Epilimnetic scavenging of Chernobyl radionuclides in Lake Constance

Radioactive debris from the Chernobyl reactor accident entered Lake Constance in southwestern Germany mainly through one rainfall episode on April 30, 1986. Nuclides scavenged by particles in a newly established epilimnion accumulated in traps deployed weekly (20 m depth) at a site in the Ueberlinger See, a northwestern bight of the lake. Activities of 137Cs and 103Ru (plus 106Ru, 125Sb, 110mAg, and 144Ce) in trapped material collected during the subsequent 21 weeks is here described by a two-stage scavenging model involving (1) nuclide transfer to 'reactive particles' with negligible mean settling rate and (2) their entrainment by large, rapidly settling particles dominated by chemically passive calcite formed seasonally in the epilimnion. The model employs first-order kinetics where forward rate coefficients depend on time-dependent concentrations of candidate 'reactive phases' such as total suspended matter (TSM), particulate inorganic matter (PIM), particulate organic matter (POM), and particulate aluminum (PAL). First-order, irreversible nuclide transfer to nonexchangeable portions or reactive phases is also included. Vertical transport is described by a time-dependent rate of particle settling through a vertically and horizontally well-mixed epilimnion of increasing depth. Model calculations reproduced observations well with PAL as the 'reactive phase' for 137Cs and POM for 103Ru. The unusual coincidence of a pulsed nuclide loading with conditions of thermal stratification and limited vertical water mass exchange, together with frequent measurement of important state variables, permitted successful evaluation of a reaction-kinetic model under markedly non-steady state conditions