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

organisms. The authors believe there may be two reasons for this. First, metal uptake routes in
soils are generally more complex than those in water, because exposure via pore water and
exposure via ingestion of soil particles may, in principle, both be important. Second, it remains
very difficult to univariately control the composition of the soil pore water and the metal
concentrations in the pore water, due to re-equilibration of the system following modification of
any of the soil properties (including addition of metal salts).
Steenbergen et al. (2005) assessed acute copper toxicity to the earthworm Aporrectodea
caliginosa using the BLM. To overcome the aforementioned problems inherent in soil toxicity
tests they developed an artificial flow-through exposure system consisting of an inert quartz sand
matrix and a nutrient solution, of which the composition was univariately modified. Thus, the
obstacles in employing the BLM to terrestrial ecosystems seem to be surmountable, and future
research may provide useful information on Pb bioavailability and toxicity to terrestrial
organisms.
AX7.1.1.5 Soil Amendments
The removal of contaminated soil to mitigate exposure of terrestrial ecosystem
components to Pb can often present both economic and logistic problems. Because of this,
recent studies have focused on in situ methodologies to lower soil-Pb RBA (Brown et al.,
2003a,b). To date, the most common methods studied include the addition of soil amendments
in an effort either to lower the solubility of the Pb form or to provide sorbtion sites for fixation of
pore-water Pb. These amendments typically fall within the categories of phosphate, biosolid,
and Al/Fe/Mn-oxide amendments.
Phosphate Amendments
Phosphate amendments have been studied extensively and, in some cases, offer the most
promising results (Brown et al., 1999; Ryan et al., 2001; Cotter-Howells and Caporn, 1996;
Hettiarachchi et al., 2001, 2003; Rabinowitz, 1993; Yang et al., 2001; Ma et al., 1995). Research
in this area stems from early work by Nriagu (1973) and Cotter-Howells and Caporn (1996), who
pointed out the very low solubilities for many Pb-phosphates (Ksp !27 to !66), particularly
chloropyromorphite [Pb5(PO4)3Cl]. The quest to transform soluble Pb mineralogical forms into
chloropyromorthite continues to be the primary focus of most studies. Sources of phosphorous
AX7-19