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Prior to the installation of the durable covers within the Project Sierra development area, COC/COEC-impacted soil will be excavated (primarily within the northern and eastern portions of the remedial boundary) to create new stormwater drainage features (i.e., detention ponds and drainage channels) and to integrate the future cover with the adjacent property. COC/COEC- impacted soil from the mass grading within the remedial boundary would be placed' in the footprint of the new Warehouse and Outdoor Storage Area, including an area south of the remedial boundary (Figure 15). Approximately 57,000 cubic yards of COC/COEC-impacted soil are anticipated to be placed and compacted under the new Warehouse and Outdoor Storage Area. The types of durable covers$ within the Project Sierra development are shown in Figure 15 and are described below: • Warehouse and Outdoor Storage Area: the durable cover would consist of a demarcation layer at its base, a minimum of 18 inches of clean soil, overlain by a minimum 4-inch-thick layer of aggregate base (AB) and concrete slabs-on-grade or concrete building foundations at the surface. • Parking and driveway areas: the durable cover would consist of a demarcation layer at its base and a minimum of 2 feet of clean soil, overlain by AB and either concrete or asphalt-concrete pavement.9 • New railroad spurs: the durable cover would consist of a minimum of 18 inches of clean soil, overlain by AB, concrete slab, and the rail spur. • Exterior concrete areas and sidewalks: the durable cover would consist of a demarcation layer at its base, a minimum of 2 feet clean soil, overlain by AB and concrete slabs-on-grade. • Stormwater detention ponds and limited landscape areas: the cover would consist of a demarcation layer at its base and a minimum of 2 feet of clean soil. Additional modifications to the drainage channel system to manage stormwater within the development footprint are described below. Outside of the Project Sierra development area, new soil covers would be installed over four distinct unpaved areas (Figure 15): 6 Excavation depths within the remedial boundary range from 2 to 6 feet below the existing ground surface. 7 COC/COEC-impacted soil will be placed above the existing ground surface(typically+1 to+2.5 feet msl)and extending up to the subgrade elevation(+4.2 feet msl within the new warehouse). These elevations are above the groundwater elevations previously measured which,as stated in Section 1.7.2,ranged between-3 and-7 feet msl depending on the season. a The combined thickness of these cover elements would be at least 2 feet;individual layer thickness(e.g.,concrete foundations)may be adjusted during the building design and will be constructed in accordance with industry standards(e.g.,ASTM International). 9 Current site development plans specify concrete pavement in areas with heavy traffic loads,and asphalt-concrete in areas with lighter traffic loads. Site 47 RAP for Soil 15 October 2023