Predicting pit lake chemistry after closure is integral for demonstrating compliance during the Environmental Impact Assessment and mine permitting process. Water quality predictions rely on multidisciplinary information from geochemists, ecologists, hydrologists, hydrogeologists and mine engineering teams to assess potential ecological risks and identify options for closure, mitigation and reclamation.
Pit lake chemistry depends on multiple factors, including geology, mineralogy, climate, and hydrogeology. Despite using state-of-the-art numerical modelling, predicting future pit lake chemistry is often a ‘best estimate’ based on the information and tools available. Furthermore, it may not be possible to verify these estimates until decades later once a pit lake has formed. To develop more accurate predictions, SRK uses water quality, limnology and mineralogy data from analogue pit lakes, where available, to refine geochemical models.
This method was successfully applied at the Copper Flat project in southern New Mexico. The project is a porphyry coppermolybdenum deposit that was mined briefly in the early 1980s before low metal prices halted mining operations. New Mexico Copper Corporation is currently undergoing permitting activities to re-open and expand the project facilities. To support the environmental studies, SRK oversaw an extensive geochemical characterisation program, including developing a predictive geochemical model to assess future pit lake chemistry.
The existing pit lake provides a valuable analogue of likely future chemistry and characterisation of precipitated mineral salts along the lake shore allows for a more detailed understanding of mineral precipitation and adsorption processes.
Calculated estimates of pit lake chemistry were developed from the results of humidity cell tests, coupled with mine plan, geologic, mineralogic, hydrogeologic, climate and hydrogeochemical information and applied in the USGS software
PHREEQC. The model was calibrated to the existing pit lake to account for all active geochemical processes. Good calibration was achieved, with predicted concentrations for most constituents being within the range of chemistry measured in the existing pit lake. This verifies the modelling method and demonstrates that future pit lake chemistry can be predicted with a good degree of accuracy.
Although the Copper Flat project is unusual in that it has an existing pit lake for comparison, published data from analogue pit lakes in a similar geologic and climatic setting can be used to indicate the expected range of pit lake chemistry for projects without a site-specific analogue. Pit lake geochemical predictions can thus be refined, allowing appropriate management and mitigation options to be identified to prevent impacts to human, environmental and ecological receptors.
