THE MARINE GEOCHEMISTRY OF SELENIUM

A fluorometric method for the determination of Se (IV) and total selenium in natural waters has been developed and optimized. The detection limit is about 20 pM for both analyses using a one liter sample. The precisions were about 2 % at 338 pM for Se (IV) and at 234 pM for total selenium.;The reduction of Se (VI) to Se (IV) by nitrate reductase (NR) extracted from D. tertiolecta was negligible. Any correlation between NR activity and the concentration ratio of Se (IV) to Se (VI) was not apparent in the surface waters at the mouth of the Chesapeake Bay.;The major fraction of dissolved inorganic selenium was in the colloidal form in river and estuarine waters. No inorganic colloidal selenium was detected in seawater. UV-oxidizable organic selenium was detected in all three types of waters, and constituted up to 60 % of dissolved selenium. Organic selenium preferentially associated with material having nominal molecular weight less than 10,000.;The concentrations of both total selenium and Se (IV) decreased with increasing salinity in the surface waters of the James River and southern Chesapeake Bay. While total selenium seemed to exhibit conservative behavior at salinities above 0.36 ('o)/oo, Se (IV) was apparently removed during estuarine mixing. The removal of Se (IV) occurred primarily at salinities below 4 ('o)/oo possibly as a result of the oxidation of Se (IV) to Se (VI). Se (IV) was the dominant inorganic species in river water. The concentration ratio of Se (IV) to Se (VI) decreased from 15 at 0.4 ('o)/oo to 0.9 at 31.8 ('o)/oo.