James M. Kaste

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 L _III –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.

Efficient and accurate measurements of uranium (U) and U-series radionuclides in earth's materials are needed to assess its environmental impact, reconstruct geochemical histories, and quantify heat production in the crust. To date, measurements of ²³⁴Th and ²³⁸U by gamma spectrometry have relied on the quantification of ²³⁴Th gamma emissions at 63 keV (absolute intensity = 3.7%) and the ^234mPa gamma at 1001 keV (absolute intensity = 0.84%). However, use of the 63 keV emissions can be hampered by ²³²Th interferences and self absorption, and the 1001 keV photon has a very low yield. Here we describe the effective use of the ²³⁴Th doublet gamma emission at 92.5 keV (total absolute intensity = 4.8%) for ²³⁴Th and ²³⁸U measurements. This doublet has been largely ignored because of its proximity to the Th K_α1 (93.3 keV) and thus its vulnerability to a Th self-fluorescence interference. We demonstrate that additions of U and ⁴⁰K to a Th ore sample do not increase the count rate at 92–93 keV above that which would be expected from the associated additions of U and ²³⁴Th. We also show that the Th self-fluorescence interference appears to be an anomaly associated only with the analysis of relatively thick (>1 mm) Th minerals, and suggest that fluorescence will not complicate the 92–93 keV region in most environmental samples. A review of decay data reveals that Th K_α1 X-rays associated with the decay of ²³⁵U and ²²⁸Ac can significantly interfere with quantification of the 92.5 keV ²³⁴Th doublet. We show that simple experimentally-derived correction factors for these X-rays can be used to accurately determine ²³⁴Th using its strongest gammas, resulting in higher count rates and smaller self-absorption corrections relative to the traditional analytical lines. Total 1σ analytical error associated with U measurements at 92.5 keV ranged from 1 to 9% and is dominated by the relative size of the ²²⁸Ac interference. Detection limits for U in environmental samples using this technique are on the order of 50 ppb.

Atmospheric deposition of lead (Pb) throughout the 1900s resulted in elevated amounts of this toxic metal even in remote forest soils of the northeastern United States. Soils can act as a net sink for metals and thus minimize groundwater and surface water contamination. Recent studies utilizing forest floor temporal data and models of total Pb in precipitation, surface soils, and streams have estimated the time scale of Pb release from soils. However, due to the limited availability and spatial variability of forest floor survey data, other methods for quantifying anthropogenic Pb movement are needed. This study uses the isotopic composition (²⁰⁶Pb/²⁰⁷Pb) of soil Pb and measurements of ²¹⁰Pb and ²²⁶Ra to directly trace the transit of atmospherically deposited Pb in the soil profile. We also report on the recovery of an enriched ²⁰⁷Pb dose applied in 1984 to the surface of a soil plot in the coniferous forest at Camels Hump in Vermont. The isotopic composition of soil Pb in low elevation deciduous forests suggests that approximately 65% of the original atmospheric Pb load has migrated from the forest floor to the upper 10 cm of the mineral soil. Higher elevation sites with coniferous vegetation have thicker forest floors, which have prevented significant amounts of Pb from entering the mineral soil. After 17 years, the soil organic horizon in the coniferous zone prevented any penetration of the applied Pb into the mineral soil. Using ²¹⁰Pb budgets in different soil compartments, we determine forest floor response times for atmospherically delivered Pb to be approximately 60 years in the low elevation deciduous forest zone and 150 years for the high elevation spruce-fir forest zone at Camels Hump. According to its distribution in the soil profile, we conclude that a dispersed release of anthropogenic Pb to groundwater and surface water is possible this century. Our results also offer independent confirmation of Pb deposition models previously generated for the region.