Hydrochemical patterns of a small lake and a stream in an uplifting area proposed as a repository site for spent nuclear fuel, Forsmark, Sweden

SummaryThe overall aim of this study was to increase the understanding of the chemical dynamics of small catchments. The focus was on a small oligotropic lake and its major inflow stream in an uplifting area in eastern Sweden (Forsmark) proposed as a repository site for spent nuclear fuel. The hydrochemical sampling campaign lasted for nearly 4 years with sample collection monthly to semi-monthly, and continuous flow measurements carried out over the last 20 months. All this was done as part of the Swedish Nuclear Fuel and Waste Management Company’s (SKBs) Site Investigation Programme. The major findings were: (1) as a result of the calcareous overburden caused by redistributed Paleozoic deposits, pH and the Ca and HCO3- concentrations were relatively high in both the stream and lake throughout the period, (2) limnic primary production resulted in decreased concentrations of Ca, HCO3-, NH4+, NO3- and Si, and increased pH and concentrations of chlorophyll a, O2, DON, POC, PON and POP in the lake in summer, while in other seasons (in winter in particular) when the production was minimal or non-existent the concentrations in the lake and the inflow stream were similar, (3) intrusion of brackish-water resulted in moderately to strongly increased concentrations of Cl−, Na, Mg, Br−, SO42-, K and Sr in the lake: the ratio versus Cl− were for Na and Br− always similar to those in sea water, for Mg and SO42- similar to those in sea water at elevated Cl− concentrations (>3 mM), while K and Sr always occurred in relative excess as compared to sea water, (4) high U concentrations in both the stream and the lake was derived most likely from reduced U-minerals in the overburden and was predicted to be carried to >90% in the form of calcium uranyl carbonate, in a model in which colloidal Fe and Al oxyhydroxides were not considered, (5) the rare earth elements (REEs) had similar concentrations and fractionation patterns in the stream and lake, unlike those found in the overburden groundwaters, and was predicted to be carried as organic complexes, (6) in terms of the area being chosen as a site for bedrock disposal of spent nuclear fuel, of particular importance is further investigations into the sources and mechanisms of the high U concentrations in the surface waters and confirmation of the importance of the calcium uranyl carbonate complex in the surface transport of this metal, and (7) the abundance of dissolved humic acids, predicted to carry the REEs, which consist of analogues for several actinides, deserve special attention in a safety assessment perspective focusing on radionuclide contamination.