Heavy Metal Concentrations and Radiogenic Isotopes
(Pb, Sr) in Recent Molluscs and Ancient Shells from a Coastal Pond (Thau, South France)
34095 Montpellier, France
Because of its ability to characterize the origins and fluxes of waters and their loads, geochemistry is being increasingly used in environmental problems. The molluscs are known to concentrate metals in a very strong manner and equilibrate rapidly (1-2 month) with their environment. They are used as bioindicators in many programs of coastal survey (NOAA, french mussel watch,...). The aims of this study are: 1- to better understand the spatial distribution and temporal variations of metals and the major sources of pollution in a greatly anthropised pond by using molluscs. For this purpose, the concentration of metals (Co, Ni, Cu, Zn, Tl, Cd, Pb,...) and the Pb isotopic ratios are determinated in the flesh and shells of mussels and clams; 2- to compare past and present metal levels in this area by analysing also ancient
(1 bc-19th century) shells.
The Thau pond (70 km2) is located in the south of France near Sete harbour. Its surroundings are greatly anthropised: the industrial Sete harbour, various industries (cement factory, fertilizers,...), agriculture (vineyards), shellfish farms, heavy road traffic, camping areas and leisure ports. Mussel (mytilus galloprovincialis) samples were collected along the pond coast near the local harbours and young mussels from the sea were introduced at various locations in the pond. Sampling is done 4 times a year. After depuration for 1-2 days, the samples, composed by 10 animals, are rinsed in distilled water and the flesh separated from the shell. The flesh is dried and dissolved in HNO3. Then the solution is filtered on 0.45µ teflon filters. After cleaning with distilled water, the shells are leach in HCl to take away the external pollution, then they are dissolved in HCl 6N and the organics removed. Concentrations of 15 elements are determined directly at the ICP-MS after proper dilution and Pb and Sr isotopes are measured on a solid source mass spectrometer.
Concentrations: The clams' fleshes tend to have higher heavy metals concentration (factor 1.5 to 3) except for Zn and Pb when compared to the mussels . In the mussels we can observe a decrease of concentration (factor 2 to 10) between the coast and the interior of the pond. Clams as well as mussels show two centers of pollution in the pond corresponding to the two important industrial areas.(around: 200 ppm Zn, 9 ppm Cu, 4 ppm Pb).
Isotopic ratios: Pond's mussels show principally the influence of road pollution (gasoline endmember). within the pond, variations are also visible with the distance to heavy trafic roads. The ancient shells are more radiogenic than the current shells. Variations of Pb ratios can be rougly correlated with age and are compared to different Pb sources used by romans (Spain, Britain,..). The Sr ratios in the shells are dominated by the sea value but in the ancient shells the ratio is sensibly lower.
Fig. 1.