SCAVENGING AND PARTICLE RESIDENCE TIMES DETERMINED FROM TH-234 U-238 DISEQUILIBRIA IN THE COASTAL WATERS OF MECKLENBURG BAY/

Activities of the naturally occurring radionuclide Th-234 were determi ned in water samples of Mecklenburg Bay (SW Baltic Sea) using a new Th -specific diatomite adsorption technique followed by liquid scintillat ion spectrometry. Activities of ''dissolved'' (operationally defined a s Th in the centrifugate) and particulate Th-234 varied in the range o f 1.4-6.9 and 0.9-9.3 mBq l(-1), respectively. A significant correlati on between K-d and SPM concentration was found. From this particle-con centration effect, the ''colloidal pumping'' model predicts that 98% o f the ''dissolved'' Th is associated with colloids rather than being t ruly dissolved. Relative to calculated activities of the parent nuclid e U-238 in the Bay, the Th-234 data yielded mean Th-234 scavenging res idence times in the range of 1.2-9.7 days. Particulate Th-234 activiti es are inversely correlated to SPM concentrations. Particle residence times ranged from a few days in winter up to 20 days in spring charact erized by less intense bottom currents. The hydrodynamic regime is the master variable controlling scavenging of Th and other similarly part icle-reactive elements in Mecklenburg Bay. (C) 1998 Elsevier Science L td. All rights reserved.