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5.3. Chemical Analytical Results Groundwater samples were submitted to ALS Environmental of Everett,Washington,for chemical analyses of the list of Site COCs modified by Ecology as follows: gasoline-, diesel- and lube oil-range petroleum hydrocarbons by NWTPH-Gx and NWTPH-Dx Methods, VOCs by EPA Method 8260, PAHs by EPA Method 8270D/SIM and arsenic, chromium, and lead (total and dissolved) by EPA Method 200.8. Groundwater sampling procedures are summarized in Appendix B. Laboratory reports are presented in Appendix C. Chemical analytical results for Site groundwater monitoring are summarized in Table 1 and discussed below. 5.3.1. First Quarter Post-Cleanup Groundwater Sampling July 2020 Diesel-and lube oil-range petroleum hydrocarbons (600 and 610 pg/L, respectively) were detected in the July 2020 sample from MW-3 at concentrations slightly greater than the MTCA Method A cleanup level of 500 pg/L. Total arsenic, chromium and lead were detected in the July 2020 sample from MW-3 at concentrations greater than the MTCA Method A cleanup levels (see Table 1). Remaining COCs either were not detected in the July 2020 samples from the monitoring wells or the detected concentrations were less than MTCA Method A cleanup levels. 5.3.2.Final Four Quarters Post-Cleanup Groundwater Sampling October 2020 to July 2021 COCs were not detected in post-cleanup compliance groundwater samples during the four quarterly samples events between October 2020 and July 2021 except as follows: ■ Total arsenic was detected in the MW-6 groundwater samples obtained in January and April 2021 at concentrations of 15 and 5.1 pg/L, respectively, which are greater than the MTCA Method A cleanup level of 5 pg/L. The corresponding dissolved arsenic concentrations in the January and April 2021 groundwater samples were 3.3 and 2.9 pg/L, respectively, which are less than the MTCA Method A cleanup level. Only the January 2421 groundwater sample result from MW-6 (15 pg/L) exceeded the natural background concentration for arsenic in the Snohomish Basin (11.8 pg/L). 6.0 SUMMARY AND CONCLUSIONS 6.1. Soil The results of completed remedial actions at the Site demonstrate Site soil cleanup is complete, as confirmed by the opinions issued to date by Ecology. 6.2. Groundwater COCs were not detected in post-cleanup compliance groundwater samples during the four quarterly samples events between October 2020 and July 2021 except fortotal arsenic in the January and April 2021 groundwater samples from MW-6; however, the corresponding dissolved arsenic results for the MW-6 groundwater samples were less than the MTCA Method A cleanup level for arsenic. 6.2.1.Evaluation of Arsenic in MW-6 Groundwater Sections 6.2.1.1 and 6.2.1.2 present a discussion regarding the arsenic results in groundwater samples from monitoring well MW-6 to evaluate whether the cleanup level exceedances for total arsenic in January GEOENGINEER� January24,2022 Page5 File No.0661-108-03