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Work Plan to Delineate Soil Contamination in the Ag Chem Area ARCADIS Former Occidental Chemical Corporation Lathrop, California and DDT, disulfoton, dicampa and dinoseb(1 location). Groundwater remediation is currently being addressed under the RWQCB. Pesticide concentrations in all soil samples were below residential screening levels(Table 4)with one exception.Toxaphene in boring PWB-1 exceeded residential screening levels but was below industrial screening levels. ARCADIS is currently supporting OCC in developing a remedial approach to reach unrestricted closure of soil in the Ag Chem Area. ARCADIS and OCC have evaluated historical data and propose additional investigation to delineate soil contamination in the Ag Chem Area. Per a March 7, 2012 meeting with Tom Olson of DTSC,ARCADIS is submitting this work plan for supplemental soil investigation. Following DTSC approval and field implementation,ARCADIS will evaluate results in the context of potential human health risks and prepare a remedial action work plan summarizing results, risk evaluation,feasibility study.An implementation report will be submitted within 30 days of completing field activities (per HAS-VCA 10/11-097). Details of the proposed supplemental investigation are provided in Section 3. 3. Field Sampling Plan The objective of this Work Plan is to collect and analyze soil samples to evaluate the extent of residual soil COCs within the Ag Chem Area. Specifically,this Work Plan proposes the following: • Collect soil samples at depths of 2, 5 and 8 feet below ground surface(ft bgs) at approximately 23 primary locations and up to 20 supplemental locations within the Ag Chem Area including the former LIST excavation area (Figure 6), Tank Farm Area (Figure 7), and the area North of the Tank Farm Area (Figure 8)where contaminated soil and asphalt was placed, graded and covered with asphalt. • Analyze samples for DBCP, ethylene dibromide(EDB), sulfolane, herbicides, organochlorine pesticides (OCPs), organophosphorus pesticides(OPPs)and benzene hexachloride(BHC)isomers. Data will be used to identify the vertical and horizontal extent of residual soil contaminants. The effect of historical and/or residual soil contaminants on groundwater quality in the Ag Chem Area is being addressed under a separate cleanup action under RWQCB oversight. To provide managers and field staff with a better understanding of equipment, field procedures, materials, health and safety considerations, and documentation procedures associated with each specific activity,ARCADIS has developed standard lathrop work plan supplemental Invest ag chem 05012012.d— 4