Executive Summary
Sites which contain contaminated soils are common. While the need to protect human health and the environment at these sites is rarely debated, there are questions about the magnitude of risk posed by the chemicals in such soils and about the cleanup levels that should be achieved. Currently, soil cleanup levels are based on chemical- or media-specific criteria or guidelines, or on limits stemming from risk-based analyses. Chemical availability-the ability of a chemical to reach and adversely affect human health and the environment-is rarely taken into consideration with these approaches.
Recently, however, knowledge about the availability of chemicals in soils-i.e. leachability, mobility, rate of release, and relative toxicity-has increased. There is now considerable weight-of-evidence information from laboratory and field data indicating that for certain common situations-i.e., after chemicals have “weathered” over time, or after bioremediation has been performed-organic chemicals in soils may not be readily available for uptake by organisms, may not have an adverse impact on human health or the environment, and may not require costly remediation.
In this study, the issues associated with chemical bioavailability are presented and discussed, along with a review of current data on the availability of organic chemicals in both treated and untreated soils. The primary conclusions of this study are:
- Increasing experience indicates that measures of chemical concentration alone are insufficient to determine the actual risk posed by the chemicals or concentrations that constitute an environmentally acceptable endpoint.
- Environmentally acceptable endpoints for soils at some sites may be determined using simple approaches such as waste- or material-specific criteria or generic, risk-based state or federal values or standards however, in other circumstances such generic criteria can result in environmentally acceptable endpoints for a specific site that are unnecessarily conservative and that may not be applicable to the conditions at that site.
- Variations in chemical availability, mobility and toxicity are important factors to consider in making decisions about the necessary degree of cleanup or remediation at a given site. Chemical availability differs for fresh and weathered chemicals: chemicals recently released to soils will be more available for leaching, degradation, and bio-uptake than will be weathered chemicals. For some sites where the chemicals have weathered for decades, and where the chemicals are held tightly by the soil and are unavailable for transport, there may be little need for remediation.
- Chemical availability can also change as soil is remediated. Though some of the chemicals originally contaminating the soil might still be detectable, they may have been naturally “stabilized” in the soil matrix. Thus, these remaining chemicals are less mobile and less available, posing a reduced risk to the environment and may require no further remediation.
- Decisions relating to soil cleanup should reflect the fact that many chemicals in soil move slowly, and may be retarded and transformed during such movement. They should also reflect the fact that only a fraction of the chemical associated with a soil is readily available to cause adverse impact. In addition, not every site will have an ultimate use that requires cleanup to background conditions.
- The weight of evidence information on chemical availablity in soils has important implications to research directions as well as to remediation and regulatory policy.