In a move that could significantly boost the odds of commuters getting a new highway to Orange County, the board of Southern California's heavyweight water provider voted unanimously Tuesday to team up with two regional transportation agencies on a study of the potential for a tunnel.

Transportation officials welcomed the development.

"This is like hitting a grand-slam home run in the bottom of the ninth in the World Series," said Robin Lowe, chairwoman of the Riverside County Transportation Commission.

Wes Bannister of Huntington Beach, chairman of the Metropolitan Water District, said in a telephone interview that the marriage was a natural one because transportation officials want to punch a highway through the mountains and his board wants to drill a water line through the range.

"We have been looking for the last 15 years at the idea of putting a tunnel beneath the Cleveland National Forest through the Santa Ana Mountains," Bannister said. "It's an extremely important project because it brings water directly from Lake Mathews and Diamond Valley Lake into Orange County, Long Beach and San Diego."

Bannister said Metropolitan, the agency that supplies half of Southern California's water by piping in river water from hundreds of miles away, wants to construct a line that would move water by gravity between Corona and south Orange County. That line could also hook up with Long Beach and San Diego County.

"We're talking about an 11- or 12-mile tunnel that would effectively eliminate the need for putting in a pipeline of about 80 miles," Bannister said, referring to a long, circuitous route around the Santa Anas that would follow Highway 91 into Orange County.

Meanwhile, Highway 91 is in the forefront of minds of officials with the Riverside County Transportation Commission and Orange County Transportation Authority, regional boards that plan roads and railroads in their respective counties. Those agencies are jointly sponsoring a $3.3 million study of potential routes for a new highway and/or commuter railroad linking the counties. Options for a long tunnel, or series of short tunnels, figure prominently in the study, which is on pace to produce a preferred transportation solution by December.

Bannister said his board wants to team up with the commission and authority on a complementary study that would examine soil, rock and groundwater that the agencies would have to bore through to lay a tunnel for a water line, a highway or both.

It could turn out that agencies or companies also would want to put fiber-optic cables, oil pipes, gas lines and telephone cables in a Riverside County-Orange County tunnel, he said.

"There are any number of entities that would absolutely benefit by the 11-mile route rather than having to take the 80-mile route," Bannister said. "It would be an economic boon to Riverside and Orange counties."

Bannister suggested the parties could take advantage of $30 million set aside for more study in that corridor, in legislation moving through Congress.

Commission Chairwoman Lowe, who is also a Hemet city councilwoman, termed the decision "good news" for long-term transportation planning efforts.

"I think it widens our options," Lowe said. "I'm much encouraged by the fact that Metropolitan has voluntarily become a partner. Let's don't spend money for getting to Orange County and use it for only one thing."

Lowe said the partnership could lower tolls or other costs to commuters for a transportation project across the forest because multiple agencies would share costs.

Lowe added that Tuesday's decision has huge implications because Metropolitan wields substantial political clout, has representation throughout the region and draws on extensive technical expertise.

Besides boosting chances for a new road, officials said Metropolitan's involvement could change the score on odds of a tunnel being built.

In recent months, consultants spearheading the transportation-routes study have suggested that a long continuous bore is unlikely, because of daunting challenges such as containing groundwater and removing the exhaust of 125,000 cars that would cross daily. Instead, consultants have said a series of short tunnels interspersed with overland sections of highway might prove more feasible.

Bannister said he expected Metropolitan's involvement to put the option of one long bore squarely back on the table.

"A tunnel would be less intrusive to the environment," he said.

Contact staff writer Dave Downey at (951) 676-4315, Ext. 2616, or ddowney@californian.com.