A long-awaited study on the costs and benefits of Gov. Jerry Brown’s Delta tunnels should be finished by next spring, a state official said Thursday after an independent audit concluded such a study should have already been done. The tunnels have been in the planning stage for 11 years, but state officials have never completed a comprehensive analysis of whether the project pencils out financially. Such a study could answer basic questions like whether the tunnels will benefit the state as a whole and whether they should be built at all.