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" (<0.45 mu m) mercury concentrations (HgTD) 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 HgTD 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 (HgRD/HgTD: 44-95%), and very low total monomethylmercury concentrations (MMHg < 8 pg l(-1)). The HgTD 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 (MNMgD) concentrations varied from 9.5 to 13.5 pg l(-1) (2 to 8% of the HgTD). Comparable levels were measured in the Seine estuary (range: 12.2- 21.1 pg l(-1); 6-12% of the HgTD). These groundwater karstic estuaries seem to be mostly characterized by the higher HgTD and HgRD concentrations than in the adjacent HTZ Seine estuary. While the HTZ of the Seine estuary acts as a dissolved mercury removal system, the low turbid mixing zone of the Pays de Caux receives the dissolved mercury inputs from the groundwater seepage with an apparent Hg transfer from the particulate phase to the "dissolved" phase (<0.45 mu m). In parallel, the soft tissue of mussels collected near the groundwater discharges, at Etretat and Yport, exhibited significantly higher values than those found in the mussel from the mouth of the Seine estuary. We observe that this difference mimics the differences found in the mercury distribution in the water, and argue that the dissolved phase of the groundwater estuaries and coastal particles are significant sources of bioavailable mercury for mussels. (C) 2007 Elsevier B.V. Al rights reserved.