Declaring that the Delta “will not be the next Owens Valley,” San Joaquin and Contra Costa counties — along with farmers and environmental groups — sued Thursday to block a Southern California water district from buying more than 20,000 acres of farmland in the heart of the estuary.

The lawsuit, filed in San Joaquin County Superior Court, charges that the Metropolitan Water District of Southern California violated the law when it determined that the $175 million land buy is exempt from environmental study.