Site Description
The SLAC site occupies approximately 426 acres of moderately sloping land that is situated within the eastern foothills of the Santa Cruz Mountain Range Geomorphic Province of Central California. Regionally, the SLAC facility is situated on an east-northeast trending ridge that is situated above the alluvial plains of San Francisco Bay. The SLAC facility consists of an approximately three-mile long accelerator that is oriented in a near east-west direction, and a main facility area housing numerous buildings, located near the eastern terminus of the accelerator. Topography at the SLAC facility varies from approximately 175 feet above mean sea level near the western border to approximately 376 feet above mean sea level near the eastern boundary.
Geology
Generally, the geologic environment at the SLAC site consists of a complex mixture of artificial fill, Quaternary (less than 1.6 million years old) alluvial deposits, and marine Tertiary Eocene and Miocene (5.3 to 57.8 million years old) sedimentary rocks that have been locally deformed by the San Andreas Fault, located approximately one mile southwest of the western boundary of the site. The Quaternary alluvial deposits and Miocene sedimentary rocks are typically encountered within the eastern portions of the site, whereas the Eocene sedimentary rocks are typically encountered within the western portions of the site. The Miocene rocks are the most widespread stratigraphic unit at the site. Artificial fill is encountered at random areas, especially within areas of development, throughout the site. Local alluvial deposits are also encountered along former and active stream beds near the eastern portion of the site. The alluvial deposits are characterized as interbedded gravel, sand, silt, and clay that is locally well cemented to unconsolidated. The Eocene sedimentary rocks are characterized as consisting of interbedded sandstone and finer-grained rocks (claystone and siltstone). The sandstone units are typically hard to strong, closely fractured to massive, and locally weak and friable. The finer-grained units are typically less hard, more fractured, and weak. The Eocene rocks are characterized as consisting of interbedded sandstone, siltstone, with one basalt flow unit. The sandstone and siltstone typically contains abundant shell fragments, is consolidated, and has low hardness. Artificial fill at the site is estimated to be a maximum thickness of 70 feet (21 meters), locally.
In general, the soils at the SLAC facility were formed as a result of alluvial deposition from the San Francisquito Creek that overlie the marine sedimentary and volcanic rocks. Most of the site is overlain by asphalt, concrete, and structures, however underlying soil characteristics may be significant factors if surface releases or spills occur. The dominant upper subsoil units typically have a higher sand content with the lower soils containing higher silt and very fine sand constituents. These soils typically have slow to moderately slow permeability rates. Figure 2.2-1 is a Regional Geologic Map of the SLAC site area.
Hydrogeology
SLAC is situated within the San Francisquito Creek watershed of the Santa Cruz Mountain Range, which covers approximately 40 square miles. All surface drainage from the SLAC site flows into San Francisquito Creek or one of its subsidiary drainages. San Francisquito Creek eventually flows into San Francisco Bay, located approximately 12.4 miles northeast of the SLAC site.
The groundwater beneath the SLAC site occurs and moves within three water-bearing units:
The most extensive water-bearing units at the SLAC facility are the marine sedimentary rocks of Tertiary age. Alluvium and terrace deposits are sparse and discontinuous and are, therefore, insignificant conveyors of groundwater.
The depth to groundwater ranges from a few feet to over 30 feet below the ground surface and water levels have been observed to fluctuate as much as ten feet, annually. A comparison of water levels and screened intervals between the monitoring wells indicates that some wells penetrate fracture-dominated water-bearing zones, where groundwater appears to be confined, whereas other wells are screened across fractures in which groundwater is unconfined. Regionally, groundwater flows to the southeast, but locally it may have complex flow patterns such as following a tortuous path along fractures and through more permeable discontinuous beds. Figure 2.2-2 shows the groundwater flow directions at SLAC.
Groundwater Use
Groundwater within the SLAC vicinity (3 to 4 miles radius) is not used for drinking water purposes. The cities of Menlo Park, Portola Valley, and the town of Woodside receive their water from the California Water Service_San Francisco Region. Similarly, the city of Palo Alto and the town of Atherton receive their drinking water supply from the Hetch-Hetchy system. Palo Alto does have emergency backup water supply wells that are used generally once a year for maintenance purposes. Stanford University and Menlo School and College utilize water wells for various purposes, occasionally for drinking water. The the university wells are not known to be near any areas of contaminant sources.
