Soils affected by acid mine waters in Galicia (N.W. Spain)

Soils and surface waters contaminated by copper mining in Galicia, Spain, were acidic and contained high concentrations of sulphate, aluminium, iron, calcium, magnesium, potassium, manganese, copper and zinc. Activity data were applied to mineral equilibrium diagrams. Solutions extracted from soils around slag heaps reflected contamination of run-off waters when the solid phase had little or no buffering capacity. This was attributed to the short residence time and a real decrease in the acid buffering capacity of the soils. Mineral neoformation processes were modified with respect to normal processes in uncontaminated environments. At pH less than 4.0, the only stable minerals were aluminium sulphate (jurbanite or alunite), quartz and chalcedony. a sulphate mineral of aluminium controlled the concentration of aluminium and the activity of dissolved aluminium(III). At pH greater than 5, aluminium concentration was controlled by microcrystalline gibbsite. Kaolinite was the most stable mineral. Between pH 4 and 5, there was an intermediate situation with saturation of kaolinite, gibbsite, smectites and sulphates such as alunite, Differences between fluid phases were related to their origin.