Lead concentrations and stable lead isotope ratios in moss in Slovenia and Switzerland

Using moss as a biomonitor is an established technique for monitoring atmospheric deposition of trace elements, including lead (Pb), a metal that is toxic for most organisms. Lead enters the environment during production and combustion processes and during the use of leaded gasoline. There are four stable isotopes of Pb, and the isotopic composition can be used to determine sources and pathways of atmospheric Pb pollution. In this study we determined Pb concentrations and isotope ratios in moss (Hypnum cupressiforme and Pleurozium schreberi) collected from 1990 to 2015 in Switzerland and from 2006 to 2015 in Slovenia. We used microwave sample digestion and inductively coupled plasma mass spectrometry (ICP-MS) for Pb concentrations and multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) for Pb isotope ratio determination. The aim was to assess the change in atmospheric Pb deposition over time and the differences between and within the two countries. Additionally, we assessed the current Pb isotope ratios for both countries and evaluated the change over time in Switzerland. The Pb concentration in moss decreased significantly over time in both countries. In 2015, at the end of the study period, concentrations were significantly higher in Slovenia compared to in Switzerland. Higher Pb concentrations in Slovenia may be related to the prolonged use of leaded gasoline or to the larger influence of industrial sources. Within Switzerland Pb concentrations differed significantly between regions. These differences are likely a consequence of different population densities and precipitation amounts. In Slovenia there were no significant differences between the regions, indicating that there are no large differences in Pb emissions. The Pb-206/Pb-207 and Pb-208/Pb-206 isotope ratios differed between Slovenia and Switzerland and between some regions within Switzerland. This finding shows that Slovenia and Switzerland, as well as the different regions within Switzerland, are influenced by different Pb sources. In Switzerland, Pb-206/Pb-207 increased and 208Pb/2 6Pb decreased over time and shifted away from the isotopic signature of leaded gasoline. Additionally, both isotope ratios became more homogeneous within Switzerland, which suggests that the sources of Pb became more similar. Overall, the Pb isotope ratios reflect the diminishing influence of leaded gasoline and indicate an increasing importance of industry, coal burning and natural bedrock as Pb sources.