SOURCES AND BURDEN OF LEAD IN ST-LAWRENCE ESTUARY SEDIMENTS - ISOTOPIC EVIDENCE

The depth distribution of the concentration of lead has been determined in sediments from seven stations located along the longitudinal axis of the Laurentian Trough in the St. Lawrence Estuary. In the sediments collected at one of the sites, we also measured the stable isotope composition of the total Pb as well as that of the Pb found in sedimentary fractions operationally defined by a sequential extraction procedure. The results reveal three isotopically-distinct sources of lead to these sediments: two natural and one from recent industrial pollution. While the two types of-natural lead most likely derive from crystalline rocks of the Canadian Shield, the industrial lead has an isotopic composition typical of that measured in the atmosphere of urban regions in Canada. The total burden of this type of lead in sediments deposited at water depths below 200 m in the entire St. Lawrence Estuary is on the order of 1.3 x 10(4) t, on average about 1.4 x 10(2) t annually since the beginning of the present century. The sedimentary lead record observed at each station indicates, however, a recent decrease in the rate at which lead has been added to this coastal marine environment. Industrial lead from sources in the United States does not have to be invoked to rationalize our observations on the Laurentian Trough sediments.