Fate and Speciation of Gasoline‐Derived Lead in Organic Horizons of the Northeastern USA

Abstract

Although legislation in the late 1970s significantly reduced atmospheric lead (Pb) inputs to ecosystems in North America, organic (O) horizons in forests of the northeastern USA still contain up to 30 kg of gasoline‐derived Pb ha−1 The residence time, geochemical behavior, and fate of this contaminant Pb in soils is poorly understood. Here we use forest floor time series data and synchrotron‐based X‐ray techniques to examine the mobility and speciation of Pb in O horizons collected from remote sites across the northeastern USA. At high elevation (>800 m) sites in Vermont and New York, samples collected from similar locations in 1980, 1990, and early 2000 had indistinguishable Pb contents, ranging (±1σ) from 11 to 29 kg Pb ha−1 However, at lower elevation and lower latitude sites with mixed vegetation, significant decreases in Pb amount were observed during the two‐decade study period. Lower elevation sites ranged from 10 to 20 kg Pb ha−1 in 1980, and from 2 to 10 kg Pb ha−1 20 yr later. Lead‐enriched soil grains were determined to be amorphous with microfocused X‐ray diffraction, and Pb concentrations correlated well with Fe on maps generated via microfocused X‐ray fluorescence. Bulk Pb LIII–edge X‐ray absorption spectroscopy of coniferous O horizon samples indicate that Pb is binding to iron‐rich soil particles by inner‐sphere complexes, most likely to amorphous Fe oxides. Based on our paired regional and microscopic observations, we conclude that Pb is strongly retained in well‐drained O horizons, and mobility is governed by decomposition and colloidal transport.