600 RIVERSIDE RD Geotech Report 2022-10-25

Id 1 Ci 1 I Landau Associates portion of the underlying silt unit. Deep excavations that penetrate the silt unit could create the potential for contamination in the upper sand unit to carry down to the lower sand unit. Site development planning and design will, therefore, need to incorporate approaches to address the potential of encountering contaminated soil and groundwater during construction and to be compatible with maintaining the long-term environmental integrity of the site. The general approach to site construction considering the potential for soil and groundwater contamination is outlined in the Riverside Business Park Development Guidelines Technical Memorandum (LAI 2014) and the Soil and Groundwater Management Plan (LAI 2017). If, during construction, it is determined that portions of the excavations penetrate through the silt unit, mitigating measures will be necessary to limit the potential for contamination in the upper sand unit from carrying down into the lower sand unit. The extent of mitigating measures required will depend on the location, depth, and nature of the excavation. Possible mitigating measures include using low -permeability soil as backfill where excavations penetrate the aquitard or low -permeability check dams placed at regular intervals along buried pipeline alignments to limit the migration of groundwater along the utility trench backfill and pipe bedding. Other environmental issues to consider during construction relate to the control of dust and the decontamination of equipment during and following construction. When working with the existing soil during periods of dry weather, the contractor should be prepared to use water or other methods to control dust and prevent the spread of contamination on and off of the site. If clearly contaminated soil is encountered during excavation activities, the contractor should establish contamination reduction and exclusion zones, to help prevent the spread of contaminated soil on the site. In order to prevent spreading contaminated soil to other sites, the contractor should decontaminate all construction equipment used onsite before moving it to another site. Wash water generated during the decontamination process should be contained and not allowed to flow into the nearby Snohomish River. The contained wash water may either be infiltrated onsite or disposed offsite at an approved disposal facility. Weyerhaeuser Company maintains and monitors a number of monitoring wells on the site for purposes of groundwater compliance monitoring as required in the consent decree between Ecology and Weyerhaeuser (Superior Court of the State of Washington 1997). ASARCO has also installed monitoring wells in some locations. Any existing monitoring wells that will be required for long-term monitoring need to be protected during and following construction. 3.2 Contaminated Soil Handling and Disposal If any soil encountered during construction visually appears to be highly contaminated (e.g., soil with oily residue, groundwater with an oily sheen, strong petroleum -like odors, etc.), it should be tested and compared to the site cleanup action levels. If the level of contamination in the soil is determined Geotechnical Engineering Report 0121030.010.011 Riverside Business Park March 22, 2018 Public Access Trails and Roadway Improvements 3-2