A Pb isotope and trace element study of rainwater from the Massif Central (France)

Abstract

Lead isotope ratios and Zn, Pb, Cu, Cd, Sb and Rb contents were measured in samples of rainwater collected over a period of 15 months from the Massif Central (France). Each sample, collected automatically at monthly intervals, represents a series of rainfall events. Rainwater chemistry was interpreted in terms of the chemical contributions from wet deposition and from different source regions for dust in the centre of France. Trace element concentrations in rainwater samples showed a wide range, particularly for Pb (1.30–465 μg/l), with variations decreasing for Cd (0.07–1.70 μg/l), Zn (1.00–54.00 μg/l), Cu (0.20–25.00 μg/l), Sb (∼0–0.33 μg/l) and Ni (∼0–15.00 μg/l). Trace element contents do not correlate with rainfall amount and no inter-element correlations are evident in the data. Lead is the most common trace metal found in the rainwater (mean value=996 μg/m 2/y) while Sb is the least common element measured (mean value=1.12 μg/m 2 /y). The composition of rainwater collected from the Massif Central shows a range in Pb isotope ratios from 17.935 to 19.22 ( 206Pb/ 204Pb), 15.578 to 15.73 ( 207Pb/ 204Pb) and 37.559 to 38.606 ( 208Pb/ 204Pb). A five-component mixing model involving contributions from the natural background, gasoline inputs from industrial and agricultural activity and a source resulting from mining waste may be used to explain both the Pb isotope signature and the fluctuations in trace metal contents of Massif Central rainwater.