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RES 07163
City of Pleasanton
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RES 07163
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6/15/2009 1:03:17 PM
Creation date
12/31/2007 1:57:28 PM
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CITY CLERK
CITY CLERK - TYPE
RESOLUTIONS
DOCUMENT DATE
12/18/2007
DESTRUCT DATE
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DOCUMENT NO
RES 07163
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City of Pleasanton <br />Final Water Supply Assessment <br />Stoneridge Drive Specific Plan Amendment and Staples Ranch Project <br />4.0 Water Agency Background <br />1977 and 1987 -1992 droughts, although 1992 water levels in many monitoring wells were <br />significantly below the previous historic lows of the 1960s. <br />Today groundwater in both aquifer zones generally follows a westerly flow pattern, like the <br />surface water streams, along the structural central axis of the valley toward municipal <br />pumping centers. The majority of subsurface inflow, however, occurs across the northern <br />boundaries of the Main Basin —in particular the Dublin and westem Camp sub basins —and <br />flows in a southerly direction. (Zone 7, 2004) These sources of groundwater commingle in the <br />Bemal and Amador sub basins and have a general flow toward municipal or gravel mining <br />company groundwater pumping wells or pits. (Zone 7, 2004) The relatively low hydraulic <br />conductivity of the aquitard layers impedes the vertical movement of groundwater between the <br />Upper and Lower Aquifer Zones. The exchange between the two aquifers, as indicated by the <br />groundwater monitoring data, varies depending upon the thickness and permeability of the <br />separating aquitard and the potential gradient. Even though the movement of water and salts <br />from the upper aquifer to the lower aquifer is slow, it is still the major sources of recharge to <br />the lower aquifer. (Zone 7, 2004) <br />The Main Basin has a storage capacity of more than 250,000 acre -feet. The Main Basin was <br />full in early 1900 and full again in 1983. Groundwater has been withdrawn down to historical <br />low storage in 1962 and 1966 with an estimated remaining storage of 128,000 acre -feet. <br />(Groundwater levels approached the "historic low' in some parts of the basin during the <br />droughts of 1977 and 1987– 1992.) In 1987, Zone 7 adopted a Groundwater Management <br />Policy that induded maintaining groundwater levels high enough to provide emergency <br />reserves adequate for the worst credible drought. For planning purposes, Zone 7 maintains <br />this reserve above historical lows. The remaining half of the groundwater (that portion above <br />historical lows) is actively managed for supply reliability and is used for water supply storage, <br />and recovery during times of drought or emergency. (Zone 7, 2004) In 2002, as part of the <br />development of Zone 7's Well Master Plan, Zone 7 further defined "historic lows" as a <br />piezometric surface used to manage groundwater levels. <br />
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