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

varies from 4:1 in rural streams to 27:1 in urban streams (Getz et al., 1977). In still waters, Pb is
removed through sedimentation at a rate determined by temperature, pH, oxidation-reduction
(redox) potential, organic content, grain size, and chemical form of Pb in the water and
biological activities (Jenne and Luoma, 1977). Since the publication of the 1986 Lead AQCD
(U.S. Environmental Protection Agency, 1986a), knowledge of the properties of Pb in aquatic
ecosystems has expanded. This section will provide further detail on the chemical species and
the environmental factors affecting speciation of Pb in the aquatic environment. In addition,
quantitative distributions of Pb in water, sediment, and biological tissues will be presented for
aquatic ecosystems throughout the United States. Finally, recent studies discussing the tracing of
Pb in aquatic systems will be summarized.
AX7.2.2.1 Speciation of Lead in Aquatic Ecosystems
The speciation of Pb in the aquatic environment is controlled by many factors. The
primary form of Pb in aquatic environments is divalent (Pb 2+ ), while Pb 4+ exists only under
extreme oxidizing conditions (Rickard and Nriagu, 1978). Labile forms of Pb (e.g., Pb 2+ ,
PbOH + , PbCO3) are a significant portion of the Pb inputs to aquatic systems from atmospheric
washout. Lead is typically present in acidic aquatic environments as PbSO4, PbCl4, ionic Pb,
cationic forms of Pb-hydroxide, and ordinary Pb-hydroxide (Pb(OH)2). In alkaline waters,
common species of Pb include anionic forms of Pb-carbonate (Pb(CO3)) and Pb(OH)2.
Speciation models have been developed based on the chemical equilibrium model developed by
Tipping (1994) to assist in examining metal speciation. The EPA MINTEQA2 computer model
(http://www.epa.gov/ceampubl/mmedia/minteq/) is one such equilibrium speciation model that
can be used to calculate the equilibrium composition of dilute aqueous solutions in the laboratory
or in natural aqueous systems. The model is useful for calculating the equilibrium mass
distribution among dissolved species, adsorbed species, and multiple solid phases under a variety
of conditions, including a gas phase with constant partial pressures. In addition to chemical
equilibrium models, the speciation of metals is important from a toxicological perspective.
The BLM was developed to study the toxicity of metal ions in aquatic biota and was previously
described in Section AX7.2.1.3. Further detail on speciation models is not provided herein,
rather a general overview of major speciation principles are characterized in the following
sections.
AX7-118