Distribution and origin of lead in stream sediments from small agricultural catchments draining Miocene molassic deposits (SW France)

Thomas Bur (1) , Jean-Luc Probst (1) , Yao Mathieu N'Guessan (1) , Anne Probst (1)

1 LEFE - Laboratoire Ecologie Fonctionnelle et Environnement

Total Pb concentrations and isotopic composition were determined in stream-bed sediments and bedrock from 29 small agricultural or forested catchments in the Gascogne area (SW France). The contribution of Pb from various natural and anthropogenic sources was investigated in this rural area which is very weakly impacted by industrial or urban emissions. Environmental parameters in catchments (importance of forest cover, organic matter and oxide content in sediments) were considered. A combination of geochemical (enrichment calculation, sequential extraction) and isotopic investigations was performed to constrain the origin of Pb and the distribution of anthropogenic Pb in sediments. Most of the sediments have low total Pb content compared to other agricultural regions more impacted by industrial or urban emissions. The results indicated a moderate but significant Pb enrichment, particularly for catchments draining forested areas. This enrichment was positively related to organic C content in sediment and catchment forest cover, whereas in entirely cultivated catchments it was related to Fe-oxide content. An average anthropogenic end-member was determined using Pb isotopes, and was supposed to be representative of background atmospheric Pb pollution, with a weak influence of Pb from recent gasoline and local fertiliser spreading. The amount of anthropogenic Pb (Pbanthr) in sediments estimated using a geochemical approach (mean 63.7 ± 20.4%) was higher than that estimated using an isotopic approach (mean 36.6 ± 17.8%), but the same trend was observed among the samples, except for low anthropogenic contributions. The distribution of Pbanthr in sediments from weakly forested catchments indicated a strong affinity for carbonates and Fe-oxides. Amorphous Fe-oxides became preferential trapping compounds as soon as Pb enrichment increased and carbonate content decreased. Finally, in cultivated catchments, organic C was not a main trapping component for Pbanthr in sediments.

Mots clés: Stream sediments; Molassic deposits

Domaines: Géochimie

Thomas Bur, Jean-Luc Probst, Yao Mathieu N'Guessan, Anne Probst. Distribution and origin of lead in stream sediments from small agricultural catchments draining Miocene molassic deposits (SW France). Applied Geochemistry, 2009, 2 (7), pp.1324-1338. ⟨10.1016/j.apgeochem.2009.04.004⟩. ⟨hal-03573346⟩