3.3.11 Other Types of Decision Units

ISM may be used to estimate background concentrations for comparisons with potentially contaminated volumes of soil (see Sections 4.4 and 7.2.4).  The term SU is more appropriate than DU to describe the area and depth for ISM sampling for background concentrations because these types of areas will not require decisions. As for any background sample, background SUs should be located in a nonimpacted area that is geologically similar to the contaminated area. Ideally, the background sampling area will be of a scale similar to the potentially contaminated areas; however, this may not be feasible if the background data is used for comparing background and contaminant concentrations across multiple DUs.