Opponents of the Delta tunnels proposal, facing a long-shot bid to kill the controversial project on environmental grounds, are now trying to undermine the plan’s financial structure. Six environmental groups filed court papers late Thursday attempting to derail the state Department of Water Resources’ plans to bankroll the tunnels with billions of dollars in bond financing. The groups said bonds can’t be issued because the tunnels violate California environmental laws and because the project has received illegal subsidies from the U.S. Bureau of Reclamation.