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

lack of agreement on the level of mSQGQs, masking of an individual chemical’s effect due to
data aggregation, lack of SQGs for all chemicals of concern, and mSQGQs were not initially
derived as a regulatory standard or criterion, thus there is a reluctance to use them in
enforcement or remediation (Long et al., 2006).
For assessing Pb effects on aquatic ecosystems, it is not truly feasible to account for metal
mixtures, because these will obviously vary highly from site to site. However, the toxicity of
metal mixtures in surface water should be considered on a site-specific basis.
AX7.2.1.6 Background Lead
Because Pb is naturally occurring, it is found in all environmental compartments
including surface water, sediment, and aquatic biota. Background Pb concentrations are spatially
variable depending on geological features and local characteristics that influence Pb speciation
and mobility. In the European Union risk assessments for metals, an “added risk” approach has
been considered that assumes only the amount of metal added above background is relevant in a
toxicological evaluation. However, this approach ignores the possible contribution of
background metal levels to toxic effects, and background metal levels are regionally variable,
precluding the approach from being easily transferable between sites. In terms of deriving
environmental criteria for Pb, background levels should be considered on a site-specific basis if
there is sufficient information that Pb concentrations are naturally elevated. As discussed
previously, the use of radiogenic Pb isotopes is useful for source apportionment.
AX7.2.2 Distribution of Lead in Aquatic Ecosystems
Atmospheric Pb is delivered to aquatic ecosystems primarily through deposition (wet
and/or dry) or through erosional transport of soil particles (Baier and Healy, 1977; Dolske and
Sievering, 1979; and Yang and Rose, 2005). A number of physical and chemical factors govern
the fate and behavior of Pb in aquatic systems. The EPA summarized some of these controlling
factors in the 1986 Lead AQCD (U.S. Environmental Protection Agency, 1986a). For example,
the predominant form of Pb in the environment is in the divalent (Pb 2+ ) form and complexation
with inorganic and organic ligands is dependent on pH (Lovering, 1976; Rickard and Nriagu,
1978). A significant portion of Pb in the aquatic environment exists in the undissolved form
(i.e., bound to suspended particulate matter). The ratio of Pb in suspended solids to Pb in filtrate
AX7-117