Facilitating Open Pit Mine Closure with Managed Aquifer Recharge

Abstract

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.

Groundwater-level recovery following closure of open-pit mines

“…In general, it is an unsuccessful management option under highly transmissive aquifer settings, as any gain in lake level quickly dissipates into the surrounding aquifer. It could be speculated, that managed aquifer recharge may have a role in rapid recovery of groundwater levels in mining regions of higher aquifer transmissivity (Cook et al 2022). In less permeable geology, added surplus water is retained much longer within the pit and elevated lake and groundwater levels as a result may last for several decades.…”

Managed Aquifer Recharge in Mining: A Review

“…Depending on climate and hydrogeological conditions, the recovery may take tens to several hundreds of years. Cook et al (2022) found that groundwater recovery post‐mine closure may be enhanced by optimizing re‐injection distances from the pit so that injected water flows back to the closed pit soon after mining and dewatering operations have ceased. Water management post‐ closure is one of the most important aspects of environmental protection, and effective water management strategies during the mining process can significantly reduce the costs associated with rehabilitation (El Idrysy and Connelly 2012).…”

“…Effective groundwater management strategies are essential for open pit mines operating below the water table. Dewatering of open‐pit or underground mine workings is implemented to create dry conditions for mining operations (Rozkowski et al 2021; Cook et al 2022). This process creates a cone of depression, where water levels decrease around the mine, resembling the shape of an inverted cone.…”

“…MAR has also been used to maintain groundwater levels for sensitive ecosystems or third‐party users that may be impacted by reduced water levels following mine dewatering (e.g., Youngs and Brown 2005; Baquero et al 2016). More recently, injecting surplus water at a target distance from the pit during mining operations has been considered as a technique for accelerating groundwater recovery following mine closure (Cook et al 2022). Mine closure may require that pre‐ mining hydrological conditions are re‐established, including the recovery of groundwater levels.…”

“…However, depending on climate and hydrogeological conditions, a full recovery that relies on the naturally occurring rainfall and recharge conditions may take tens to hundreds of years, if it occurs at all. Through the controlled strategic injection of excess mine water into suitable sections of the subsurface during the operating phase of the mine, post-closure groundwater level recovery can be accelerated (Cook et al 2022). Water needs to be injected distant enough to prevent excessive re-circulation back to the pit, but close enough that water is stored within the drawdown cone to facilitate groundwater recovery once mining has ceased (Figure 1).…”