SCAVENGING OF TH-234 AND BE-7 IN LAKE CONSTANCE

Particulate and dissolved concentrations of Th-234 and Be-7, along wit h fluxes of total mass add particulate Th-234 and Be-7 into sediment t raps, were measured in Lake Constance (Bodensee) from March to Decembe r 1993. During the same period, atmospheric deposition rates of Be-7 a nd Pb-210 were determined. Variarions in atmospheric fluxes of Be-7 an d Pb-210 were found to reflect variability in rainfall. The mean atmos pheric flux for Be-7 was 13.5 dpm cm(-2) yr(-1) and for Pb-210 it was 0.65 dpm cm(-2) yr(-1). Total Th-234 activities in surface water were generally high in winter and tow in summer, primarily in response to t he seasonality of biological activity. About 50-70% of Th-234 was foun d in particulate form, depending on particle concentration and particl e residence time. Scavenging rates for dissolved Th-234 ranged from 0. 14 to 0.88 d(-1), having increased slightly with total particle concen tration. Be-7 activities in surface water exhibited large variations b ecause of variable atmospheric input and particle flux. During periods of high biological productivity, particulate Be-7 in surface water in creased from similar to 20 to similar to 60% of the total Be-7 concent ration. The scavenging rates for dissolved Be-7 (0.01-0.12 d(-1)) were generally lower than those for dissolved Th-234 because of the lower particle reactivity of Be-7. Although the Be-7 flux into sediment trap s at 50 m did not differ significantly from the flux at 120 m, the Th- 234 flux increased with depth because of the further adsorption of wat er column-produced Th-234 onto sinking particles. The range of residen ce times for the total radionuclide inventories estimated from an irre versible scavenging model was 33-83 d for Th-234 and 40-340 d for Be-7 (box depth, 50 m). The residence times of both nuclides were controll ed by the particle flux, which, in turn, was determined by biological productivity in the surface water.