7.2.1 Decision Mechanism 1: Comparison of One ISM Sample from the DU to the Action Level

The simplest decision mechanism is the comparison of a single ISM sample result for a DU to an action level, which is typically a threshold value derived through risk assessment, regional background estimate, or other regulatory means. Sometimes, more than one set of action levels may apply to a site because they reflect different objectives (e.g., protection of acute and chronic health end points). Because ISM yields estimates of mean concentrations within a DU, it is important to note the spatial and/or temporal scale that was originally intended in the development of the action level.

This decision mechanism is simple and straightforward. The result of the decision is immediately apparent; a failure is indicated if the ISM sample result exceeds the action level. This approach is frequently used with individual discrete samples under the CERCLA preliminary assessment process; however, when the action level is intended to represent a mean concentration for a risk assessment exposure area, it is more logical to compare an estimate of the mean concentration (e.g., 95% UCL) in the DU from an ISM sample to the action level or, similarly, the mean (or 95% UCL) of multiple discrete sample results.

Comparison of a single ISM sample result to an action level is useful when the ISM result is either well above or well below the action level and error in the estimate of the mean is unlikely to lead to an incorrect decision.

A single ISM sample provides one estimate of the mean concentration, which may be above or below the actual mean concentration in the DU. Unless replicate ISM samples are taken, there is no indication of the extent to which estimates of the mean vary, and consequently, it is difficult to predict how far from the actual mean a single ISM sample result might be. This uncertainty greatly limits the scientific defensibility of this approach. Use of a single ISM result might be acceptable when the estimated mean concentration obtained is much greater than, or much less than, the action level such that even a great deal of error in the mean estimate could be tolerated without making a decision error. In this situation, the ISM sample provides confirmation of what may have already been strongly suspected—that the DU clearly passes or fails. However, when the ISM sample result is close to the action level, this decision mechanism is unreliable, and decision errors in both directions are possible (i.e., concluding that the DU fails when the average concentration is in fact below the action level, or vice versa). How big a difference from an action level is big enough to conclude confidently that a DU passes or fails? The problem with this approach is that there is no clear answer. Because only one concentration is available, there is no indication of the potential magnitude of error, and a decision that a concentration difference is large enough that a decision error will not be made is arrived at subjectively. Obviously, uncertainty about making the right decision increases as the ISM sample result gets closer to the action level. Comments from many states suggest that the uncertainty associated with making decisions with only one ISM sample would make this approach unacceptable.

Decision Mechanism 1 example

A single, 5000 ft2 DU is established across an area suspected to be a former transformer dump site. The objective of the investigation is to determine whether the mean concentration of PCBs in surface soil (0–4 inches bgs) exceeds an action level of 0.22 mg/kg for residential land use. Forty increments of soil are collected using systematic random sampling and combined into a single ISM sample for sample preparation and analysis. The reported concentration of PCBs in the sample is 6.2 mg/kg. The result is substantially higher than the action level, and the planning team comes to consensus that plausible error in estimating the mean would not likely change the ultimate decision that the DU fails.