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

horizons by 2001 (Kaste et al., 2003). In a spruce-fir stand, containing a thicker organic forest
floor layer, penetration of pollutant Pb into the mineral soil was much lower.
This recent research has resulted in a reevaluation of the turnover time of Pb in forest
floor soils. The Camel’s Hump data suggest that Pb resides in the forest floor of deciduous
stands for about 60 years and about 150 years in coniferous stands (Kaste et al., 2003). These
values are somewhat greater than those published previously by Miller and Friedland (1994),
who used a Pb budget approach. Extremely rapid turnover of Pb was observed in some
hardwood forest floor soils in south-central Ontario (Watmough et al., 2004). Their estimated
turnover times of 1.8 to 3.1 years are much lower than any other published values, which they
attribute to the mull-type forest floor at their sites. Mull-type forest floors are normally underlain
by organic-rich A horizons, capable of immobilizing Pb released from the forest floor. Indeed,
at the same site in Ontario, Watmough and Hutchinson (2004) found that 90% of the pollutant Pb
could be found in this A horizon.
The time period over which the accumulated Pb in soils may be released to drainage
waters remains unclear. If Pb moves as a pulse through the soil, there may be a point in the
future at which problematic Pb concentrations occur. However, several authors have argued
against this hypothesis (Wang and Benoit, 1997; Kaste et al., 2003; Watmough et al., 2004),
contending that the strong linkage between Pb and DOM will result in a temporally dispersed
release of Pb in the form of Pb-DOM complexes. Thus, the greatest threat is likely to be in the
most highly contaminated areas surrounding point sources of Pb, where the amount of Pb
accumulated in the soil is high, and the death of vegetation has resulted in reduced soil organic
matter levels.
AX7.1.4.4 Summary
Atmospheric Pb pollution has resulted in the accumulation of Pb in terrestrial ecosystems
throughout the world. In the United States, pollutant Pb represents a significant fraction of the
total Pb burden in soils, even in sites remote from smelters and other industrial plants. However,
few significant effects of Pb pollution have been observed at sites that are not near point sources
of Pb. Evidence from precipitation collection and sediment analyses indicates that atmospheric
deposition of Pb has declined dramatically (>95%) at sites unaffected by point sources of Pb, and
AX7-105