Health & Saf. Code, § 116405

(a) In counties with a population not exceeding 500,000 persons as shown by the 1970 federal decennial census, any public water system supplying both domestic and untreated irrigation water in separate pressurized systems that were in existence prior to January 1, 1990, and that is operated by an incorporated or unincorporated association of users, shall not require protection against backflow into the domestic water system from premises receiving both the water services and having available no other source of water, except where interconnection between the systems has taken place. It shall be a misdemeanor for any person to knowingly interconnect the water services on a user’s premises without installing a backflow protection device approved by the state department.

(b) Regulations of the state department requiring the installation of backflow protection shall not be continued to require the installation of the protection in any public water system described in subdivision (a), except as provided in that subdivision.