C.1.2.2 DU Designation and Investigation Objectives

The footprint of the former landfill area was designated as a spill area DU, based on the past history of the site and the approximate extent of PCB-contaminated soil identified in the earlier investigations. An 80 × 180 foot DU was established, covering an area of approximately 15,000 ft2. The targeted depth interval of the DU was 3 feet, although in some cases samples were collected to a depth of 5 feet. The total volume of the soil incorporated by the DU was approximately 2,700 yd3.

The objective of the investigation was to estimate the representative (i.e., mean) concentration of PCBs for the designated DU mass of soil. Identification of the maximum concentration of PCBs for any given aliquot mass of soil within the DU or sample-size hot spots was determined to be feasible or, more importantly, necessary. The area and volume of the DU were considered to be small enough for evaluation of potential risks to ecological and human receptors. Risk-based decisions on the need for additional remedial actions at the site would be made for the mass of soil incorporated within the spill area DU as a whole. Incorporating these objectives into the design of the investigation was intended to help minimize the need for additional, follow-up investigation and to avoid confusion over the need to investigate and address smaller, sample-size hot spots within the DU as a whole.