Beryllium-7 as a tracer to study mechanisms and rates of metal scavenging from lake surface waters

The removal of Be-7 from lake surface waters (Lake Lugano, Switzerland/Italy) was studied from September 1995 to May 1996. During this period the atmospheric input and sedimentary flux of Be-7, as well as concentrations of dissolved (1 mm) Be-7 in the epilimnion were measured. The separation of dissolved, colloidal, and particulate fractions of Be-7 was carried out using continuous flow centrifugation (CFC) and tangential flow filtration (TFF) techniques. Lacustrine colloids (log Kc =6) were shown to be much more efficient sorbents for Be-7 than the suspended particles (log Kp 4–5). Particle concentrations Cp ranged from 0.5 to 2 mg/L. The ratio of colloid (>10 kD) to particle concentration Cc/Cp was close to 0.09 during winter months when detrital inputs are dominant, but was higher (0.16) in summer and in spring after an algal bloom. The Be-7 data were used to estimate coagulation rates of colloids. The highest coagulation rates (lambda coag = 0.4 d-1) along with the lowest Kc (log Kc = 5.2) were found after an algal bloom in spring