Assessing Bioaccessibility of Lead in Urban Soils: The Effect of Amendments.

Twenty-one pairs of urban garden soil samples were selected from an urban soil sample repository composed of nearly 3000 soil samples submitted to the Brooklyn College Soil Lab from home and community gardens in New York City since 2009. Each pair was from one urban residence, with one of them relatively “original” (undisturbed) while the other had documented history of amendments in the past. The bioaccessible lead in these samples were evaluated following the standard US EPA protocol (EPA 9200.2-86: Standard Operating Procedure for an In Vitro Bioaccessibility Assay for Lead in Soil), in which 0.4-M glycine acid at pH=1.5 was used. Standard Reference Material 2710a was used as the external standard for both total Pb and the percentage of bioaccessible Pb. Total Pb in the undisturbed soils ranged 150-4100 ppm and percent bioaccessible Pb ranged 37-78%; while for the amended soils the ranges were 56-3000 ppm and 39-97%, respectively. There is no obvious correlation between the percent bioaccessible Pb and soil organic content (determined by loss on ignition) – a probable indictor of amendments. In many cases, the amended soil had higher percentage of bioaccessible Pb than the undisturbed soil in the same pair. There is a weak positive correlation observed between the percentage bioaccessible Pb and total P, contradict to the common assumption that P would help bind the lead and make it less bioaccessible. The lack of systematic differences for the percent of bioaccessible Pb between undisturbed and amended soils could suggest that the existing in vitro assay might not be adequate for assessing bioaccessibility of Pb in urban soils, which are known to contain relatively high carbonate and phosphate contents. It is also possible that the effects of amendments on reducing lead bioaccessiblity are highly variable for urban soils, due to the high degree of heterogeneities in soil properties.