Wat. Code, § 13281

(a) In making a determination pursuant to Section 13280, except as specified in subdivision (b), the regional board shall consider all relevant evidence related to the discharge, including, but not limited to, those factors set forth in Section 13241, information provided pursuant to Section 117435 of the Health and Safety Code, possible adverse impacts if the discharge is permitted, failure rates of any existing individual disposal systems whether due to inadequate design, construction, maintenance, or unsuitable hydrogeologic conditions, evidence of any existing, prior, or potential contamination, existing and planned land use, dwelling density, historical population growth, and any other criteria as may be established pursuant to guidelines, regulations, or policies adopted by the state board.

(b) (1) To the extent that resources are available for that purpose, the regional board shall prohibit the discharge of waste from existing or new individual disposal systems on parcels of less than one-half acre that overlie the Mission Creek Aquifer or the Desert Hot Springs Aquifer in Riverside County, if a sewer system is available.

(2) For parcels of one-half acre or greater that overlie the aquifers described in paragraph (1), the maximum number of equivalent dwelling units with individual disposal systems shall be two per acre. For the purpose of this paragraph, the term “equivalent dwelling unit” means a single family dwelling as defined in Section 221.0 of the 1997 edition of the Uniform Plumbing Code of the International Association of Plumbing and Mechanical Officials.

(3) For the purposes of this subdivision, a sewer system is available if a sewer system, or a building connected to a sewer system, is within 200 feet of the existing or proposed dwelling unit, in accordance with Section 713.4 of the 1997 edition of the Uniform Plumbing Code of the International Association of Plumbing and Mechanical Officials.

(4) To the extent that resources are available for the purposes of this subdivision, the regional board shall achieve compliance with this subdivision on or before January 1, 2004.