One day after the largest water district in America pulled out of a $17 billion state project to build twin tunnels under the Delta, a water supplier for 220,000 Alameda County residents supported the plan and said it wants to join in. In a 5-2 decision Wednesday night, Alameda County’s Zone 7 Water Agency endorsed the California WaterFix , which proposes to build tunnels under the Delta as a means of making state water supplies more reliable for buyers like Zone 7. The agency also agreed to commit up to $250,000 more toward state planning of the project.