Regional Groundwater – Frequently Asked Questions
1. How does groundwater storage and recovery (conjunctive use) work?
Groundwater storage and recovery—also called “conjunctive use”—is the coordinated management of surface
water and groundwater supplies to maximize the sustainable yield of the overall water resource. Conjunctive use is
becoming an important part of the state’s overall water management strategy. As part of the proposed project, we
would provide additional surface water to Partner Agencies (City of Daly City, City of San Bruno, and California Water
Service Company) during normal or wet years when sufficient surface water supplies are available. The Partner
Agencies would reduce their pumping by a comparable amount, allowing the groundwater basin to recharge
naturally during these periods. In dry years or during drought conditions, when less surface water is available,
groundwater would be pumped from the expanded underground storage using the new recovery well facilities.
2. What is meant by “in-lieu recharge” and how does it work?
In-lieu recharge is the process of temporarily decreasing the amount of groundwater pumped from an aquifer, in
combination with increasing surface water deliveries. Typically the decrease in groundwater pumping occurs in wet
years when surface water supplies can be delivered “in-lieu” of groundwater pumping. Decreasing pumping allows
the natural recharge to accumulate in the underground aquifer for use during dry years. In-lieu recharge of an
aquifer does not include artificial recharge through ponds or injection wells.
3. How is the South Westside Basin groundwater aquifer managed?
Groundwater is a shared resource in the South Westside Basin, and is managed by local agencies overlying the basin.
In July of 2012, a groundwater management plan was completed for the Basin. The purpose of the Plan is to provide
a framework for regional groundwater management in the South Westside Basin that sustains the beneficial use of
the groundwater resource. Currently, groundwater pumping in the basin is monitored through metering for
municipal pumping and metering or estimating for irrigation pumping. Additionally, San Bruno, Daly City, and Cal
Water jointly participate with us in a monitoring program for the Westside Basin to provide information regarding
groundwater levels and quality for the different aquifers in the basin.
4. How do we know storage and recovery will work in the Westside Basin aquifer?
As a precursor to this project, and to determine the potential for conjunctive use in the South Westside Basin, we
conducted a pilot study to characterize aquifer properties and test the aquifer yield potential. Results of the pilot
study indicated that natural recharge of the groundwater basin can successfully be accomplished by reducing
pumping, using additional surface water to replace the reduced pumping, and allowing ongoing recharge to
accumulate in the basin. The study showed that as a result of the varying reductions in pumping throughout the
southern Westside Basin, the total increase in storage during the fall 2002 to spring 2005 recharge period was
approximately 13,000 acre-feet. Test results varied between locations, and we will continue to manage this project
by monitoring changes in groundwater water levels and quality.
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5. Where would groundwater for this project be pumped from?
The proposed Regional Groundwater Storage and Recovery Project would pump previously stored groundwater
from the South Westside Basin which extends from Daly City to Burlingame. The South Westside Basin is located
within the larger Westside Basin groundwater aquifer, which extends from Golden Gate Park in San Francisco south
to Burlingame. The trough-shaped basin is filled with sand, silt and clay sediments. The layers of sand, which can
readily yield water to wells, are called “aquifers.” South Westside Basin wells typically draw water from depths of
between 300 and 700 feet below the ground surface.
6. Is there potential for mobilization of contaminants in the aquifer?
If contaminants are present, they are normally found close to the ground surface. For the GSR project, water stored
in the aquifer is very deep and is below the depth that is typically subject to contamination. Potential for
mobilization of contaminants will be evaluated in the EIR, and the EIR will propose mitigation measures if impacts
are anticipated.
7. What facilities would be constructed as part of the project and what would they look like?
The facilities proposed as part of the Regional Groundwater Storage and Recovery Project would consist of new
groundwater production well facilities in Northern San Mateo County. Up to 16 new groundwater well facilities
would be constructed at 16 of the 19 proposed sites. Well facilities would be connected to Daly City, San Bruno, Cal
Water, or our water distribution systems. Each groundwater well facility site would contain a groundwater
production well, pump station, underground distribution piping and utility connections. The facilities would provide
disinfection and additional treatment as needed. The above ground features of the groundwater production well
facilities would be housed within a pump station building or fenced enclosures. The facilities would be single story
with a footprint ranging between approximately 700 square feet to 3,000 square feet.
8. How will the Regional Storage and Recovery Project be paid for?
The Regional Storage and Recovery Project is part of the $4.6 billion Water System Improvement Program (WSIP).
The goal of the WSIP is to reinforce vulnerable portions of the Hetch Hetchy Regional Water System to withstand an
earthquake and ensure a reliable supply of water for customers. All rate payers who benefit from the Hetch Hetchy
Regional Water System help pay for the WSIP. Costs are passed on to wholesale and retail customers in proportion
to the amount of water that is delivered to them. Since 26 regional Bay Area wholesale customers receive about
two-thirds of the water delivered from the system, they are responsible for approximately two-thirds of the
program costs. The remaining one-third is paid by San Francisco retail customers.
The City of San Francisco issues revenue bonds to finance this capital investment in our regional water system. The
bonds are paid back through water rate increases passed on to customers. Some water providers have already
begun raising rates to cover the cost of these capital improvements. Others will do so in the future on an as-needed
basis.
9. Where else are groundwater storage and recovery projects being implemented?
Conjunctive use projects such as this one have been implemented by many water agencies in California and across
the U.S. On a local level, several Bay Area agencies have been successfully operating groundwater storage and
recovery projects for many years include the Santa Clara Valley Water District, the Alameda County Water District
and Zone 7 Water Agency serving Livermore, Pleasanton and Dublin. In Southern California, conjunctive use and
groundwater management programs have been in place since the 1950s.
10. How do I get more information on the Regional Groundwater Storage and Recovery Project?
For more information on the project, or to sign up for the privacy protected project mailing list for the proposed
Regional Groundwater Storage and Recovery Project, contact us at: [email protected].
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