Physics
Scientific paper
Jun 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56.2339r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 6, pp.2339-2361
Physics
4
Scientific paper
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 Überlinger See, a northwestern bight of the lake. Activities of 137 Cs and 103 Ru (plus 106 Ru, 125 Sb, 110m Ag, and 144 Ce) in trapped material collected during the subsequent 21 weeks is here described by a two-stage scavenging model involving 1. (1) nuclide transfer to "reactive particles" with negligible mean settling rate. 2. (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), paniculate inorganic matter (PIM), paniculate organic matter (POM), and paniculate aluminum (PAL). First-order, irreversible nuclide transfer to nonexchangeable portions of 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 137 Cs and POM for 103 Ru. Calculated reaction rates for all nuclides were sufficiently high that activity changes were dominated by temporal variations in pertinent state variables. Selective chemical extraction of Chernobyl 137 Cs from sediments and study of uptake kinetics by addition of radiocesium to fresh sediment suspensions supported model results indicating its negligible affinity for calcite, probable transfer to clay minerals (for which PAL is a surrogate), particle concentration-independent distribution coefficient, and significant transfer to nonexchangeable sites. For 103 Ru, model calculations implied no transfer to nonexchangeable sites and a forward rate term proportional to the square root of POM concentration, a result in accord with the Honeyman-Santschi "Brownian pumping" model. 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.
Frenzel P.
Kleiner J.
Lindner G.
Pfeiffer Walter
Robbins John A.
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