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

there is little evidence that Pb accumulated in soils at these sites represents a threat to
groundwaters or surface water supplies.
The highest environmental risk for Pb in terrestrial ecosystems exists at sites within about
50 km of smelters and other Pb-emitting industrial sites. Assessing the risks specifically
associated with Pb is difficult, because these sites also experience elevated concentrations of
other metals and because of effects related to SO2 emissions. The concentrations of Pb in soils,
vegetation, and fauna at these sites can be two to three orders of magnitude higher than in
reference areas (see Sections AX7.1.2 and AX7.1.4.2). In the most extreme cases, near smelter
sites, the death of vegetation causes a near-complete collapse of the detrital food web, creating a
terrestrial ecosystem in which energy and nutrient flows are minimal. More commonly, stress in
soil microorganisms and detritivores can cause reductions in the rate of decomposition of detrital
organic matter. Although there is little evidence of significant bioaccumulation of Pb in natural
terrestrial ecosystems, reductions in microbial and detritivorous populations can affect the
success of their predators. Thus, at present, industrial point sources represent the greatest Pbrelated
threat to the maintenance of sustainable, healthy, diverse, and high-functioning terrestrial
ecosystems in the United States.
AX7.2 AQUATIC ECOSYSTEMS
AX7.2.1 Methodologies Used in Aquatic Ecosystem Research
As discussed in previous sections, aerial deposition is one source of Pb deposition to
aquatic systems. Consequently, to develop air quality criteria for Pb, consideration must be
given to not only the environmental fate of Pb, but also to the environmental effects of Pb in the
aquatic environment through consideration of laboratory toxicity studies and field evaluations.
Perhaps the most straightforward approach for evaluating the effects of Pb is to consider extant
criteria for Pb in aquatic ecosystems, i.e., water and sediment quality criteria. A key issue in
developing Pb water and sediment criteria that are broadly applicable to a range of water bodies
is properly accounting for Pb bioavailability and the range in species sensitivities. This section
summarizes how these criteria are derived, the types of toxicity studies considered, and key
factors that influence the bioavailability of Pb in surface water and sediment to aquatic life.
Because Pb in the aquatic environment is often associated with other metals (e.g., cadmium,
AX7-106