Arsenic speciation in waters and sediment of ephemeral floodplain pools, Ríos Agrio–Guadiamar, Aznalcóllar, Spain

The April 1998 breaching of the Aznalcóllar tailings dam at Boliden Apirsaʹs Spanish Aznalcóllar/Los Frailes Ag–Cu–Pb–Zn mine resulted in the flooding of the Ríos Agrio and Guadiamar with arsenic (As)-rich tailings and water. Mineralogical and geochemical characterisation of water and sediment from five ephemeral pools on the Agrio–Guadiamar floodplain has been carried out to understand how the As is sequestered and to evaluate the long-term stability of the As-bearing phases. Dissolved As concentrations in pool waters range from an extreme of 85,000 μg l− 1 down to 5.5 μg l− 1. XANES, EXAFS, XRD, X-ray mapping, electron microprobe analysis and single chemical extractions suggest that As in the sediments is sequestered in sulphide (As0 in arsenopyrite and pyrite, coordinated by 1 S and 3 Fe) and in Fe oxide (AsV in arsenate, coordinated by 4 O, bound to Fe oxide and Fe sulphate, probably jarosite). The pools evolved as a result of the interaction of rainwater with the sediments, and As concentrations are controlled by evaporation and by the amount of tailings (and specifically, arsenopyrite) in the sediments that remained even after clean-up efforts. The arsenopyrite remaining in the floodplain sediment is potentially highly reactive under the prevailing pH and Eh conditions on the floodplain, and will probably continue to provide As to ephemeral pools and possibly, Agrio–Guadiamar river water.