Marina Shores Village Project Draft EIR City of Redwood City 11. Soils and Geology March 5, 2003 Page 11-1 11. SOILS AND GEOLOGY This EIR chapter describes existing geologic and soil conditions at the project site and immediate vicinity, identifies associated potential geotechnical impacts related to development of the proposed project, and sets forth measures designed to mitigate identified significant adverse impacts. Data sources used to complete these descriptions include a preliminary geotechnical investigation of the Marina Shores Village project site performed for the applicant by Treadwell & Rollo, Environmental and Geotechnical Consultants, dated June 21, 2001; the assessment of project hydrologic implications completed for this EIR by Clearwater Hydrology (see chapter 9); a wetlands investigation and a biological assessment performed for the applicant by the Huffman-Broadway Group, both dated February 2002; published reference materials produced by the Department of the Interior, U.S. Geological Survey (USGS); environmental documents prepared for previous development projects on the site (e.g., the Peninsula Marina and Office Park) and in the immediate vicinity (e.g., the "Villas at Bair Island" and the Bair Island Marina); and the Redwood City Strategic General Plan. The Treadwell & Rollo preliminary geotechnical investigation, much of which is presented in this EIR chapter, describes the limitations and preliminary nature of its conclusions by stating, "The conclusions presented in this report are preliminary and intended to address general geotechnical conditions of the site. The report has not been prepared to meet the need of design professionals, contractors, or any other parties in preparation of final design or construction documents. Final conclusions and recommendations will be presented after the proposed future studies have been performed and the data collected has been analyzed." 1 These future study needs are further described in section 11.3 herein (Impacts and Mitigation Measures). 11.1 SETTING 11.1.1 Regional Geologic Setting 1Treadwell & Rollo. Preliminary Geotechnical Investigation, Bair Island Project (Project No. 3147.01), June 21, 2001, p. 15. WP9.0\622\DEIR\11.622 Marina Shores Village Project Draft EIR City of Redwood City 11. Soils and Geology March 5, 2003 Page 11-2 The project site is within a broad structural trough of the California Coast Ranges that extends from an area south of Hollister northward beyond San Pablo Bay. This structural depression, which contains San Francisco Bay, has been subsiding and filling with alluvium during the past five to eight million years. At the site, bedrock of the Franciscan Assemblage underlies the alluvial deposits at a depth of approximately 400 feet. The most recent underlying deposit is soft Bay mud, which has been deposited in the last 10,000 years.2 The approximately 46.45-acre project site is located on flat-lying, estuarine sediments along the southwest shoreline of San Francisco Bay. The site is generally bounded by Redwood Creek on the north and east; Smith Slough and the Bair Island National Wildlife Refuge on the northwest and west; townhouses, auto retail, and offices on the west; and Redwood Creek (with a marina of live-aboard vessels and an aquatic club) and U.S. 101 on the south. Redwood Creek and Smith Slough are tributaries to San Francisco Bay. The project site is comprised of two noncontiguous properties: the Peninsula Marina property, which is currently comprised of approximately 33.24 acres, including 14.1 acres of water area; and the Pete's Harbor property, which currently encompasses approximately 13.21 acres, including 2.9 acres of water. 11.1.2 Site Topography and Drainage (a) Site Drainage. Stormwater runoff from the Peninsula Marina property discharges to Redwood Creek, either directly via overland flow or by the storm drain system serving the existing commercial complex and parking area. Review of storm drain plat maps on file with the City Department of Public Works confirmed that the storm drain system on the Peninsula Marina property has no link with the Bair Island Road storm drain system. Field observations indicated that all runoff from the Pete's Harbor property flows overland and discharges directly into either the inner harbor, Smith Slough, or Redwood Creek. The adjacent Bair Island Road system conveys runoff generated over the roadway and parcels flanking it on the north to a 6,400 gallon-per-minute (gpm) stormwater pumping station that is maintained by the City. The pumping station is located immediately south of the junction of Bair Island Road and East Bayshore Road. Drainage from the project site has no connection to or effect on this pumping system. 