Assessment of Airborne Lead Provenance in Northern Ontario, Canada, Using Isotopic Ratios in Snow and Cladonia rangiferina Lichens

Abstract

Fresh snow and lichens (Cladonia rangiferina) were collected along two ~300-km transects in a regional survey around two smelter areas in Northern Ontario, i.e., the Sudbury and Timmins areas, in 2009–2010, as a part of a monitoring program. The samples were analyzed for Pb isotope ratios in snow from 47 sites, and in lichens from 28 sites, for background airborne lead and to determine the influence of distance versus local sources of Pb on a regional scale. Moreover, the lichen samples were split into two portions, the top portion, corresponding to recent growth (2–4 years), and the lower portion (up to 10+ years old) to determine the regime of recent versus old deposition. The study also investigated whether the isotopic signature in fresh snow (~1–2 weeks old) could correlate to the lichens’ recent growth. The Pb isotope signatures, reported as Pb isotope ratios 208Pb/206Pb and 207Pb/206Pb, in both snow and lichen recent growth were uniform over the region, except for localized influences near Sudbury and Timmins, suggesting that the background Pb represented remote sources for most of the region. Three-isotope plots 208Pb/206Pb versus 207Pb/206Pb of snow and lichens followed a linear model typical of mixing lines between two sources. As there are no statistical differences of Pb isotope ratios between fresh snow and recent lichen growth at neighboring stations, either type of sample may be used to describe the short-term record of airborne inputs. Finally, we found a strong relationship between the Pb isotope ratios of the recent growth (upper part) and old growth (lower part) of the lichens, with a slope of 0.9 between the lichen parts. This observation suggests either an isotopic segregation or the older parts of the lichens developed under different Pb source inputs in earlier times.