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

the forested Lehstenbach catchment in Bavaria, about six times the efflux in runoff of 2.82 g ha !1
year !1 .
Lead pollution has resulted in the accumulation of large Pb burdens in terrestrial
ecosystems (see also Section AX7.1.2). Despite reductions in emissions, this accumulation of Pb
continues, though at markedly lower rates. The large pool of Pb bound in soils may potentially
be a threat to aquatic ecosystems (see Section AX7.2), depending on its rate of release from the
soil. Early estimates of the residence time of Pb in the forest floor ranged from 220 to 5,000
years (Benninger et al., 1975; Friedland and Johnson, 1985; Turner et al., 1985). However, more
recent literature suggests that Pb is transported more rapidly within soil profiles than previously
believed. The pool of Pb in forest floor soils of the northeastern United States declined
significantly in the late 20th century. Friedland et al. (1992) reported a 12% decline in the
amount of Pb in forest floor soils at 30 sites in the region between 1980 and 1990, a much greater
decline than would be expected for a pool with a residence time of 220 to 5,000 years.
At Hubbard Brook, the pool of Pb in the forest floor declined by 29% between 1977 and 1987,
an even more rapid rate of loss than reported by Friedland et al. (1992). More recently, Evans
et al. (2005) reported significant declines in the Pb content of forest floor soils in the
northeastern United States and eastern Canada between 1979 and 1996. The magnitude of the
decrease in Pb content was greatest at their sites in southern Vermont, and smallest at sites on the
Gaspe Peninsula in Quebec, reflecting the historic gradient in Pb deposition in the region. At the
Vermont site, the Pb concentration in the litter layer (Oi horizon) was 85% lower in 1996 than in
1979. In the Gaspe peninsula of Quebec, the decrease was only 50%.
Since drainage water Pb concentrations remain low, the Pb released from forest floor soils
in the past has been largely immobilized in mineral soils (Miller and Friedland, 1994; Johnson
et al., 1995a; Johnson and Petras, 1998; Watmough and Hutchinson, 2004; Johnson et al., 2004).
This is supported by evidence from Pb-isotope analyses. Gasoline-derived Pb has a 206 Pb: 207 Pb
ratio that can be easily discriminated from Pb in the rocks from which soils are derived. Using
isotopic mixing models with gasoline-Pb and Pb in soil parent materials as end members,
a number of researchers have documented the accumulation of pollutant Pb in mineral soils
(Bindler et al., 1999; Kaste et al., 2003; Watmough and Hutchinson, 2004; Bacon and Hewitt,
2005; Steinnes and Friedland, 2005). In a hardwood stand on Camel’s Hump Mountain in
Vermont, as much as 65% of the pollutant Pb deposited to the stand had moved into mineral
AX7-104