EVALUATING THE FATE OF ATMOSPHERICALLY DEPOSITED LEAD IN MONTANE FOREST SOILS WITH ISOTOPES AND XANES SPECTROSCOPY

Fossil fuel combustion during the 20th century resulted in deposition of 1 to 4 g of lead (Pb) per square meter in forests of the northeastern United States. Despite its widespread dispersal and potential toxicity, the fate of Pb and other heavy metals in watersheds remains unclear. Here we trace the transport and speciation of atmospheric Pb in forest soils using Pb isotopic analyses and X-ray absorption near edge structure (XANES) spectroscopy. We sampled soil profiles from Camels Hump Mountain in Vermont, U.S.A. in the low elevation deciduous forest (600 m) and in the higher elevation coniferous forest (1200 m). The forest floor (i.e., the organic or mor layer overlying the mineral soil) retains atmospherically deposited Pb on timescales of 60 to 150 years, depending on forest zone. Lead originating from the combustion of leaded-gasoline has penetrated 20 cm or less in both deciduous and coniferous forest soils, where total soil thickness is on the order of 60 cm. Lead extracted from forest floor samples with rinses of de-ionized water is entirely >5000 molecular weight cut-off (MWCO), and associated with organic carbon, which suggests that the movement of organic colloids or other particulate phases govern Pb transport. XANES spectroscopy reveals that Pb speciation changes with depth and is preferentially retained in more decomposed organic matter. Thus, Pb retention is linked to the cycling of organic carbon in these soils.