Facilitating Open Pit Mine Closure with Managed Aquifer Recharge.

Dewatering of open pit mines can lower the regional water table for distances of several kilometers from the pit. When the mine is closed, dewatering operations usually cease, and the water table near the pit begins to rise. If the pit is backfilled, the water table will eventually recover, but this recovery may take several hundred years. However, if the extracted water is re-injected into the subsurface, then this may accelerate recovery of the water table. We show that there is an optimal distance for re-injection, which is sufficiently far from the mine to minimize the amount of groundwater that flows back to the pit during mine operations (and hence necessitate additional pumping) but is still close enough to speed up the water table recovery post-mine closure. The optimal injection distance increases with the aquifer hydraulic diffusivity and the mine life (duration of dewatering and injection), and typically ranges between about two and nine times the radius of the mine pit. Where the mine pit is not backfilled, the relative reduction in drawdown due to injecting all the pumped water at the optimal distance is between approximately 10% and 50% after a recovery time equal to the mining period, increasing to 30% to 90% after a recovery time five times the mining period. The relative drawdown reduction due to managed aquifer recharge will be even greater for a pit which is backfilled when mining ceases.