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Addendum to the Well Installation and Hydraulic Testing Report and Additional fARCADIS Well Installation Work Plan Former Occidental Chemical Corporation Lathrop, California Aquifer Parameter Site-Specific Value Units Distance beyond which the pumping well has no effect 65 m 800 ft Radius of influence 0.1 ft drawdown, 65 m 750 ft Radius of influence 0.1 ft drawdown, 30 m 700 ft The hydraulic testing data from EW-09 indicated that although the well would not likely sustain an extraction rate of 65 gpm, 30 gpm would likely be sustainable.This information was used to confirm that multiple water table zone wells would be needed to efficiently remove residual sulfolane from the sulfolane investigation and treatment area and to verify a proposed design extraction rate of 30 gpm for each water table zone well. ARCADIS also used the aquifer parameter estimates from the hydraulic testing to assess the groundwater model input parameters.The groundwater model was used as a guide for the approximate number and placement of water table zone wells. In addition,the groundwater model was used to test the overall effectiveness of the expanded extraction and injection well network in capturing COCs and maximizing COC mass removal.The groundwater model is described further in Section 3 of this Addendum.A comparison of hydraulic testing parameters and model input parameters is presented in Section 4. 3. Basis of Extraction and Injection Well Network Design The groundwater model prepared by ARCADIS is a numerical fate and transport model. Input parameters are based on site-specific and regional hydrogeologic data. Groundwater model simulations were used to guide the proposed locations of new extraction and injection wells.The model was also used to assess the speed and overall effectiveness of the expanded groundwater extraction and injection well network in reducing COC concentrations over time.The preliminary modeling results were presented to the RWQCB during a meeting at RWQCB offices on October 6, 2011 (Appendix B).The groundwater model was subsequently updated in August 2012 using updated Site information and summarized in this Addendum. Updates to the groundwater modeling assumptions are summarized below: • Minor changes in the location of several proposed extraction wells(10 to 50 ft from the initial proposed location)for feasibility/convenience of access for piping and installation • Moving the location of proposed injection well INJ-12 midway between the Simplot BSW and Site extraction wells • Assessing the design flow rates proposed for each of the extraction and injection wells and adjusting them if needed 2012-09-13 addendum well install w k plan_hy test fpt.d— 5