Our Reliability "Bank Account"
Unlike most other Bay Area communities, the Valley benefits from local water-storage capacity in an underground basin that provides us with increased water-supply reliability.
Operating the basin as a bank account, Zone 7 during wet years uses a portion of its State Water Project water coming through the Bay-Delta, along with local surface runoff water stored in Del Valle Reservoir, to recharge the groundwater basin. This is done via release of water into local streams that then percolates into the underground aquifer. We draw stored water to augment imported water supplies, especially during the summer when seasonal water demands are the highest, and in times of drought. Recharge also helps dilute the natural hardness of groundwater.
Zone 7 had been in its infancy when the first State Water Project deliveries were made in 1962, and it marked the beginning of sustainable groundwater management locally. Zone 7 initially used all of its early State Water Project deliveries to replenish the groundwater basin that had been over-pumped over the previous decades. Groundwater recharge using imported surface water supplies continued even after Zone 7's treatment plants opened, and has been a critical component of Zone 7's water resources management ever since.
Regularly monitoring groundwater levels helps Zone 7 protect against overdraft of the Valley's underground storage basin, and ensure its long-term reliabilty as a drinking-water supply. Shown here is monitoring at a Pleasanton wellfield.
Protecting the Groundwater Basin's levels
The Valley's main groundwater basin has an estimated storage capacity of 250,000 acre-feet (an acre-foot is about 326,000 gallons, enough to supply two households per year). To prevent overpumping, the basin is cooperatively managed by Zone 7 and its retailers so that, even during multiyear droughts, groundwater levels do not drop below historic low levels of 130,000 acre-feet.
Protecting the Quality/Reliability of Groundwater Basin Supplies
- To learn about Zone 7's Salt Management Plan update, click here.
- For information on our Mocho Groundwater Demineralization Plant, which will reduce the buildup of salts and minerals in our groundwater basin and improve delivered water quality, click here.
- To view the annual Groundwater Management Program Annual Report, click here.
A Long-Term Asset
Zone 7 is working to boost its surface-water treatment capacity so it can rely less on groundwater to meet demand in normal rainfaill years. Nevertheless, the basin remains a critical component of reliability planning:
- Through our future Chain of Lakes, created from a series of abandoned gravel quarries between Livermore and Pleasanton, Zone 7 plans to increase groundwater recharge during wet years with imported water supplies.
- As part of its Well Master Plan, the agency is planning new wells -- two that came on line in the Chain of Lakes area in 2010 -- to ensure sufficient production during surface-water shortages.
Offsite Groundwater Banking Programs
Zone 7 also has groundwater-banking rights in Kern County, allowing us to store surplus state water supplies during wet years to draw upon when needed during a drought. We have obtained 65,000 acre-feet of groundwater storage capacity in the Semitropic Water Storage District and another 120,000 acre-feet of capacity from the Cawelo Water District.