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4 Sampling Design and Rationale As discussed in previous sections of this FSP, the START reviewed available site information to determine past site activities and previously identified contaminants potentially present at the subject property. This information was used to determine the areas of concern at the site where sampling is required,as well as to determine the COPCs. 4.1 Analytes of Concern The COPCs are listed in Tables 1 through 3 (Appendix B). The COPCs are based on data from previous investigations and historical land uses. The soil COPCs consist of TPH-g,TPH-d, TPH- mo,VOCs(including BTEX and MTBE)and metals. Soil gas COPC's consist of VOCs and groundwater COPCs consist of TPH-g,TPH-d,TPH-mo, and VOCs(including BTEX and MTBE). The analytical methods for the COPCs will be modified EPA Method 8015 for TPH-g,TPH-d, and TPH-mo; EPA Method 8260 for VOCs(including BTEX and MTBE); and EPA Method 6010/7473 for metals. Method TO-15 will be used for VOC analyses for soil gas. A summary of the analyses that will be conducted as part of this assessment is presented for soil, soil gas, and groundwater,respectively, in Tables 4 through 6 (Appendix B). 4.2 Sampling Design and Rationale Previous soil sampling and the sampling of groundwater monitoring wells at the site did indicate that contaminants of concern are present beneath the Stockton Fire Department Station 3 LUST site, additional soil and groundwater sampling is warranted to identify the extent of groundwater contamination on site and to fill in data gaps in potential soil contamination outside of the two UST areas. Groundwater beneath the site is documented to flow in an easterly to southeasterly direction,based on the previous monitoring well network. However the location of these previous well locations are cross-gradient of the potential contamination from the USTs and further groundwater monitoring for hydrocarbon contamination is wan-anted. A triangular-grid-based systematic sampling approach will be used for the El Concilio No. 2 site. Grid-based sample locations were modified to account for the placement of the USTs,buildings, and other obstacles to drilling. The identified data gaps include surface and near surface soils as well as groundwater beneath the site. 4.2.1 Soil Sampling A total of twelve grid-based borings are proposed for the El Concilio No. 2 site. Visual Sample Plan(VSP) software(Batelle Memorial Institute,version 5.9,2009)was used to confirm that using an approximate 125-foot triangular grid across the 1.98 acre site results in seven sampling locations with a 95%probability of detecting a hotspot with a radius of 65 feet (Battelle Memorial Institute 2009). 002693.1501.OIBR 4-1 EI Concilio No.2