Tracing the retention and redistribution of lead and other atmospheric fallout in soils

A basic understanding of how atmospherically-derived elements behave in soils is of concern in contaminant transport, nutrient cycling, and carbon sequestration studies. Here, I use stable and radioactive tracers to evaluate the fate of atmospherically-delivered Pb, which is perhaps the most widely dispersed contaminant in the world. I also examine the redistribution of fallout in soils resulting from atmospheric weapons testing during the 1950's and 1960's.; At Camels Hump Mountain, the forest floor in the coniferous forest zone completely retained a 207Pb enriched tracer applied to the surface in 1984. I use natural 210Pb budgets and determine forest floor Pb response times of 60 years in the deciduous zone and 150 years in the coniferous zone. Using 206Pb/207Pb measurements, I show that in the deciduous zone, 65% of the atmospheric Pb from the combustion of leaded-gasoline during most of the 20th century has migrated to the underlying mineral horizons. This pollutant Pb is limited to the upper 21 cm of soils in both forest zones. Selective chemical extractions demonstrate that Pb is removed from the forest floor with extractions specific to organic matter. Lead is not exchangeable; salt solutions with pH 3.5 leach <5% of Pb from soils. I use dialysis experiments and show that Pb transport from the forest floor is in colloidal form (>5000 MWCO). Spodosols in the northeastern United States will retain atmospherically-delivered Pb on timescales of centuries.; In Spodosols of the northeastern United States, 137Cs and 241Am have different distributions in soils, despite identical source terms. This results from the lack of vermiculite which can trap Cs. Cesium is cycled by vegetation, and transport is dominated by chemical leaching, while 241Am is bound strongly to soil organics. At the Nunnock River Watershed (NR), in southeastern Australia, fallout 210Pb and 137Cs are both strongly retained by soil particles, and show similar distributions in the profile. Consequently, physical soil mixing, resulting from bioturbation, can dominate the redistribution of particle-reactive elements delivered from the atmosphere even on timescales of decades.