Standard Operating Procedure

1.0 Purpose

An increasingly important property of contaminated media found at environmental sites is the bioavailabilty of individual contaminants. Bioavailability is the fraction of a contaminant that is absorbed by an organism via a specific exposure route. Many animal studies have been conducted to experimentally determine oral bioavailability of individual metals, particularly lead and arsenic. During the period 1989-97, a juvenile swine model developed by USEPA Region VIII was used to measure the relative bioavailability of lead and arsenic in approximately 20 substrates (Weis and LaVelle 1991; Weis et al. 1994). The bioavailability determined was relative (RBA) to that of a soluble salt (i.e. lead acetate trihydrate or sodium arsenate). The tested media had a wide range of mineralogy, and produced a range of lead and arsenic RBA values. In addition to the swine studies, other animal models (e.g. rats and monkeys) have been used for measuring the RBA of lead and arsenic from soils. However, to-date the swine model is still considered the most appropriate for measuring child exposure.

Several researchers have developed in vitro tests to measure the fraction of a chemical solubilized from a soil sample under simulated gastrointestinal conditions. The in vitro tests consist of an aqueous fluid, into which the contaminant is introduced. The solution than solubilizes the media under simulated gastric conditions. Once this procedure is complete, the solution is analyzed for lead and/or arsenic. The mass of the lead and/or arsenic found in the filtered extract is compared to the mass introduced into the test. The fraction liberated into the aqueous phase is defined as the bioaccessable fraction of lead or arsenic in that media (IVBA). To date, for lead-bearing materials tested in the USEPA swine studies, this in vitro assay has correlated well (R² = 0.83, p= .0001) with relative bioavailability. Arsenic has yet to be fully validated but shows a promising correlation with in vivo results.