Rainfall-driven, high-resolution assessment of water acidification in a planned reservoir within an unreclaimed historical mining basin.

Acid mine drainage (AMD) from historical mining in the Iberian Pyrite Belt severely degrades water quality in southwest Spain. The planned Alcolea Reservoir, intended to supply water from the Odiel River basin in response to growing demands, is widely recognized as being critically constrained by AMD, as existing studies indicate that, without upstream remediation, its impoundment would lead to inevitable deterioration and acidification of water quality. In this sense, a high-resolution sampling campaign was conducted over a hydrological year in the Odiel and Oraque rivers to quantify pollutant fluxes entering the reservoir and to assess the potential risk of water acidification. Despite dilution- and neutralization-driven fluctuations, the inflows showed mostly persistent acidic conditions and high metal concentrations during the monitored year. Floods were the main driver of pollutant transport, with annual net acidity and sulfate loads about 31247 tons eq. CaCO3 and 73598 tons, respectively. During the autumn floods, the highest amounts of acidity and metals were released due to flush-out processes (i.e., redissolution of evaporitic sulfate salts and mobilization of sulfide oxidation products). Estimated reservoir water quality (pH 3.71; 131 mg/L eq. CaCO3 of acidity; and 307 mg/L of sulfate) mostly fails to comply with water guidelines, showing conditions similar to the nearby acidified Sancho Reservoir. In fact, the planned reservoir would receive more (∼24.0%) acidity load than the Sancho Reservoir, proving its potential acidification after construction. Due to the severe pollution by AMD, decision makers should prioritize reclamation of the Odiel River basin before completing the Alcolea Reservoir, ensuring adequate water quality for end users.