Abstract
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.
Highlights
Mining can have severe impacts on the natural environment in the vicinity of a mineral resource (Bradshaw and Chadwick 1980; Cooke and Johnson 2002), including the contamination of local streams with acidic mine waters (Lottermoser 2003)
20.1 near neutral in Tunakohoia Stream (Table 1) and the addition of lime in Adit 5 has only had a small effect on pH and little effect on electrical conductivity (EC) or sulfate concentrations when compared with data of Sharplin (2008)
Temperature and dissolved oxygen (DO) levels are sufficient to support a healthy ecology, with the possible exception of the elevated temperature at Tuna 5, a shallow section of this stream where it passes through open farm land
Summary
Mining can have severe impacts on the natural environment in the vicinity of a mineral resource (Bradshaw and Chadwick 1980; Cooke and Johnson 2002), including the contamination of local streams with acidic mine waters (Lottermoser 2003). Formed Fe-, Mn- and Al (oxy)hydroxides readily adsorb many trace elements (Johnson and Hallberg 2005; Watzlaf et al 2004). Under favorable conditions, such (oxy) hydroxides will coagulate and settle into the sediment,. Spatial or temporal changes in pH, redox conditions, and water chemistry influence trace element adsorption and mineral precipitation processes. PH and redox conditions when the oxide precipitate is forming can affect how effectively trace elements are scavenged from solution (Dale et al 2015)
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