California Code, Health and Safety Code - HSC § 116595

(a) A public water system under the jurisdiction of a local primacy agency shall reimburse the local primacy agency for any enforcement cost incurred by the local primacy agency related to any of the following relating to that water system:

(b) The local primacy agency shall submit an invoice to the public water system that requires payment, before September 1 of the fiscal year following the fiscal year in which the costs were incurred. The invoice shall indicate the total hours expended, the reasons for the expenditure, and the hourly cost rate of the local primacy agency. The invoice shall not exceed the total costs to the local primacy agency of enforcement activities specified in this subdivision. Notwithstanding the reimbursement to the state board of enforcement costs, if any, pursuant to Section 116577, any public water system under the jurisdiction of the local primacy agency shall also reimburse the local primacy agency for enforcement costs incurred by the local primacy agency pursuant to this section. The local primacy agency shall not be entitled to enforcement costs pursuant to this subdivision if a court determines that enforcement activities were in error. “Enforcement costs” as used in this subdivision does not include “litigation costs” as used in Section 116585.

(c) Payment of the invoice shall be made within 90 days of the date of the invoice. Failure to pay the invoice within 90 days shall result in a 10-percent late penalty that shall be paid in addition to the invoiced amount.

(d) The local primacy agency may, in its sole discretion, waive payment by a public water system of all or any part of the invoice or the penalty.