Application of Physiologically Based Extraction Tests to Evaluate Metal Bioaccessibility In a Continental-Scale Soil Geochemical Survey

In 2004 the U.S. Geological Survey and the Geological Survey of Canada sampled soils along two transects across the two countries to test and refine sampling and analytical protocols recommended for the planned soil geochemical survey of North America.

Samples were collected at 221 sites selected randomly at approximately 40-km intervals. Physiologically based extraction tests (PBETs), an inexpensive in vitro leach test designed to estimate the bioaccessibility of metals, were conducted on a subset of 20 samples of soils collected from a depth of 0-5 cm from these two transects. Two separate size fractions (<2 mm and <250 mm) were extracted using a simulated human gastric fluid consisting of a solution of hydrochloric acid and glycine adjusted to a pH of 1.5. The standard size fraction used for bulk chemical analysis of soils is <2 mm; however, current research makes use of a <250 mm particle size for PBETs, as these particles are most likely to adhere to the hands of children and be ingested. Due to the large number of samples anticipated, one goal of this study was to identify if there are appreciable differences in gastric bioaccessibility between the <2 mm and <250 mm size fractions that would require the additional costly step of sieving to the smaller size. Our results indicate that in general, the <2 mm size fraction was not substantially different than the <250 mm size fraction in the concentrations of potentially toxic metals and metalloids leached by simulated gastric fluids for these uncontaminated transect soils. Bioaccessibility of the metals decreased in the order Cd> Pb> Ni> As> Cr. For these samples, gastric leachate values appear to be principally correlated with the exchangeable soil fraction and total concentration of metal in the soil.