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Work Plan to Delineate Soil Contamination in the Ag Chem Area ARCADIS Former Occidental Chemical Corporation Lathrop, California pavement surfaces with asphalt slurry. Remaining underground drainage piping was abandoned in-place using concrete grout. • North of Former Tank Farm Area: Placing soils from the tank farm berm in the area north of the Tank Farm Area, consolidating the soils, grading the surface to drain, covering the area with laterally extensive asphalt pavement designed for truck traffic loading, and sealing the pavement surfaces with asphalt slurry. Results are documented in an Interim Remediation Report(Geomatrix, 1997). Approximately 280 cubic yards of soil and asphalt exceeding industrial soil screening levels were left in place at the site,as well as some underground drainage piping (Geomatrix, 1997; EPA, 1996). OCC, Simplot and Geomatrix presented a path forward for risk-based final closure per a human health risk assessment. The human health risk assessment indicated that hazard indices and carcinogenic risks were below acceptable risk levels for maintenance and utility workers in the Former Tank Farm Area. The hazard index for the Former UST Area was also below acceptable risk levels; carcinogenic risks ranged from 1 x 10-5 to 6 x 10-5, primarily due to one sample (B-40)(Geomatrix,2006). In response to DTSC comments,AMEC Geomatrix prepared an addendum to the human health risk assessment stating that a land use covenant with industrial use restrictions and site management plan would be prepared (AMEC, 2009). In 2010, OCC considered taking a different approach, based on discussions with BioTech Restorations regarding bioremediation of COCs within the former Tank Farm Area and UST Area. BioTech Restorations submitted a draft Remedial Action Workplan to DTSC in December 2010. The Remedial Action Workplan did not meet DTSC requirements and was not approved. In June 2011, OCC entered into a Voluntary Cleanup Agreement with DTSC.Also in 2011,ARCADIS began parallel efforts on behalf of OCC to expedite remediation of contaminated groundwater at the Site, under a separate cleanup action with the Central Valley California Regional Water Quality Control Board (RWQCB). In March 2012,ARCADIS advanced seven borings within the Ag Chem Area to collect and analyze soil and groundwater samples to evaluate the potential contributions of residual soil contaminants within the Ag Chem Area to underlying groundwater (Figure 5). This work was completed in accordance with the Work Plan for Supplemental Investigation in the Ag Chem Area (ARCADIS,2011 a)approved by the RWQCB. All samples were analyzed for pesticides. Results are summarized in Table 2 (groundwater)and Table 3(soil)and will be included in the Supplemental Investigation in the Ag Chem Area Summary Report and Well Installation Work Plan (ARCADIS 2012). The only pesticides detected in site groundwater were DBCP, ethylene dibromide(EDB), and BHC isomers(all 7 locations);toxaphene(5 locations); Iathropwork pan s pgementai n—t ag chem 05012012.docx 3