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

Abstract

Stable Pb isotope ratios were used to trace thesources and pathways of Pb between the atmosphere,surficial sediment fractions, the white water-lilyNymphaea odorata, and waters collected at 26littoral sites in 23 Ontario lakes in summer 1993,three years after alkyl Pb additives were finallyeliminated from Canadian gasoline. Based onsimilarities of isotopic composition, the exchange ofPb between lakewater and sediment 'carbonate', andsubsequently between 'carbonate', 'oxide' and othersediment fractions, was the most likely water-sedimentpathway of Pb movement. pH controlled Pb fractionationwithin surficial sediments, with the 'organic' poolcomprising 80–97% of total Pb in most acidic lakesand 15–60% in alkaline lakes. About 28% of the Pb inN. odorata shoots was accumulated directly fromwater, whereas there was no evidence of root uptake ofPb from sediments. The Pb in plant tissues wasisotopically homogeneous and dissimilar to thevariable composition exhibited in ambient waters andsediments. Plant Pb isotopes strongly resembled thehistorical Canadian atmospheric (alkyl Pb) signature.A possible explanation is that, like essential tracemetals, historically-accumulated Pb was highlyconserved during the annual growth cycle of thislong-lived, clonal macrophyte, being storedover-winter in underground rhizomes and recycled intospring growth. Given the low rate of 'new' Pb uptake,historical alkyl Pb may continue to dominate planttissues for some time, even though it was notdetectable in littoral waters and sediments.