Cal. Pub. Util. Code § 125551

The district may contract with the Board of Administration of the Public Employees’ Retirement System, and in that case the board of administration shall enter into a contract with the district, to enter all, or any portion, of the employees of the district under that system; provided, that no employees of the district in a bargaining unit that is represented by a labor organization shall be included in the contract except as authorized by a collective-bargaining agreement.