Groundwater-level recovery following closure of open-pit mines

Open-pit mining has increased substantially over the past two decades. Many currently operating open-pit mines are facing the end of mine-life over the next few decades and, increasingly, focus is shifting towards mine-closure planning that provides evidence on available closure options under the given geological, hydro(geo)logical and climatic conditions. This study uses synthetic groundwater modelling to build basic process understanding of closure options and how these will determine the formation of pit lakes. This governs the long-term pit lake water quality and how postmining landscapes may be utilised. Simulations show that the recovery time of postmining groundwater levels increases with decreasing aquifer transmissivity. Final postmining water tables are predominantly controlled by the implemented mine closure options and climatic conditions. The most important decision is, thereby, whether to backfill the pit to above the water table or allow a pit lake to develop. Under moderately transmissive aquifer settings, backfilling of pits leads to rapidly rising groundwater levels within the first decade after mining, with water-table recoveries of above 70%. If mine voids remain unfilled, evaporation from the pit lake surface becomes a governing factor in determining whether the unfilled mine pit becomes a terminal sink for groundwater. Lake levels may remain subdued by several 10s of metres in arid to semiarid climates. If surplus surface water can be diverted into open pits, rapid filling can accelerate groundwater recovery of open pits in regions of low permeability. This is a less successful management option in transmissive aquifers. L’exploitation minière à ciel ouvert s’est accrue considérablement au cours des deux décennies écoulées. Beaucoup de mines à ciel ouvert opérationnelles à l’heure actuelle sont confrontées à une fin de vie dans les quelques décennies prochaines et l’attention se porte de plus en plus sur la planification de leur fermeture, qui fournit des preuves sur les options de fermeture disponibles dans des conditions géologiques, hydro(géo)logiques et climatiques données. Cette étude utilise une modélisation synthétique des eaux souterraines pour acquérir une compréhension de base des méthodes relatives aux options de fermeture et de la manière dont celles-ci détermineront la formation des lacs de mine. Ceci détermine la qualité de l’eau du lac de la mine sur le long terme et la façon dont les paysages de l’après-mine peuvent êt