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

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 mine closed in 2000, but after the recovery of the copper price in international markets, it reopened in 2015, with the commitment to eliminate part of the acid water discharges from the waste generated by old mining. In this paper, we analyze the effect of the reopening of the Riotinto mine in 2015 on the concentrations of dissolved elements in the water of the Tinto River based on hydrochemical data obtained from 2008 to 2021. The SWAT model was used to generate stream discharge data at the hydrochemical sampling point. The pH at the control point remained very constant at around 2.5, while dissolved concentrations were very variable, with median values of 3.45 g/L of sulfate, 834 mg/L of Fe or 59 mg/L of Zn. The effect of mine reopening was not evident. There was a slight downward trend in concentrations for most elements, while Pb concentrations remained unchanged and Fe and As showed a slight upward trend. However, the 2015-2021 period was clearly drier than the previous one, which suggests improved conditions and a lower contribution of pollutants from the mining area, as concentrations should have increased during a dry period. Concentration-discharge (CQ) relationships were statistically significant and, except for Pb, showed a clear dilution pattern, with lower values for the period 2015-2021, confirming the improvement of the water quality from 2015 onwards. Fe, As and Pb, showed the most complex behavior as they are affected by precipitation/coprecipitation processes. These results demonstrate that combining hydrological modelling with long-term hydrochemistry is effective for evaluating remediation under highly variable climatic regimes.