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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 5.3.4.2. Groundwater Basin Management In years prior to construction of the SWP and other surface water deliveries, groundwater was the sole source of water for the entire Livermore-Amador Valley. The Main Basin experienced several periods of extended withdrawals and subsequent recovery in normal or wet years. In the 1960's, approximately 110,000 acre-feet of groundwater was extracted for supply uses; this resulted in an estimated historic low of 130,000 acre-feet remaining in storage. Now, under all circumstances, this historic low is considered snot-to-exceed threshold in any given year. Significant recovery of the Main Basin to pre-1960's levels was achieved in the twenty year span of 1962 to 1983. It was during this era that Zone 7 initiated a series of programs to build reliability into their water supply system. First, Zone 7 conducted a program of groundwater replenishment by artificially recharging via local streams with imported surface water for storage in the Main Basin; secondly, Zone 7 commenced using treated surface water to augment groundwater supplies; and finally, Zone 7 began regulating groundwater pumping throughout their service area by contractually establishing annual Independent Quotas (IQ) now known as a Groundwater Pumping Quota (GPQ) for each municipality. Groundwater levels in the Main Basin are routinely monitored by Zone 7 staff. Two independent methods are used to estimate groundwater storage: 1) Hydrologic Inventory and 2) Nodal Water Level. The Hydrologic Inventory method computes storage change each quarter from basin supply and demand data. This method can also be used to forecast future water storage conditions. The Nodal Water Level method computes storage from hundreds of water level measurements. According to the City of Pleasanton's UWMP, the correlation between these two distinct methods ensures confidence in Zone 7's ability to measure, monitor and even forecast groundwater storage changes in the Main Basin." Zone 7 operates the groundwater basin to maximize conjunctive use of regional water supplies. Currently, the Main Basin is managed so that under non-emergency conditions, including several multi-year droughts, groundwater elevations do not drop below historic low levels of 130,000 AF. Present day operational plans consistently maintain approximately 130,000 AF [above the historic low] for extreme emergency storage at all times. Under Zone 7's Basin Management Plan (Plan), in wet and normal years, groundwater is replaced through artificial recharge in the Arroyo Valle stream system using imported surface water. Operational plans call for seasonal storage through artificial recharge of 15,000 to 20,000 AF of water within the groundwater basin. In other words, Zone 7 replaces seasonal Main Basin extractions with an equal quantity of artificially recharged water. This seasonal replacement program helps maintain at least 110,000 AF for "drought storage" in the Main Basin. Zone 7's operational policy is to maintain the balance between the combination of natural and artificial recharge and groundwater withdrawals. This preserves approximately 240,000 AF of total storage in the Main Basin for drought and emergency use - 110,000 AF for drought storage and 130,000 AF to be used only in case of extreme emergency. The Plan allots a portion of 110,000 AF of Main Basin "drought storage" for extraction in dry years to make up for any SWP shortages that may occur due to hydrologic conditions or environmental constraints placed on the SWP system, such as the California aqueduct pump shutdown in June 2007.12 In terms of reliability, this amount is sufficient when used in conjunction with other water supplies and [Out-of-Basin] groundwater storage in 11 2005 Zone 7 Water Agency Urban Water Management Plan, page 19. 12 City of Pleasanton letter dated June 8, 2007: Regarding the Department of Water Resources Pump Shutdown June 2007 Zone 7 provided adequate water to Pleasanton by utilizing groundwater storage in addition to supplies in the Del Valle Reservoir. P:\Projects - WP OnIyW1162.01 Staples Ranch WSA\Final\Final WSA.doc 5-12