5.3.2.1 Subsurface ISM samples using core sampling

If a coring device is used, samples should be collected from targeted depth intervals in a manner that ensures the best coverage of the interval. For example, the selected subsurface DU investigation strategy may require the collection of soil samples from specified 15 cm (6-inch) intervals over a 1 m depth (see Figure 5-6). In other cases, the mean concentration of a targeted contaminant over the entire 1 m DU (or larger) interval may be desired for risk assessment or remedial purposes (see Figures 3-7, 3-8, and 3-9).

Schematic of a procedure to collect an ISM profile sample where two depths have been selected.

Figure 5-6. Schematic of a procedure to collect an ISM profile sample where two depths have been selected.



Collection of the entire core interval depth as the increment is the recommended subsurface ISM procedure.

Ideally, to be representative, the entire core depth interval should be considered as an increment, collected, combined with additional increments for an ISM sample and submitted to the laboratory. Collection of the complete core interval as an increment is the recommended subsurface ISM procedure. This method can result in large ISM samples (approximately 510 kg), making logistics, such as field storage and shipping, problematic. Additionally, the selected laboratory must have facilities available to store, dry (if required), and process these large amounts of soil mass. Consequently, depending on the core diameter and interval depth, inclusion of the entire core increment across a targeted depth interval in an ISM sample may be impractical. In such cases, individual cores may be subsampled to reduce the final mass of the ISM sample. Two options are described below.

The preferred option for collecting a representative subsample from a subsurface core increment for nonvolatile contaminants is to collect a “core wedge” sample.

The preferred option for collecting a representative subsample from a subsurface core increment for nonvolatile contaminants is to collect a "core wedge" sample. The simplest approach is to split the core in half vertically along the axis, reducing the increment mass by half. Alternatively, a single wedge of soil is taken from the entire length of the targeted depth interval. Removing a wedge of soil across the length of a larger core to encompass the entire depth interval rather than collecting the entire core depth interval as a whole, constitutes the mass of an individual increment of an ISM sample (see Figure 5-7). Individual wedges from 30 or more separate DU cores are then combined to form the complete subsurface ISM sample. This option results in a more biased and less precise estimate of the DU mean as compared with collecting the entire core. However, since the mass of each increment (and thus the ISM sample mass) is reduced, some of the practical constraints associated with handling full core increments are addressed.

Figure 5-7. Example of removing a wedge from the entire length of a soil core.



Replicate(s) can be collected from the same core, combined with other wedge increments, and submitted as separate ISM sample(s) to assess the precision of this subsampling strategy. However, core wedge replicates are not the same as ISM field replicates because ISM field replicates require completely separate incremental locations. Thus, core wedges should not be used as a measure of DU or overall sampling and analysis variability. Core wedge replicates evaluate only the variability in the subsampling process as opposed to collecting the entire core interval as the increment. The variability of wedge subsamples from alternative areas of the core is evaluated, e.g., replicate wedge collected 180° opposite the initial wedge subsample. ISM field replicates provide information on spatial variability and the variance in the estimate of the mean without specifically separating out the contribution of field and/or laboratory sample processing/subsampling from other sources of variance. ISM field replicates are discussed in Section 5.3.5. Core wedge replicates may also be collected when COPCs require separate laboratory processing procedures (see Section 6.2.2.2).

This approach is not appropriate when VOCs are of concern since they can be quickly lost from an exposed surface (Hewitt, Jenkins, and Grant 1995). For VOCs, multiple “plugs” representative of the desired core depth are collected and immediately preserved in methanol (see Section 5.4.2).

Subsurface ISM increment collection techniques in recommended order are as follows:
  1. Collect entire core interval
  2. Core wedge subsample
  3. Core slice subsample

The least preferred option for subsampling individual subsurface cores for nonvolatile contaminants is to collect a “core slice” from the targeted DU layer (see Figure 5-8). In this approach, a randomly selected perpendicular “slice” from within the larger targeted depth interval is collected as the ISM increment. For example, if the targeted depth interval was 2 feet in length (e.g., 8–10 feet bgs), a 4-inch perpendicular slice is randomly selected from within the targeted depth interval of each individual core and collected as the ISM increment. Individual, randomly selected core slices from 30 or more separate DU cores are then combined to form the complete subsurface ISM sample. This option introduces more bias than whole-core increment or core-wedge approaches. However, by reducing the increment mass, some of the logistical issues associated with handling the full core or the wedge increments are addressed. This is the least recommended approach for subsurface ISM core sampling since it is least likely to accurately represent the complete vertical length of the targeted DU layer.

Figure 5-8. Examples of "core slice" sample.
Source: Illinois EPA LUST FAQ and BIOTREE websites.