Studie och riskanalys av sulfidleror i Uppsala stad
Most of the city of Uppsala rests on postglacial clay with a varying content of sulphur. The sulphur occurs naturally in the clay in reduced form as sulphide and the clay is for that reason usually called ?sulphide clay?. Excavation during construction activities gives rise to large amounts of excavation material, of which the sulphide clay is a considerable part.When the clay is exposed to air and water, an oxidation of the sulphides occurs which may produce an acid leachate and the mobilisation of heavy metals bound in the clay or underlying material. The aim of the work has been to improve our understanding of the behaviour of sulphide clays and produce a basis for risk assessment in the future handling of excavation material with high sulphide content. After sampling had been carried out in two places, batch tests were performed to examine the long-term acidification potential of clays with different calcium carbonate content. The tests were performed with continuous air supply and during the experiment leachate water was analysed with respect to pH, alkalinity, dissolved sulphate and mobilised metals. The results from the laboratory study showed that a clay with a high calcium carbonate content and thus a high net neutralisation potential (NNP) will consume the generated acid and prevent against a lowering of the pH.In clay lacking calcium carbonate, the pH dropped significantly and caused a mobilisation of chiefly Cd, Mn, Co, Ni, Ca and As. Regardless of the changes in pH over time, a large production of sulphur was observed as a result of the sulphide oxidation. In a risk assessment, it is suggested that the NNP should be determined by methods agreed upon to facilitate future comparisons and because different methods may give different results. A clay with a NNP<5 kg CaCO3/ton should be further analysed with respect to metal content to establish the leachate generation potential and estimate the future environmental influence of the excavation material.