ANTIOCH -- As details of the state's proposal to move water south from the Sacramento-San Joaquin Delta continue to spill out, leaders here received a tutorial this week on the potential impacts of the plan on local water.

The City Council was brought up to speed by city staff on the history of Antioch's ability to draw river water and how the salinity of that water has changed over time. Staff also has been monitoring and analyzing possible impacts of state and federal activities related to the Delta, namely the Bay Delta Conservation Plan to divert northern water through two massive tunnels to farms and cities in other parts of the state.

"We're going to continue to protect our water rights. This is our story right here," Mayor Wade Harper said.

The future financial impact of the state's latest proposal on Antioch is unknown, Public Works Director Ron Bernal said. However, negative impacts to the water supply, such as increased salinity, could cost the city, and ultimately ratepayers, millions of dollars each year to purchase replacement supplies and equipment, he said.

"It could substantially increase costs to the water users in our city. It's an issue that the citizens of Antioch should be very concerned about," Councilman Gary Agopian said.

Antioch could also move to the point where it draws in water farther north, Bernal said. Either way, the city would pursue compensation from the state "to be made whole" for increased costs, he said.

Antioch, the westernmost city or agency to draw water from the Delta, has rights dating back to before 1914 to pump without the need for a state permit.

The once-fresh water has become saltier over time as state and federal agencies moved water from the region to the San Joaquin Valley and Southern California, said Matt Emrick, an attorney hired by Antioch to handle its water issues.

"Antioch is sort of the canary in the coal mine with respect to the Delta," Emrick said. "If Antioch's water supply is healthy, the Delta is healthy."

Because of the salinity increase, the state Department of Water Resources entered into an agreement with the city in 1968 in which it pays one-third of Antioch's costs to purchase substitute water when its normal supply is not usable.

Though implementation is years away, the city is trying to examine now how much saltier the latest proposal would make its water.

Gov. Jerry Brown's plan calls for building two side-by-side underground tunnels, each 40 feet high, that would carry fresh water 35 miles from the state's largest river, the Sacramento, under the Delta to federal and state pumps near Tracy.

There it would flow into canals run by the State Water Project and the Central Valley Project, which deliver Delta water to 25 million Californians, from the Bay Area to San Diego, and to irrigate 3 million acres of farmland.

Construction would start in 2017, and the project would be completed by 2026.

Antioch analyzed and publicly commented on the plan's modeling and flow impacts, while also informing the plan's crafters about Antioch's water rights and the salinity history, Emrick said.

"Many of the folks working on these processes had no idea Antioch had its own water rights," he said. "They thought we all got our water from (Contra Costa Water District) and as long as (they) were taken care of, Antioch was taken care of."

Once more environmental documents and analysis come out for the plan, Antioch can further study possible effects, Emrick said.

Contact Paul Burgarino at 925-779-7164. Follow him at Twitter.com/paulburgarino.