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City of Pleasanton 4.0 Water Agency Background Final Water Supply Assessment Stoneridge Drive Specific Plan Amendment and Staples Ranch Project Between the early 1900s into the early 1960's, the Main Basin has been relied upon to provide for much of the Valley's local water supplies and continues to be a reliable and significant local groundwater supply. Between about 1980 and 1988, this area was called the central basin. Since 1988, the central basin, except for the eastern portion of Livermore, has been referred to as the Main Basin. Several subsurface barriers to lateral groundwater movement form the boundaries of the Main Basin. Observations and investigations by Zone 7 and others continue to confirm the existence of these groundwater barriers. Faults are the major structural features known to have marked effects on the movement of groundwater in this region. Faults in this region tend to act as barriers to the lateral movement of groundwater. The Main Basin is comprised of the Castle, Bernal, Amador and Mocho II Sub-Basins and is bounded on the: • north by the Parks Boundary (which was initially considered to be fault-related, but may actually be a depositional boundary between recent alluvium and older material); • east by shallow bedrock separating Mocho I from Mocho II sub-basins; • south by shallow bedrock and the Livermore Uplands; and west by the Coastal Ranges and the Calaveras Fault. The portion of the groundwater basin that is outside the Main Basin is called the fringe basin. The majority of the connectivity between the fringe and Main Basins is through the Upper Aquifer Zone. Subsurface inflow from the Lower Aquifer Zone is considered negligible. Particular Sub-Basin boundaries and features are described in more detail below. Sub-Basins of the Main Basin Castle Sub-Basin The Castle sub-basin is a thin strip that extends along the southwestern portion of the Main Basin. It is bounded to the south, west, and north by marine sediments of the Coastal Range and to the east by the Calaveras Fault. While usually included in the Main Basin, this sub- basin is not used for municipal groundwater production. Only small production wells are located in this area. Bernal Sub-Basin The Bernal sub-basin is located in the southwestern portion of the groundwater basin and is bounded to the west by branches of the Calaveras Fault, to the east by the Pleasanton Fault, to the north by the Parks Boundary, and to the south in part by contact with non-water-bearing formations and partly by contact with the Verona Fault. Both unconfined and confined aquifers exist in the water-bearing sediments. Waters from the northern and central portions of this sub-basin are of fair to excellent quality. However, much of the upper aquifer water has high TDS exceeding 600 mg/I. The water from the northern and southern portions of the sub-basin are of sodium bicarbonate nature, while the central portion is of the magnesium bicarbonate type and the western and south-central portions are of calcium bicarbonate character. The area overlying the Bernal sub-basin is the point of convergence for all major streams that drain the Livermore Valley. The area overlying the sub-basin is subsequently drained by the Arroyo de la Laguna. Like surface water, groundwater also historically converges in this sub- basin, which allows for the mixing of the dominant cations of sodium, magnesium, and calcium. The Quaternary alluvium is estimated to have a thickness of at least 800 feet in this sub-basin and overlies the Livermore Formation. Well production (primarily by Zone 7) in this sub-basin currently ranges up to 3,500 gallons per minute (gpm), and specific capacities range from 3 to 260 gpm per foot of drawdown. Other basin pumpers include the City of Pleasanton (although much of City of Pleasanton's pumping has shifted to the West Amador sub-basin, discussed below), San Francisco Public Utilities Commission (supplying the Castlewood area) and the Alameda County Fairgrounds. Historically (pre-1960's), this Sub-Basin was overdrafted but has since been partially refilled P:\Projec[a - WP OnIyU1182.01 Staples Ranch WSA\FinallFinal WSAdoc 5-O