Testing Geochemical Predictions of Trace Element Toxicity and Bioavailability at a Rehabilitated Mine Site

The speciation, toxicity and bioavailability of trace elements in mine drainage environments can be readily predicted using geochemical modelling, and this is frequently the basis for assessing the likely impacts of mine effluents and efficacy of rehabilitation plans. However, such predictions are rarely validated against observed trace element characteristics after mine rehabilitation is complete. In this study of a former Pb-Zn mine in New Zealand, PHREEQC was used to predict dissolved trace element and sediment-bound speciation for the rehabilitated mine site, and the results were compared to the observed water and sediment quality. For Fe, Mn, Al, Cu, Pb, Zn, Cd, Ni, As, and Sb, it was predicted that only Zn2+ and Cd2+ concentrations would exceed recommended guideline values for ecosystem health. PHREEQC indicated that the pH would have to be raised to > 9.5 to reduce these toxicants to a level fit for ecosystem health. Modelling of potential mineral formation indicated that the waters were saturated with respect to a variety of Fe-, Mn- and Al (oxy)hydroxides at and immediately downstream of the mine site, but were not saturated with respect to any trace element-bearing minerals, or sulfide or carbonate phases. This was consistent with X-ray diffraction and scanning electron microscopy (SEM) observations of the sediment. Sequential extraction of the sediment showed strong associations of Zn, Cu, Pb, As and Sb with iron (oxy)hydroxides. Modelling trace element adsorption onto only hydrous ferric oxide surfaces accurately predicted the adsorption of Zn, Cd, Cu, and Ni, but predictions of Pb and As adsorption were less reliable. Additionally, a strong association between Zn and Mn oxyhydroxide was observed in SEM analysis.