The San Diego County Water Authority filed suit Monday to stop the rural Fallbrook and Rainbow water districts from leaving the county system, citing environmental harm under the California Environmental Quality Act.

The lawsuit filed in Superior Court challenges a decision by the San Diego Local Agency Formation Commission to allow the two districts to join Riverside County without paying what the Water Authority says is their fair share of water-reliability investments.

The “detachment” effort is the first of its kind in California and would shift approximately $140 million in costs to the rest of the Water Authority’s customers.