In mining areas, residues of mineral processing are deposited as waste dump or tailings. These mining site materials are often polluted with arsenic (As) and heavy metals. Due to internal dissolution and exposure to the weather microbial and abiotic oxidation processes begin and the oxidation state of arsenic, metals and sulphur change. As a result the material turns acidic and the contaminants can be easily mobilized [HIRNER ET AL 2000). These materials therefore act as a source of contaminants, which may pollute especially ground and surface waters.

Important factors controlling the mobility of As in the source material include the pH value, redox conditions and the characteristics of the source material [MASSCHELEYN ET AL 1991, IRGOLIC 1994, BERGFELDT 1995, HIRNER ET AL. 2000, HAURY 2001). Although of great environmental importance, little is known about the role of pH-redox conditions in the mobility and speciation of As in mine waste materials. Moreover, there are disagreements in the literature data.

The aim of this study is to clarify the effects of different pH and redox conditions on the mobility and speciation of As by using a tailing waste from former tin-ore processing in Gottesberg (Erzgebirge, Germany). These flotation residues (approx. 400,000 t) are poor in sulphur compounds but highly polluted by As. The As concentration in the solid ranges from 500-1 ,500 mg/kg, and As is released into the ground and surface waters in concentrations up to 1 mg/I.