The dynamics of moisture content in cover systems constructed on mining wastes were monitored at the pilot scale using 2D autonomous, remote, and non-invasive time-lapse electrical resistivity tomography combined with conventional point sensors. A methodology was proposed to process the daily hydrogeophysical datasets from 23 m-long instrumented sections of covers with capillary barrier effects (CCBEs) designed to act as oxygen barriers, and covers with low saturated hydraulic conductivity layers (LSHCCs) designed to limit the water infiltration rate. Hydrogeophysical monitoring suggested that CCBEs were able to maintain high degrees of saturation in the moisture-retaining layer throughout the one-year monitoring period, which would make it an efficient oxygen barrier. Larger spatio-temporal changes in moisture content were observed in LSHCCs and most of the low hydraulic conductivity layers remained below 85% saturation, which was attributed to the combined effect of low precipitation, rapid vegetation development, and water percolation through the cover. The methodology proposed in this pilot-scale "proof-of-concept" study allowed the hydrogeological behavior of the cover systems to be monitored in the 23 m-long instrumented sections using continuous geoelectrical data, which demonstrated that this innovative monitoring technique could be useful for geochemical and geotechnical monitoring programs in large-scale mining waste storage facilities.

Accurate and large-scale assessment of volumetric water content (VWC) plays a critical role in mining waste monitoring to mitigate potential geotechnical and environmental risks. In recent years, time-lapse electrical resistivity tomography (TL-ERT) has emerged as a promising monitoring approach that can be used in combination with traditional invasive and point-measurements techniques to estimate VWC in mine tailings. Moreover, the bulk electrical conductivity (EC) imaged using TL-ERT can be converted into VWC in the field using petrophysical relationships calibrated in the laboratory. This study is the first to assess the scale effect on the accuracy of ERT-predicted VWC in tailings. Simultaneous and co-located monitoring of bulk EC and VWC are carried out in tailings at five different scales, in the laboratory and in the field. The hydrogeophysical datasets are used to calibrate a petrophysical model used to predict VWC from TL-ERT data. Overall, the accuracy of ERT-predicted VWC is [Formula: see text], and the petrophysical models determined at sample-scale in the laboratory remain valid at larger scales. Notably, the impact of temperature and pore water EC evolution plays a major role in VWC predictions at the field scale (tenfold reduction of accuracy) and, therefore, must be properly taken into account during the TL-ERT data processing using complementary hydrogeological sensors. Based on these results, we suggest that future studies using TL-ERT to predict VWC in mine tailings could use sample-scale laboratory apparatus similar to the electrical resistivity Tempe cell presented here to calibrate petrophysical models and carefully upscale them to field applications.

Abandoned underground mine galleries or excavations in general pose important environmental and economic problems when a site is reactivated after being abandoned, because they might create risks for land stability. At the request of a need for a solution for the reopening of a mine (Horne 5 project of Falco Resources in Rouyn-Noranda, Quebec, Canada) future exploitation would start from 600 m deep. Before that reopening, it is very important to determine old mining excavation locations and conditions. The present study is targeting a technological challenge of detecting old and deep mining infrastructure. There is to date no application allowing the detection of underground tunnels or mining galleries at a depth of >80 m due to their depth and small size. Electrical Resistivity Tomography (ERT) is promising given its ability to image the contrasts between rocks and voids, whether these cavities are empty or filled with water or backfill materials. Several 2D and 3D ERT surveys were carried out to explore the abandoned old mining excavations. These ERT measurements highlighted the existence of old mining stopes, confirmed by the geotechnical boreholes performed on site, as well as two known galleries located at an approximate depth of 100 m. Overall, the results of this study highlight the potential of ERT approaches for the characterization of deep underground excavations.

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.

