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

contributions of dry deposition may have changed in the last few decades. If the major source of
Pb to a terrestrial ecosystem is resuspended particulates from transportation corridors, then the
particle size fraction that dominates deposition may be relatively coarse (>50 µm) relative to
other atmospheric sources (Pirrone et al., 1995; Sansalone et al., 2003).
Total contemporary loadings to terrestrial ecosystems are approximately 1 to 2 mg m Γ2
year Γ1 (Wu et al., 1994; Wang et al., 1995; Simonetti et al., 2000; Sabin et al., 2005). This is a
relatively small annual flux of Pb if compared to the reservoir of approximately 0.5 to 4 g m Γ2 of
gasoline-derived Pb that is already in surface soils over much of the United States (Friedland
et al., 1992; Miller and Friedland, 1994; Erel and Patterson, 1994; Marsh and Siccama, 1997;
Yanai et al., 2004; Johnson et al., 2004; Evans et al., 2005). While vegetation can play an
important role in sequestering Pb from rain and dry deposition (Russell et al., 1981), direct
uptake of Pb from soils by plants appears to be low (Klaminder et al., 2005). High elevation
areas, particularly those near the base level of clouds often have higher burdens of atmospheric
contaminants (Siccama, 1974). A Pb deposition model by Miller and Friedland (1994) predicted
2.2 and 3.5 g Pb m Γ2 deposition for the 20th century in the deciduous zone (600 m) and the
coniferous zone (1000 m), respectively. More recently, Kaste et al. (2003) used radiogenic
isotope measurements on the same mountain to confirm higher loadings at higher elevation.
They measured 1.3 and 3.4 g gasoline-derived Pb m Γ2 in the deciduous zone and coniferous
zones, respectively. Higher atmospheric Pb loadings to higher elevations are attributed to
(1) the higher leaf area of coniferous species, which are generally more prevalent at high
elevation; (2) higher rainfall at higher elevation; and (3) increased cloudwater impaction at high
elevation (Miller et al., 1993).
Although inputs of Pb to ecosystems are currently low, Pb export from watersheds via
groundwater and streams is substantially lower than inputs. Therefore, even at current input
levels, watersheds are accumulating industrial Pb. However, burial/movement of lead over time
down into lower soil/sediment layers also tends to sequester it away from more biologically
active parts of the watershed (unless later disturbed or redistributed, e.g., by flooding, dredging,
etc.). Seeps and streams at the HBEF have Pb concentrations on the order of 10 to 30 pg Pb g Γ1
(Wang et al., 1995). At a remote valley in the Sierra Nevada, Pb concentrations in streamwaters
were on the order of 15 pg Pb g Γ1 (Erel and Patterson, 1994). Losses of Pb from soil horizons are
assumed to be via particulates (Dörr and Münnich, 1989; Wang and Benoit, 1996, 1997). Tyler
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