Hydrogeochemical characteristics of the Tinto and Odiel Rivers (SW Spain). Factors controlling metal contents

The Tinto and Odiel Rivers are strongly affected by acid mine drainage (AMD) due to the intense sulphide mining developed in their basins over the past 5000 years. This study aims to analyze the hydrogeochemical characteristics and the factors controlling metal contents in both rivers through a comprehensive evaluation of weekly sampling conducted over three and a half years before reaching the Ría of Huelva. The findings reveal that concentrations of sulphates, Al, Cd, Co, Li, and Zn in the Tinto River are significantly higher compared to the Odiel River, attributed to lower dilution effects. Conversely, the concentration of Fe in the Odiel River is notably lower due to the more intense precipitation of Fe oxyhydroxysulphates from neutralizing processes. Other elements, including As, Cr, Cu, and Pb, showed lower concentrations in the Odiel River, influenced by sorption and co-precipitation with Fe. Seasonal trends in metal concentration were observed, peaking in autumn as evaporitic salts dissolved, while the Tinto River exhibited a seasonal increase in Ca, Na, and Sr during summer, likely linked to interactions with marly materials. Different behaviors of barium, Fe, As, and Pb were noted, suggesting complex interactions influencing metal solubility in each river.