Geochemistry of natural wetlands in former uranium milling sites (eastern Germany) and implications for uranium retention

Discharge from former uranium mining and milling areas is a source of elevated uranium contents in wetlands worldwide. In this work, the efficiency of organic-rich wetland environments for entrapment and accumulation of uranium was assessed using hydrogeochemical field studies of natural small-sized wetlands in Thuringia and Saxony, Germany. The objective was to estimate if artificial wetlands can be used in a similar way: as a sustainable ‘passive’ treatment methodology. Worldwide, a dozen such systems have been implemented for uranium-bearing mine waters as experiments, primarily aiming at uranium reduction and precipitation.Pore water and solid phase samples were collected from the upper decimetres of substrate profiles in minerotrophic ‘volunteer’ wetlands and natural fen-type wetlands that facilitate uranium accumulation. Elemental analyses, correlation techniques and sequential chemical extraction were applied to evaluate retention mechanisms (e.g., reduction, mineral sorption). No process was dominant but the bulk of uranium is retained in moderately labile forms, predominantly as operationally defined organically bound or acid soluble (‘specifically adsorbed’) phases. Macrophyte intracellular uranium accumulation (‘phytoaccumulation’) is not responsible for the high uranium concentrations in the wetland substrates. Although there is no evidence for stable U(IV) mineralisation via ‘reductive precipitation’, high accumulation efficiency of the wetlands results from processes involving species regarded as more labile. According to the findings, the previous concept for treatment wetland that was commonly designed for uranium reduction needs modification.