10 years per Iowa Department of Natural Resources; per the EPA Manual for the Certification of Laboratories Analyzing Drinking Water: Criteria and Procedures, Quality Assurance, 5th ed., chap. 3, sec. 15: “Record Keeping: The certification program manager should ensure that records for on-site laboratory assessments and certification program reviews be maintained in an easily accessible central location for a period of 6 years to include the last two onsite audits, or longer if required by specific State regulations. This includes records/correspondence used to determine compliance with the requirements in this manual. Records may include checklists, corrective action reports, final reports, certificates, PT study results and related documents.”