Temporal evolution of hydrochemical changes in an acid mine drainage–impacted river using SWAT modeling and concentration–discharge relationships

Abstract

The Tinto River is heavily polluted by historical sulfide mining, showing acidic conditions and extremely high concentrations of metals and metalloids along its 101 km course. The Riotinto mine closed in 2000 and was reopened in 2015, with a commitment to reduce acidic discharges from legacy mining wastes. This study assesses the effect of the 2015 reopening on pH and dissolved sulfate, metal and metalloid concentrations in the Tinto River, using a 2008–2021 dataset from a monitoring point 13 km downstream of the main mining area. Because no gauging station exists at this site, the Soil and Water Assessment Tool (SWAT) was applied to reconstruct continuous daily discharge and support concentration–discharge (C-Q) analyses. The pH remained consistently low (≈2.5), whereas dissolved species varied widely, with median concentrations of 3.45 g/L sulfate, 834 mg/L Fe and 59 mg/L Zn. The 2015–2021 period after the mine reopening was clearly drier than previous years. This should have produced an increased in dissolved concentrations through time. Nevertheless, the dissolved concentration of most elements showed slight downward trends, indicating reduced pollutant inputs from the mining areas, whereas Pb remained unchanged and Fe and As showed slight upward trends. For most elements concentration-discharge (CQ) relationships showed a dilution pattern with lower concentrations during 2015–2021, also suggesting improved water quality. Fe, As, and Pb showed the most complex behavior, as their concentrations are controlled by precipitation and coprecipitation processes. These results demonstrate that integrating hydrological modelling with long-term hydrochemistry datasets provides a robust framework to assess remediation effectiveness while accounting for the variability of climatic regimes.