Statistical Approach to Meeting Soil Cleanup Goals

The establishment of health-protective soil remediation levels often relies on the results of a risk assessment, which provides a way to equate a permissible risk to a target soil contaminant concentration. Inherent in such risk assessments is the assumption that the target concentrations are representative averages. Unfortunately, soil cleanup levels thus calculated are typically misapplied on a point by point basis rather than on an average. This is not cost-effective because it results in post-remedy conditions that overshoot the target risk goals. Because environmental contamination is characterized by a distribution of concentrations, some exceedances of target averages, average risk, or average concentration can be allowed in the post-remediation distribution. This work presents a mathematical model for calculating this allowable higher than average concentration, termed the confidence response goal (CRG), which places a limit on concentrations requiring remediation while ensuring that target average concentrations are satisfied overall. The CRG is site-specific becauses it depends on the contaminant concentration distribution. The strength of the approach lies in its ability to handle typical data uncertainties quantitatively because it relies on the upper confidence limit as a measure of the mean concentration (in a manner similar to its use in risk assessment), hence the term “confidence” in the CRG. The advantages of the approach are significant. An example is given of a Superfund site where excavation volumes were reduced by 66% and $40 million was saved, about half of which could be attributed to the CRG approach.