Acid mine drainage in the Iberian Pyrite Belt (Odiel river watershed, Huelva, SW Spain): Geochemistry, mineralogy and environmental implications

This work reports the physical properties and water chemistry of 64 AMD discharges from 25 different mines in the IPB draining to the Odiel river watershed, which have been investigated during the hydrologic year 2003-2004. These AMD solutions vary largely in flow rate and chemical composition both spatially (between the different mine sites, suggesting a strong geologic control on AMD chemistry) and seasonally (due to marked hydrologic variations), and include cases with very low pH (mostly in the range 1.4-4), and extreme sulphate (up to 44 g/L SO42-) and metal content (e.g., up to 7.7 g/L Fe, 2.6 g/L Al, or 1.4 g/L Zn). Different hydrogeochemical facies of AMD (namely, Fe(II)/anoxic, Fe(III)/suboxic, and aluminous/oxic) are recognized in the field, as a response to the continuous oxidation and hydrolysis of dissolved Fe. Relevant geochemical aspects of these AMD environments are discussed, including: (i) the redox chemistry of the Fe(II)/Fe(III) couple, (ii) the reaction rates for bacterially catalyzed oxidation of Fe(II) and hydrolysis of Fe(III), (iii) the role played by dissolved Fe and Al in the acidity and chemical buffering of the AMD systems, and (iv) the solubility and trace metal retention capacity of the Fe oxyhydroxysulphate and hydrated sulphate minerals commonly associated with AMD. In addition, the mineralogy and chemistry of the Fe precipitates (schwertmannite, jarosite, goethite, ferrihydrite), Al phases (e.g., basaluminite) and Mg-Fe-Al efflorescent SO4 salts (e.g., epsomite, hexahydrite, copiapite, halotrichite, rozenite, coquimbite) present in the AMD-generating mine sites, have also been studied. The mineralogy of the Fe precipitates is well correlated with the water pH (with jarosite at pH ∼ 2, schwertmannite at pH 2-4, basaluminite at pH 4-5, and ferrihydrite at pH > 6). Schwertmannite appears to be the most important mineral phase, both in controlling the Fe solubility at pH 2-4, and as sorbent of trace elements (As, Cu, Zn), which favours natural attenuation. Finally, a basin-scale environmental perspective is given in order to evaluate the impact of AMD on the water quality, including calculation of metal loadings transported by AMD from the most important mine districts in the province.