The hydraulic and chemical evolution of groundwater-fed pit lakes following mine closure

Abstract

When an open pit mine closes, dewatering to keep the pit dry ceases, causing the water table to start recovering. In arid and semi-arid climates, hydraulic recovery is often predominantly governed by groundwater inflow into the former mine pit and evaporation from the developing pit lake. Low hydraulic conductivities and low hydraulic background gradients as well as high net evaporation rates can cause pit lakes to remain ‘terminal sinks’, i.e., groundwater enters the pit while outflow only occurs via evaporation. With time the ambient hydraulic gradient re-establishes and may cause pit lake water to exit into the adjacent aquifer, transforming the pit into a ‘throughflow system’. Due to prolonged residence times and evapoconcentration, the pit lake water quality may deteriorate as the salinity increases, a specific problem in arid and semi-arid regions, such as the extensive mining regions of Western Australia. Knowing whether a pit lake remains a terminal sink or transitions into throughflow systems is important for mine closure management as the pit lake water quality might decline to a lesser extent under throughflow conditions compared to terminal sinks as residence times of pit lake water decrease with higher outflow rates. Nevertheless, downgradient aquifers might be affected by exiting pit lake water in throughflow systems. Terminal sinks, however, may, over time affect local aquifer systems also: with increasing salinity due to evapoconcentration, the pit lake water becomes denser and may leak along the density gradient into the less dense groundwater. This work improves the basic understanding of processes that occur during the hydraulic and geochemical evolution of pit lakes after mine closure. The timeframe for transformation from terminal sink to throughflow system, the salinity concentrations in pit lakes and the shape of salinity plumes are investigated through numerical modelling. Furthermore, the impact of density-driven flow under varying environmental factors is explored. Opposed to what often is intuitively expected to happen in arid climates, it was found that the transformation from terminal sinks to throughflow systems happens frequently and quickly (< 20 years) after the mining operation ceases under a wide range of hydrogeological site conditions. This is while the groundwater heads are still recovering. The evolution of salinity in the evolving pit lake and surrounding aquifers is thereby largely dependent on the initial concentration of surrounding groundwater. When the initial concentrations are low, density-driven flow has no substantial effect over the simulation time of 500 years. However, when the initial groundwater concentration is higher (e.g., TDS > 1500 mg/l) the impact of density-driven flow is significant and higher concentrations in the pit lake see larger impacts on the adjacent aquifer. Forecasting the hydraulic and geochemical conditions of pit lakes post-mining is essential for mine closure planning and fundamental knowledge to determine any potential use of post-mining landscapes.