What is residual concentration?

CALCULATION OF AVERAGE RESIDUAL CONCENTRATION (ARC)ARC is defined as an area-weighted statistical estimation of the average residual concentration of PCBs

left in the surface soil in the characterized area of the site following excavation and prior to backfill.

Information used for the ARC estimates was site characterization surface soil data and the

computer-generated/hand calculated surface areas. In addition, classical deterministic statistical

methods and the Statistical Analysis SystemTM (SAS) computer software package were utilized to

calculate the ARCS values. The step-by-step procedure to calculate the ARC for a particular site is as

follows:

Surface soil data with concentrations greater than or equal t o 1 ppm total PCBs were

obtained from the TlMS database.

Property surface areas for areal extent of PCBs were calculated for four areas (A, 8, C,

D) of each site:

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Area within 25 ppm contour lines (Area A).

Area between 10 and 25 ppm contour lines (Area B).

Area between 5 and 10 ppm contour lines (Area C).

Area between 1 and 5 ppm contour lines (Area D).