Fractional Bio-Accessibility: A New Tool with Revised Recommendations for Pb Risk Assessment for Urban Garden Soils and Superfund Sites.

Pb contamination of urban soils and its association with elevated blood Pb concentrations in children has raised concerns about whether gardening should be recommended for urban areas. Reliable evaluation of the potential hazard to the public from exposure to urban garden soils greatly depends upon the bioavailability of Pb in soil which may be ingested. Several researchers have shown that in vivo soil Pb bioavailability tests can be replaced by in vitro or bioaccessibility tests to save cost and time. However, present bioaccessibility methods are expensive and laborious. Thus there is a need for an easier and lower cost bioaccessibility test method that could be widely adopted by agricultural and environmental laboratories. The authors have tested a simplified soil Pb bioaccessibility extraction test method (extraction of 5 g of <2 mm soil with 50 mL of 0.4 M glycine hydrochloride solution adjusted to pH 2.5 using 4 M NaOH (=0.38 M glycine hydrochloride), at room temperature; shaking for 2 hours @ 100 rpm) which is 10-20 times less expensive than previous Pb-bioaccessibility established methods. The method not only shows a high correlation with the reduction in Pb bioavailability results from feeding Joplin, MO, control and remediated smelter contaminated soils to humans and rats, but was also applied to Pb rich urban garden soils. The method revealed that fractional bioaccessibility (bioaccessible compared to total) of Pb in urban garden soils is only 2-15% (median=10%) of total Pb in soils containing 37-2400 ppm Pb, far lower than the 60% as bioavailable as food-Pb presumed by US-EPA in evaluating risk from soil Pb. EXAFS examination of Pb speciation in the urban garden soils will also be reported. It seems likely that repeated tillage, fertilization, and time have promoted inactivation of Pb in most garden soils. These findings suggest that urban gardening has much lower potential for Pb risk than is currently assumed, and that authorities could recommend urban gardening more freely than previously believed. The findings also support revised recommendations from US-EPA and USDA for Pb contaminated urban gardens.