“黄河”入海流 | 矿山开采活动流域留下的水文地球化学遗迹

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.

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Outlook and future work The Tararu Stream exhibits an unusual degree of hydrochemical and isotopic alteration. While this study suggests that dissolved metal concentrations are moderate by comparison to AMD-impacted streams in other settings ( Winterbourn et al., 2000 ), we suggest that the degree of isotopic alteration points to a significant burden of tailings within the catchments’ steep hillsides, but this needs to be confirmed by a more extensive, longer-term study of isotope anomalies in the watershed. Given the propensity of the Tararu (and its northern siblings) to debris flow events ( Longstaff 2014 ) and the rising incidence of extreme rainfall events in Aotearoa-NZ's warming climate ( Reid et al., 2021 ; Shu et al., 2021 ), further investigations into the distribution and stability of extant tailings in the Tararu and comparable catchments along the western seaboard of the Coromandel Peninsula are also recommended. In addition to a more comprehensive geochemical and isotopic analysis of stream and groundwater samples and an assessment of tailings mineralogy, the analysis of sulfur and oxygen isotopes in dissolved sulfate would greatly enhance delineation of sulfur sources within the catchment ( Mayer et al., 1995 , 2010 ; Migaszewski et al., 2018 ).

Stream water stable isotope ratio analysis (δ18O vs δ2H) revealed a pronounced degree of water-rock interaction in the Tararu watershed, consistent with the oxidative weathering of sulfides in extant mine waste and oxygen isotope exchange (18O) with secondary, likely carbonate, mineral phases. This interpretation is also consistent with the generally well-buffered conditions (pH > 7.5) in groundwater contributing to Tararu Stream baseflow in spring/summer. Outside of localised acidification and high metal loadings in one tributary, surface water acidification was not pervasive. However, elevated metal concentrations are of concern for freshwater ecology. Of greatest concern is extensive streambank erosion and the propensity of this catchment for slips. Further studies of tailing stability are therefore warranted to understand the potential for mass flow events to carry tailings into Tikapa Moana-o-Hauraki, the Firth of Thames.