40 CFR §141.91

Any system subject to the requirements of this subpart shall retain on its premises original records of all sampling data and analyses, reports, surveys, letters, evaluations, schedules, State determinations, and any other information required by §§ 141.81 through 141.88, 141.90, 141.92, and 141.93. Each water system shall retain the records required by this section for no fewer than 12 years.