Field study on the application of time-lapse electrical resistivity tomography to assess the performance of an inclined multi-layer cover system reducing water infiltration

Post-mining can raise environmental issues, including water contamination in tailings storage facilities. Contaminated mine drainage can occur in these facilities when oxygen and water come into contact with tailings containing sulfides. In the past 20 years, various reclamation methods have proven to be effective in preventing potential contamination, such as the use of multi-layer cover systems. These engineered covers consist of successive layers with different hydrogeological properties to prevent water from reaching tailings. One way of assessing the effectiveness of these covers on the field is to monitor the flow of water within the cover over time, using time lapse electrical resistivity tomography (TL-ERT) in conjunction with hydrogeological instruments. This method allows to recover the spatio-temporal distribution of the soil electrical conductivity, and thus providing an image of the water flow in the near subsurface.

The objective of this project is to monitor water flow within a mine cover system which acts as a barrier to water infiltration into tailings using TL-ERT. This approach involves the use of numerical models, combined with field and laboratory data processing.

This study presents preliminary results from the two-weeks field campaign that was conducted in Fall 2024 at a tailing storage facility in Quebec where a multi-layer cover system is installed on a 7% slope. The cover configuration consists of four layers: 30 cm of silt, 20 cm of gravel, 30 cm of moisture-retaining silt, and 20 cm of gravel as a capillary break (from top to bottom). A 32 m-long ERT profile was installed along the slope of this cover with 64 electrodes and a spacing of 0.5 m. A 20 cm-high, 30 cm-wide and 2.75 m-long trench was excavated perpendicularly to the ERT profile, one-third along the profile. An infiltration test was performed, during which a total of 2000 L of a 1000 μS/cm saline tracer was injected into the trench over a period of 4 hours. TL-ERT monitoring consisted of acquiring a dataset of 65 ERT images using the Wenner configuration, every hour during the infiltration test, and every 6 hours thereafter for a week.

Preliminary results from field data inversion showed a spatio-temporal variation in resistivity associated with the start of the infiltration test. Near the trench, the inverted conductivity increased by a factor of two soon after the start of the injection, and a slightly conductive bulb appeared along the slope in the hours following the test. In addition, over the course of the two-week recording period, the surface of the cover became increasingly resistive, which can be associated to a significant drop in temperature between the beginning and end of the monitoring period (no rain was monitored during the monitoring period). The future steps of the processing will include a temperature correction to ensure that resistivity variations are only attributed to water inflow. Finally, thermo-hydrogeological modeling of the multilayer cover system during the infiltration test will allow to compare the geophysical results with modeled water dynamics.