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Geosyntec'` Geosyntec Consultants, Inc. (Geosyntec)has prepared this In-Situ Bioremediation Pilot Test Work Plan (Work Plan) on behalf of Glenn Springs Holdings, Inc. (GSH) for the Former Occidental Chemical Corporation (OCC) Lathrop Facility (Site) located at 16777 Howland Road in Lathrop, California(Site; Figure 1). Site work is being completed under the oversight of the Central Valley Regional Water Quality Control Board (CVRWQCB). Historical groundwater monitoring data suggest migration of sulfolane from its source area to the eastern portion of the Site, beyond the capture zone of the existing groundwater extraction and treatment system (GETS). In recent years, sulfolane has been reported at the water supply wells operated by J. R. Simplot (Simplot) located in the eastern portion of the Site. Supplemental characterization work is currently underway to evaluate the lateral and vertical extent of sulfolane impacts in groundwater in the eastern area, and to better understand migration pathways from the source area to the eastern portion of the Site. The final report for these investigations is expected to be available in October 2018, however, Geosyntec has evaluated the investigation results obtained to date in the preparation of the work plan. Coincidental with the characterization activities, Geosyntec has undertaken a preliminary evaluation of potential remedial alternatives to reduce sulfolane concentrations in the eastern Site groundwater,and to prevent further migration of sulfolane towards the water supply wells. Based on the fact that aerobic degradation of sulfolane in the on-site groundwater treatment system has long been recognized as an effective means to remove sulfolane from recovered groundwater and that sulfolane in un-preserved water samples collected from the Simplot Backup Supply Well (BSW) for evaluation of remedial options appears to have biodegraded while being shipped for laboratory analysis, in-situ bioremediation (ISB) may be a feasible alternative for sulfolane remediation in groundwater in the eastern portion of the Site. Geosyntec is currently conducting focused laboratory microcosm studies to confirm and optimize the observed sulfolane degradation behavior in groundwater. Concurrent with the microcosm studies, this Work Plan has been prepared to guide design, installation, operation, and performance monitoring of an ISB pilot test to treat sulfolane in groundwater and to prevent further sulfolane migration towards the water supply wells. It is anticipated that the data generated from the pilot test may be used to develop a full-scale remedial alternative, as appropriate. The Work Plan includes details on design of the pilot test and the associated implementation activities, including planning, permitting, monitoring, and reporting. 1.1 Document Organization Following this section, this Work Plan is organized as follows: • Section 2 "Background" presents a summary of the relevant Site background information including Site location, use, Site geology and hydrogeology, the nature and extent of In-Situ Bioremediation Pilot Test Work Plan—GSH Lathrop 1