4.2.3 Nondetects

While nondetects are relatively common for discrete sampling data sets due to spatial heterogeneities in the mechanisms of release of contaminants, it is less likely that an ISM result will be below the reporting limit because some small percentage of the increments are expected to capture source areas. In other words, while individual increments may be nondetect, it is unlikely that the mean of the increments (given by an ISM result) will be nondetect. Exceptions may include constituents that are below reporting limits under ambient (background) conditions and are unrelated to site activities or post-remediation confirmation samples. In both cases, so long as the reporting limits do not approach action levels, nondetect results should not introduce decision errors. If reporting limits approach action levels, users should consider alternative analytical procedures, revisions to the sampling design to characterize the DU, and other lines of evidence that concentrations might be below action levels. If replicate results include a mix of detects and nondetects, then the only option with small sample sizes is to apply substitution methods such as one-half reporting limits to ISM results and to qualify results as potentially biased due to the use of substitution methods. A variety of substitution methods may be applied and the consequences of those options should be explored.