phosphorus (P) was measured using the Mehlich-3 extraction .... Select properties and particle-size distribution of study soils. ..... doi:10.1081/CSS-120025208.
Journal of Environmental Quality
TECHNICAL REPORTS Urban Pollutants
Using the Mehlich-3 Soil Test as an Inexpensive Screening Tool to Estimate Total and Bioaccessible Lead in Urban Soils K. K. Minca, N. T. Basta,* and K. G. Scheckel
R
ecently, large-scale urban redevelopment and demolition of abandoned housing has created large areas of vacant land in old industrial cities in the United States, including Cleveland, OH, Detroit, MI, and Chicago, IL. City planners, community groups, and other stakeholders seek to convert some of the newly created vacant land into urban agriculture and gardening, parks, playgrounds, and other common areas. Because this type of land use involves high amounts of exposure to the soil, it is important to assess historical legacy soil contaminants in these areas before redevelopment. All soils contain Pb with background levels at relatively low concentrations due to naturally occurring soil lead minerals. Background Pb in 3045 uncontaminated surface soil samples in the United States ranged from 5 mg dL-1) (CCBH, 2010). Children in urban areas are at risk of high blood lead because they are exposed to Pb through the ingestion of household dust as well as ingestion of soil. Most risk from Pb is associated with the soluble forms of Pb in the gastrointestinal tract that are biologically available for absorption across the intestinal epithelium, or “bioavailable” to humans. Bioavailable Pb is the portion of Pb chemical dose that enters the systemic circulation from the gastrointestinal tract (i.e., absorbed dose) from an administered dose. Animal models have been used to predict the bioavailability of Pb and other metal(loid)s in soil to humans via incidental ingestion of soil (Casteel et al., 1997, Freeman et al., 1993; Roberts et al., 2007; Ruby et al., 1999). Because of the cost and complexity of these in vivo methods, several in vitro methods have been developed to predict the bioaccessibility of Pb in soil (Chaney et al., 2011; Ruby et al., 1999; Scheckel et al., 2009). Bioaccessibility is the amount of contaminant (e.g., Pb) in ingested soil that dissolves under simulated gastrointestinal conditions and becomes available for potential absorption (Hooda, 2010). To protect human health, the USEPA set the maximum Pb concentration in residential bare soil used for children’s play at 400 mg Pb kg-1 soil as a soil screening level (SSL). Soils that exceed the maximum concentration are recommended for further investigation to determine restrictions on land use. The soil test used to assess the potential human health risk of Pb in urban soil is designed to measure most of the total Pb content of soil. The most commonly used methods involve partial dissolution of soil using strong acids and/or pressure-assisted microwave acid digestion such as USEPA Method 3051a (USEPA, 1997). The amount of ingested soil Pb dissolved in the gastrointestinal tract and absorbed by human into the blood (i.e., bioavailable) is less than the total content of Pb in the soil. Information on bioavailable Pb in soil is more important to human health effects than is total Pb in the soil. Testing of Pb bioavailability can be performed by using expensive animal dosing trials or using an in vitro bioaccessibility (IVBA) method, such as the Relative Bioavailability Leaching Procedure (RBALP) (Drexler and Brattin 2007), to predict bioavailable Pb. Neither total nor bioaccessible Pb provides information about the soil and both are expensive (>$300 per soil sample). Furthermore, rarely do soil testing laboratories analyze total Pb, and less than 10 laboratories www.agronomy.org • www.crops.org • www.soils.org
in the United States analyze bioaccessible Pb soil tests. Most soils used for agriculture and gardening in urban areas are not tested for Pb because of the cost and unavailability of highly specialized in vitro soil tests. Most soil testing laboratories routinely test agricultural soils for plant available nutrients to make fertilizer recommendations. The Mehlich-3 soil test is one of the most commonly used method in the United States. (Wolf and Beegle, 2011). This soil test is widely available to the public and inexpensive (
