December 10, 2016

Will desalination plants protect South Bay against drought?

Would a ring of desalination plants around the Bay and on the coast be the best way to meet South Bay water needs for coming decades?

The Bay Area Water Supply and Conservation Agency (BAWSCA) is a super-district composed of 24 water agencies, one private company (California Water Services Company), and one university (Stanford) that serve most of the populated areas of San Mateo and Santa Clara Counties, plus Hayward and the Tri-Cities in Alameda County.

On July 3 BAWSCA released a report on water-supply issues through 2035.

Most of BAWSCA’s water comes from the Tuolumne River, purchased from the San Francisco Public Utilities Commission (SFPUC). Because water conservation has increased since 2005, BAWSCA is not currently purchasing all of its allocation of SFPUC water. The surplus allotment amounts to 13 million gallons per day (mgd). In 2010 BAWSCA served a population of 1.7 million. With a projected growth of 0.4 million people by 2035–to be offset by modest increases in conservation, recycled water, and groundwater–the report projects a normal-year deficit in 2035 of 4 mgd. That amount could easily be made up through slightly more-aggressive conservation measures.

But in a three-year drought, the gap between projected supply and demand in 2035 would be much greater. Droughts causing a 10% reduction in SFPUC water deliveries to BAWSCA have occurred on average every 10 years. During the historic six-year 1986-1992 drought, SFPUC supplies to BAWSCA were twice cut by 29%. In a 20% drought-reduction scenario, the difference between supply and demand in 2035 could be as large as 62.5 mgd.

BAWSCA is considering four types of projects to fill the gap:

  • recycled water—1.7 mgd;
  • rainwater—0.5 mgd;
  • greywater—2.4 mgd;
  • desalination—54 mgd.

(Figures represent maximum deliveries. Note that they do not total to 62.5; there would still be a deficit even with all the proposed projects.)

The desalination total is made of five separate projects:

  • west end of the Dumbarton Bridge–4 mgd;
  • west end of the San Mateo Bridge–8 mgd;
  • on the Bay in South San Francisco–16 mgd;
  • on coast near Pacifica–6 mgd;
  • Bay Area Regional Desalination Facility on Mallard Slough near Pittsburg–the BAWSCA share would be 20 mgd.

In this scenario, desalination would make up 86% of the projected 20%-drought-year deficit.

Desalination on enclosed bays and estuaries can damage marine ecosystems. Because of the large initial capital investment, if the plants are run only every 10 years, i.e. only when needed in severe drought, desalinated water is extremely expensive. If the plants are run continuously, the water becomes much cheaper (though still much more expensive than SFPUC water), but then, to meet costs without drastically raising the water rates to local customers, the agencies financing the plants need to sell a large surplus of water in normal years, and to do so, they are tempted to aggressively promote unsustainable housing growth.

As an alternative, BAWSCA ought to give more serious consideration to its greywater, recycled-water, and conservation efforts. Current (2010) per-capita use of water for BAWSCA stands at 132 gallons per day; projected for 2035 is 150 gallons per day, a 13.3% increase. If BAWSCA instead holds per-capita water use at current levels through 2035, it would have a 37 mgd surplus in normal years, and a 20%-drought-year deficit of only 25 mgd. If normal-year surpluses could be stored in local aquifers, there would be no need to build multiple desalination plants on the Bay.

The Sierra Club is definitely opposed to the Mallard Slough project. We will study the other projects as more details become available. Sierra Club national policy opposes desalination on enclosed bays and estuaries, and Sierra Club California opposes all desalination on the California coast.

In January BAWSCA will select a consultant to develop a “more robust and consistent” method for projecting BAWSCA members’ water needs. A key component of these projections will be forecasts of population growth; the growth projections will drive the water-needs forecast, and such projections have long been controversial. The Bay Chapter Water Committee, working in conjunction with the Loma Prieta Chapter, is preparing to participate in the development of the water-needs forecasting.


At the Water Committee meeting, at 7 pm on Mon., Jan. 21, at the Chapter Office, 2530 San Pablo Ave. in Berkeley, we will hear a presentation by Dick Schneider, former chair of the Chapter Population Committee, on how ABAG and California state law set growth targets for Bay Area cities. Come join us and get involved in this key planning process. For more information contact committee co-chair Charlotte Allen at

For a copy of BAWSCA’s report, go to

Charlotte Allen


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