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 behaviour of Pb, Zn and Cd in the watershed of the Thau pond (southern France), using Pb isotopes and Pb, Zn and Cd contents. The study was carried out at two different sites. The first (sub-watershed of the Vène river) presents a high density of potential sources of anthropogenic pollution (liquid urban wastes, road traffic, industries) coupled with a karstic-type water supply. The second (Pallas zone) is located in a mainly agricultural area distant from industrial activities. Several main sources of lead pollution are distinguished using the 206Pb207Pb 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 occurs essentially during rainfall; in the Vène area, the urban anthropogenic lead component dominates while the ‘rock-derived’ lead is the main source in the rural area of the Pallas. Relationships existing between Pb concentrations, PbZn ratios and 206Pb207Pb ratios make it possible to determine the various transfer cycles of these different elements. Metal concentrations in water remain very low: Pb < 9 ppb (μg/l), Zn < 35 ppb and Cd < 0.4 ppb, much lower than the norms accepted for drinking water.