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and April 2021 are significant in relation to MTCA compliance. Based on the evaluation presented in Section 6.2.1.2 and Figure 8,the January 2421 total arsenic exceedance at MW-6 is a statistical outlier.Given that arsenic is not a soil COC for the Site (see Section 6.2.1.1), arsenic in MW-6 groundwater is consistent with background levels when discounting the statistical outlier value, and dissolved arsenic concentrations are less than the arsenic cleanup level, a case is made that MW-6 groundwater complies with MTCA. 6.2.1.1.Background Arsenic in Soil There are no known potential sources of arsenic associated with the Site and soil sampling and chemical analysis completed as a part of the RI and soil cleanup indicate that arsenic concentrations in soil beneath the Site are consistent with natural background concentrations in the Snohomish Basin. Arsenic was not detected in seven of twelve soil samples collected at the Site from depths between 3 and 10 feet bgs, and the detected concentrations in the remaining five samples were less than the MTCA Method A cleanup level of 20 mg/kg (RI Table 2; GeoEngineers 2018a). The only possible non-background potential source of arsenic that is nearest to monitoring well MW-6 was the former sump that was removed during Site cleanup. However, concentrations of arsenic in the sample of residual product collected within the sump at time of removal and in five soil confirmation samples collected following sump removal, were all less than the MTCA Method A cleanup level. 6.2.1.2.Background Arsenic in Groundwater and MW-6 Statistical Evaluation for Outliners Total arsenic has been detected in groundwater samples obtained from monitoring wells MW4, MW-3, MW-5, and MW-6 during at least two of four groundwater monitoring events completed following soil cleanup, indicating arsenic is naturally occurring in the Site aquifer. The April 2021 arsenic concentration of 5.1 pg//L at MW-6 only slightly exceeds the MTCA Method A cleanup level of 5 pg/L and is significantly lower than the published background concentration for arsenic in the Snohomish Basin (11.8 pg/L). Ecology concluded in their draft publication that the current MTCA Method A cleanup level for arsenic in groundwater 5 pg/L is likely too low (Ecology 2021). Based on a statistical analysis of all arsenic groundwater data collected at the Site using EPA's ProUCL software, the January 2421 detection of arsenic at MW-6 is statistically a data outlier, suggesting an anomalous result(possible field or analytical causes). The results of the statistical analysis are presented graphically in Figure 8. 6.3. Request for NFA Determination The scope of groundwater compliance monitoring completed at the Site was approved by Ecology under VCP opinion letters. The results of groundwater compliance monitoring and the discussion above indicate the Site cleanup is protective of groundwater. On this basis, the City requests a No Further Action determination for the Site. Following receipt of a NFA determination the groundwater compliance monitoring wells will be decommissioned by a licensed well driller in accordance with WAC 173-160-381. 7.0 LIMITATIONS This report has been prepared for use by the City of Everett and their authorized agents.This report may be provided to regulatory agencies for review. No third parties should place legal reliance on this report. GeoEngineers has performed this work in accordance with the scope and limitations of our Agreement with the City of Everett dated October 5, 2016 and associated Contract Amendment No. 3 dated June 9, 2021. Within the limitations of scope, schedule and budget, our services have been executed in accordance with GEOENGINEER� January24,2022 Page6 File No.0661-108-03