Evaluation of Traditional Soil Testing Methods to Estimate Lead (Pb) Bioaccessibility.

Lead (Pb) is an insidious and ubiquitous soil contaminant in urban areas. Blood Pb is associated with impaired cognitive function including increased incidence of ADHD, reduced test scores, and behavioral issues even at very low blood Pb concentrations (e.g. <5 µg/dL). Chemical immobilization of Pb with phosphorus is a common Pb remediation strategy due to the low bioavailability of Pb phosphates as well as the logistical obstacles that impede realization of alternative remediation strategies (e.g. extraction and replacement).

Although Pb hazard is contingent on Pb bioavailability, public access to bioaccessibility assays (e.g. the Relative Bioaccessibility Leaching Procedure) is severely limited because the procedures are expensive and unavailable at state or commercial laboratories. To improve public access to Pb bioaccessibility testing, this study explores the ability of five routine soil nutrient tests (DTPA, Mehlich 1, Mehlich 3, Bray P1, and Olsen) and one experimental assays (0.4 M glycine) to estimate Pb bioaccessibility as determined by EPA Method 1340.

A silt loam soil was incubated with Pb(NO₃)₂ and KCl solutions to target concentrations of 2000 mg Pb kg^-1 and 288 mg Cl kg^-1. The spiked soil was then divided and treated with differing amounts of H₃PO₄. Target P:Pb molar ratios of the treatments were 0:1, 1:1, 2:1, 5:1, 10:1, and 25:1. Following Pb, P, and Cl incubation, Ca(OH)₂ was added to raise the soil pH to 6.

Results show that the Mehlich 3 correlates well with the Pb bioaccessibility procedure EPA Method 1340. Bray P1, DTPA, and Olsen tests correlate with EXAFS spectra. These findings suggest that common, inexpensive tests may accurately predict soil Pb bioaccessibility. However, results from the RBALP and EXAFS spectra are incongruent. In vivo validation is necessary to ultimately determine which, if any of these tests is suitable to determine Pb bioaccessibility in a P amended soil.