At the former Daisy mine gold heap leach operations in Nevada, long term solution draindown is c. 0.25 m3/day (0.045 gpm) and hosts arsenic concentrations in the order of 0.27 – 0.61 mg/L. This draindown solution is infiltrated into the ground where it eventually contacts groundwater. To assess the potential for natural attenuation of arsenic in the subsurface and to evaluate the interaction of the draindown solutions with groundwater, a series of predictive geochemical scenarios were undertaken using PHREEQC. These scenarios included varying the draindown rate to assess how this affects groundwater chemistry and varying the proportion of reactive sorbent mineral phases in the sub-surface to mimic the range observed in soil mineralogy.

The results of the geochemical predictions demonstrate that arsenic is effectively removed from draindown solutions by adsorption onto soil particles with the net effect that groundwater arsenic concentrations are not increased above baseline levels. The results denote a mass solubility control on solute concentrations during the interaction of heap draindown with groundwater without significant mineral precipitation. Draindown rates measured for the Daisy Heap continue to decline every year and are approaching steady state conditions. Therefore, at the current negligible draindown rates, this geochemical assessment indicates that the interaction of Daisy Heap draindown with groundwater will not alter baseline groundwater chemistry in the vicinity of the operations.

Supplementary material: PHREEQC files that support the modelling are available at https://doi.org/10.6084/m9.figshare.c.3587243

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