Extensive dispersion of metals from hemiboreal acid sulfate soil into adjacent drain and wetland

Abstract

Extensive red/brown precipitates of unknown origin and composition have caused ecological degradation of a wetland nature reserve (the Water Kingdom Biosphere Reserve) in the hemiboreal zone in south Sweden. Chemical analyses of samples containing the precipitates showed strong dominates of Fe and elevated levels of rare earth elements (REEs), Be, and U. In addition, synchrotron-based analyses indicated that the Fe in these precipitates was bound largely in akageneite and/or schwertmannite. Under nearby farmlands, acid sulfate soils, developing on sulfide-bearing sediments and notorious for abundant export of metals, were identified and found to be widespread, deep (down to the sampling depth of 180 cm or deeper), and very acidic (minimum pH range for soil profiles: 2.8–3.5). In-between the farmland and wetland was a central drain that can act as both a transporter and sink of elements leached from the acid sulfate soils. In the drain had accumulated sediments that had strongly elevated concentrations of Al (15%), ∑REE (2725 mg/kg), Be (15 mg/kg), and U (37 mg/kg). Based on these data and features, a conceptual model for the areas was proposed. The acid sulfate soil releases several major and trace elements, including Fe2+, which are transported in acidic waters via drainpipes to the central drain where pH increases, causing extensive precipitation of Al, REEs, Be, and U as well as Fe2+ oxidation and formation of Fe oxyhydroxides and oxyhydroxysulfates. A substantial part of the Fe2+ in the drain water, however, remains in solution, so when this water is ultimately pumped to the wetland, large amounts of Fe2+ together with significant amounts of Al, REEs, Be, and U and transported to the wetland where Fe2+ is finally oxidized, precipitated and retained. Yet several other metals, leached abundantly from the acid sulfate soils (Mn, Zn, Ni, Co, and Cd), were not found in elevated levels in any of the recipients and therefore most likely have been transported beyond our sampling sites and has thus reached further out in the ecologically sensitive wetland.