Interaction of science, policy, and law in risk assessment: Unreal risk assumptions in cleanup of contaminated media

Calculation of a risk or science-based standard for cleanup of contaminated soil and groundwater usually assumes a constant concentration (steady-state) of a contaminant in these media. However, the cleanup of a contaminated site to this standard defines a fixed mass of a contaminant at the site that must decrease over time due to transport of the contaminant and natural degradation processes. The standard cannot be correct, as it is based on a fixed toxic dose that cannot be achieved because contaminant mass must decrease over the exposure duration (not steady-state). Thus, this standard will always be numerically smaller than reality would allow. Some may consider this bias as “conservative” for protection of public health even if derived from an assumption impossible for the conditions. This raises three issues regarding this impossible assumption: Is there a policy reason for it? Is it permissible within the legal framework for rule-making? Is there a scientific reason for it?; A review of regulatory policy for setting cleanup requirements for contaminated sites showed no reason or need to use this constant concentration assumption. In fact, plausible conservative assumptions used throughout the process of setting risk-based cleanup requirements provide a multiple and compounding conservatism for these requirements.; A study of court cases showed that environmental requirements based on impossible assumptions were consistently arbitrary and capricious, and vacated or remanded. These assumptions could be categorized using an uncertainty taxonomy that simplifies their identification in regulatory requirements. The constant concentration assumption used in setting risk-based cleanup criteria or standards for contaminated soil and groundwater is an impossible assumption that would likely be found arbitrary and capricious if appropriately presented and tested in court.; An analysis of contaminant fate and transport processes showed that the constant concentration assumption was both untenable and unnecessary for defining a soil or groundwater cleanup standard. Algorithms were developed to account for contaminant mass-loss processes and correct for the constant concentration assumption. These algorithms are robust as they distinguish between contaminants that may pose a substantive risk in soil and groundwater and those that may not.