Tracing acid mine drainage and estuarine Zn attenuation using Cd and Zn isotopes

It has been estimated that the acid mine drainage (AMD) impacted Odiel river basin in southern Spain supplies 0.37% and 15% of the global riverine fluxes of Cd and Zn to the oceans, respectively (Sarmiento et al., 2009). However, the behaviour of Cd and Zn in the Ria of Huelva estuary, which connects the Odiel and Tinto wa-tersheds with the Gulf of Cadiz, has yet to be fully investigated. Furthermore, very few studies have investigated Cd and Zn isotope behaviour in estuaries worldwide. This study presents Cd and Zn concentrations and isotopic compositions for the Ria of Huelva estuary and surrounding watersheds, sampled in 2017 and 2019. Sulfide-rich rock samples extracted from three mines yield Cd and Zn isotope compositions that range from -0.14% to +0.07% (n = 4) for delta 114Cd and -0.01% to +0.29% (n = 4) for delta 66Zn. However, a uniform riverine signal of about +0.02% for Cd and +0.17% for Zn indicates that tracing of individual mining regions using Cd and Zn isotopes is challenging. Limited variability was observed in dissolved delta 114Cd values throughout the watershed, including AMD, the estuary, and the Gulf of Cadiz, with a mean value of +/- 0.00 +/- 0.13% (n = 25, 2 SD; excludes one AMD outlier, at +0.48%), including both 2017 and 2019 data. By contrast, delta 66Zn values ranged from -0.12% to +0.35% (n = 28) for the same geographical and temporal scope. In May 2017, a large spill from an abandoned mine, La Zarza, resulted in a drastic increase in the concentrations of trace metals reaching the es-tuary compared to 2019, but no impact of this mine spill on Cd or Zn isotope compositions is observed. In 2019, an increase in delta 66Zn values in the estuary coincided with high pH values (up to pH = 8.8) and chloride con-centrations (2.73%), which may reflect an alkaline anthropogenic input from the active neighbouring industrial complex. Overall, Cd concentrations and isotope compositions in the estuary are largely consistent with con-servative mixing behaviour. By contrast, Zn behaviour is non-conservative, with removal of 49-97% of dissolved riverine Zn in the estuary during the period 2017-2019, associated with a relatively small isotopic shift to lighter Zn isotope compositions. Removal of Zn to the particulate phase in the Ria of Huelva estuary therefore largely attenuates high riverine Zn fluxes from AMD, indicating that previously estimated Odiel river basin Zn fluxes were overestimated. Nevertheless, the variable but generally light AMD Cd and Zn isotope compositions, coupled with high dissolved metal concentrations, suggest that Cd and Zn isotopes may be useful tracers of regionally averaged AMD inputs to the Gulf of Cadiz and beyond.