Temperate, macrotidal, turbid estuarine behavior of mercury species. The case of the Loire river

We have examined the distribution and spectiation mercury and trace metals under contrasted season, river flow conditions and tidal amplitudes with a series of  cruises taking place between 2012 and 2015. Our laboratory had alredy undertaken a similar study in the early 1990's. The latest data acquisition efforts are an opportunity to examine the changes that occurred within that time span for mercury and its species, and to discuss the causes of the observed changes. In particular, we will replace the  observations in the perspectives of environmental regulation, and of field and laboratory practices observed for environmental chemical characterization.

Early 1980's estuarine transects show total dissolved Hg levels near 5nM in the Loire river (Frenet, 1981). In the early 1990's, a careful and detailed study shows estuarine levels between 1 and 6pM (Coquery, 1994) with a strong dependency on the hydrological regime. New results confirm the riverine clevels in the low pM range. Concentrations decrease progressively during mixing with seawater and the oceanic end-member concentrations are near 0.5pM. Monitoring of mercury levels and speciation in the Loire river and the upper reaches of the estuary show that these Hg levels and species fluctuate with water flow and areal flooding upstream, AND on the history of that hydrological regime.