Scoping level post-closure pit water quality predictions were prepared for an Australian iron ore operation. Conceptual models of the expected controls on the water balance and water quality were developed for a final pit lake.
The water balance calculations for the pit indicated that the lake would remain a permanent water sink, with water elevations remaining below the regional water table. As such, the water would become progressively more saline over time. Due to significant alkalinity in the inflowing groundwater, acidic conditions, should they develop, were predicted to be transient.
Flooding that occurred during a cyclonic event during a period when mining in the pit was suspended (though dewatering was maintained) allowed an opportunity to assess the validity of the predictive modelling. The large cyclonic event resulted in a pulse of acid water to the pit void. Dewatering was interrupted and, within a short time, groundwater influx resulted in the pit lake pH rapidly recovering to alkaline values. Water level and water quality were monitored over time until dewatering could recommence. The data collected during this event have been to test some of the pit lake water quality predictions.
This case study describes the overall assessment approach and includes comparisons between modelled and measured data.
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