Predictive geochemical modelling has been carried out for the Turquoise Ridge and Valmy pits, Getchell gold mine, Nevada (USA) as part of an updated evaluation of waste rock management. The purpose of the modelling exercise was to predict the concentrations of solutes that may emanate from the waste rock backfilled pits and to assess the impact of any seepage from the pits on underlying groundwater chemistry. The results of the geochemical modelling will be used to assess the level of cover required for the backfilled pits in order to ensure that no degradation of groundwater occurs.

The assessment of geochemical reactivity was undertaken using laboratory column leaching utilizing alternating wet and dry cycles to simulate accelerated weathering in humidity cell tests (HCTs). The chemistry of the leaching solutions was used to develop source term solutions that were scaled from laboratory to field conditions and used as inputs for the geochemical models.

Geochemical modelling was carried out using the USGS-developed software PHREEQC. Seepage water quality predictions were made based on the mass load mixing of waters contacting different lithologies in each of the backfilled pits. Water quality predictions were then made for a number of seepage scenarios, including 1%, 5%, 10%, 50% and 100% of mean annual precipitation that will infiltrate the backfill and report as seepage to groundwater.

The results of the modelling exercise demonstrate that the impact of the pits on the underlying groundwater is largely dependent on the amount of seepage through the backfill. In general, significant changes in groundwater chemistry were noted when there was greater than 10% seepage from both the Turquoise Ridge and Valmy pits. Under the 100% seepage scenario, groundwater is likely to be characterised by elevated concentrations of arsenic, selenium, sulphate and manganese. However, the impact to groundwater will be negligible if less than 10% seepage occurs. This can be attained through placement of a thick alluvium cover on the backfill material and re-contouring of the area to natural topography.

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