C.4.1.2 Pesticide Mixing Area Investigation

The pesticide mixing area was ringed with 33 1000 ft2 and 5000 ft2 DUs to verify that the boundary of heavy contamination had been adequately identified, based on previous discrete sample investigations (Figure C.4-2, ESTC 2007). A 0–6 inch surface ISM sample was collected from each DU (total 33 samples and 990 increments, plus replicates).

ISM investigation of a 400-acre former sugarcane field and 59 hypothetical, lot-size (5000 ft2) DUs characterized.

Figure C.4-1. ISM investigation of a 400-acre former sugarcane field and 59 hypothetical, lot-size (5000 ft2) DUs characterized.




ISM investigation of a -acre pesticide mixing area DU within the former sugarcane field.

Figure C.4-2. ISM investigation of a -acre pesticide mixing area DU within the former sugarcane field.




The interior of the mixing area was divided into 15 DUs (ESTC 2010). A suspected area of especially heavy contamination was subdivided into three small spill-area DUs (<2000 ft2), with the remaining area divided into 12 DUs equal to or less than the default, residential-lot exposure area of 5000 ft2. Each of the 15 DUs was subdivided into up to four “sampling unit” (SU) layers to investigate the vertical distribution of contaminants. An ISM sample was collected within each surface and subsurface SU (total 31 SUs), with triplicates collected in three units. Twenty direct-push borings were installed in the three spill-area DUs to characterize subsurface contamination (i.e., one 20-increment ISM sample per subsurface SU layer). Subsurface SUs in the outer DUs were accessed and sampled by trenching.

A total of 64 ISM samples composed of 2000+ increments, plus replicates, was collected. Significant dioxin contamination was identified in all 15 DUs (maximum 650,000 ng/kg TEQ dioxins) and heavy triazine contamination within the targeted spill areas (see Figure C.4-2). Both the lateral and vertical extent of contamination was significantly greater than estimated based on earlier, discrete sample data, increasing the volume of contaminated soil by a factor of at least 3. The ring DU ISM samples also identified a 15,000 ft2 area of dioxin-contaminated soil on the south side of the mixing area that was likewise missed by earlier discrete samples (see Figure C.4-2).

In 2009, the USEPA collected 83 surface and subsurface discrete samples around the perimeter of the mixing area to confirm that the extent of contamination had been adequately identified (USEPA 2009, unpublished). The discrete samples similarly suggested that contamination around the mixing area was below target action levels. The samples failed to identify the outer area of contamination identified in ring DUs to the south, however. The investigations confirm that ISM samples, essentially very good “composite” samples with additional lab requirements, are better able to capture small hot spots and overall contaminant heterogeneity within a targeted area.