Public water supply systems as defined by RCW 70A.120.020 that the state board of health or local health department determines do not comply with the water quality standards applicable to the system shall immediately initiate preparation of a corrective plan designed to meet or exceed the minimum standards for submission to the department of health. The owner of such system shall within one year take any action required to bring the water into full compliance with the standards. The department of health may require compliance as promptly as necessary to abate an immediate public health threat or may extend the period of compliance if substantial new construction is required: PROVIDED FURTHER, That the extension shall be granted only upon a determination by the department, after a public hearing, that the extension will not pose an imminent threat to public health. Each such system shall include a notice identifying the water quality standards exceeded, and the amount by which the water tested exceeded the standards, in all customer bills mailed after such determination. The notification shall continue until water quality tests conducted in accordance with this chapter establish that the system meets or exceeds the minimum standards.