PB ISOTOPES AND PB, ZN AND CD CONCENTRATIONS IN THE RIVERS FEEDING A COASTAL POND (THAU, SOUTHERN FRANCE) - CONSTRAINTS ON THE ORIGIN(S) AND FLUX(ES) OF METALS

The aim of this study was to identify the various sources and the beha viour of Pb, Zn and Cd in the watershed of the Thau pond (southern Fra nce), using Pb isotopes and Pb, Zn and Cd contents. The study was carr ied out at two different sites. The first (sub-watershed of the Vene r iver) presents a high density of potential sources of anthropogenic po llution (liquid urban wastes, road traffic, industries) coupled with a karstic-type water supply. The second (Pallas zone) is located in a m ainly agricultural area distant from industrial activities. Several ma in sources of lead pollution are distinguished using the Pb-206/Pb-207 ratio: lead from gasoline (1.109-1.111), natural lead leached out of rocks and soils (1.198-1.200) and lead from treated or untreated urban wastes (1.147-1.162). The transfer of gasoline lead to the rivers occ urs essentially during rainfall; in the Vene area, the urban anthropog enic lead component dominates while the 'rock-derived' lead is the mai n source in the rural area of the Pallas. Relationships existing betwe en Pb concentrations, Pb/Zn ratios and Pb-206/Pb-207 ratios make it po ssible to determine the various transfer cycles of these different ele ments. Metal concentrations in water remain very low: Pb < 9 ppb (mu g /l), Zn < 35 ppb and Cd < 0.4 ppb, much lower than the norms accepted for drinking water.