Extending the use of the stable lead isotope ratios as a tracer in bioavailability studies

In this study, the Pb-206/207 Stable isotope ratio tracer was used to decipher the biogeochemical pathway leading to the Pb contamination of freshwater macrophytes and periphyton in a small agricultural watershed of southern Quebec (Canada). Lead ratio and concentration were measured in atmospheric bulk deposition, soils, freshwater (dissolved and particulate) as well as in different plant materials. The Pb-206/207 ratio measured for the two main sources of lead contamination, bulk deposition and agricultural soils, was significantly different. This allowed a direct apportionment of lead found in plant materials throughout the watershed. In the agricultural streams, a preferential uptake of atmospheric Pb over eroded soil Pb was observed, suggesting a higher degree of mobility and bioavailability for atmospheric Pb. This preferential uptake was affected by the suspended solids concentration. In contrast, >95% of the Pb tissue content in an urban stream with a low suspended solids concentration was from anthropic origin. Apportionment calculated for two freshwater invertebrate species were more variable. However, results obtained for minnows showed that the isotopic Pb signature is preserved during trophic transfer of Pb from phyton to nekton. Therefore, this natural tracer could become a powerful tool for managers of ecoregion or hydrologic units in determining the extent of responsibility for up to four distinct sources of Pb contamination and in addressing priority remedial actions.