5.6 Westside5.6 Groundwater 5.6 Westside Groundwater Basin Resources 5. WSIP Water Supply and System Operations – Setting and Impacts 5.6 Westside Groundwater Basin Resources This section describes the potential effects of the WSIP water supply and system operations and associated WSIP projects on the Westside Groundwater Basin and related water resources, including Lake Merced. The proposed water supply sources under the WSIP include 10 million gallons per day (mgd) of supply every year in all years (including nondrought periods) from implementation of conservation, water recycling, and groundwater supply programs in San Francisco; in addition, the proposed water supply option includes a long-term conjunctive-use program in the San Mateo County portion of the Westside Groundwater Basin, referred to as the South Westside Groundwater Basin, as part of the drought-year water supply for the regional system. The recycled water and groundwater components of this supply would be achieved through two WSIP projects, the Local and Regional Groundwater Projects (SF-2) and the Recycled Water Projects (SF-3), which are described in Chapter 3. The potential effects of the WSIP on the Westside Groundwater Basin and related resources are discussed in the context of ongoing activities in this area occurring among the SFPUC, City of Daly City, California Water Service Company (Cal Water, the municipal water purveyor to South San Francisco), and the City of San Bruno. 5.6.1 Setting 5.6.1.1 Westside Groundwater Basin The Westside Groundwater Basin extends from San Francisco south to San Mateo County (Figure 5.6-1). With an area of about 45 square miles, this groundwater basin is the largest in San Francisco. The Westside Groundwater Basin is separated from the Lobos Basin to the north by a northwest-trending bedrock ridge through the northeastern part of Golden Gate Park (DWR, 2006). San Bruno Mountain and San Francisco Bay form the eastern boundary, and the San Andreas fault and Pacific Ocean form the western boundary. The southern limit of the Westside Groundwater Basin is defined by an area of high bedrock that separates it from the San Mateo Plain Groundwater Basin. The basin opens to the Pacific Ocean on the northwest and San Francisco Bay on the southeast. The portion of the Westside Groundwater Basin north of the San Francisco/San Mateo County line is referred to as the North Westside Groundwater Basin. The portion of the Westside Groundwater Basin south of the San Francisco/San Mateo County line is referred to as the South Westside Groundwater Basin. Geology The four major geologic units in the Westside Groundwater Basin are the Mesozoic-age Franciscan Complex, Pleistocene-age Merced and Colma Formations, and the Pleistocene to recent Dune Sands, as illustrated in Figure 5.6-2 (Luhdorff and Scalmanini, 2006). There are also minor but widespread units of recent alluvium along historical stream channels. SF Planning Department Case No. 2005.0159E 5.6-1 PEIR on SFPUC Water System Improvement Program / 203287 GOLDEN GATE BRIDGE BRIDGE AY 101 B 80 1 SAN FRANCISCO San Golden Gate Park Francisco North Westside Bay Groundwater Basin P a c 1 i f i c 280 101 35 San Francisco Harding 280 Zoo Park LAKE MERCED Olympic San Francisco Lake Merced Golf Club Golf Club Golf Club SAN FRANCISCO COUNTY FORT FUNSTON SAN MATEO COUNTY 101 DALY CITY Daly City Muncipal Wells O 35 c e BRISBANE a 280 Colma n Cemeteries COLMA 1 Cal Water Municipal Wells 280 South Westside Groundwater Basin California Golf Club SOUTH 01 SAN FRANCISCO Golden Gate 380 National Cemetery Mile 101 Municipal Production Well SAN FRANCISCO General Basin and Conjunctive Use Monitoring Well INTERNATIONAL AIRPORT Coastal Monitoring Well San Bruno Municipal Lake Merced Monitoring Well Wells Lake-Aquifer Monitoring Well MILLBRAE Basin Boundary SAN ANDREAS SF Zoo Major Historic or Current Groundwater User RESERVOIR SFPUC Water System Improvement Program . 203287 SOURCE: Luhdorff & Scalmanini, 2006; ESA Figure 5.6-1 Westside Groundwater Basin Monitoring Network and Major Production Areas 5.6-2 NORTH SOUTH SF INTERNATIONAL GOLDEN AIRPORT GATE GOLDEN SUNSET SOUTH 400 GATE PARK DISTRICT LAKE MERCED DALY CITY COLMA SAN BRUNO 400 FT. MILEY SAN FRANCISCO SF GOLDEN GATE BAY 200 SUNSET IMPOUND BAYSHORE NATIONAL 5000’ 200 WELL FIELD SF ZOO N LAKE S LAKE LAKE CEMETARY FREEWAY Jk 0 Qc 0 Q/H Shallow Aquifer Q/H Q/H -200 unk 100’ Clay Primary Production Aquifer -200 ? ? Qmu Qmu -400 Qmu? “W” Clay ? -400 ? Elevation (feet) ? Jk -600 Deep Aquifer ? ? -600 Jk ? Qm? Qm? ? -800 -800 -1000 Jk -1000 5.6-3 -1200 -1200 STRATIGRAPHIC UNITS* Q/H Bay Clays Q/H unk Unknown Correlation Qc Colma Formation Qmu Upper Merced Formation Qm? Older Merced Formation-Middle, Lower Jk Franciscan Bedrock * Surficial units not shown CROSS SECTION LEGEND Blue Clay Blue & Gray Clay with Sand & Clay and Sand Beds 0 5000 Clay & Sand Red & Brown Soil Zone Feet Sand, Sand & Gravel or Vertical Scale Exaggerated 12.5 times Sandy Gravel Bedrock SFPUC Water System Improvement Program . 