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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 • Extraction from Simplot's primary water supply well at approximately 500 gpm and annual average extraction rate from the Simplot BSW at 250 gpm based on process water needs and seasonal historic pumping of the Simplot BSW • Installation and operation of Lathrop Water District well LWD-21 to the south of the Site and updated average extraction rates for all Lathrop Water District wells, based on the past six months of operation (January through June 2012). Total LWD system extraction rates were comparable to annually-averaged flow rates • Addition of proposed water table zone well EW-17 near the Ag Chem Area at an annual average extraction rate of 25 gpm (ARCADIS, 2012b) • Incorporation of new COC occurrence data collected in 2012, particularly in the Ag Chem Area (ARCADIS, 2012b), Simplot BSW, and at other locations east of South Howland Road • A recharge rate of 43%, totaling approximately 107 gpm,was assigned to the field south of Simplot ponds/lagoons to better represent recharge of the irrigation water.This estimate is based on ARCADIS'experience at a site in Southern California where extracted groundwater was used for agricultural purposes. A summary of the updated groundwater modeling results is presented in Appendix C.The overall goal of the expanded extraction and injection well network is to increase COC mass removal, in addition to continuing to maintain plume capture. Extraction wells were therefore placed in areas with high COC concentrations.The effectiveness of optimization of the treatment system and installation of EW-09 has already increased sulfolane mass removal and treatment(Figure 5). Over the past year, sulfolane mass removal has increased by two-fold from approximately 3 pounds per day(lbs/day)to over 6 lbs/day. The goal of expedited COC mass removal guided the placement and design flow rates of extraction and injection wells,within the 700 gpm flow rate limitation of the groundwater treatment system.The optimized groundwater extraction and injection well network was designed to be dynamic; extraction well and injection well flow rates will be periodically adjusted in response to measured trends in COC concentrations and groundwater elevations. As the system continues to operate over the next few years, additional extraction or injection wells may be necessary or existing wells may be turned off. A discussion of the detailed plans for extraction and injection within each aquifer zone is provided in Section 6. 2012-09-13 addendum well install work plan_hy test rpt.doca 6