7.2.5 Decision Mechanism 5: Combining DUs

There are circumstances when it may be advantageous to combine results from two or more DUs into a larger DU. This situation might occur when there are multiple sampling objectives for a given area. For example, delineation of source areas might necessitate creation of several small DUs, while evaluation of risk from exposure is based upon a DU that encompasses two or more of these DUs. DUs can be constructed in such a way as to meet both objectives efficiently if results from smaller DUs can be combined to produce an estimate of the mean for a larger, “super” DU. In constructing the “super” DU, each of the smaller, component DUs is in a sense like a SU. However, all are true DUs in that a decision must be reached for each, based upon one site objective or another.

Combining results from two or more small DUs to estimate the overall mean concentration in a larger combined DU is advantageous when the data must support more than one decision (e.g., overlapping exposure units for ecological and human health receptors).

Another example is a situation in which sampling objectives require assessment of exposure of different receptors or scenarios such that differently sized, superimposed exposure areas must be evaluated. Here again, the ability to combine results from small DUs to estimate mean concentrations for larger DUs would be advantageous.

Operationally, the mechanism requires a stratified sampling plan. The overall mean of the larger DU can be calculated using replicate data from the smaller, component DUs using formulas described in Section 4. These formulas take into account the size of the smaller DUs, weighting their contribution to the larger DU accordingly. The ability to combine DUs extends vertically as well as horizontally; that is, results from DUs from different soil depths can be combined if needed to meet sampling objectives.

Decision Mechanism 5 example

An elementary school is divided into three DUs based on anticipated exposure of students and maintenance workers to soil. The kindergarten children have their own playground that is designated as DU1. The older children have another playground that is designated as DU2. School maintenance workers come in contact with soil from both DUs equally, and their area of exposure is DU3, which consists of DU1 + DU2. DU1 and DU2 are each sampled using systematic random sampling with a total of three ISM samples from each. The results from the six ISM samples are combined, with appropriate weighting as described in Section 4, to derive the average concentration for DU3. The weighting factors applied to each DU result should reflect the assumptions in the CSM.