Quantifying the Bioavailability of Toxic Metals in Soils
Considerable research and other evaluative efforts have been ongoing in recent years to identify environmentally acceptable endpoints (EAE) in soil, develop protocols that can be used to determine EAEs, and make site-specific decisions using EAE data. When applied effectively, these efforts have provided useful descriptions of risk. EAEs for soil most commonly are defined as concentrations of chemicals or other measures of contamination (e.g., biological response or leachability) that are judged acceptable by a regulatory agency or an appropriate entity and are derived either from standard guidelines or following an analysis of site- or chemical-specific information and/or testing. There is a need to supplement the current lack of information regarding metals-contaminated soils, which are of particular relevance to the Department of Defense (DoD).
The overall objective of this project was to investigate the relative bioavailability of toxic metals in soils, primarily in relation to the human health risk posed by soil ingestion, which often controls the degree of cleanup required at metal-contaminated sites. Specific objectives included: (1) measure changes in relative bioavailability over time in a wide range of soil types that may be encountered at DoD sites within the United States, (2) develop a predictive capability to quantify toxic metal bioavailability on the basis of soil properties, and (3) investigate the fundamental relationship between molecular-level speciation and bioavailability to enhance the understanding and predictive capability of the fate of toxic metals in soil.
In this research, soil-metal bioavailability was measured with an in vitro protocol: a physiologically based extraction test to estimate the bioavailability of soil-bound metals in the human gastrointestinal tract. The bioavailability of lead, cadmium, arsenic (As), and chromium (Cr) was measured as a function of time in metal-spiked soils with a wide range of soil properties. The research also featured the use of synchrotron-generated X-ray absorption spectroscopy (XAS), a powerful technique to monitor molecular-level speciation in unaltered soil samples.
The results of this project have been documented in a number of peer-reviewed papers published in leading journals, including Environmental Science & Technology, Journal of Environmental Quality, and Soil and Sediment Contamination. Major conclusions include that the bioaccessibility of both Cr and As is significantly reduced when added in a soluble form to soils. Since these results are due strictly to soil-metal interactions as opposed to metalspecific speciation, they are valid as long as the major soil properties do not change. The bioaccessibility of As in soils depends mainly on the soil’s pH and iron oxide content, while the bioaccessibility of Cr in soils depends mainly on total carbon (both organic and inorganic) and clay content. Models were developed and published to predict both As and Cr bioaccessibility based on the major soil properties.
Results provide site managers and risk assessors with tools to make better initial estimates of site risk and EAEs. Although site-specific data always will need to be considered in final cleanup decisions, the models developed can be used to prioritize sites and to justify more definitive site-specific bioavailability studies, such as detailed soil speciation investigations and in vivo studies. These results will contribute to mission readiness by avoiding unnecessary diversion of DoD funds for unwarranted site cleanup. (Project Completed - 2003)
Points of Contact
Mr. Philip Jardine
University of Tennessee
SERDP and ESTCP
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