9.1 Underground Mines

Following cessation of operations, dewatering systems at an underground mine are shut down and the workings allowed to flood as the water table rises toward a new equilibrium condition. The rate at which the water level will recover depends upon the hydraulic conductivities of the subsurface geologic units and the specific yield of the units in the region where the bedrock was dewatered and the specific storage of the deeper bedrock that was depressurized but not dewatered. Groundwater enters storage in the rock mass, the open mine voids, and in any backfill materials placed in the mine workings. While the changed hydraulic conditions in the subsurface will modify the flow regime on a local scale from that existing prior to mine development, it is usually assumed that the post-closure flow system would have the same general configuration as that prior to mine development. However, this is not always the case as groundwater flow divides can shift with subsurface disturbance and groundwater that prior to development reported to one valley might report to an adjacent valley and discharge to surface after mine closure.

An assessment of recovery rates in hydraulic head and water table elevation, changing flow paths during the recovery period, and the ultimate geometry of the post-recovery flow system is usually undertaken using a three-dimensional, transient groundwater flow model. Underground mine workings are more complex than can normally be incorporated in a numerical model grid; simplified representations of the workings must be adopted. In mines that have been operated for decades, early mining records might not have been retained and access to older workings not safe, so there can be significant uncertainty in defining the geometry of the mine workings and the open void volume, and thus accurate quantitative predictions may not be achievable.

Dewatering of the underground workings creates a zone of drawdown centered about the mine workings. This drawdown cone may be somewhat circular but more commonly has a roughly elliptical shape due to the influence of geologic structure and geometry of the mine workings. A common question asked when evaluating mine closure plans concerns the time period over which the drawdown cone will be in place to maintain an inward hydraulic gradients toward the mine workings. If the walls of the mine workings hold soluble salts generated from mineral weathering, or if reactive waste rock stockpiles are located above the footprint of the drawdown cone, then for the duration of the recovery period there will be hydrodynamic containment of mine-affected waters. However, once the drawdown cone has dissipated, groundwater flow through the mine workings becomes part of the regional groundwater flow regime and contaminants might eventually move beyond the boundaries of the mine site. Surface outflows from mine adits carrying elevated concentrations of metals may require a collection and treatment system that must be operated for many decades after mine closure.

The rate of mine re-flooding will depend on the total drawdown that developed during mine operations, the void volume of the mine workings, and the hydraulic conductivity, specific storage and specific yield of the geologic units, and groundwater recharge rates in the area. The recovery rate is greatest in the initial period after dewatering wells are shut down. The recovery rate decreases through time in the same manner observed in the water level recovery curve following the termination of a pumping test. Mine workings located in semi-arid to arid climatic regimes will generally require longer recovery periods than at mine sites located in higher rainfall areas. On one hand, hydrodynamic containment in the vicinity of the mine workings might be maintained for decades to many hundreds of years following mine closure. On the other hand, if the hydrogeological conditions lead to the drawdown cone dissipating relatively quickly, off-site migration of solutes in the groundwater system could become a concern early in the closure period.