Citation:

Bradham, K, S L. Harper, AND E A. Coppedge. BIOAVAILABILITY OF METALS IN CONTAMINATED SOIL AND DUST. Presented at 19th Annual Regional Risk Assessors Meeting, Boston, MA, May 3-7, 2004.

Impact/Purpose:

The objective of this task is to develop state-of-the-science methods for evaluating toxic elements. These methods will include measuring bioavailable concentrations of toxic elements as well as enhanced traditional methods. Once established, the performance of these methods will be demonstrated on a small set of samples. Procedures will be documented such that they are readily transferable to EPA and non-EPA researchers.

Specific objectives of this task include:

1) Development of more sensitive and rapid quantitative procedures for evaluating exposure to toxic elements.

2) Assessment of the bioavailability of toxic elements in environmental media using bioavailability tools.

3) Application of this data to improve the scientific basis of site and source specific risk assessments.

4) Application of these methods to fill critical data gaps identified in the aggregate/cumulative exposure and risk research area of the Human Health Risk Assessment research plan.

Description:

Due to widespread metal contamination, it is necessary to characterize soils suspected of metal contamination and determine if the metal levels in these soils pose a hazard. Metal toxicity is often not directly related to the total concentration of metals present due to a number of modifying factors that depend, in part, on soil physicochemical properties. Soil organic matter, pH, and clay content are a few of the soil physicochemical properties that influence metal toxicity and bioavailability. Current routine practices of investigating the nature and extent of contamination at metal-contaminated sites involve determining total metal content in media, such as soil and dust. Approximately 60-80% of household dust is tracked-in soil. The quantity of toxic element to be used as exposure concentrations in the risk calculation is an assumed percentage of the total content, which may be the bioavailable concentration. Often, very high percentages (near 100%) of total metals are assumed bioavailable. While this assumption is conservative in terms of being protective of human health and the environment, it may not be a reasonable estimate of site conditions because the actual bioavailability of metals has not been assessed. The resulting risk calculations may overestimate the true risk of exposure to site media. Use of the bioavailability approach in conjunction with the total concentration analysis will reduce uncertainty in the risk assessment, thus avoiding unnecessary risk-reduction expenses.

The data from research showing the effect of soil properties on the bioavailability and toxicity of metals in 21 contaminated soils with a wide range of physicochemical properties (pH, % organic carbon, % clay, and amorphous iron and aluminum oxides) will be presented. The combined relationship between biological endpoints (lettuce and earthworms) and soil properties were examined using multiple regression and structural equation modeling (path analysis). Structural equation models proved useful for providing a quantitative causal influence of soil properties on metal bioavailability and toxicity.

In addition, an overview of on-going research projects within the National Exposure Research Laboratory (NERL), Human Exposure and Atmospheric Sciences Division (HEASD) will be presented. These projects include investigating the bioavailability of arsenic from select soil samples contaminated with chromated copper arsenate (CCA) in a controlled field study as well as a sub-set of soils and house dusts collected in the U.S. Department of Housing and Urban Development's (HUD) nationwide "American Healthy Homes Survey". Both projects will produce data under varying environmental conditions to help establish relationships between total concentrations and bioavailable concentrations to yield a better understanding of the absorbed dose of toxic elements.

Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.

Record Details: