A $42 million project that doubled the capacity of an innovative groundwater desalination plant in Chula Vista is now complete, another big step for a region in dire need of diversification of its water supply. The expansion doubles the facility’s production of drinking water from 5 million gallons a day to 10 million by adding five new wells. The drought-resistant water source — brackish, or saline, groundwater that’s been cleaned using reverse-osmosis technology — will be shared evenly between the Sweetwater Authority and the city of San Diego, which split the costs of the project not covered by $31 million in state and federal grants.