Recently proposed regulations on the federal and state levels establish uniform cleanup standards to provide objectives for remediating contaminated sites. The purpose of these standards is to restore contaminated sites to levels that ensure protection of human health and the environment. These regulations may result in consistent cleanup decisions and may expedite the remediation of contaminated sites by eliminating lengthy negotiations over the extent of cleanup. However, this "consistency" may result in overconservative remediations and excessive expense or, more important, inadequate remediation.
As part of a remedial investigation, a baseline risk assessment is conducted to determine if there is any potential for adverse health effects due to hazardous substances released from a site in the absence of controls. The risk assessment identifies constituents of interest (COIs) exposure scenarios, and likely receptors (residents, construction workers, etc.) and quantifies the amount of chemical intake by a receptor at the site. The amount of intake can be translated into a risk value that indicates the extent to which public health may be affected. The results of the baseline risk assessment document the magnitude of risk and identify its primary cause. The results also provide a basis for a decision as to whether remedial action is necessary.
The U.S. Environmental Protection Agency (EPA) has established guidelines that provide a consistent process for evaluating and documenting threats to the public health and the environment. Consequently, risk assessment provides a means to determine site-specific cleanup goals that are adequately protective of human health. Other scenarios consider the environmental fate and transport of chemicals to determine whether chemicals in one medium might have an impact on another medium. These approaches consider factors such as the spatial distribution of chemicals, interim measures currently in place to control chemical transport, plausible exposure scenarios, likely receptors to chemical exposure, and the physical and chemical properties of the chemicals. Therefore, the results generated from these approaches may span several orders of magnitude yet still result in protection of human health and the environment. More important, these approaches derive levels that are technologically and economically feasible to attain.
In this chapter we present four case studies that consider the factors previously discussed to establish cleanup levels. These cases illustrate various means to derive a cleanup level that is technically defensible. However, these levels may not necessarily be the optimum for site remediation. Usually, more than one approach should be explored to establish the optimum cleanup level.
Case I explores an innovative approach to the derivation of risk-based cleanup levels for carcinogenic polycyclic aromatic hydrocarbons (cPAHs). Using relative potency factors and specific exposure scenarios and considering apparent remedial strategies, a cleanup level for cPAHs in pond sediments was determined. The target cleanup level was developed by establishing a correlation between the cPAH concentration in a pond sediment sample and the corresponding risk. Plotting these data to generate a regression equation and selecting a target risk level allows one to identify a cleanup level.
Case II employs a similar statistical and graphical approach to deriving cleanup levels for cPAHs, but the approach is more complex because cleanup levels are also being derived for pentachlorophenol (PCP) and arsenic. Risk-based cleanup levels can be derived for exposures to single constituents by back-calculating a soil constituent concentration corresponding to a target risk level using risk assessment methodology. However, when the risks result from exposure to more than one constituent, the target risk level should be apportioned among the individual constituents. The available site data were evaluated to determine the relative distribution of the chemicals across the site. Target risk levels were then apportioned among the individual chemicals according to these relative distributions.
Case III uses EPA's recently developed Multimedia Exposure Assessment Model (Multi-med) to derive soil cleanup levels for a source area of chemical release that would be protective of groundwater at a designated downgradient receptor location. In this study, Multimed was used to model the unsaturated and saturated zone fate and transport of PAHs. Groundwater investigations revealed that the PAHs may leach from source soils and migrate horizontally through an underlying aquifer to a downgradient well location and an adjacent estuarine river. The cleanup levels were developed to be protective of humans consuming water at the receptor well location, organisms inhabiting the river, and humans consuming organisms from the river. Soil cleanup levels were calculated by using the dilution-attenuation factors derived by the model in combination with partition coefficients to meet applicable and appropriate performance standards in the water.
Finally, Case IV employs other environmental fate and transport models, specifically EPA's Organic Leachate Model (OLM) and Vertical and Horizontal Spreading (VHS) model, to determine a soil cleanup goal for carcinogenic PAHs that is protective of underlying groundwater quality. These two models account for the leaching of cPAHs into groundwater and their dilution in the shallow saturated zone afforded by dispersion. Use of these analytical solutions to derive a soil cleanup level provides a timely, cost-effective alternative to complex numerical modeling. Furthermore, the conservatism of the results may account for some of the temporal and spatial variations inherent in even the simplest groundwater flow domains not accounted for in complex models.
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