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Addendum to the Well Installation and Hydraulic Testing Report and Additional r�'-;_' ARCADIS Well Installation Work Plan Former Occidental Chemical Corporation Lathrop, California Updated groundwater modeling results predict that the installation and start-up of extraction wells EW- 10 and EW-11 and EW-17, in concert with EW-09, will create a zone of influence that extends outwards approximately 2,000 ft(Appendix C). Based on actual conditions measured following extraction well installation and start-up, an additional water table zone extraction well may be added in a subsequent phase if needed to capture sulfolane-impacted groundwater within the water table zone towards the southeast. Shallow Zone (50 to 100 ft bqs) Within the shallow zone, groundwater is currently extracted from well EW-08A. EW-03 is also screened in the shallow zone-, however, sulfolane, DBCP and EDB concentrations in EW-03 have decreased to levels at or below Site cleanup standards in this well. Several additional extraction wells are proposed to be screened in the shallow zone, including EW-12A, EW-14A, EW-15A and contingency well EW-16. These wells would be located within the areas of highest sulfolane concentrations, with the exception of EW-16, which is currently designated as a contingency well. Well locations are shown on Figure 8. Extraction rates from the shallow zone are expected to be approximately 25 to 40 gpm for each new extraction well (Appendix C). In addition, injection of treated groundwater into the shallow zone can occur at INJ-11, INJ-12 or INJ-13/EW-03. Injection of oxygenated treated water into INJ-11 and INJ-12 will stimulate in-situ biodegradation of sulfolane, increase the hydraulic gradient towards the extraction wells EW-8A, EW-12A, EW-14A and EW-15A. Injection of oxygenated treated water into INJ-12 will also facilitate in-situ degradation of sulfolane and extraction of sulfolane-affected groundwater at well EW-15A and the Simplot BSW. From a historical perspective, the footprint of sulfolane contamination in the shallow zone has decreased over time to current conditions (Figure 8). Sulfolane was historically detected in the following wells that are currently non-detect: PW04-058, PW05-063, PW06-080, PW09-080, PW13-090, PW16- 083, PW19-092 and PW22-071. Based on recent analytical data collected from the shallow zone provided by Simplot from March and July 2012 sampling events, sulfolane was detected in shallow well LP-2 (570 pg/L). The current plume footprint interpretation shown on Figure 8 and the updated groundwater modeling runs are consistent with these results. Intermediate Zone (100 to 150 ft bqs) Groundwater is currently extracted from the intermediate zone from wells EW-03, EW-05, EW-06, and EW-08A. As discussed above, sulfolane, EDB and DBCP concentrations in EW-03 have decreased to levels at or below Site cleanup standards in this well. Several additional extraction wells are proposed to be screened in the intermediate zone, including EW-12A, EW-13, EW-14A, and EW-15A. These wells are proposed to be located in the areas of highest sulfolane concentrations (Figure 9). Extraction rates from the intermediate zone are expected to be approximately 10 to 50 gpm from each well (Appendix 2012-09-13 addendum well install work plan_hy test rpt.docx 12