203287 SOURCE: Luhdorff & Scalamanini, 2006 Figure 5.6-2 Regional Cross Section Through Westside Groundwater Basin 5. WSIP Water Supply and System Operations – Setting and Impacts 5.6 Westside Groundwater Basin Resources Exposed in the low hills east and northeast of Lake Merced, the Franciscan Complex forms the basement rock for the aquifer system.1 The surface of the bedrock slopes southwestward to Daly City, occurring at depths of almost 600 feet near the center of Lake Merced and nearly 1,000 feet beneath the southern portion of Daly City (SFPUC, 2005). The Merced Formation comprises three units (lower, middle, and upper) and is the deepest water- bearing formation overlying the basement rock. The upper unit consists of a sequence of thin- bedded beach, dune, estuarine, and fluvial deposits of weakly consolidated fine sandstone with some gravel and mudstone beds. This unit is up to approximately 500 feet thick and is the primary water-producing aquifer in the basin (the primary production aquifer). The middle and lower units of the Merced Formation form the deep aquifer in the basin within the San Francisco and Daly City areas and are composed of fine sandstone, siltstone, and mudstone. The majority of the surficial geologic units in the North Westside Groundwater Basin are composed of the Colma Formation and Dune Sands, which form the basin’s shallow aquifer system. The Colma Formation is a surficial unit consisting of fine-grained sand with some clay, sand, and gravel beds of fluvial, floodplain, alluvial fan, and dune sand origin. Dune Sands are also a surficial unit of fine-grained sands with some clay soil horizons. The separation between these units and the Merced Formation is not clearly defined, thus preventing an accurate measurement of their thickness. Aquifer System The portion of the Westside Groundwater Basin beneath San Francisco (the North Westside Groundwater Basin), has an area of approximately 14 square miles; it extends from Golden Gate Park to the San Francisco/San Mateo County line in the vicinity of Lake Merced and from the Pacific Ocean to inland bedrock exposures generally associated with Mount Sutro and Mount Davidson (SFPUC, 2005). This portion of the basin is characterized by relatively shallow depths to groundwater (5 to 60 feet) and, in the vicinity of Lake Merced and the San Francisco Zoo, is comprised of three aquifers2 (see Figure 5.6-2). The shallow, unconfined aquifer in the Lake Merced area extends from the water table to the top of the “-100 ft clay” -- a clay layer at approximately 100 feet below sea level that separates the shallow aquifer from the underlying primary production aquifer in the Lake Merced area (Luhdroff and Scalmanini, 2006). The elevation of the water table in this area varies between 10 and 20 feet above mean sea level (msl).3 The primary production aquifer (the main target for municipal and irrigation pumping in the basin) overlies the W-Clay, and the deep aquifer underlies the W-Clay. The -100-foot clay and W-Clay are aquitards4 and appear to thin and pinch out beneath the Sunset District. 1 Basement rock is impermeable bedrock that restricts groundwater flow, forming the vertical boundaries of a groundwater basin, and sometimes the lateral boundary. 2 An aquifer is a geologic unit, typically composed of sand and gravel, that transmits and stores water and yields a substantial quantity of water to a well. In the Westside Groundwater Basin, aquifer materials are typically medium sand to fine sand. 3 Under a program of managed lake levels, future conditions are expected to be closer to the higher value in the range (i.e., 20 feet above msl). 4 An aquitard is a fine-grained unit (such as clay or silt) that restricts the vertical movement of groundwater. Where groundwater occurs beneath an aquitard, the aquifer is considered confined. SF Planning Department Case No. 2005.0159E 5.6-4 PEIR on SFPUC Water System Improvement Program / 203287 5. WSIP Water Supply and System Operations – Setting and Impacts 5.6 Westside Groundwater Basin Resources Two surface water features, Lake Merced and Pine Lake, are incised in the shallow aquifer. The lakes are in hydraulic continuity with the shallow groundwater, and water levels in the lakes generally reflect the shallow groundwater level. In the vicinity of Lake Merced, the primary production aquifer is confined. It is separated from the shallow aquifer by the -100-foot clay, and lower water levels in the primary production aquifer indicate the potential for flow from the shallow aquifer to the primary production aquifer. The South Westside Groundwater Basin has an area of approximately 31 square miles (SFPUC, 2005) and is effectively the portion of the Westside Groundwater Basin that underlies Daly City, Colma, South San Francisco, San Bruno, Millbrae, and parts of Burlingame and Hillsborough.