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4.0 CONTAMINANTS OF CONCERN AND GROUNDWATER CLEANUP STANDARDS The initial list of contaminants of concern (COCs) identified for Site groundwater were gasoline, diesel and lube oil-range petroleum hydrocarbons, VOCs, PAHs, and metals including arsenic, cadmium, chromium, lead and mercury. These groundwater COCs were proposed in the MTCA Cleanup Report (GeoEngineers 2020) and approved by Ecology via email (Ecology 2020). However, Ecology subsequently noted that cadmium and mercury could be eliminated as groundwater COCs because they were not previously detected in groundwater samples collected at the Site. Additionally, Ecology requested that analyses of groundwater for arsenic,chromium and lead include both total and dissolved analyses. Changes requested by Ecology were incorporated into the groundwater compliance monitoring program as described in this report. The pathway of potential concern for groundwater is direct contact. The standard point of compliance for groundwater is throughout the Site from the uppermost level of the saturated zone extending vertically to the lowest depth that could potentially be affected by Site contaminants. Groundwater cleanup standards for gasoline-,diesel-and lube-oil petroleum hydrocarbons, naphthalenes,cPAHs,and metals are presented below and included in the groundwater data table presented as Table 1. Cleanup standards for individual PAHs and VOCs not listed below are presented in Table 1 for analytes detected in one or more samples. Contaminant of Concern Cleanup Standard Cleanup Level Gasoline-range petroleum hydrocarbons MTCA Method A cleanup level' 1,000 pg/L Diesel-range petroleum hydrocarbons MTCA Method A cleanup level 500 pg/L Lube oil-range petroleum hydrocarbons MTCA Method A cleanup level 500 pg/L Naphthalenes MTCA Method A cleanup level 160 pg/L cPAHs(benzo[a]pyrene TEC) MTCA Method A cleanup level 0.1 pg/L Arsenic(total and dissolved) MTCA Method A cleanup level 5 pg/L Chromium (total and dissolved) MTCA Method A cleanup level 50 pg/L Lead (total and dissolved) MTCA Method A cleanup level 15 pg/L Notes: 1 Benzene was not detected in the groundwater samples collected at the Site.Therefore,the MTCA Method A cleanup level used when benzene is not present(1,000 pg/L)is the appropriate cleanup level. pgIL=micrograms per liter,TEC=toxicity equivalent concentration The published background value for naturally occurring arsenic in groundwater in the Snohomish basin of is 11.8 pg/L(Ecology 2021). 5.0 GROUNDWATER COMPLIANCE MONITORING The post-cleanup groundwater compliance monitoring was completed in accordance with the compliance monitoring plan presented in the MTCA Soil Cleanup Report(GeoEngineers 2020), as modified by Ecology as described in Section 4.0 and outlined in Ecology's written approval (received via email; Ecology 2020). Groundwater sample chemical analytical results for Site CDCs were compared to groundwater cleanup levels listed in Section 4.0 and Table 1. GEOENGINEER� January24,2022 Page3 File No.0661-108-03