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Work Plan to Delineate Soil Contamination in the Ag Chem Area ARCADIS Former Occidental Chemical Corporation Lathrop, California laboratory report sheets, a tabular summary of the analytical results, data analysis, and recommendations.The report will be submitted to the DTSC approximately six weeks after receiving the final laboratory data from APPL and validated data set from CRA. 4. Methodology for Assessing Results Using data generated from the proposed field activities along with historical data from the Tank Farm Area and UST Area, risk-based cleanup goals will be identified using a risk assessment process that broadly includes the following: 1. Identifying exposure areas based on human exposure scenarios and chemical distribution in soil, calculating exposure point concentrations, and calculating risk for human exposure scenarios using standard risk assessment equations 2. Identifying risk drivers by chemical and by exposure area 3. Calculating risk-based cleanup goals for risk drivers for each exposure area, assuming an unrestricted land use The exposure point concentration will be based on the 95`h percentile upper confidence limit(UCL)of the mean concentration(95% UCL)for a set of observations. A 95% UCL indicates that there is less than 5%chance that the true mean of the data set is greater than the UCL concentration. Data for a particular chemical will be grouped together using the EPA ProUCL software(Version 4.1). Based on the distribution of chemical concentrations, a mean value, upper, and lower confidence limits on that mean value will be identified. The 95% UCL will be used to calculate risks(and also to compare to cleanup goals). Risks will be calculated for each exposure area and compared to the target risk levels for unrestricted land use, i.e., cancer risk of less than or equal to 1x10-6 excess lifetime cancer risk and non-cancer hazard index of less than or equal to 1. Risk drivers are defined as either having a chemical-specific cancer risk of greater than 1x10-6 and/or non-cancer hazard quotient of 1 or higher. For each exposure area with potentially unacceptable risk at the site(i.e., risks for unrestricted land use above target risk limits), datasets for the risk drivers will be visually reviewed. Professional judgment will be used to identify inflection points in the distribution of detected concentrations. Soil samples with concentrations above the inflection point will be considered to potentially contribute significantly to unacceptable risk. These sample locations will be mapped and visually evaluated for spatial patterns or groupings. Iathropwork pan supplemental Invest ag chem 05012012 docx 8