rocks contain reactive minerals (mainly sulfides), water and air circulation can lead to the generation of contaminated drainage. An experimental WRP was built at the Lac Tio mine (Canada) to validate a new disposal method that aims to limit water infiltration into reactive waste rocks. More specifically, a flow control layer was placed on top of the pile, which represents a typical bench level, to divert water toward the outer edge. Hydrogeological sensors and geophysical electrodes were installed for monitoring moisture distribution in the pile during infiltration events. A three‐dimensional (3D) time‐lapse hydrogeophysical monitoring program was conducted to assess water infiltration and movement. Readings from the 192 circular electrodes buried in the WRP were used to reconstruct the 3D bulk electrical resistivity (ER) variations over time. A significant effort was devoted to assessing the spatiotemporal evolution of water ER because the bulk ER is strongly affected by water quality (and content). The water ER was used as a tracer to monitor the infiltration and flow of resistive and conductive waters. The results indicate that the inclined surface layer efficiently diverts a large part of the added water away from the core of the pile. Local and global models of water infiltration explaining both bulk and water ER variations are proposed. The results shown here are consistent with hydrogeological data and provide additional insights to characterize the behavior of the pile.

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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.

Mining operations generate large amounts of wastes which are usually stored into large-scale storage facilities which pose major environmental concerns and must be properly monitored to manage the risk of catastrophic failures and also to control the generation of contaminated mine drainage. In this context, non-invasive monitoring techniques such as time-lapse electrical resistivity tomography (TL-ERT) are promising since they provide large-scale subsurface information that complements surface observations (walkover, aerial photogrammetry or remote sensing) and traditional monitoring tools, which often sample a tiny proportion of the mining waste storage facilities. The purposes of this review are as follows: (i) to understand the current state of research on TL-ERT for various applications; (ii) to create a reference library for future research on TL-ERT and geoelectrical monitoring mining waste; and (iii) to identify promising areas of development and future research needs on this issue according to our experience. This review describes the theoretical basis of geoelectrical monitoring and provides an overview of TL-ERT applications and developments over the last 30 years from a database of over 650 case studies, not limited to mining operations (e.g., landslide, permafrost). In particular, the review focuses on the applications of ERT for mining waste characterization and monitoring and a database of 150 case studies is used to identify promising applications for long-term autonomous geoelectrical monitoring of the geotechnical and geochemical stability of mining wastes. Potential challenges that could emerge from a broader adoption of TL-ERT monitoring for mining wastes are discussed. The review also considers recent advances in instrumentation, data acquisition, processing and interpretation for long-term monitoring and draws future research perspectives and promising avenues which could help improve the design and accuracy of future geoelectric monitoring programs in mining wastes.

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