Bioavailability of lead in contaminated soil depends on the nature of bioreceptor.

Long-term lead (Pb) contaminated soils from two lead-zinc smelters and a shooting range, along with freshly spiked control soil, were studied by means of chemical, biological or a physiological method to examine the effect of ageing on Pb bioavailability. The freshly Pb spiked control soil was subjected to an earthworm toxicity test to observe the avoidance and mortality response of the earthworms. Meanwhile, an extractable fraction of Pb on the spiked soil as a result of ageing was examined and further compared with physiologically based in vitro bioaccessibility extraction tests. Their differences in lethal concentration, LC(50), to the earthworm population from spiked soils varied substantially as a function of soil pH. The strong effect of ageing on toxicity was also reflected in the extractability of Pb which was far greater in acidic soil, labelled AC, compared to the alkaline soil, labelled BC. This demonstrates that the bioavailable fraction causing toxicity to earthworms was achieved at a much lower total Pb content for acidic soils relative to alkaline soils. Moreover, the effect of ageing also exhibits that a marked decline in bioavailable Pb results in lowering toxicity. Significant amounts of weight loss in earthworms during an acute toxicity test in long-term contaminated soils at a relatively low Pb concentration suggested that other metal or combined metal toxicity may also play a significant role. This study demonstrates that the soil characteristics and ageing period greatly influence the bioavailable fraction of Pb which is related to the bioreceptor.