BRENTWOOD — Millions of Bay Area residents could get extra drought insurance against water shortages and quality problems from a proposed $800 million expansion of the Los Vaqueros Reservoir that may have up to 10 water suppliers as partners.

Ten water agencies serving San Jose, Fremont, Oakland, Concord, Richmond, Antioch, San Francisco and other communities have negotiated preliminary deals to contribute a combined $1 million for feasibility studies on expanding the reservoir south of Brentwood.

Boards of some of those agencies recently agreed to share the costs. Others are considering approval soon, including the Santa Clara Valley Water District board on Tuesday and the East Bay Municipal Utility District board on Nov. 22.

Bay Area water managers said interest has grown in having more places to stash away water reserves after the last two years of drought lead to widespread water restrictions and steep prices.

Also, the state is dangling a huge carrot to would-be reservoir builders. The state will give out grants to pay up to 50 percent of the cost of a new reservoir out of Proposition 1 bond funds approved by California voters in 2014. That maximum would allow a state grant of up to $400 million of the $800 million cost of expanding Los Vaqueros.

Many water districts have gone solo on reservoirs in the past. The Contra Costa Water District looked but found no partners to help plan and build the original Los Vaqueros in the 1980s, and expand it in 2012.

But now many agencies are ready to contribute to the study on expansion options, costs and benefits. The information will help them decide whether to become a project partner.

“The project would expand water supplies that can be drawn upon in dry times to deal with shortages or water quality problems,” said Jennifer Allen, a spokesman for the Contra Costa Water District, the operator of Los Vaqueros. “With the last drought, suppliers are increasingly seeing regional cooperation as an important part of our options.”

Extra water stored away could be used to ease shortages among Contra Costa Water’s 500,000 customers in droughts and also be used to dilute salts and minerals that can build up in Delta water in dry spells.

“An expanded reservoir could provide us with another option in dry years for our 1.4 million customers,” said Mike Tognolini, the East Bay Municipal Utility District manager of water improvements.

Water stored in Los Vaqueros could be especially useful if one or more of the EBMUD aqueducts delivering Sierra water to Contra Costa and Alameda counties were knocked out of service by an earthquake in the Delta, Tognolini said.

Other water suppliers that rely on Delta water from state and federal pumps near Tracy could benefit by taking some of their water from Los Vaqueros, according to a report by the Contra Costa Water staff.

An expanded Los Vaqueros could make it easier for the Santa Clara Valley, Alameda County Water and Alameda County Zone 7 Water agencies to get Delta water. Diversions from the Tracy pumps is limited at times because of environmental restrictions to protect fish, such as the endangered Delta smelt.

CCWD’s water diversion points for Los Vaqueros have modern fish screens and slower moving water, making them less likely to be turned down or off to protect Delta fish, water planners said.

Jill Duerig, Zone 7’s general manager, said her agency might be able to get water through Los Vaqueros more quickly when it needs it in drought years, and recharge its groundwater basins more readily.

“It gives us another option so we don’t have to put our eggs in one basket,” she said.

Contra Costa Water on Wednesday hired consultants to prepare an environmental impact report on the expansion options and help prepare a state grant application due in June 2017.

Public meetings on the expansion report will be held next year before Contra Costa Water submits its grant applications.