Cal. Pub. Util. Code § 105143

The district may contract for management services with any public agency or person and may contract for operations and maintenance services with the district organized pursuant to Part 3 (commencing with Section 27000) of Division 16 of the Streets and Highways Code. However, the district may not contract out the performance of services performed by or fairly claimable by employees of a bargaining unit without the agreement of the accredited representative of that bargaining unit’s employees.