Scavenging of 234Th and 7Be in Lake Constance

Abstract

Particulate and dissolved concentrations of 234Th and 7Be, along with fluxes of total mass and particulate 234Th and 7Be into sediment traps, were measured in Lake Constance (Bodensee) from March to December 1993. During the same period, atmospheric deposition rates of 7Be and 210Pb were determined. Variations in atmospheric fluxes of 7Be and 210Pb were found to reflect variability in rainfall. The mean atmospheric flux for 7Be was 13.5 dpm cm−2 yr−1 and for 210Pb it was 0.65 dpm cm−2 yr−1. 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−1, having increased slightly with total particle concentration. 7Be activities in surface water exhibited large variations because of variable atmospheric input and particle flux. During periods of high biological productivity, particulate 7Be in surface water increased from ~20 to ~60% of the total 7Be concentration. The scavenging rates for dissolved 7Be(0.01−0.12 d−1) were generally lower than those for dissolved 234Th because of the lower particle reactivity of 7Be. Although the 7Be 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 column‐produced 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 7Be (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.