EVASION OF SELENIUM TO THE ATMOSPHERE VIA BIOMETHYLATION PROCESSES INTHE GIRONDE ESTUARY, FRANCE

Dissolved volatile selenium species were monitored in the Gironde estu ary (south-west France) during three cruises along with hydrological p arameters (salinity, temperature, suspended particulate matter, dissol ved organic matter fluorescence). Volatile selenium species presented a strong seasonal variability with concentrations ranging from non-det ectable levels (<0.9 pmol/l) in December 1992 to a few pmol/l in the S eptember 1992 and May 1993 cruises. Dimethyl selenide [(CH3)(2)Se] was the main volatile selenium compound detected (84 to 100%). Low concen trations of dimethyl diselenide [(CH3)(2)Se-2] and methane selenol [(C H3)SeH] were also observed in May 1993 with sensitive equipment. The v olatile selenide concentration presented a positive correlation with t he water temperature and a negative correlation with the rivers freshw ater discharge. This temperature dependence allowed us to obtain, by t he use of the Arrhenius plot the apparent activation energy of the dim ethyl selenide formation in estuarine waters (147 +/- 51 kJ/mol). The nature of the mechanisms involved in the formation of volatile seleniu m species are suggested to be both related to microbial and algae's ac tivity in the rivers and estuary environments. The production and rele ase of biomethylated volatile selenium compounds in the estuarine wate rs seem also strongly dependent on the hydrodynamic stability and the water residence time in the Gironde estuary. Considering the seasonal variability of the volatile selenium species in estuarine waters, the selenium fluxes to the atmosphere from the Gironde estuary ecosystem w ere then estimated to range between 0.6 and 2.6 X 10(5) g of Se per ye ar. (C) 1997 Elsevier Science B.V.