A large-scale effort to purify wastewater and inject it into groundwater basins could feasibly produce enough water to serve 335,000 homes, according to a study released Monday by the Metropolitan Water District of Southern California. The project, jointly proposed by MWD and the Sanitation Districts of Los Angeles County, would use wastewater treated at a Carson plant, but instead of discharging it into the ocean, the water would be further purified using techniques such as reverse osmosis then stored in four groundwater basins in Los Angeles and Orange counties.