BEDROCK AQUIFERS CHARACTERIZATION BASED ON UNDERGROUND MINE SITE INVESTIGATION: RESULTS AND OPPORTUNITIES

Underground mine operation requires groundwater pumping to dewater the excavations, and discharging that water in surface water bodies. This drainage often generates environmental and economic impacts, but it also provides unique data set for characterizing bedrock aquifers and it presents interesting research opportunities on fundamental hydrogeological processes.

Besides direct hydrogeological effects, mining also generates significant perturbations of the geomechanical stress field in the rock mass surrounding the excavations. All of these perturbations produce hydrogeological (H), geomechanical (M) and geochemical (C) phenomena occurring simultaneously, and interacting with each other. Important phenomena include: 1) the geomechanical stress perturbations resulting from the excavations; 2) the effects of these perturbations on rock mass permeability; 3) the important decrease in groundwater pressure at depth; 4) the variety of types and ages of the groundwater extracted at operating mine sites; 5) the generation of an important volume of desaturated rock mass due to pumping; 6) the percolation of infiltrating groundwater through acid-generating backfill material placed in upper level excavations in operating mines. All of these factors are acting simultaneously at operating mine sites, creating real field laboratories to conduct research on the coupling of H-M-C processes. At mine closure, existing mine excavations may affect adversely ground stability up to the surface, as well as the quantity and the quality of groundwater flowing through the system.

A number of analytical models developed to predict inflow into underground workings are used with input data collected at underground mines in order to estimate the hydraulic conductivity of the regional bedrock aquifer. Mine excavations also provide unique opportunities to characterize the fracture networks and to collect water samples at various locations and depths, allowing for a better structural and hydrogeochemical characterization of the regional aquifer.

Many of these factors will be illustrated with data from a number of mine sites in Canada and elsewhere, advocating for more hydrogeological research taking advantage of increasing mineral resource extraction.