2Bay Area Geotechnical Group (BAGG). Report of Geotechnical Investigation for Bair Island Marina, 700 Bair Island Road, Redwood City, California, February 9, 1998, p. 8. WP9.0\622\DEIR\11.622 Marina Shores Village Project Draft EIR City of Redwood City 11. Soils and Geology March 5, 2003 Page 11-3 (b) Site Topography. The project site is relatively flat; existing ground surface elevations range from approximately 105 to 110 feet (or +5.0 feet to +10.0 feet NGVD).3 The site and surrounding area were originally tidal marshlands with meandering sloughs. During the early 1900s, the area was diked and filled. The upper soil dried over time, and a crust formed on the surface. Between approximately 1950 and 1964, salt evaporation ponds were developed in the surrounding area by constructing interior levees.4 The peninsula upon which Peninsula Marina and Pete's Harbor are built was filled in stages, with Pete's Harbor dredged (marina) and filled (parking lots) over two decades from the early 1960s through the mid-1980s. The 1973 revised USGS 7.5-minute topographic maps for the Redwood Point and Palo Alto quadrangles indicated that the Peninsula Marina site had been filled; however, the marina was not shown on the map. According to aerial photo evidence, the marina dredging and construction took place sometime between 1975 and 1979.5 11.1.3 Site Soils (a) USSCS Soil Survey. The project site has undergone human disturbance, including filling, vegetation removal, soil compaction, altered runoff conditions, and road and building construction. The tidal condition of the adjacent reach of Redwood Creek results in a relatively stable and shallow on-site groundwater elevation; mid-winter groundwater elevations could rise somewhat closer to the ground surface. Due to the presence of nearly impervious Bay mud and the tidal zone beneath and adjacent to the project parcels, the perched groundwater underlying the site does not represent a significant groundwater resource. A review of the U.S. Soil Conservation Service (SCS) Soil Survey map for San Mateo County Eastern Part and San Francisco indicates the presence of one soil series on the project site: urban land--orthents, reclaimed complex, 0 to 2 percent slopes. In the project vicinity, this soil series is located in areas that were once part of San Francisco Bay and adjacent tidal flats. Urban land soils generally consist of deep, poorly drained fill. The soil texture varies greatly, and is typically composed of native soil, gravel, broken cement and asphalt, Bay mud, and solid waste. Field 3All elevations are referenced to City of Redwood City Datum, which is equal to National Geodetic Vertical Datum (NGVD) plus 100 feet. Treadwell & Rollo. 4 Treadwell & Rollo, p. 3. 5 Mark Peppercorn, Glenborough-Pauls, LLC, personal communication, May 2002. WP9.0\622\DEIR\11.622 Marina Shores Village Project Draft EIR City of Redwood City 11. Soils and Geology March 5, 2003 Page 11-4 investigation has confirmed that the SCS soils mapping is generally accurate throughout the project site.6 6The Huffman-Broadway Group, Inc., 2002. Biological Assessment: Marina Shores Village Project, Redwood City, California. Larkspur, California. Prepared for Glenborough-Pauls, LLC, February 2002, pp. 9-10. WP9.0\622\DEIR\11.622 Marina Shores Village Project Draft EIR City of Redwood City 11. Soils and Geology March 5, 2003 Page 11-5 (b) Site-Specific Investigation. To evaluate subsurface conditions, five exploratory borings and five cone penetration tests (CPTs) were performed in May 2001 by Treadwell & Rollo, environmental and geotechnical consultants for the applicant.7 The on-site test borings indicate that the project site is covered by approximately 5 to 9.5 feet of sandy and clayey fill. The fill generally is underlain by about 15 to 22 feet of weak, compressible marine clay known as Bay mud, except at the south end of the site, where the Bay mud decreases to approximately 8 feet in thickness. Interlayered stiff clay and thin lenses of medium-dense sand are present below the Bay mud to the maximum explored depth of approximately 100 feet. Soil samples obtained during the field investigation were re-examined in a laboratory to confirm field classifications. Representative samples were tested to measure moisture content, dry density, fines content, shear strength, and compressibility. Tests for pH, sulfate, chloride, and resistivity were performed on a sample of the fill and a sample of the Bay mud recovered from the test borings in order to provide preliminary indications of the potential for chemical conditions in the soil to adversely affect concrete and steel. The results of these tests in relation to proposed construction of the Marina Shores Village Project are discussed in upcoming section 11.3 (Impacts and Mitigation Measures). (c) Vegetation. Vegetation on the project site is limited to some ornamental landscaping around the perimeter of the commercial buildings, the Uccelli residence, and the fill slope flanking the east side of Bear Island Road, and a narrow band of salt marsh vegetation (e.g., pickleweed and salt grass) along the west bank of Redwood Creek. (d) Groundwater. The levels of measured groundwater in the borings and CPTs ranged from approximately 3 to 6.5 feet below ground surface.