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

calculations indicate that only a small fraction of leaded gasoline fallout from the late 1980s has
been washed out of the San Joaquin and Sacramento rivers’ drainage basin by 1995 and
consequently freshwater inputs remain a Pb source to the bay (Steding et al., 2000). The authors
suggest that the continuous source of Pb from the river systems draining into the bay, coupled
with benthic remobilization of Pb, indicates that historic gasoline deposits may remain in the
combined riparian/estuarine system for decades.
AX7.2.2.3 Tracing the Fate and Transport of Lead in Aquatic Ecosystems
The following section presents a generalized framework for the fate and transport of Pb in
aquatic systems (Figure AX7-2.2.14). The primary source of Pb in natural systems is
atmospheric deposition (Rickard and Nriagu, 1978; U.S. Environmental Protection Agency,
1986a). Estimated median global atmospheric emission for anthropogenic and natural sources
are 332 Η 10 6 kg/year and 12 Η 10 6 kg/year, respectively (summarized by Giusti et al., 1993).
Inorganic and metallic Pb compounds are nonvolatile and will partition to airborne particulates
or water vapors (Syracuse Research Corporation., 1999). Dispersion and deposition of Pb is
dependent on the particle size (U.S. Environmental Protection Agency, 1986a; Syracuse
Research Corporation., 1999). More soluble forms of Pb will be removed from the atmosphere
by washout in rain.
In addition to atmospheric deposition, Pb may enter aquatic ecosystems through industrial
or municipal wastewater effluents, storm water runoff, erosion, or direct point source inputs
(e.g., Pb shot or accidental spills). Once in the aquatic environment, Pb will partition between
the various compartments of the system (e.g., dissolved phase, solid phase, biota). The
movement of Pb between dissolved and particulate forms is governed by factors such as pH,
sorption, and biotransformation (see Section AX7.2.2.1). Lead bound to organic matter will
settle to the bottom sediment layer, be assimilated by aquatic organisms, or be resuspended in the
water column. The uptake, accumulation, and toxicity of Pb in aquatic organisms from water
and sediments are influenced by various environmental factors (e.g., pH, organic matter,
temperature, hardness, bioavailability). These factors are further described in Section
AX7.2.3.4). The remainder of this section discusses some methods for describing the
distribution of atmospheric Pb in the aquatic environment.
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