Seasonal and decadal variations in lead sources to eastern North Atlantic mussels

The concentration of Pb and its stable isotope composition were measured in 216 composite samples of 50 blue mussels (Mytilus edulis (M. edulis)) collected quarterly between 1985 and 2005 at three sites along the French Atlantic coast, one in the La Fresnaye Bay and the others in the Loire and Seine River estuaries. Depending on the sites and time periods, Pb concentrations were 5-66 times higher than the natural background value for the North Atlantic. Even for the samples with the lowest Pb concentrations, the isotopic signature of Pb is very different than that of the regional natural Pb, suggesting that most of the bioaccumulated Pb is anthropogenic in origin. Stable Pb isotope ratios measured in the mussels differ markedly from that of Pb emitted in Western Europe as a result of leaded gasoline combustion, which was still a dominant source of contaminant Pb to the atmosphere during most of our study period. The isotope composition of Pb in the mussels was instead more typical of that of the Pb released to the environment by wastewater treatment plants, municipal waste incinerators, and industries such as metal refineries and smelters. Continental runoff, rather than atmospheric deposition, is therefore identified as the leading transport pathway of Pb along the French Atlantic coast. From the strong seasonal variations in (206)Pb/(208)Pb ratios in the mussels from the Seine Estuary site we also conclude that the resuspension of contaminated sediments, triggered by high river runoff events, is a chief factor affecting the bioaccumulation of Pb in M. edulis . The value of this organism as a biomonitor of coastal contamination is thus further demonstrated.