Persistent Acid Mine Drainage and Iron-Associated Arsenic Storage at the Pintor Mine, Portugal: Historical Evidence for Rehabilitation

Acid mine drainage (AMD) produces iron (Fe)-rich precipitates that retain arsenic (As), forming contaminant stores susceptible to disturbance during mine rehabilitation. We re-examined aqueous As partitioning and solid-phase geochemical evidence from a 1996–1998 investigation at the abandoned Pintor W–As mine, Portugal. During two 1996 sampling campaigns, adit drainage had pH 3.1–3.9 and operationally dissolved As concentrations of 1126–2208 µg L−1. These concentrations decreased by 99.5–99.8% over 1.8 km, but corresponding changes in As loads could not be determined without concurrent flow measurements. Streambed coatings contained 2.9–4.0 times more As than paired sediments, and an Fe-rich outlet crust had 3.85% aqua regia-extractable As. Dark-oxalate extraction accounted for 48–82% of recovered As across the analysed solids. The concentration gradient coincided with As enrichment in Fe-rich deposits, consistent with Fe-associated retention. Later investigations commissioned by Empresa de Desenvolvimento Mineiro, S.A. (EDM) confirmed that acidic, As-bearing drainage remained present in 2007 and 2022–2024. The historical evidence identifies priorities for pre-intervention characterisation of As-bearing deposits, monitoring particulate As export during excavation, evaluating dissolved and particulate As loads through the proposed treatment system, and assessing As accumulation in channel deposits and treatment residues.