Effects of weathering on metal releases from an engineered deposit for alum shale leaching residues

A pile of ca 15,000 t of crushed alum shale leaching residues from uranium refining was deposited in Ranstad, Sweden, in 1972 as a part of a pilot study of various waste storage concepts. A field study has been performed in order to evaluate the efficiency of the engineered barriers (bentoniteltill or crushed limestone) for the prevention of weathering of the leaching residues and subsequent release of metal rich leachates. The concentration levels of Fe (from pyrite in the alum shale) as well as metals associated with the pyrite (e.g. Cu and Ni) indicate that the weathering of leaching residues underneath the cover has progressed to less than 4 cm in 15 yr. No breakthrough of precipitation is indicated from the composition of the percolation water coming from the deposit. Thus, the weathering rate is reduced by ca 3 orders of magnitude in comparison with leaching residues freely exposed to air and precipitation.