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DRAFT Geosyntec <br /> consultants <br /> Code Section 12220. UST 817B is located in the central portion of RRI on Davis Avenue <br /> (Figure 1). <br /> RRI is underlain by Unconsolidated Deltaic Deposits to a depth of approximately 1,300 feet <br /> below ground surface (bgs). The Deltaic Deposits are comprised of interbedded clay, silt, sand, <br /> gravel, and organic soils (Geosyntec, 2018a). There is a surface layer consisting of peat or <br /> mineral-rich silt and clay to a depth of 10 to 15 feet bgs. Fill materials, which consist of silty fine <br /> to medium grained sand with some fine gravel, may overlay the surface layer in some areas. <br /> Underlying the surface layer of organic-rich soils are fluvial sediments corresponding to the <br /> Unconsolidated Deltaic Deposits. Lenses of fine to coarse silty sand and sand are present in a <br /> matrix of silt and clay. <br /> RRI is surrounded by surface waters at elevations greater than the surface of RRI. A system of <br /> both exposed and subsurface drainage channels conveys surface water runoff and any surfacing <br /> groundwater to a pump station located on the west side of RRI where the water is discharged to <br /> the Burns Cutoff Channel under National Pollutant Discharge Elimination System(NPDES) <br /> permit number CAS0084077 (Geosyntec, 2018a). Shallow groundwater gradients in the interior <br /> of RRI are generally toward the drainage channels (Geosyntec, 2018a). <br /> 2.2.2 Site-Specific Geology and Hydrogeology <br /> The subsurface under Building 817B has characteristics of deltaic sediment deposits. These <br /> deposits contain unconsolidated clay, silt, and sand. Boring logs from Geosyntec's January 2019 <br /> investigation indicate that a fill layer that extends from ground surface up to 5 feet bgs. <br /> Unsaturated clay was encountered below the fill layer(Appendix A). <br /> Review of historical boring logs indicate that an unsaturated, organic, silty clay layer is present <br /> from approximately 4 to 9 feet bgs,underlain by an inorganic, silty clay layer from <br /> approximately 9 to 18 feet bgs. Below this clay layer there is silty sand and sand from <br /> approximately 18 to 27 feet bgs, silty clay from approximately 27 to 34 feet bgs, and well-sorted, <br /> fine-to medium-grained sand from approximately 34 to 40 feet bgs (ERS, 2008). <br /> Draft Soil and Soil Vapor Investigation 3 August 30,2019 <br />