EXPOSURE POINT CONCENTRATION

The essence of the cumulative risk calculation is the determination of the appropriate exposure point concentration (EPC). In the Land Recycling Program (LRP) an acceptable site assessment must be completed before EPCs can be determined. Methods for determining the appropriate EPCs used to evaluate compliance with cumulative risk criteria have not been specified by rule. This guidance outlines two procedures that may be used to determine EPCs for assessment of cumulative risk in the LRP. Other procedures may be used with the approval of the DNR.

1. Maximum concentrations. The maximium value for each contaminant in each medium from multiple samples of each medium of concern.
2. 95% Upper Confidence Limit (95% UCL) of the mean contaminant concentration may be used as follows.
   • Groundwater. The 95% UCL may be computed based on the up-to-8 most recent samples from the most contaminated monitoring well where a drinking-water well is possible and will not be precluded by an institutional control. When risk is being assessed for an actual drinking-water well, the 95% UCL may be computed based on the up-to-8 most recent samples from that well. In either case, if contaminant levels are not stable or improving, and are assured of remaining so in the future, a prediction of future concentrations must be made in accordance with subrule 567 IAC 137.9(4).
   • Soil. The maximum area over which the 95% UCL may be determined for soil sample results is ¼ acre for a site resident or 1 acre for a site worker or construction worker. If the site is larger than the maximum resolvable area, the resolvable areas shall be laid out to maximize the average concentrations using rectangles with the length not to exceed twice the width.
     
     NOTE: Soil samples used for determining compliance should be from the soil profile for which exposure is being assessed. Separate cumulative risk assessments should be made for each soil profile of concern.
   • Air. Generally contaminants in air will be a potential issue only in confined spaces. The 95% UCL from air samples from a confined space may be used, if this provides a reasonable representation of air concentrations throughout the confined space. Frequently, air samples will not be available from the point of exposure (e.g., confined space). Soil-gas samples will often be collected in lieu of indoor air samples. When soil-gas samples are utilized to assess risks, soil-gas concentrations may be used directly or a conservative estimate of resultant indoor air concentrations may be used. When an existing confined space exists, the soil-gas samples should be collected as close to the confined space as practical. When risk is being assessed for a future confined space, the 95% UCL of soil-gas samples should be determined over a maximum resolvable area that is no larger than footprint of potential buildings at the site.