Scavenging and particle residence times determined from 234Th/238U disequilibria in the coastal waters of Mecklenburg Bay

Activities of the naturally occurring radionuclide 234Th were determined in water samples of Mecklenburg Bay (SW Baltic Sea) using a new Th-specific diatomite adsorption technique followed by liquid scintillation spectrometry. Activities of “dissolved” (operationally defined as Th in the centrifugate) and particulate 234Th varied in the range of 1.4–6.9 and 0.9–9.3 mBq l−1, respectively. A significant correlation between Kd and SPM concentration was found. From this particle-concentration effect, the “colloidal pumping” model predicts that 98% of the “dissolved” Th is associated with colloids rather than being truly dissolved. Relative to calculated activities of the parent nuclide 238U in the Bay, the 234Th data yielded mean 234Th scavenging residence times in the range of 1.2–9.7 days. Particulate 234Th activities are inversely correlated to SPM concentrations. Particle residence times ranged from a few days in winter up to 20 days in spring characterized by less intense bottom currents. The hydrodynamic regime is the master variable controlling scavenging of Th and other similarly particle-reactive elements in Mecklenburg Bay.