High-resolution study of pollutant mobility in an acid mine drainage-impacted estuary under changing tidal and river contributions

Abstract

The present study investigates the behaviour of acid mine drainage (AMD)-derived contaminants in the Tinto River estuary (SW Spain) using five high-resolution (hourly) fixed-point sampling campaigns conducted under contrasting hydrological and tidal regimes: low-riverflow neap and spring tides, and high-riverflow events following rainfall across different tidal coefficients. Water chemistry and mineral phases were characterised using established analytical techniques and electron microscopy, while aqueous speciation and mineral saturation indices were evaluated through PHREEQC geochemical modelling. The results indicate that Mn, Zn, Co, Ni and Cd exhibit quasi-conservative behaviour, with low partition coefficients (Kd) and predominance in the dissolved phase. In contrast, Fe and Al sequentially precipitate as schwertmannite and basaluminite, respectively, dominating the suspended particulate matter (SPM) and acting as sinks for other metal(oid)s (As, Pb and Cu). Arsenic is initially removed under acidic conditions and remobilized when pH exceeds 6 due to desorption from schwertmannite. Lead exhibited a singular response, with episodically high dissolved concentrations under specific high-flow conditions, likely linked to sulfate-mineral solubility and evaporite washout in the fluvial basin. Low-flow conditions, particularly during spring tides, and highly diluted high-flow events reduce metal mobility by enhancing partitioning into SPM and deposition in the upper estuary (except for As). Conversely, less diluted high-flow conditions promote downstream transport and export to the Atlantic Ocean. This work provides the first tide-resolved and hydrologically contrasted assessment of contaminant dynamics in an AMD-impacted estuary and offers a transferable framework for predicting metal(loid) fluxes to coastal systems under increasing hydrological variability.