Cal. Wat. Code § 35470.5
The district may, by resolution, provide that a penalty not in excess of 10 percent shall be added to water, standby, facility, or other charges which are delinquent, and the delinquent charges shall bear interest at a rate not in excess of 11/2 percent per month. For purposes of this section, the district shall establish the period or date after which the charges shall become delinquent if they remain unpaid. The delinquency dates established in Part 7 (commencing with Section 36550) and Part 7.5 (commencing with Section 37200) for unpaid assessments, which may include standby or other charges for the use of district water that has been made a part of the assessment, shall not apply to the addition of penalties and interest to delinquent charges, pursuant to this section.
Ca. Water Code § 35470.5