STATISTICAL APPROACH TO MEETING SOIL CLEANUP GOALS

The establishment of health-protective soil remediation levels often r elies on the results of a risk assessment, which provides a way to equ ate a permissible risk to a target soil contaminant concentration. Inh erent in such risk assessments is the assumption that the target conce ntrations are representative averages. Unfortunately, soil cleanup lev els thus calculated are typically misapplied on a point by point basis rather than on an average. This is not cost-effective because it resu lts in post-remedy conditions that overshoot the target risk goals. Be cause environmental contamination is characterized by a distribution o f concentrations, some exceedances of target averages, average risk, o r average concentration can be allowed in the post-remediation distrib ution. This work presents a mathematical model for calculating this al lowable higher than average concentration, termed the confidence respo nse goal (CRG), which places a limit on concentrations requiring remed iation while ensuring that target average concentrations are satisfied overall. The CRG is site-specific becauses it depends on the contamin ant concentration distribution. The strength of the approach lies in i ts ability to handle typical data uncertainties quantitatively because it relies on the upper confidence limit as a measure of the mean conc entration (in a manner similar to its use in risk assessment), hence t he 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 w hich could be attributed to the CRG approach.