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

Leach et al. (1998), Stacey et al. (1968), Zartman (1974), Cannon and Pierce (1963), Graney
et al. (1996), Unruh et al. (2000), James and Henry (1993), Rabinowitz (2005), and
Small (1973).
The use of Pb isotopes to quantitatively apportion source contributions follows the simple
mixing rule when only two sources are possible (Faure, 1977). Once multiple sources need to be
considered, a unique solution can no longer be calculated (Fry and Sherr, 1984). Phillips and
Gregg (2003) have designed a model to give feasible source contributions when multiple sources
are likely. Many studies have demonstrated the usefulness of this apportionment technique.
Media of all types have been studied: water (Flegal et al., 1989a,b; Erel et al., 1991; Monna
et al., 1995), ice (Planchon et al., 2002), dust (Adgate et al., 1998; Sturges et al., 1993), and
soil/sediments (Hamelin et al., 1990; Farmer et al., 1996; Bindler et al., 1999; Haack et al., 2004;
Rabinowitz and Wetherill, 1972; Rabinowitz, 2005; Ketterer et al., 2001).
AX7.1.1.2 Speciation in Assessing Lead Bioavailability in the Terrestrial Environment
One of the three processes defined by the National Research Council in its review on
bioavailability (NRC, 2002) is “contaminant interactions between phases”, more commonly
referred to as “speciation.”
A wide variety of analytical (XRD, EPMA, EXAFS, PIXE, XPS, XAS, SIMS) and
chemical speciation modeling (SOILCHEM, MINTEQL, REDEQL2, ECOSAT, MINTEQA2,
HYDRAQL, PHREEQE, WATEQ4F) tools have been used to characterize a metal’s speciation
as it is found in various media. Currently, for risk assessment purposes (not considering
phytotoxicity), where large sites with numerous media, pathways, and metals must often be
characterized in a reasonable time frame, electron microprobe analysis (EMPA) techniques
provide the greatest information on metal speciation. Other techniques such as extended X-ray
absorption fine structure (EXAFS) and extended X-ray absorption near edge spectroscopy
(EXANES) show great promise and will be important in solving key mechanistic questions.
In the case of phytotoxicity, the speciation of metals by direct measurement or chemical models
of pore-water chemistry is most valuable. Further work needs to be done in developing
analytical tools for the speciation of the methyl-forming metals (Hg, As, Sb, Se, and Sn) in soils
and sediments.
AX7-3