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

Sites Affected by Long-Range Lead Transport
Because the effects of anthropogenic Pb emissions tend to be restricted in geographic
extent, most natural terrestrial ecosystems in the U.S. sites have Pb burdens derived primarily
from long-range atmospheric transport. Pollutant Pb represents a large fraction of the Pb in
many of these ecosystems. In particular, many of these sites have accumulated large amounts of
Pb in soils. For example, at the Hubbard Brook Experimental Forest in New Hampshire, the
amount of Pb in the forest floor was estimated to have increased from about 1.35 kg ha !1 in 1926
(before the introduction of alkyl-Pb additives in gasoline) to 10.5 kg ha !1 in 1977 (Johnson et al.,
1995a). They also estimated the atmospheric Pb deposition from 1926 to 1987 to be 8.7 kg ha !1 ,
an amount that could account for nearly all of the increase in Pb in the forest floor during the
period. The input of precipitation Pb to the Hubbard Brook ecosystem in the six decades
spanning 1926 to 1987 was more than half of the total Pb estimated to have been released by
mineral weathering in the entire 12,000- to 14,000-year post-glacial period (14.1 kg ha !1 :
[Johnson et al., 2004]). Other studies employing Pb budgets (Miller and Friedland, 1994;
Watmough et al., 2004), and Pb isotopes (Bacon et al., 1995, 1996; Watmough et al., 1998;
Bindler et al., 1999; Hansmann and Köppel, 2000; Kaste et al., 2003), have also shown that
pollutant Pb, primarily from gasoline combustion, represents a quantitatively significant fraction
of labile Pb in temperate soils, especially in the upper, organic-rich horizons.
Despite years of elevated atmospheric Pb inputs and elevated concentrations in soils, there
is little evidence that sites affected primarily by long-range Pb transport have experienced
significant effects on ecosystem structure or function. Low concentrations of Pb in soil
solutions, the result of strong complexation of Pb by soil organic matter, may explain why few
ecological effects have been observed. At Hubbard Brook, for example, the concentration of Pb
in soil solutions draining the Oa horizon is