For a large number of landfills, basic knowledge about extent, waste composition or environmental impact is incomplete. Considering the large number of non-sanitary landfills located in semi-urban areas subject to increased land use pressure plus the high cost for remediation, it is crucial to develop efficient characterization tools suitable in landfill contexts. Such tools are required on a broader level to enable the identification of landfills with high priority for remediation or high potential in terms of waste valorisation (landfill mining) and, on a more detailed level, to enable planning of remediation or landfill mining projects. Due to the high heterogeneity and complexity of landfills, the application of different geophysical methods in combination with targeted sampling has proven to be a highly favourable approach. In contrast to conventional ground truth methods, geophysical techniques provide the possibility to characterize large portions of the landfill volume in a non-invasive and relatively efficient way. Furthermore, the application of complementary geophysical techniques reduces the risk of misinterpretation, and by verifying/calibrating the results with targeted sampling a relatively detailed landfill model can be built. However, building a landfill model from data measured at different resolution, coverage and with different uncertainties is a challenge. We present a case study from Emersons Green (UK) where we completed multiple geophysical surveys on a former landfill site prior to its full excavation. The excavation works provided nearly continuous information on the waste and cover layer thickness as well as information on material composition from several locations. This enabled us to validate the geophysical measurements and to test different approaches for model building, as well as testing virtual sampling strategies in order to assess how the number and location of ground truth samples affects the model quality. The case study has highlighted the advantage of a multi-geophysical approach where Electromagnetics (EM) and Magnetics (Mag) were able to provide a rapid overview of the landfill structure and its lateral extent. In contrast, Induced Polarization Tomography (IPT) and Multichannel Analysis of Surface Waves (MASW) were most suitable to delineate the bottom interface of the waste layer. IPT was in addition able to delineate the cover layer thickness and Electrical Resistivity Tomography (ERT) seemed more sensitive to changes in moisture content. For the model building, a probabilistic approach has proven to be efficient. In terms of sampling strategy a minimum number of samples are required co-located with the geophysical measurements to train the probability model. Furthermore, additional sampling points at locations where geophysical methods are only sparsely available increase the model certainty.

Summary For a large number of landfills, basic knowledge about extent, waste composition or environmental impact is incomplete. Therefore, effective tools are required for landfill characterization to be able to identify landfills with high priority for remediation or high potential for landfill mining. In contrast to conventional ground-truth methods, geophysical methods are relatively inexpensive and since they are minimally invasive, they entail less risk for environmental contamination. With our case study on the landfill in Emersons Green UK, we tested the effectiveness of multiple geophysical methods for landfill characterization. Electromagnetics (EM) and Magnetics (Mag) were able to provide a rapid overview of the landfill structure, and detected areas with higher metal content. Induced Polarization (IP) was the most suitable method to delineate the extent of the waste layer whereas Electrical Resistivity Tomography (ERT) seemed more sensitive to changes in moisture content. This study has shown that a multi-method geophysical approach in combination with targeted sampling is essential to avoid misinterpretations. Furthermore, planning the profile location of the more time-consuming methods such as IP and ERT based on the results of the rapid mapping methods such as EM and Mag can improve the survey efficiency.

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.

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.

Several 3D Electrical Resistivity Tomography (ERT) surveys were carried out on an experimental waste rock pile to characterize water infiltration. The pile was built at the hemo-ilmenite Lac Tio mine (QC, Canada) to test a proposed technique for the disposal of waste rocks that could considerably reduce their impact on the environment. The pile has been instrumented with multiple hydrogeological sensors including tensiometers, moisture probes, lysimeters and geophysical electrodes permanently installed to study the hydrogeological behavior of the pile. This paper presents the results of the geophysical surveys done in September 2016 for imaging the resistivity distribution in the 60m-long, 10m-wide and 7m-high pile and reconstructing the water content distribution. 3D time-lapse ERT measurements were carried out to image water flow before, during and after infiltration tests. 192 electrodes were disposed at the bottom and the top of the pile using 2m electrode spacing in both horizontal directions. The authors have analyzed and selected ERT protocols that would provide the required spatial resolution in the core of the pile where hydrogeological information is not directly available from the geophysical measurements. Each ERT protocol contains approximately 4000 measurements and a total of 230 000 measurements were recorded over a period of 15 days. This large database is currently being processed. The main goal of this project is the interpretation of resistivity values with regards to volumetric moisture content, so considerable attention was given to providing reliable hydrogeological results. Laboratory measurements were carried out on waste rock samples from the pile: the mineralized core (60-70 % ilmenite) is conductive (200 Ω.m) while non-mineralized rocks (anorthosite) are resistive (3000 Ω.m). Preliminary interpretation was carried out to reconstruct 3D images of the resistivity distribution in the pile: geometry and resistivity values are consistent with the known structure of the pile. The work performed in close collaboration with hydrogeologists and geochemists to improve the global understanding of the environmental behavior of the waste rock pile.