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Geosyntec° Geosymtec° cnnsultants consultants 4. SCOPE OF WORK engineers I scientists I innovators 4.1 Rationale and Approach The Supplemental Guidance recommends a four-step evaluation process for conducting VI evaluations.The four steps consist of the following: VAPORINTRUSION PILOT STUDY 1. Prioritizing buildings in the proximity to source contamination for a VI assessment 2. Collect exterior soil vapor samples to determine if buildings have potential for VI WORKPLAN 3. Collecting indoor air,sub-slab soil vapor and outdoor air samples if buildings have potential VI risks LINCOLN CENTER, 4. Evaluating the need to manage current and future VI risk based on both indoor air concentrations and soil vapor concentrations STOCKTON, CALIFORNIA The scope of work has been developed to follow the stepwise approach recommended by the Supplemental Guidance. There are two areas proposed for additional investigation activities:1)A vacant suite located at 858 West Benjarnin Holt Drive,and 2)an occupied suite located at 240 Lincoln Center(Figure Prepared for 2).These suites are located on-Site,in the Lincoln Center Shopping Center.The vacant suite is Lincoln Center Environmental Remediation Trust located near the former source area where some of the more elevated PCE impacts have been 3043 Gold Canal Drive,Suite 100 observed in shallow groundwater.This suite is located near the location where recent ISCO Rancho Cordova,California 95670 injection activities have occurred along West Benjamin Holt Drive,as described in Section 2.4. The occupied suite is located north of the main former source area where PCE impacts to Prepared by shallow groundwater have been observed to be slightly elevated.This suite is located north of West Benjamin Holt Drive,away from the recent ISCO injections and known soil source.In Geosyntec Consultants,Inc. accordance with the Step I of the Supplemental Guidance,these locations were selected because 3043 Gold Canal Drive,Suite 100 of their proximity to the potential sources of VI. Rancho Cordova,California 95670 The Supplemental Guidance recommends the collection of exterior soil vapor samples,indoor Project Number:SAC 1 I I H air,sub-slab soil vapor,and outdoor air samples to evaluate the VI potential(Step 2 and 3).This data can also be used to develop site-specific attenuation factors.Collocated groundwater and April 2020 soil vapor samples will be collected from the two study areas.In addition,a paired sub-slab soil vapor samples and indoor samples will be collected to develop building-specific attenuation factors.The two proposed locations for this study have in-part been selected because elevated concentrations of VOCs have been reported in monitoring wells screened where first encountered groundwater is present during recent sampling events.These areas are therefore anticipated to have the greatest potential for vapor intrusion at the Site. .,� According to the 2011 DTSC Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air(Vapor Intrusion Guidance;DTSC,2011),it is recommended that © groundwater samples be collected from a permanent monitoring well that is screened across the top of water and the saturated thickness in the well should be 10 feet or less.Recent groundwater �4 x monitoring data(refer to Section 2.3.5)indicated the top of water elevation is approximately 10 feet msl.However,the aquifer is semi-confined,as noted in Section 2.3.1.Historical boring logs \\ � for monitoring wells in the areas of interest were reviewed to identify where saturated soils were 202003 Liu oln Center VI Pilot Study NP.dmx 9 April 2020 V �t