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

Since the movement and fate of Pb in terrestrial ecosystems is strongly related to the
organic matter cycle (Section AX7.1.2), stressors that could lead to disruption or alteration of the
soil organic matter pool are of particular concern in assessing effects of ecosystem stress on Pb
cycling. By binding soluble Pb, soil organic matter acts as a barrier to the release of Pb to
drainage waters (Wang et al., 1995; Kaste et al., 2003; Watmough and Hutchinson, 2004). As a
result, concentrations of Pb in soil solutions and drainage waters tend to be low (Driscoll et al.,
1988; Wang et al., 1995; Bacon and Bain, 1995; Johnson et al., 1995a). Through decomposition
and leaching, soluble organic matter is released to solution, and with it, some Pb is also
mobilized. Wang and Benoit (1996) found that essentially all of the Pb in soil solutions in a
hardwood forest in New Hampshire was bound to dissolve organic matter (DOM). This release
of soluble Pb does not typically result in elevated surface water Pb concentrations, because
(1) organic matter has a relatively long residence time in most temperate soils (Gosz et al., 1976;
Schlesinger, 1997), so only a small fraction of the organic matter pool is dissolved at any time;
(2) DOM-Pb complexes solubilized in upper soil horizons may be precipitated or adsorbed lower
in the soil profile; (3) the DOM to which Pb is bound may be utilized by microbes, allowing the
associated Pb to bind anew to soil organic matter. Together, these factors tend to moderate the
release of Pb to surface waters in temperate terrestrial ecosystems. However, stressors or
disturbances that result in increased release of DOM from soils could result in the unanticipated
release of Pb to groundwater and/or surface waters.
Acidification
The effect of acidification on ecosystem cycling of Pb is difficult to predict. Like most
metals, the solubility of Pb increases as pH decreases (Stumm and Morgan, 1995), suggesting
that enhanced mobility of Pb should be found in ecosystems under acidification stress. However,
Pb is also strongly bound to organic matter in soils and sediments. Reductions in pH may cause
a decrease in the solubility of DOM, due to the protonation of carboxylic functional groups
(Tipping and Woof, 1990). Because of the importance of Pb complexation with organic matter,
lower DOM concentrations in soil solution resulting from acidification may offset the increased
solubility of Pb and hence decrease the mobility of the organically bound metal.
In a study of grassland and forest soils at the Rothamsted Experiment Station in England,
long-term (i.e., >100 years) soil acidification significantly increased the mobility of Pb in the soil
AX7-90