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

these studies have provided compelling evidence that the biomineralization of Pb phosphates by
soil organisms can play an important role in the biogeochemistry of Pb. Surface soils sampled
relatively recently demonstrate that the upper soil horizons (O + A horizons) are retaining most
of the industrial Pb burden introduced to the systems during the 20th century. The migration and
biological uptake of Pb in ecosystems is relatively low. The different biogeochemical behaviors
of Pb reported by various studies may be a result of the many different analytical techniques
employed, or they may be a result of natural variability in the behavior of Pb in different
systems.
Lead Uptake into Plants
Plants take up Pb via their foliage and through their root systems (U.S. Environmental
Protection Agency, 1986a; Påhlsson, 1989). Surface deposition of Pb onto plants may represent
a significant contribution to the total Pb in and on the plant, as has been observed for plants near
smelters and along roadsides (U.S. Environmental Protection Agency, 1986a). The importance
of atmospheric deposition on above-ground plant Pb uptake is well-documented (Dalenberg and
Van Driel, 1990; Jones and Johnston, 1991; Angelova et al., 2004). Data examined from
experimental grassland plots in southeast England demonstrated that atmospheric Pb is a greater
contributor than soil-derived Pb in crop plants and grasses (Jones and Johnston, 1991). A study
by Dalenberg and Van Driel (1990) showed that 75 to 95% of the Pb found in field-grown test
plants (i.e., the leafy material of grass, spinach, and carrot; wheat grain; and straw) was from
atmospheric deposition. Angelova et al. (2004) found that tobacco grown in an industrial area
accumulated significant amounts of Pb from the atmosphere, although uptake from soil was also
observed. The concentration of Pb in tobacco seeds was linearly related to the concentration of
Pb in the exchangeable and carbonate-bound fractions of soil, as measured using sequential
extraction (Angelova et al., 2004). Lead in soil is more significant when considering uptake into
root vegetables (e.g., carrot, potato), since, as was noted in the 1986 Lead AQCD (U.S.
Environmental Protection Agency, 1986a), most Pb remains in the roots of plants.
There are two possible mechanisms (symplastic or apoplastic) by which Pb may enter the
root of a plant. The symplastic route is through the cell membranes of root hairs; this is the
mechanism of uptake for water and nutrients. The apoplastic route is an extracellular route
between epidermal cells into the intercellular spaces of the root cortex. Previously, Pb was
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