Lead biogeochemistry in the littoral zones of south-central Ontario lakes, Canada, after the elimination of gasoline lead additives

Stable Pb isotope ratios were used to trace the sources and pathways of Pb between the atmosphere, surficial sediment fractions, the white water-lily Nymphaea odorata, and waters collected at 26 littoral sites in 23 Ontario lakes in summer 1993, three years after alkyl Pb additives were finally eliminated from Canadian gasoline. Based on similarities of isotopic composition, the exchange of Pb between lakewater and sediment 'carbonate', and subsequently between 'carbonate', 'oxide' and other sediment fractions, was the most likely water-sediment pathway of Pb movement. pH controlled Pb fractionation within surficial sediments, with the 'organic' pool comprising 80-97% of total Pb in most acidic lakes and 15-60% in alkaline lakes. About 28% of the Pb in N. odorata shoots was accumulated directly from water, whereas there was no evidence of root uptake of Pb from sediments. The Pb in plant tissues was isotopically homogeneous and dissimilar to the variable composition exhibited in ambient waters and sediments. Plant Pb isotopes strongly resembled the historical Canadian atmospheric (alkyl Pb) signature. A possible explanation is that, like essential trace metals, historically-accumulated Pb was highly conserved during the annual growth cycle of this long-lived, clonal macrophyte, being stored over-winter in underground rhizomes and recycled into spring growth. Given the low rate of 'new' Pb uptake, historical alkyl Pb may continue to dominate plant tissues for some time, even though it was not detectable in littoral waters and sediments.