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Addendum to the Well Installation and Hydraulic Testing Report and Additional ARCADIS Well Installation Work Plan Former Occidental Chemical Corporation Lathrop, California ARCADIS proposed installing extraction well EW-17 in the water table zone at the Ag Chem Area (ARCADIS, 2012b). Additional discussion of the hydraulic testing data and additional technical rationale for well network expansion was requested in the RWQCB letter(RWQCB, 2012). Specifically, the RWQCB requested the following additional information: • Analysis of Hydraulic Testing Data: Necessary discussion, technical rationale and conclusions (including a capture zone analysis)for the hydraulic testing data and how it was used to design the proposed extraction and injection well network. • Basis of Extraction and Injection Well Network Design:Any calculations and modeling results that were used for the design of the proposed extraction and injection well network. These data will be used as a basis for evaluating the recommendation of converting existing extraction wells EW- 03 and EW-04 to injection wells INJ-13 and INJ-14. • Comparison Between Hydraulic Testing Data and Model Predictions:A discussion of a comparison between the hydraulic testing data from EW-09/observation wells with the predictions made by the groundwater model which was presented to the RWQCB in October 2011. • Groundwater Quality Near Proposed Injection Well INJ-12: A discussion of the potential water quality effect of injection well INJ-12 on the backup water supply well (BSW) used by the current Site owner, J.R. Simplot(Simplot). In light of the recent discovery of elevated sulfolane at the Simplot BSW, as well as an evaluation of the potential effect of injection well INJ-12 on sulfolane- impacted groundwater in the vicinity of the Simplot BSW(i.e., the potential to spread the sulfolane plume vertically and laterally). The requested information is provided in Sections 2 through 5 of this Addendum and in the accompanying Appendices. For additional background, please refer to the Report and Work Plan (ARCADIS, 2012). 2. Analysis of Hydraulic Testing Data The objective of hydraulic testing at EW-09 was to collect data that could be used to guide the design of a groundwater remedy for sulfolane in the water table zone within the investigation and treatment area, following excavation. Groundwater extraction from EW-09 began on January 11, 2012 at approximately 5 gallons per minute(gpm). Drawdown was measured using transducers in the extraction well (EW-09) and at three observation wells (PW27-40, PW28-40, and PW29-40) (Figure 3). Groundwater extraction continued at 5 gpm until groundwater levels had stabilized in the observation wells. The extraction rate 2012-09-13 addendum well install work plan_hy test rpt.docs 2