Speciation and sorption of mercury in two macro-tidal estuaries

Abstract

Mercury speciation and partition were determined in two macro-tidal estuaries, the Loire and the Seine estuaries (France) during several cruises 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 freshwater, average total ‘dissolved’ (< 0.8 μm) mercury concentration [(HgT)D] was 4.1 ± 2.1 pM in the Loire and 11.5 ± 9.7 pM in the Seine. Mean particulate mercury concentration [(HgT)p] was 0.95 ± 0.63 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 are interpreted in terms of changes in the various water sources during flood events and variable contribution of colloidal materials. In the estuarine parts, (HgT)D concentrations generally varied from 2 to 6 pM in the Seine and from 1 to 3 pM in the Loire, while (HgT)P concentrations varied between 0.5 and 5 nmol g−1 in the Loire and between 2.5 and 10 nmol g−1 in the Seine. Because of its high distribution coefficient (log Kd = 5–6) and the high turbidity of these environments, mercury is mainly transported in particulate form in freshwaters as well as in mixing zones. Relationships between Kd and concentrations of suspended particles, dissolved and particulate organic carbon allow to describe the partition of mercury between solid and dissolved phases in terms of colloids, organic complexation in solution and sorption processes. Chlorocomplex formation does not play the major role in the speciation of Hg during the estuarine mixing. Organic associations, through sulphydryl groups, seems to be the dominant processes governing mercury distribution and partition in the two macro-tidal estuaries studied. Primary production appears to play an important role in the mercury dynamic. (HgT)P is correlated with the chlorophyll content of the particles in high flow regime of the Loire river, while elemental mercury (Hg0) was found to be related to the oxygen saturation rate in both estuaries. The production of Hg0 was sufficient to sustain a significant evasion flux to the atmosphere. Dimethylmercury was detected at fentomolar level during one cruise in the Baie de la Seine during an hypoxia episode. Highest monomethylmercury concentrations (up to 4% of the HgT) were associated with warmer and organic rich freshwater. Candidate sources for methylated mercury species are discussed. Both estuaries appear to be affected by local anthropogenic inputs. However, the Seine river is more deeply impacted by human activities. Watershed yields of the total Hg were calculated for the two drainage basins. 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 dissolved mercury inputs to the adjacent coastal waters (la Baie de la Seine and le Grand estuaire de la Loire) have been estimated to be half of the corresponding atmospheric depositions.