Bioaccessibility Estimates for Soil Pb Should Correlate with Human Bioavailability of Treated Soils.

Potential for risk from Pb in contaminated soils ingested by humans is affected by the chemical forms of Pb present in the soil, the Pb adsorptive properties of the soil, and the presence of food in the stomach and intestine. Researchers have been trying to develop chemical extraction methods (designated "bioaccessibility" to distinguish it from bioavailability which must be based on feeding test results) which are well correlated with results from animal feeding tests of soil Pb absorption. Methods were developed based on feeding tests using rabbits, rats, swine, and humans. Ultimately, human feeding tests are the "gold standard" for soil Pb bioavailability/bioaccessibility to humans. The "Drexler" bioaccessibility method adopted by US-EPA is based on feeding of mine waste contaminated soil materials to swine. But when untreated and phosphate treated soils from Joplin, MO (smelter contaminated urban lawn), were fed to human volunteers, the Drexler method substantially underestimated the reduction in soil Pb bioavailability to humans due to the in situ phosphate treatment. We used the Joplin soils to develop a bioaccessibility extraction method well correlated with the human feeding test results for these soils (Ryan et al., 2004, ES&T), trying to make the method as simple and inexpensive as practicable. The method uses a common lateral shaker, conducted at room temperature rather than 37°C, and pH 2.5 adjusted 0.4 M glycine-HCl buffer solution rather than the pH 1.5 of the Drexler method. For many soils extracting the < 2 mm sieve fraction gives results similar to extraction of the <250 μm sieve fraction, substantially reducing the effort needed to prepare soils for extraction. Because of the simplicity of the method, it can be conducted at much lower cost than the Drexler method, including use of atomic absorption or ICP-AES rather than the more expensive ICP-MS or graphite furnace AAS. A Pb-bioaccessibility extraction method is needed to provide information to urban gardeners whose soils are commonly contaminated with Pb such that they need to know whether their children would be at risk from incidental ingestion of their garden soil or housedust contaminated by soil. Testing many urban garden soils from Baltimore, MD and Washington, DC revealed that garden soil Pb bioaccessibility (5-10%) was considerably less than that of mine wastes evaluated by US-EPA indicating that many garden soils comprise lower risk than presently assumed by EPA (30%) in development of present Federal guidance regarding Pb in urban soils including gardens.