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Geosyntec consultants 3.2 RATIONALE In order to determine the data needs for this Work Plan, an analysis of the chemical and stratigraphic data was performed to develop a current understanding of the off-site subsurface area. To conduct the analysis existing information was compiled for the downgradient groundwater area (east of the site and Pacific Avenue) including boring logs, cone penetration testing logs, membrane-interface probe logs, well construction details, cross sections, laboratory data, groundwater elevation data, and aquifer parameter data. In addition to the analysis of chemical and stratigraphic data, a two-dimensional groundwater flow model prepared. The flow model was used to assess the performance of the GWETS and to provide any recommendations for system optimization. As discussed in Section 2.2.1.1, the flow model was used to adjust the extraction rates of individual wells to in an effort to achieve capture of VOCs in the A- and B-Zones downgradient of the site. Prior to these system adjustments, it is unclear if the GWETS achieved complete capture of the site impacts. Some additional investigation work is proposed in the Work Plan in response to the results of the flow model. In conjunction with the flow model, the analysis of chemical and stratigraphic data generally confirmed the hydrogeology as discussed in Section 2.1.2 with the following data gaps or qualifications: • There is a scarcity of subsurface data in the area between Pacific and Inglewood Avenues with regards to the transition between the A- and B-hydrostratigraphic units and for identifying the base of the B-Zone. As described above, an anomalous, water,bearing sand interval is found east of Pacific Avenue within the inferred depth range of the A/B-Zone. The A/B-Zone in other areas of the site has been conceived as an aquitard; however, the more permeable character of this sand interval could facilitate the continued migration of the A-Zone impacts into the B-Zone. • In general, the existing B-Zone wells are inconsistent in regards to their screen intervals. Wells classified as B-Zone are generally constructed with a 20-foot screen interval; however, the screen intervals range between 81 and 147 feet bgs (66 to 136 feet below msl). This variation in the B-Zone well screen intervals must be considered when evaluating GWETS performance and the extent of COPC impacts. Pre-Design Investigation Work Plan 11 18.05.12