EVALUATION OF MEHLICH-3 TEST FOR PREDICTION OF TOTAL AND BIOACCESSIBLE LEAD CONCENTRATIONS IN PAINT-CONTAMINATED RESIDENTIAL SOILS

Lead (Pb), a probable human carcinogen, is the second most hazardous substance in the ATSDR list. Children under 6 are most affected by Pb, which severely damages their developing neurological systems. Although USEPA banned use of Pb-based paints in 1978, still there exists more than 21 million homes in the US with Pb-based paints. In due course, Pb from the exterior paints leaches into yard soils, which eventually make way to the inside of houses as Pb-rich dust, creating the primary exposure route for children. Hence, human health risk from Pb exposure in residential settings can be minimized only if Pb in the soils is treated. It is very important to thoroughly characterize yard soils for Pb before taking any remedial action. As total soil-Pb analysis is expensive, researchers are searching for more affordable alternatives that can predict both total Pb concentrations in soils, as well as the Pb fraction that is bioaccessible. It has been proposed by some that the relatively inexpensive Mehlich3 (Mh3) soil test is a good indicator of total Pb concentration in soils, irrespective of their physic-chemical properties. The objective of our study was to evaluate the potential of Mh3-Pb as a surrogate of total Pb and bioaccessible Pb concentrations in residential soils as a function of soil properties. Soil samples were collected from 39 residential sites in San Antonio, TX (alkaline soils) and Baltimore, MD (acid soils). Soil pH ranged between 5.02 and 8.02, and total soil-Pb concentration ranged between 18 mg/kg and 5751 mg/kg. Soils were extracted using Mh3 and the Olsen soil test method (which, based on the chemistry of the extractant, is supposed to work better for alkaline soils) and correlated with total soil-Pb and bioaccessible soil-Pb. The regression factor (R²) for Mh3-Pb to total-Pb and bioaccessible Pb were 0.75 and 0.73, respectively for all soils combined (n=39). The Mh3 test was particularly good in predicting total and bioaccessible Pb in acidic soils (MD) with R² of 0.90 and 0.91, respectively (n=20), but was not as good in alkaline soils (TX) (n=19) with R² values of 0.73 and 0.71, respectively. The Olsen test did better than Mh3 while predicting total and bioaccessible Pb in alkaline soils. Our study showed the need to consider soil properties while choosing the most appropriate soil test for Pb in paint-contaminated soils.