8.5.4.2 Meeting regulatory requirements for average and maximum concentration estimates

EPCs that present a spatial average are often required to be the upper 95% confidence limit (95% UCL) on the mean. If only a single ISM sample is taken from a DU, a UCL on the mean cannot be calculated to satisfy regulatory requirements. In this situation, a sampling strategy involving replicate measurements is required. (See Section 4 for a discussion of calculation of 95% UCL values from ISM data.) Also, some programs that accept an expression of the mean for risk and cleanup evaluation also specify an upper percentile or a maximum concentration that can remain on site. For example, some states currently require for most sites that if the 95% UCL is below the soil criterion, contamination is still not within acceptable limits unless the maximum concentration is at or below three times the criterion. Because ISM provides no direct indication of the maximum concentration, its ability to demonstrate compliance with this regulatory requirement is questionable. Approaches to estimate maximum concentrations within a DU using ISM data are discussed in Section 4 and might be useful to address this issue but have not been widely used.