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 watersh ed of southern Quebec (Canada). Lead ratio and concentration were meas ured in atmospheric bulk deposition, soils, freshwater (dissolved and particulate) as well as in different plant materials. The Pb-206/207 r atio measured for the two main sources of lead contamination, bulk dep osition and agricultural soils, was significantly different. This allo wed 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 d egree of mobility and bioavailability for atmospheric Pb. This prefere ntial uptake was affected by the suspended solids concentration. In co ntrast, >95% of the Pb tissue content in an urban stream with a low su spended 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 sig nature is preserved during trophic transfer of Pb from phyton to nekto n. Therefore, this natural tracer could become a powerful tool for man agers of ecoregion or hydrologic units in determining the extent of re sponsibility for up to four distinct sources of Pb contamination and i n addressing priority remedial actions.