Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

Compared to their estimated 20th-century atmospheric Pb deposition of 0.9 g Pb m Γ2 , the forest
floor has retained 83% of the atmospheric Pb loadings (Johnson et al., 1995a). Johnson et al.
(2004) noted that gasoline-derived Pb was a significant component of the labile Pb at the HBEF.
They calculated that Pb fluxes to the HBEF by atmospheric pollution were essentially equivalent
to the Pb released by mineral weathering over the past 12,000 years. Marsh and Siccama (1997)
used the relatively homogenous mineral soils underneath formerly plowed land in Rhode Island,
New Hampshire and Connecticut to assess the depth-distribution of atmospheric Pb. They
reported that 65% of the atmospheric Pb deposited during the 20th century is in the mineral soil
and 35% is in the forest floor. At their remote study site in the Sierra Nevada Mountains, Erel
and Patterson (1994) reported that most of the anthropogenic Pb was associated with the humus
fraction of the litter layer and soils sampled in the upper few cm.
Atmospherically delivered Pb is probably present in ecosystems in a variety of different
biogeochemical phases. A combination of Pb adsorbtion processes and the precipitation of Pb
minerals will typically keep dissolved Pb species low in soil solution, surface waters, and
streams (Sauvé et al., 2000a; Jackson et al., 2005). While experimental and theoretical evidence
suggest that the precipitation of inorganic Pb phases and the adsorption of Pb on inorganic
phases can control the biogeochemistry of contaminant Pb (Nriagu, 1974; Ruby et al., 1994;
Jackson et al., 2005), the influence of organic matter on the biogeochemistry of Pb in terrestrial
ecosystems cannot be ignored in many systems. Organic matter can bind to Pb, preventing Pb
migration and the precipitation of inorganic phases (Manceau et al., 1996; Xia et al., 1997; Lang
and Kaupenjohann, 2003). As the abundance of organic matter declines in soil, Pb adsorption to
inorganic soil minerals and the direct precipitation of Pb phases may dominate the
biogeochemistry of Pb in terrestrial ecosystems (Ostergren et al., 2000a,b; Sauvé et al., 2000a).
Conclusions
Advances in technology since the 1986 Lead AQCD have allowed for a quantitative
determination of the mobility, distribution, uptake, and fluxes of atmospherically delivered Pb in
ecosystems. Among other things, these studies have shown that industrial Pb represents a
significant fraction of total labile Pb in watersheds. Selective chemical extractions and
synchrotron-based X-ray studies have shown that industrial Pb can be strongly sequestered by
organic matter and by secondary minerals such as clays and oxides of Al, Fe, and Mn. Some of
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