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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 1977 and 1987-1992 droughts, although 1992 water levels in many monitoring wells were significantly below the previous historic lows of the 1960s. Today groundwater in both aquifer zones generally follows a westerly flow pattern, like the surface water streams, along the structural central axis of the valley toward municipal pumping centers. The majority of subsurface inflow, however, occurs across the northern boundaries of the Main Basin-in particular the Dublin and western Camp sub-basins-and flows in a southerly direction. (Zone 7, 2004) These sources of groundwater commingle in the Bernal and Amador sub-basins and have a general flow toward municipal or gravel mining company groundwater pumping wells or pits. (Zone 7, 2004) The relatively low hydraulic conductivity of the aquitard layers impedes the vertical movement of groundwater between the Upper and Lower Aquifer Zones. The exchange between the two aquifers, as indicated by the groundwater monitoring data, varies depending upon the thickness and permeability of the separating aquitard and the potential gradient. Even though the movement of water and salts from the upper aquifer to the lower aquifer is slow, it is still the major sources of recharge to the lower aquifer. (Zone 7, 2004) The Main Basin has a storage capacity of more than 250,000 acre-feet. The Main Basin was full in early 1900 and full again in 1983. Groundwater has been withdrawn down to historical low storage in 1962 and 1966 with an estimated remaining storage of 128,000 acre-feet. (Groundwater levels approached the "historic low" in some parts of the basin during the droughts of 1977 and 1987- 1992.) In 1987, Zone 7 adopted a Groundwater Management Policy that included maintaining groundwater levels high enough to provide emergency reserves adequate for the worst credible drought. For planning purposes, Zone 7 maintains this reserve above historical lows. The remaining half of the groundwater (that portion above historical lows) is actively managed for supply reliability and is used for water supply storage, and recovery during times of drought or emergency. (Zone 7, 2004) In 2002, as part of the development of Zone 7's Well Master Plan, Zone 7 further defined "historic lows" as a piezometric surface used to manage groundwater levels. P:\Projecls - WP OnIyU1162.01 Staples Ranch WSA\FinaflFinal WSA.doc 5-10