Mines to moana: Hydrochemical legacies in a historically mined watershed

Abstract

This study investigates contemporary freshwater contamination originating from old mine workings in the Thames-Coromandel region of Te Ika-a-Māui (North Island), Aotearoa New Zealand. We employed a multi-method approach, including stable H and O isotope determinations of surface water, geochemical modelling, and diffusive gradients in thin films (DGT) deployments. Atypically, spring-summer (Sept–Nov) surface water stable oxygen isotope ratios (δ¹⁸O) were negatively displaced from meteoric values (ca. −0.28 ± 0.13 ‰), indicating a remarkable degree of low-temperature water-rock interaction, likely enhanced by dry antecedent conditions. Isotope ratios in tributaries showed less alteration than the main stem, suggesting greater meteoric inputs and shorter residence times in areas of high topographic relief. Conversely, in the main stem, isotope ratios revealed higher mineral weathering, accompanied by elevated dissolved metal concentrations, consistent with dominant inputs from shallow groundwater. Weathering of primary sulfides contributed pronounced acidity in one tributary (pH ca. 3.8), but overall, carbonate buffering ameliorated acid mine drainage across the catchment (pH ca. 7–8). Nevertheless, our results confirm exceedances of ecological guideline values (>80% protection threshold) for several toxic metals including Al, Zn, Cd and Pb; with implications for fresh and coastal water quality in Tikapa Moana-o-Hauraki, the Firth of Thames.