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Revised Work Plan for Additional Site Investigation Raymond Investment Corporation Former Helena Chemical Facility February 27,2018 Page 6 5.2 SAMPLE LOCATIONS AND RATIONALE 5.2.1 Proposed Borings Near Northern Property Boundary Condor proposes drilling and collecting soil samples from five (5) locations (B42 through B46) in the area where 1,2,3-TCP was detected in the groundwater sample collected from boring B39/GW12 at a concentration of 12 micrograms per liter °lg./LI which exceeds the Maximum Contaminant Level (MCL) of 0.005 ug/L, which was recently established by State Water Resources Control Board. The locations of proposed borings are presented on Figure 5 (Appendix A). Soil samples in this area will be collected for the purpose of characterizing the lithology of the subsurface soils and for laboratory analysis to define the lateral and vertical extent of soil impacted by 1,2,3-TCP. Soil samples will be collected at depths of 2 feet, 5 feet and 10 feet below ground surface (bgs) at each boring location. In addition, deeper soil samples (at depths of 15 feet, 20 feet, and 30 feet bgs) will be collected from two of the five drilling locations. The deeper soil samples will be collected from the proposed borings located near previously sampled locations B39 (where 1,2,3-TCP was previously detected) and B38 (where historical Site photos show unpaved storage). Up to twenty-one (21) soil samples will be collected from the five (5) borings and submitted to a California-certified laboratory for analyses. Condor will also collect groundwater samples from each of the proposed borings (B42 through B46). A "grab" groundwater sample will be collected at each boring location from the shallow water-bearing zone, which is expected to occur at a depth of approximately 5 to 6 feet bgs. Although the depth to the static water level in Site monitoring wells has varied from approximately 4.64 feet bgs to 8.11 feet bgs, groundwater was initially encountered at a depth of approximately 12 feet bgs, which implies that shallow groundwater beneath the Site is semi-confined within the shallow fine-grained soil material. Consequently, the target depth for shallow groundwater samples is between approximately 10 and 12 feet bgs. The approximate locations of proposed boring are shown on Figure 5 (Appendix A). In addition to the proposed shallow groundwater samples, deeper groundwater samples will be collected from two of the five drilling locations: from the proposed borings located near previously sampled locations B39 (where 1,2,3-TCP was previously detected) and B38 (where historical Site photos show unpaved storage). The target depth for the proposed deeper groundwater samples is approximately 30 feet bgs, where poorly graded sand (SP) was encountered during the drilling of boring B41/GW13. 5.2.2 Proposed Borings Near Well MW-3 Condor proposes drilling two (2) locations (B47 and B48) near well MW-3 to collect soil and groundwater samples for analysis of 1,2,3-TCP. One boring (B47) will be placed in the area between B39-GW12 and MW-3 (northeast of MW-3) to verify whether or not a 1,2,3-TCP plume exists between well MW-3 and boring B39/GW12, where historical groundwater samples at each location have contained dissolved 1,2,3- TCP at concentrations above the MCL of 0.005 ug/L. Boring B48 is proposed as a step-out boring south- southwest of MW-3, near the former hazardous materials storage area on the north side of the warehouse. No step-out boring is proposed toward the northwest at this time, due to consistent non-detect results of 1,2,3- TCP in well MW-1. Soil samples in this area will be collected at depths of approximately 5 feet, 10 feet, and 15 feet bgs at each boring location and submitted to a California-certified laboratory for analyses. Condor will also collect groundwater samples from each location. A "grab" groundwater sample will be collected at each boring location from both the shallow water-bearing zone and from a depth of approximately 30 feet bgs, as described in Section 5.2.1, above. The approximate locations of proposed boring are shown on Figure 5 (Appendix A).