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Geosyntec consultants performed in this zone (Geomatrix, 2005). The results of previous investigations indicated that COCs in groundwater or soil (or both) included volatile organic compounds (VOCs), fumigants, chlorinated herbicides, organophosphates, triazines, carbamates and general inorganics (Geomatrix, 2005). Several phases of remedial activities have been undertaken and groundwater monitoring wells have been installed in the shallow and intermediate zones both on- and off-Site for monitoring purposes. Wells have been installed(i)on-Site, (ii) in the adjacent residential area to the east, and (iii) in the industrial property to the west. Monitoring well locations are shown in Figure 2. Remediation at the Site has included: (i) excavation of soil containing pesticides, (ii) on-Site treatment of excavated soils using low-temperature thermal desorption, (iii) injection of carbon substrate into the subsurface to enhance bioremediation, and (iv) phytoremediation to address nitrogen and potential residual pesticides in soil and groundwater beneath the Site and off-Site monitored natural attenuation to reduce the concentrations of COCs in groundwater downgradient of the Site (i.e., beneath the residential neighborhood east of the Site [Geomatrix, 2005 and Apex, 2018]). Phytoremediation and off-Site monitored natural attenuation are presently the only remedial activities in use at the Site. At this time, the phytoremediation system is comprised of approximately 1,200 eucalyptus trees (primarily red gum eucalyptus, Eucalyptus camuldulensis) planted on a 5- to 10-foot grid pattern that covers about 2.3 acres of the Site. Initially 1,500 trees were planted in 2003, with a further planting of approximately 250 additional trees in 2013. Since 2013, the number of trees at the Site has decreased through a combination of natural loss and deliberate removal as part of the plantation management strategy. In 2005, Geomatrix submitted, and the CRWQCB conditionally approved, the Revised Contingency Plan, which established performance criteria to monitor the effectiveness of phytoremediation and off-Site monitored natural attenuation. The Revised Contingency Plan also established Cleanup Levels for COCs in groundwater and soil and Threshold Concentrations for CDCs in groundwater at the Site perimeter. For several COCs, the Cleanup Levels are presented as a range of concentrations (referred to as "Range of Cleanup Levels"). As a condition of approval, the CRWQCB required several modifications to the Cleanup Levels and Threshold Concentrations, which are described in the CRWQCB's February 22, 2006 letter. 2.4 Off-Site COC Concentrations and Potential Vapor Intrusion Risk Results from Q1-2020, Q3-2020 and Q 1-2021 groundwater monitoring events indicate that concentrations of the VOCs 1,2-dichloropropane (1,2-DCP) and 1,2,3-trichloropropane (1,2,3- TCP) in samples collected from one off-Site monitoring well (MW-27) located in the adjacent residential area exceeded the upper range of the Cleanup Levels in each of the three monitoring events. However, the 1,2-DCP and 1,2,3-TCP concentrations did not exceed the Threshold Concentrations in any of these three monitoring events. Based on these results,the CRWQCB has requested a work plan to evaluate the potential for vapor intrusion in the adjacent residential area. The enclosed Work Plan is intended to meet this request. Work Plan for Vapor Intrusion Investigation 3 July 30,2021