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Groundwater compliance monitoring events were conducted in July 2020, October 2020, January 2421, April 2021, and July 2021. The groundwater compliance monitoring program included three existing monitoring wells installed during the RI (MW-1, MW-2, and MW-31) and two new monitoring wells installed following Site cleanup (MW-5 and MW-6), as described below. Of the five monitoring wells utilized as a part of the groundwater compliance monitoring program,four of the wells (MW-2, MW-3, MW-5, and MW-6) are located on the downgradient margin of the Site. Depths to groundwater and groundwater elevations measured in the five compliance groundwater monitoring wells between July 2020 and July 2021 are presented in Table 1;groundwater elevation contours for each event are presented as Figures 3 through 7. 5.1. Monitoring Well Installation Two monitoring wells(MW-5 and MW-6)were installed at the Site on July 7, 2020.The wells were installed to total depths of 40 feet bgs and constructed of 2-inch-diameter Schedule 40 PVC with 10 feet of 0.010-inch slot PVC screen placed between 30 and 40 feet bgs. The monitoring wells were developed using purge and surge methods and allowed to settle for 2 days prior to the first quarterly groundwater sampling event. Soils observed during drilling generally consisted of fill material comprised of silty sand with occasional gravel and debris (asphalt) overlying consolidated silty sand or sand interpreted as glacial deposits. Soils observed during compliance monitoring well installation were consistent with the RI explorations completed previously at the Site. Soil samples obtained from the borings were field screened for evidence of contamination using visual examination,water sheen screening and vapor headspace screening with a photoionization detector(PID). The results of headspace and sheen screening are included in the boring logs. No soil samples from the borings were submitted for chemical analysis. Boring and well installation logs and groundwater sampling procedures are presented in Appendix B. Boring and monitoring well installation logs for previously installed monitoring wells MW-1, MW-2 and MW-3 are also included in Appendix B for reference. 5.2. Groundwater Occurrence Groundwater in Site monitoring wells generally occurs at depths between approximately 22 and 29 feet bgs (approximate elevations 38 to 26 feet [NAVD88]), within the native glacial deposits beneath the fill, except for MW-2. Groundwater in monitoring well MW-2, which has the shallowest screened interval (5 to 15 feet bgs) of the five wells on Site, appears to be seasonally perched above the fill-native soil contact. This well was observed to be dry during all compliance groundwater monitoring events with the exception of January 2421 when groundwater was observed at a depth of approximately 12 feet bgs (approximate elevation 42 feet NAVD88;see Table 1). Observed groundwater elevations were relatively static across all five groundwater compliance monitoring events, with seasonal fluctuations on the order of approximately 1 to 2 feet. Groundwater elevations were highest duringthe April 2021 monitoring event and lowest duringthe October 2020 event.The groundwater flow direction was generally to the east toward the Snohomish River during all five compliance monitoring events(Figures 3 through 7)with a calculated horizontal hydraulic gradient ranging between approximately 0.04 and 0.05 feet per foot. 1 Monitoring well MW-4 was installed during the RI and was decommissioned during Site cleanup;refer to the MTCA Soil Cleanup Report for additional information. GEoENGINEER� lanuary24,2022 I Page File No.0661-108-03