La contamination de la Gironde par le cadmium : origine, extension, importance

Abstract

In 1979, the French monitoring program RNO (Réseau National d'Observation de la qualité du milieu Marin) pointed out a dramatic cadmium contamination in the oysters from the Gironde estuary (up to 100 ug -1, dry weight). The wild oyster-beds upstream the estuary were shown to be the most contaminated. This result tends to point to the river as the source of the cadmium contamination. At the same rime, uncontaminated oysters from Arcachon were transplanted in the Gironde estuary (La Fosse, Dau). This ex periment confirmed the importance and proved the permanence of the contamination. Very high concentrations of dissolved cadmium (400 ng. 1-1) were measured in the estuarine mixing zone. This phenomenon can be explained by the desorption of cadmiumfrom cadmium rich par tic/es inf/owingfrom the drainage basin. A mass balance calculation indicates that 23 x 103 kg of cadmium reach the estuary each year associated with partides and 103 kg as the dissolved species. Most of this cadmium input (> 21 x 103 kg) is exported to the adjacent ocean as dissolved species, 1,2 x 103 kg are trapped in the sediment, and 103 kg exported with particles. A mining area several hundred kilometers upstream in the river lot's drainage basin has oeen found to be responsible for this contamination. The cadmium enriched plume does not spreadfar on the continental shelf. Nevertheless, it may reach the straight of Maumusson.and eventually affect the Marennes-O/eron oyster farming area. This hypothesis needs further considerations.