C.4.1.1 Field Area Investigation

The field was investigated through the characterization of 59 hypothetical, residential lot-size DUs (5000 ft2) randomly located within the 400-acre field (Figure C.4-1, TTEMI 2007). Using the terminology proposed in the ITRC ISM document, the entire field could be alternatively considered to be the DU, and in the individual lots, DUs as SUs. Previous suggestions to characterize the field using a similar number of discrete samples were rejected due to poor coverage of individual DU areas (e.g., a single discrete sample at each of the 59 target locations vs. a 30-point ISM sample).

Testing a minimum of 59 hypothetical lots ensured that contaminant levels in 95% of the lots not tested were no higher than in the most contaminated DU identified (HDOH 2008). After locating the center point for a DU in the field, a 5000 ft2 area was marked off and a 40-point ISM sample collected (total 59 ISM samples and 2360 increments; see Figure C.4-1). Triplicate samples were collected in 10% (six) of the DUs. Samples were collected over a 5-day period. Reported concentrations of targeted contaminants in all the DUs were below environmental action levels for residential use and the fields were cleared for development (e.g., maximum 100 ng/kg toxicity equivalent [TEQ] dioxins).