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

Lead (Pb) contaminated soils are nearly ubiquitous in urban areas as a result of the former use of leaded gasoline and Pb-based paint. Pb is toxic to many systems in the human body and can inflict permanent brain damage. Soil Pb hazard is related to Pb mineralogy with some forms more easily absorbed (i.e. bioavailable) than others. Phosphorus amendments are often recommended for Pb contaminated soils because Pb phosphates are among the least soluble of Pb compounds. However, Pb hazard is currently evaluated using only total soil Pb because a rapid, inexpensive soil test that is sensitive to Pb hazard (i.e. able to detect decreased Pb bioavailability upon amending soils with phosphorus) has yet to be identified. This study explored the ability of five routine soil nutrient tests (DTPA, Mehlich 1, Mehlich 3, Bray P1, and Olsen) and two experimental assays (0.4 M glycine and 0.01 M CaCl₂) to estimate Pb bioaccessibility.

A silt loam soil was incubated with Pb(NO₃)₂ to attain to soil Pb concentration of 2000 mg kg^-1. After Pb incubation, the soil was split into six treatment groups. Phosphoric acid was applied to the soils at P:Pb ratios of 25:1, 10:1, 5:1, 2:1, 1:1, and 0:1. 300 mg kg^-1 KCl was applied to all soils to induce the formation of chloropyromorphite (a low solubility Pb phosphate mineral).  Soils were returned to their original pH using NaOH.  

Preliminary results show that the soil test extractions were different in their sensitivity to P and Cl additions. Average reductions in extractable Pb following P/Cl additions were as low as 6% (Mehlich 1), as high as 82% (Bray P1), with other tests displaying intermediate reductions (e.g. 40% with Mehlich 3). In vivo trials are needed to indicate which test best approximates reductions in animal Pb uptake in P amended soils. These findings are significant because they illustrate that at least one or more of the existing soil nutrient extraction methods are likely to be useful to accurately estimate Pb bioaccessibility from contaminated soils.