Laserfiche WebLink
<br />7A - ATTACHMENT NO.3 <br />Saltworks Proposal- Water Group Summary Report (22 January 2010) Page 29 <br /> <br />100 feet in places. The site is composed of about 60 percent artificial fill and 40 percent <br />bay mud. The salt evaporator ponds are covered with a layer of residual salt crystals. <br /> <br />Subsurface geology is composed of multiple-coalescing alluvial fans that drape the <br />eastern flanks of the Santa Cruz Mountains and San Mateo uplands. The portions of the <br />alluvial fans that are located closest to the source materials of the Santa Cruz Mountains <br />consist of more permeable coarse-grained sediments (sands and gravels). These <br />sediments are usually at higher elevations and receive more annual rainfall and <br />recharge than areas near the bay. The portion of the alluvial fans located farthest from <br />the source materials and closer to the Bay contains more fine-grained, less permeable <br />sediments (fine sands, silts, and clays). In tectonically active areas such as this <br />subbasin, the fan area closer to the Bay typically has a greater thickness of sediments <br />than the area near the mountains (Reading, 1981). <br /> <br />The Saltworks site is located on the portions of the alluvial fans closest to the Bay and, <br />as such, aquifers are expected to be finer-grained. In addition to alluvial fan sediments, <br />the subsurface sediments include inter-beds or alternating layers of fine-grained <br />estuarine and marine organic silts and clays from San Francisco Bay, particularly during <br />the recent depositional history of the site. <br /> <br />Groundwater recharge to the underlying aquifers is from direct precipitation on the <br />contributory watershed area of Redwood Creek and adjacent watersheds west of the <br />site. The contributory area is about 12,000 acres or about 19 mi2. Most groundwater <br />recharge is probably occurring near the upper portion of the alluvial fan along the flanks <br />of the Santa Cruz Mountains. Recharge from direct rainfall to replenish groundwater <br />beneath Redwood City ranges from about 900 to about 1,800 AFY (Todd Engineers, <br />2003). <br /> <br />3.5.1.2 Groundwater Development and Wells <br /> <br />OMB has not installed site-specific wells on the Saltworks property to test freshwater <br />aquifers directly beneath the site. Some information was available from shallow onsite <br />borings (<20 feet), but not on potential drinking water aquifers. Accordingly, data from <br />nearby wells were compiled and reviewed to interpret the hydrogeology and <br />groundwater conditions in close proximity to the site. <br /> <br />Since 1924, at least 14 irrigation and industrial wells have been installed just northwest <br />of the Saltworks site near the Port of Redwood City. Three sets of wells have been <br />installed for different projects near the Port of Redwood City: Bayport Center <br />(GeoConsultants, 1991a and 1991 b), Pacific Shores (Bohley and Maley, 1993 and <br />Maggiora Brothers, 2001), and Pacific Portland Cement Company (Poland and Garrett, <br />1943). General well locations are shown on Figure 5 and well details are summarized in <br />Table 4. Table 4 also includes data from three additional wells in the area. <br /> <br />Collectively, these wells provide relevant information on the subsurface stratigraphy and <br />the hydrogeology beneath the Saltworks site. According to well data, the area around <br />the Saltworks site is underlain by a sequence of alternating layers of fine-grained <br />sediments (silts and clays) and coarse-grained sediments (sand and gravel). The wells <br />tap confined aquifers between 130 to 330 feet below ground surface. <br /> <br />22 <br />