If tunnels under the Sacramento-San Joaquin River Delta actually get built, more than 2 million people in the Bay Area will see their water bills increase to help pay for them. That's because three water agencies in the Bay Area receive a significant amount of their supply from the delta, via pumps near Tracy, and construction costs of a new water routing system will fall on those users and others across the state who rely on that supply. Leaders of those three Bay Area agencies - the Santa Clara Valley Water District, the Alameda County Water District and the Zone 7 Water Agency in southeastern Alameda County - said they expect rates to increase from 10 to 20 percent, but opponents of the project say ratepayers should expect much higher charges. Water rates are relatively low compared with other utility costs, like energy or even cable television, so if the leaders of the agencies are correct in their forecasts, the average household would see bills increase by a few dollars a month. The plan still has many unanswered questions, though, especially on how costs will be divided among farmers and urban water users statewide, and the final numbers could be much different. Leaders of the Bay Area agencies said they have yet to tell their customers about the potential costs of the project, as it has been unclear whether it would ever come to fruition. But they said the announcement last week of a preferred plan means they will have to start advising customers.