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DATA GAP ASSESSMENT WORK PLAN AND PATH TO CLOSURE REVIEW Chase Chevrolet, 424 N. Van Buren Street LTCP Closure Evaluation January 2, 2018 from MW-15, that source material was excavated at the site between April and May 2004, and an additional biodegradation has likely occurred in the 13 years since the last time groundwater samples were collected at the site. Presence of Free Product: Free product was historically reported in well MW-3; however, free product was last reported in well MW-3 in 1996 and did not recur up until the well was destroyed in 2004. Additionally, the area directly surrounding well MW-3 was overexcavated in 2004. Stantec does not recommend any further evaluation of the potential presence of free product as part of the release. Presence of Sensitive Receptor to Groundwater (Surface Water Body or Water Supply Well): The nearest surface water body is McLeod Lake, which is located approximately 750 feet south of the historical site boundary (historical upgradient direction). Per the 1996 receptor survey, the closest municipal water supply well to the site was located 1,100 feet west of the site (historical cross-gradient direction). Based on the age of the last sensitive receptor survey, Stantec recommends the performance of an updated sensitive receptor survey (scope of work presented in Section 4.0 below). Presence of Dissolved Phase Benzene and MTBE: During the last groundwater monitoring and sampling event performed at the site in October 2003, the presence of benzene was reported at a maximum concentration of 6,600 pg/L from well MW-3. The next highest concentration of benzene reported at the site was 48 pg/L from well MW-22. Methyl tertiary butyl ether (MTBE) was last sampled at the site in May 2002, and was reported as non-detect in site wells. MTBE is not a contaminant of concern at the site. Given that the area surrounding well MW-3 was overexcavated between March and April 2004, it is not likely that concentrations of dissolved-phase benzene are still elevated in the area surrounding the former UST pit. Stantec recommends additional grab groundwater sampling at the site to evaluate current dissolved phase conditions at the site. The proposed scope of work below in Section 4.0 addresses how additional data will be collected to evaluate the current site conditions. 3.2.2 Petroleum Vapor Intrusion to Indoor Air Historical assessment activities at the site were not designed to collect an appropriate data set for the evaluation of site conditions against the petroleum vapor intrusion to indoor air criteria of the LTCP. Historical soil analytical data is not available to demonstrate that total petroleum hydrocarbon (gasoline and diesel range combined) concentrations are less than 100 mg/kg in the vadose zone and current dissolved-phase benzene concentrations in the shallow groundwater are not currently known. Additionally, direct measurements of the soil vapor concentrations at 5 feet bgs have not been performed historically. Based on the current site use as a multi-floor parking garage with ground level retail units, whose foot print overlies the historical dissolved phase plume (Appendix D), Stantec recommends the Stantec bm \ \us1312- f02 \workgroup\ I 857 \ active \ 185703766 \ 05_report_deliv \ deliverable \work_plan \ I 85703766_20180102_424_van_buren_work_plan_final.docx 3.3