1.3 How ISM Addresses Traditional Investigation Approach Limitations
The fundamental question with all soil sampling, discrete or incremental, is representativeness. In reviewing sampling results, environmental professionals often find themselves asking, "What does the sample concentration we get back from the lab represent?" With incremental sampling that question is purposefully rephrased as, "What does the (incremental) sample have to represent?" and that question is used to shape the project planning and establishment of DQOs well before any sample is actually collected.
The major problem with discrete soil sampling is the extreme magnitude between the mass of the subsample analyzed by the laboratory and the mass of the target population (area to be investigated or sample volume collected), which can be on the order of 1 in 10 million or more. This increases the chance that the sample misses contamination, which will consequently not be represented in the analytical results at all. ISM builds a sample from increments to provide a good representation of the DU and so is more likely to capture even heterogeneous contamination.
ISM requires that the project team address the spatial dimensions associated with the analyte concentration that is of interest. That is, the project team must define the DU to be represented by each incremental sample. This requirement is inherent in any soil sampling effort but must be addressed head-on and with great deliberation in ISM.
Early on the project life cycle, ISM forces the project team to confront the inherent heterogeneity in soil by defining the scale at which heterogeneity will be addressed.
Early in the project life cycle, ISM forces the project team to confront the inherent heterogeneity in soil by defining the scale at which heterogeneity will be addressed. ISM does this early in the project life cycle by getting stakeholder agreement on the dimensions of the DUs from which samples will be collected. The scale issue is present for all sampling approaches but has typically not been made an integral part of the sampling strategy as it has with ISM. Furthermore, once the scale of the DU has been decided, the concept of hot-spot delineation within the DU should be moot. If it is not, then the DU may not be appropriately sized and should be reevaluated (see Section 3.5 for further discussion on hot spots).
ISM addresses common errors associated with sampling soils. As such, ISM embeds the concept of quality assurance (QA)/quality control (QC) in a meaningful way into planning, design, field sampling and sample processing, as well as laboratory work, by explicitly addressing all of the activities necessary to build an ISM sample that will be representative of the DU of interest. Traditional QA/QC approaches have focused primarily on laboratory procedures, particularly those that take place after a subsample of soil has been extracted, and do not address the major sources of error that occur well before an extract solution is introduced into an analytical instrument.