The Mussel Watch program conducted along the French coasts for the last 20 years indicates that the highest mercury concentrations in the soft tissue of the blue mussel (Mytilus edulis) occur in animals from the eastern part of Seine Bay on the south coast of the English Channel, the "Pays de Caux". This region is characterized by the presence of intertidal and submarine groundwater discharges, and no particular mercury effluent has been reported in its vicinity. Two groundwater emergence systems in the karstic coastal zone of the Pays de Caux (Etretat and Yport with slow and fast water percolation pathways respectively) were seasonally sampled to study mercury distribution, partitioning and speciation in water. Samples were also collected in the freshwater-seawater mixing zones in order to compare mercury concentrations and speciation between these "subterranean" or "groundwater" estuaries and the adjacent macrotidal Seine estuary, characterized by a high turbidity zone (HTZ). The mercury concentrations in the soft tissue of mussels from the same areas were monitored at the same time.
The means of the "dissolved" (b 0.45 μm) mercury concentrations (HgT D ) in the groundwater springs were 0.99 ± 0.15 ng l -1 (n = 18) and 0.44 ± 0.17 ng l -1 (n = 17) at Etretat and Yport respectively. High HgT D concentrations were associated with strong runoff over short water pathways during storm periods, while low concentrations were associated with long groundwater pathways. Mean particulate mercury concentrations were 0.22 ± 0.05 ng mg -1 (n = 16) and 0.16 ± 0.10 ng mg -1 (n = 17) at Etretat and Yport respectively, and decreased with increasing particle concentration probably as a result of dilution by particles from soil erosion. Groundwater mercury speciation was characterized by high reactive-to-total mercury ratios in the dissolved phase (HgR D /HgT D : 44-95%), and very low total monomethylmercury concentrations (MMHg b 8 pg l -1 ). The HgT D distributions in the Yport and Etretat mixing zones were similar (overall mean concentration of 0.73 ± 0.21 ng l -1 , n = 43), but higher than those measured in the adjacent industrialized Seine estuary (mean: 0.31 ± 0.11 ng l -1 , n = 67). In the coastal waters along the Pays de Caux dissolved monomethylmercury (MMHg D ) concentrations varied from 9.5 to 13.5 pg l -1 (2 to 8% of the HgT D ). Comparable levels were measured in the Seine estuary (range: 12.2-21.1 pg l -1 ; 6-12% of the HgT D ). These groundwater karstic estuaries seem to be Marine Chemistry 106 (2007) 352 -364 www.elsevier.com/locate/marchem ☆ The publisher regrets that this article was previously published in an issue of Marine Chemistry, for citation purposes please use the original publication details; Laurier, F.J.G., Cossa, D., Beucher, C., Brévière E., 2007. The impact of groundwater discharges on mercury partitioning, speciation and bioavailability to mussels in a coastal zone. Mar. Chem. 104, 143-155.