Geochemistry and partitioning of trace metals in acid sulphate soils in Sweden and Finland before and after sulphide oxidation

Abstract

Acid sulphate soils (ASS) are formed on sulphidic sediments along the Baltic coasts of Sweden and Finland. The sulphides in the fine-grained sediments were once formed in an anoxic environment some centimetres below an oxic sediment–water interface. In earlier investigations, high concentrations of many trace elements have been observed in streams draining such areas. We compared the trace element contents in soils formed in sulphidic and non-sulphidic clays. The results show that the total contents of Cd, Ni, Mn, Co and Cu in the sulphidic sediments are not higher than in non-sulphidic clays from adjacent areas. However, the mobilisation of Cd and Ni is higher from acid sulphate soils than from soils formed on sulphide-free clays. To understand the processes causing the mobilisation, we studied the partitioning of trace metals before and after oxidation of sulphidic sediments. Cadmium, Ni, Mn, Co, Zn, and to some extent Cu, were found to be mobilised from acid sulphate soils due to increased weathering caused by the low pH in these soils. Iron and Cr are only mobilised to a limited extent. Copper, Mo and Fe mobilised from weathering are precipitated in oxides that form in the oxidised Bg-horizons. Nickel, Co, Mn, Mo and Cu were shown to be partly bound in pyrite and are mobilised from acid sulphate soils during the pyrite oxidation. However, mobilisation of trace elements from pyrite was not found to be an important quantitative contributor to trace element leaching. Any elements that are mobilised are likely to be transported down through the characteristic cracks, which occur frequently in the oxidised horizons. Evidence of this was found at the groundwater table, where the most easily extractable fraction of mobilisable elements occurred at higher concentrations.