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Pit Lake Model Validation – Understanding Impact of Mine Dewatering
Issue
Enpoint was engaged to validate modelled predictions on the dewatering radius of influence, pit lake recovery and pit lake salinity. The key outcomes of the models were as follows:
- Pit inflow was estimated to be < 1kL/day (10-5 m/day) to 100 kL/day (10-3 m/day) during pit dewatering.
- Pit water levels would recover to an elevation of between ~310 mAHD to ~320 mAHD within a period of 7 to 11 years. The pit lake was therefore predicted to become a groundwater ‘sink’.
- Salinity of pit water was predicted to increase over a 100-year period to 55,000 mg/l (low K scenario) and 8,600 mg/l (high K scenario).
The objective of the assessment was to review measured groundwater elevations, pit water elevations and salinity to validate the model predictions of for the pit lake.
The Enpoint Solution
To validate the modelled predictions, Enpoint completed the following:
- A comparison of the observed to predicted head data to estimate the actual radius of influence from dewatering;
- An assessment of the changes in pit lake water levels, with inference to groundwater level, during recovery; and
- An assessment of salinity concentration estimates following pit lake recovery.
The outcomes
In summary, the validation of the initial model indicated the following:
- The pit lake recovered to a level comparable to the local water table elevation within approximately 2 years of mine completion. The pit lake was therefore deemed to be a surface expression of the water table, which is in direct hydraulic connection with the superficial aquifer and considered a flow through system rather than a sink.
- The pit lake salinity is freshening with time rather than becoming more saline as predicted by the model.