Superfund Research Program
The Effect of Contaminant Aging in Soils on Bioavailability
Many decisions related to the risk assessment and remediation of Superfund sites are based on analytical results that report the total concentration of a contaminant detected in environmental samples. These results are typically obtained from vigorous extraction procedures that are designed to remove as much of the contaminant as possible from samples. Part of the reason why remediation requirements have been based on such data is that it has often been assumed that all of a contaminant detected in an environmental sample was available for biological assimilation. However, evidence is accumulating that contaminant concentration is not necessarily an accurate predictor of potential risks and biological effects of contaminants in soils. Scientists are beginning to learn that the biological availability (bioavailability) of some organic contaminants changes substantially as the compounds reside in soil over long periods of time.
In recent years important advances have been made in understanding the concept of contaminant bioavailability and its significance to the risk assessment and clean up of contaminated soils on Superfund sites. During prolonged exposure of contaminants to soils, the compounds are subject to many dynamic physical and chemical processes. For instance, contaminants can diffuse deeper into the soil micropore structure, or possibly move through the solid phase itself by solid-state diffusion. This sequestration of organic contaminants to soil is termed "aging," and it is a process that ultimately makes contaminants less bioavailable.
Because the bioavailability of contaminants has important implications for predicting the health and ecological risks of contaminated sites, there is a need for a greater understanding of the factors affecting bioavailability. To fill in some of the important knowledge gaps surrounding contaminant bioavailability in soils, researchers at Cornell University are investigating the process of pollutant aging, studies that are providing a more realistic understanding of the risks of contaminated soils. In particular, they are investigating the interactions of model organic compounds in different soil types, as well as the properties of these soils.
Sixteen different types of soil were collected from various locations of the United States and were characterized for their clay content and type, organic matter percentage, nanoporosity, pH, and surface area. Two test contaminants, phenanthrene and atrazine, were aged in the soils. The aging process involved sterilizing the soils, adding the contaminants, and incubating the amended soils for 200 days. The rates and extents of sequestration of the chemicals were determined throughout the aging process by measuring the amount of contaminant completely degraded by microbes, and the amount extracted by a mild extraction technique.
The scientists found that the rates and extents of sequestration of phenanthrene and atrazine varied markedly among the 16 soils, which differed greatly in physical and chemical properties. Both chemicals were sequestered in all 16 soils, but the extent and rate of sequestration varied greatly among the soils. In some soils, sequestration occurred rapidly, while in others it took place slowly. In addition, the two test chemicals exhibited different rates of sequestration over the range of soil types.
These studies were extended to determine the role that organic matter content may have in contaminant aging in soils. According to the principal investigator, Dr. Martin Alexander, based on preliminary observations "it appears that soils having less than about 2% organic carbon support slow and not extensive sequestration, whereas soils with greater quantities or organic matter will support more rapid and more extensive sequestration."
This work is highly significant because it shows that equal amounts of a pollutant in dissimilar soils result in different degrees of bioavailability, findings that may improve the predictive capabilities of contaminant bioavailability based on soil types.
More recent studies show that aging occurs, reducing the toxicity of pesticides to insects and plants and the uptake of various other compounds by earthworms, even without changing the total concentration.
From a remediation and risk assessment perspective, these studies are important for demonstrating a decline in the bioavailability of some organic compounds in soils. This work suggests that the toxicity of some environmental contaminants may diminish over time simply by aging in soil and that current analytical methods for organic pollutants overestimate, often grossly so, the actual exposure and risk.
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To learn more about this research, please refer to the following sources:
- Chung N, Alexander M. 1998. Differences in sequestration and bioavailability of organic compounds aged in dissimilar soils. Environ Sci Technol 32:855-860.
- Robertson BK, Alexander M. 1998. Sequestration of DDT and dieldrin in soil: Disappearance of acute toxicity but not the compounds. Environ Toxicol Chem 17:1034-1038.
- Kelsey JW, Alexander M. 1997. Declining bioavailability and inappropriate estimation of risk of persistent compounds. Environ Toxicol Chem 15:582-585.
- Alexander M. 1995. How toxic are toxic chemicals in soil?. Environ Sci Technol 29:2713-2717. PMID:22206515
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