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Addendum to the Well Installation and r Hydraulic Testing Report and Additional f ARCADIS Well Installation Work Plan Former Occidental Chemical Corporation Lathrop, California C). In addition, injection of treated groundwater into the intermediate zone can occur at INJ-11, INJ-12, INJ-13/EW-03 or INJ-14/EW-04. Injection of treated water into INJ-11 and INJ-12 will 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 stimulate in-situ biodegradation of sulfolane, increase hydraulic gradient and facilitate extraction of sulfolane-affected groundwater at well EW-15A and the Simplot BSW. Injection into INJ-113/EW-03 and INJ-14/EW-04 at approximately 30 to 50 gpm will stimulate in-situ biodegradation of sulfolane, and increase the hydraulic gradient towards extraction wells(Figure 9). From a historical perspective, the footprint of sulfolane contamination in the intermediate zone has been stable over time, similar to current conditions (Figure 9). Sulfolane was historically detected in PW05-125 but is currently non-detect. Deep Zone 050 to 200 ft bgs) Groundwater is currently extracted from the deep zone from wells EW-02, EW-04A, EW-05, EW-06, EW-0813, Simplot's PSW, and Lathrop Water District wells to the east, southeast and northeast of the Site. In past years, the Simplot BSW has reportedly operated on a contingency basis and seasonally (March through October)to supplement water supply. Several additional extraction wells are proposed to be screened in the deep zone, including EW-1213, EW-14B, and EW-15B. These wells would be located within the areas of highest sulfolane concentrations (Figure 10). Extraction rates from each of the new deep zone wells are expected to be approximately 20 to 30 gpm (Appendix C). In addition, injection of treated groundwater into the deep zone can occur at INJ-11, INJ-12, or INJ-14/EW-04. Injection of oxygenated treated water into INJ-11 and INJ-12 will enhance in-situ biodegradation of sulfolane and increase the hydraulic gradient towards the deep zone extraction wells within the plume. Injection of treated water into INJ-12 will enhance in-situ biodegradation of sulfolane and increase hydraulic gradient, facilitating extraction of sulfolane-affected groundwater at well EW-15A and the Simplot BSW. Injection into INJ-14/EW-04, expected to be approximately 100 gpm,will increase the hydraulic gradient towards extraction wells from areas outside of the plume(Appendix C). Sulfolane, EDB and DBCP concentrations in INJ-14/EW-04 have decreased to levels at or below Site cleanup standards. From a historical perspective, the footprint of sulfolane contamination in the deep zone appears to be fairly stable and has decreased slightly over time to reach current conditions(Figure 10). Sulfolane was historically detected in wells PW03-218, PW05-196, PW08-180, PW10-195, PW11-195, PW13-165, PW16-216, PW19-196 and PW21-190 but is currently non-detect or below Site screening levels. Recent sampling of BSW detected up to 570 pg/L sulfolane during a pumping test. The current plume footprint interpretation shown on Figure 10 and the updated groundwater modeling runs in Appendix C are consistent with these results. 2012-09-13 addendum well install work plan_hy test rpt.docx 13