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

Protection Agency, 1986a). In fact, the main issue identified by the EPA (U.S. Environmental
Protection Agency, 1986a) related to invertebrate uptake of Pb was not toxicity to the
invertebrates, but accumulation of Pb to levels that may be toxic to their consumers. Several
authors suggest that shrews are a good indicator of metal contamination, because they tend to
accumulate higher levels of metals than herbivorous small mammals (see data summary in
Sample et al. (1998)). Shrews accumulate higher levels of metals in contaminated habitats,
because their diet mainly consists of detritivores (i.e., earthworms) and other soil invertebrates in
direct contact with the soil (Beyer et al., 1985).
Physical/Environmental Factors
Plants
The uptake and distribution of Pb into higher plants from the soil is affected by various
chemical and physical factors including the chemical form of Pb, the presence of other metal
ions, soil type, soil pH, cation exchange capacity (CEC), the amount of Fe/Mn-oxide films
present, organic matter content, temperature, light, and nutrient availability. A small fraction of
Pb in soil may be released into the soil water, which is then available to be taken up by plants
(U.S. Environmental Protection Agency, 1986a).
The form of Pb has an influence on its toxicity to plants. For example, Pb-oxide is less
toxic than more bioavailable forms such as Pb-chloride or Pb-acetate. In a study by Khan and
Frankland (1983), radish plants were exposed to Pb-oxide and Pb-chloride in a loamy sand at pH
5.4, in a 42-day study. In a tested concentration range of 0 to 5000 mg/kg, root growth was
inhibited by 24% at 500 mg/kg for Pb-chloride and an EC50 of 2400 mg/kg was calculated from a
dose-response curve. Plant growth ceased at 5000 mg/kg and shoots exhibited an EC50 of
2800 mg/kg. For Pb-oxide exposure (concentration range of 0 to 10,000 mg/kg), reported results
indicate an EC50 of 12,000 mg/kg for shoot growth and an EC50 of 10,000 mg/kg for root growth.
There was no effect on root growth at 500 mg/kg and a 26% reduction at 1000 mg/kg Pb oxide.
Soil pH is the most influential soil property with respect to uptake and accumulation of Pb
into plant species. This is most likely due to increased bioavailability of Pb created by low soil
pH. At low soil pH conditions, markedly elevated Pb toxicity was reported for red spruce
(P. rubens) (Seiler and Paganelli, 1987). At a soil pH of 4.5, ryegrass (Lolium hybridum)
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