In determining the rates to be charged by each water company subject to its jurisdiction, the commission may provide for the funding of a reserve account exclusively for the purpose of making capital improvements approved by the department of health as a part of a long-range plan, or required by the department to assure compliance with federal or state drinking water regulations, or to perform construction or maintenance required by the department of ecology to secure safety to life and property under RCW
43.21A.064(2). Expenditures from the fund shall be subject to prior approval by the commission, and shall be treated for rate-making purposes as customer contributions.