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Particulate and dissolved concentrations of 234Th and 'Be, along with fluxes of total mass and particulate 234Th and 'Be into sediment traps, were measured in Lake Constance (Elodensee) from March to December 1993. During the same period, atmospheric deposition rates of 7Be and 210Pb were determined. Variations in atmospheric fluxes of 'Be and 210Pb were found to reflect variability in rainfall. The mean atmospheric flux for 'Be was 13.5 dpm crned2 yr-' and for 2LoPb it was 0.65 dpm cm-2 yr-I. Total 234Th activities in surface water were generally high in winter and low in summer, primarily in response to the seasonality of biological activity. About 50-70% of 234Th was found in particulate form, depending on particle concentration and particle residence time. Scavenging rates for dissolved 234Th ranged from 0.14 to 0.88 d-l, having increased slightly with total particle concentration. 'Be activities in surface water exhibited large variations because of variable atmospheric input and particle flux. During pericds of high biological productivity, particulate 'Be in surface water increased from -20 to -60% of the total 'Be concentration. The scavenging rates for dissolved 'Be (0.0 1-O. 12 d-l) were generally lower than those for dissolved 234Th because of the lower particle reactivity of 'Be. Although the 'Be flux into sediment traps at 50 m did not differ significantly from the flux at 120 m, the 234Th flux increased with depth because of the further adsorption of water columnproduced 234Th onto sinking particles. The range of residence times for the total radionuclide inventories estimated from an irreversible scavenging model was 33-83 d for 234Th and 40-340 d for 'Be (box depth, 50 m). The residence times of both nuclides were controlled by the particle flux, which, in turn, was determined by biological productivity in the surface water.Natural and man-made radionuclides are useful in the study of the geochemical cycle of some particle-reactive metals and other contaminants in both surface waters and groundwaters (Santschi and Honeyman 1989;Murray 1987). The radiotracer methodology is well established in the marine sciences for processes with various time scales. Lakes are often more dynamic and variable than oceans, which results in shorter time scales for transport processes. The residence time of particle-reactive radionuclides in lakes is controlled mainly by particle flux (Santschi 1984). Nevertheless, hydrodynamic factors, such as horizontal or vertical diffusion and advection, as well as the chemical sorption behavior of the element with respect to specific particle surfaces, need to be considered to understand the geochemical pathways of an element.Although several investigators have studied seasonal cycling of 210Pb (half-life TP/, = 22.1 yr) and 210Po (TP/, = 138 d) in lakes (e.g. Talbot and Andren 1984;Stiller and Imboden 1986; Mangini et al. 1990), data for the shortlived radionuclides 7Be (75/Z = 53.3 d) and 234Th (7,/, = 24.1 d) in freshwater environments are rare (Robbins and Eadie 199...

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