5.3.2.2 Additional subsurface ISM considerations

Collecting fewer increments generally increases the grouping and segregation error and results in a less precise and more biased estimate of the mean contaminant concentration.As with surface ISM samples, it is recommended that a minimum of 30 increments be collected for each DU. In some cases, collecting the recommended minimum of 30 soil increments per subsurface DU may not be feasible or practical. Reducing the number of increments collected per sample may be the only viable option. In this situation, it is important to recognize that collection of a reduced number of sample increments generally increases the GSE and results in a less precise and more biased estimate of the mean contaminant concentration. Depending on the degree of data variability that can be tolerated within the project-specific DQOs, a significant reduction in the number of increments may result in a decision error. A sample containing fewer increments than required to estimate the DU mean concentration within the project-specific uncertainty level may not be considered a defensible ISM sample. Consequently, in these circumstances careful review of DQOs as well as any other sampling options that may be available is warranted. The subsurface sampling strategy chosen, the sampling constraints, and potential impacts on data quality should also be identified in the DQOs in the SAP and or Quality Assurance Project Plan (QAPP).

Increments from the same depth interval throughout the DU can be combined and used to create a single ISM sample for that depth interval. This is a useful approach for the characterization of vertically stacked DUs (see Figures 3-7, 3-8, and 3-9). Data for each ISM sample can be used to create a 3-D map of contaminant levels in the DU. This procedure can be especially useful where a large number of side-by-side DUs are designated for the investigation of large areas (e.g., redevelopment of a former golf course contaminated with pesticides).