600 RIVERSIDE RD Geotech Report 2022-10-25

11 Landau Associates I 2.6 Soil Contamination Historical operations associated with the Weyerhaeuser Everett East Site (East Site) resulted in environmental impacts to soil and groundwater that were addressed by a final cleanup action conducted by Weyerhaeuser under a consent decree with the Washington State Department of Ecology (Dalton, Olmsted & Fuglevand 1997). The final cleanup action addressed the following identified contaminants of concern (COCs): total petroleum hydrocarbons (TPH), carcinogenic polycyclic aromatic hydrocarbons (cPAHs), polychlorinated biphenyls (PCBs), and pentachlorophenol (PCP). The East Site cleanup removed the COCs identified in the consent decree from soil located above the groundwater table at concentrations greater than the East Site cleanup levels from 20 specific areas on the Property. In 12 of these areas, groundwater and/or a concrete slab were encountered prior to achieving the East Site cleanup levels. As a result, soil containing COCs at concentrations above the East Site cleanup levels may remain at depth in these areas. The 12 areas where residual contamination above the East Site cleanup levels may be present are shown on Figure 3. Because the East Site cleanup was based on cleanup to industrial standards, East Site COCs may be present throughout the East Site at concentrations below the East Site cleanup levels, but exceeding the Model Toxics Control Act (MTCA) cleanup levels for unrestricted site use. Arsenic has also been encountered in soil at the project site due to smelter slag and possibly arsenic affected fill material that was imported from the former ASARCO Smelter Site (Smelter Site) to the project site when Weyerhaeuser owned the property. Additionally, the surface of the mud flat deposit present below the fill material that was the former exposed surface at the project site during the period that the Smelter operated, may have been affected by direct aerial deposition of arsenic, although the soil quality of this prior exposed surface has not been evaluated. 2.7 Groundwater Contamination Two groundwater aquifers are present at the project site. The shallow aquifer is present in fill overlying the former mud flat deposits, which acts as an aquitard that separates the shallow aquifer from a deeper aquifer that underlies the mud flat deposits. Potential COCs in the shallow aquifer beneath the project site include the East Site COCs (TPH, PCP, cPAHs, and PCBs), and dissolved arsenic associated with the Smelter Site. The former mud flat aquitard has inhibited the migration of East Site groundwater COCs from reaching the deeper aquifer. However, dissolved arsenic groundwater contamination originating from the Smelter Site is present within a portion of the deeper aquifer underlying the project site. Dissolved arsenic groundwater concentrations have not been thoroughly evaluated for the Smelter Site Lowlands Area for more than 10 years; therefore figures showing dissolved arsenic concentrations for the shallow and deep aquifers are not provided as part of this document. 1 I 1 I 1 I 1 1 1 �i Geotechnical Engineering Report 0121030.010.011 , Riverside Business Park March 22, 2018 Public Access Trails and Roadway Improvements 2-4