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

floor) time-series data to evaluate the movement of gasoline-derived Pb in the soil profile. These
studies have concluded that the distribution of Pb in the upper soil horizons has changed over the
past few decades. Yanai et al. (2004) documented a decline in Pb from the Oie horizon between
the late 1970s to the early 1990s in remote forest soils in New Hampshire. Johnson et al. (1995a)
and Friedland et al. (1992) demonstrated that some fraction of Pb had moved from the O horizon
to the mineral soil during the 1980s at Hubbard Brook and at selected remote sites in the
northeastern United States, respectively. Evans et al. (2005) demonstrated that Pb concentrations
in the litter layer (fresh litter + Oi horizon) sampled in a transect from Vermont to Quebec
decreased significantly between 1979 and 1996, reflecting a decrease in Pb deposition to forests
and upper soil horizons during that time period. Miller et al. (1993) and Wang and Benoit
(1997) used forest floor time-series data to model the response time (e folding time, the time it
takes a reservoir to decrease to the 1/e, (ca. 37%) of its original amount) of Pb in the forest floor.
Miller et al. (1993) calculated O horizon response times of 17 years for the northern hardwood
forest and 77 years in the spruce-fir zone on Camel’s Hump Mountain in Vermont. Wang and
Benoit (1997) determined that the O horizon would reach steady state with respect to Pb
(1.3 µg g Γ1 Pb) by 2100. Both suggested that the vertical movement of organic particles
dominated Pb transport in the soil profile.
AX7.1.2.3 Inputs/Outputs of Atmospherically-delivered Lead in Terrestrial Ecosystems
The concentration of Pb in contemporary rainfall in the mid-Atlantic and northeastern
United States is on the order of 0.5 to 1 µg/L (Wang et al., 1995; Kim et al., 2000; Scudlark
et al., 2005). Long-term trends in nine elemental concentrations (including Pb) in wet deposition
were measured on the Delmarva Peninsula near Lewes, Delaware, over the period from 1982 to
1989 (Figure AX7-1.2.1) (Scudlark et al., 1994). Of the nine elements measured, only Pb (and
possibly Al and Cd), indicated a decreasing trend over time (3 µg/L in 1982 to