Most of the Palo Alto wells are located downgradient of the SLAC site. Similarly, most of the Menlo Park wells are cross gradient of the SLAC site, with
The Stanford wells serve approximately 25,000 people, and the Menlo College wells serve approximately 1,500 people. The remaining private wells located in Palo Alto and Menlo Park serve either a single household or a small group of people. Documentation from conversations with city representatives from Menlo Park and Palo Alto indicates that using private wells for drinking water is not allowed in Menlo Park, and very few people, if any, use their private wells for drinking water purposes in Palo Alto.
Surface Waters
Surface water from the SLAC site flows into San Francisquito Creek, located parallel to and south of the site. Prior to entering the creek, much of the surface water flows into a storm drain system. Runoff from watershed properties located upgradient of the SLAC site generally bypasses it through streams bordering the facility. SLAC also rests on a topographically high ridge, limiting its drainage to San Francisquito Creek. No surface water from the SLAC facility is used as drinking water. Surface water flow across the SLAC site is generally consistent with groundwater flow, which is to the southeast.
Surface waters near the SLAC facility include Searsville Lake, located approximately 0.8 miles south of the western terminus of the accelerator; Bear Gulch Reservoir, located approximately 2.8 miles northeast of the western terminus of the accelerator; Lagunita Lake, located approximately 3 miles east of the eastern terminus of the accelerator; and Felt Lake, located approximately 4 miles southeast of the eastern terminus of the accelerator.
Surface water from Searsville Lake is diverted via pipeline so that it does not commingle with surface water from SLAC. Water from Searsville Lake is used to irrigate the Jasper Ridge Biological Preserve, located approximately 0.3 miles south of the accelerator. Lagunita Lake is used for recreational purposes by Stanford University and is seasonally filled by San Francisquito Creek when Felt Lake has excess water.
Climate
Climate in the SLAC area is mediterranean. Winters are warm and moist, and summers are mostly cool and dry. Daily mean temperatures are seldom below 32°F or above 86°F. The rainfall at SLAC varies from just under 20 inches (51 cm) per annum at the eastern property line to slightly over 25 inches (64 cm) at the western property line, based on information compiled in 1963. Over 75 percent of the precipitation occurs between December and March.
Vegetation and Wildlife
The SLAC facility is distinguished by the presence of many mature oak trees standing singly and in groves. The land is further characterized by broad areas of open grassland. The facility is bordered, in part, on the south by San Francisquito Creek, the headwater area of which is dammed to form Searsville Lake, located at the western terminus of the linear accelerator.
Most of the plants now found in and around the SLAC area were planted as landscaping following the original construction of the facility. These landscaped
Francisquito Creek, the headwater area of which is dammed to form Searsville Lake, located at the western terminus of the linear accelerator.
Most of the plants now found in and around the SLAC area were planted as landscaping following the original construction of the facility. These landscaped plants include Bishop pines, varieties of eucalyptus, coastal live oaks, pyracantha, and a number of low-growing ornamental shrubs.
The area provides habitat for various species of wild animals including deer, foxes, rabbits, and rodents (gophers, mice, and squirrels). Many types of birds inhabit the site, including doves, finches, redwing blackbirds, hawks, quail, turkey vultures, killdeer, and owls.
Sensitive environments were explored from the probable point at which surface runoff enters San Francisquito Creek from the SLAC site to the point at which the creek enters the Bay (approximately 12.4 miles downstream). Six threatened or endangered species (plants and animals) have been recorded for the general area around SLAC, but not on SLAC property. The following is a list of those endangered or threatened species:
Wetlands
A field survey and evaluation was conducted in FY91 as part of an environmental assessment to determine if areas within SLAC should be designated as wetland. The survey concluded that JR-8 showed characteristics of wetlands, but the natural hydrology of the area would probably not be capable of supporting the wetlands community due to seasonal drought, even under normal conditions. The portion of JR-8 drainage channel that represents the great majority of the potential wetland is approximately 4,000 square feet (less than one-tenth of an acre).