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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 Location Zone Date Sampled Sulfolane /L) DBCP /L EDB(N /L) LP010 Water table 1/19/2012 ND <20 ND <0.01 ND <0.02 3/29/2012 28 -- -- 7/19/2012 22 -- -- LP011 Water table 1/19/2012 ND <20 ND <0.01 ND <0.02 7/19/2012 ND <20 -- -- PW01-48 Water table 9/09/2005 ND <20 -- -- 1/19/2012 ND <20 ND <0.01 ND <0.02 7/19/2012 ND <20 -- -- PW01-129 Intermediate 9/01/2005 75 -- -- PW02-40 Water table 9/07/2005 ND <20 -- -- PW02-80 Shallow 9/07/2005 ND <20 -- -- PW03-49 Water table 9/06/2005 ND <20 -- -- PW03-142 Intermediate 9/06/2005 ND <20 -- -- Notes: ND=not detected above the laboratory reporting limit. The reporting limit is presented in parentheses. Supply wells and LP wells are Simplot wells; PW wells are GSH wells. These results were taken into account when contouring COC isoconcentration maps(Figures 7 through 10) and are consistent with the current conceptual site model. Furthermore,these results provide new information regarding the nature and extent of the COC plume east of South Howland Road.Additional sampling may be needed to delineate the extent of sulfolane in this area to inform extraction well placement. Following additional investigation and monitoring, additional extraction wells can be installed or extraction rates adjusted to remove sulfolane-affected groundwater.These data also highlight the benefits of using an adaptive, phased approach to the placement and operation of extraction and injection wells. BSW Operation Currently the Simplot BSW is not being used. Based on information provided by Simplot,this well serves as a temporary replacement for the PSW, providing for process water, potable water, emergency safety showers, and emergency fire suppression if needed. During the summer months, Simplot historically allowed a neighboring farmer to use water from the Simplot BSW to irrigate alfalfa. It is GSH's understanding that Simplot intends to maintain this arrangement in the future. In light of the recent sulfolane detection in the Simplot BSW,ARCADIS is constructing a temporary treatment system to remove sulfolane from groundwater that is pumped from the BSW.The design and operation of the temporary treatment system will be described in a memorandum to be submitted to the RWQCB in September 2012.The temporary Simplot BSW treatment system will be used on an emergency basis to supply process water if the PSW needs to be shut off temporarily.A permanent Simplot BSW treatment system will be constructed to treat an minimum of 250 gpm and up to 700 gpm 2012-09-13 addendum well install work plan_hy test rpt.doex 9