Importance of lacustrine physical factors for the distribution of anthropogenic 60Co in Lake Biel
Limnol. Oceanogr., 44(1), 1999, 196-206 | DOI: 10.4319/lo.1999.44.1.0196
ABSTRACT: The fate of particle reactive substances in a surface aquatic system is controlled by both geochemical and physical parameters. Scavenging processes in Lake Biel (Switzerland) have been studied using the anthropogenic tracer 60Co. This isotope originates mostly in dissolved form from the Muhleberg nuclear power plant and is transported 18 km down by the River Aare to the lake. Radionuclide data from sediments, combined with inventory calculations, indicate that between 30% and 55% of the discharged 60Co is transferred to sediments. Scavenging efficiency is higher during the winter period than during summer, in spite of higher particle fluxes during the summer. An intrusion model, based on buoyancy calculations, predicts that Aare water, including its radionuclide load, remains in the epilimnion during the warm period; thus, mixing is restricted to only a fraction of the lake volume and the residence time is reduced to 1 week. Buoyancy in the summer is controlled mostly by temperature. During winter, when the lake and Aare waters are close to 4oC, the higher dissolved load of Aare water increases its density and forces it to intrude into deeper sections of the lake, increasing the residence time and thus the time available for adsorption and particle settling. The predictive capability of the model was tested for the summer period using a fluorescent dye (uranin) to track Aare water from the inflow to the outflow of the lake. Higher fluxes of both particles and associated radionuclides in deep sediment traps confirm the results of the intrusion model for the winter period. Comparison between modeled and measured 60Co activities in lake water at the end of the winter validates the credibility of the intrusion model further. This combined geochemical and physical approach may be applied to transport modeling of radionuclides and of any particle reactive substance, inorganic or organic.