SPECIATION AND SORPTION OF MERCURY IN 2 MACRO-TIDAL ESTUARIES

Mercury speciation and partition were determined in two macro-tidal es tuaries, the Loire and the Seine estuaries (France) during several cru ises covering various hydrological conditions. Additional measurements were performed in freshwaters in order to assess temporal Variations of mercury inputs to the estuaries during an annual cycle. In freshwat er, average total 'dissolved' (< 0.8 mu m) mercury concentration [(Hg- T)(D)] was 4.1 +/- 2.1 pM in the Loire and 11.5 +/- 9.7 pM in the Sein e. Mean particulate mercury concentration [(Hg-T)(P)] was 0.95 +/- 0.6 3 nmol g(-1) in the Loire and 5.4 +/- 2.6 nmol g(-1) in the Seine. The seasonal variation in the mercury concentrations in the two rivers ar e interpreted in terms of changes in the various water sources during flood events and variable contribution of colloidal materials. In the estuarine parts, (Hg-T)(D) concentrations generally varied from 2 to 6 pM in the Seine and from 1 to 3 pM in the Loire, while (Hg-T)(P) conc entrations varied between 0.5 and 5 nmol g(-1) in the Loire and betwee n 2.5 and 10 nmol g(-1) in the Seine. Because of its high distribution coefficient (log K-d = 5-6) and the high turbidity of these environme nts, mercury is mainly transported in particulate form in freshwaters as well as in mixing zones. Relationships between K and concentrations of suspended particles, dissolved and particulate organic carbon allo w to describe the partition of mercury between solid and dissolved pha ses in terms of colloids, organic complexation in solution and sorptio n processes. Chlorocomplex formation does not play the major role in t he speciation of Hg during the estuarine mixing. Organic associations, through sulphydryl groups, seems to be the dominant processes governi ng mercury distribution and partition in the two macro-tidal estuaries studied. Primary production appears to play an important role in the mercury dynamic. (Hg-T)(P) is correlated with the chlorophyll content of the particles in high flow regime of the Loire river, while element al mercury (Hg-0) was found to be related to the oxygen saturation rat e in both estuaries. The production of Hg-0 was sufficient to sustain a significant evasion flux to the atmosphere. Dimethylmercury was dete cted at fentomolar level during one cruise in the Bale de la Seine dur ing an hypoxia episode. Highest monomethylmercury concentrations (up t o 4% of the Hg-T) were associated with warmer and organic rich freshwa ter. Candidate sources for methylated mercury species are discussed. B oth estuaries appear to be affected by local anthropogenic inputs. How ever, the Seine river is more deeply impacted by human activities. Wat ershed yields of the total Hg were calculated for the two drainage bas ins. The Seine yield (41 mmol km(-2) a(-1)) is in the range of what is found in other urban areas, while for the Loire (9 mmol km(-2) a(-1)) it is close to values found for agricultural watersheds. In spite of the anthropogenic inputs within the two estuaries, the riverine dissol ved mercury inputs to the adjacent coastal waters (la Bale de la Seine and le Grand estuaire de la Loire) have been estimated to be half of the corresponding atmospheric depositions. (C) 1997 Elsevier Science B .V.