Which document must be maintained when taking UST systems out of service?

The maintenance of a permanent closure report is essential when taking Underground Storage Tank systems out of service because it serves as a formal record of the closure process, including details of the tank's removal or decommissioning. This report typically includes documentation of how the tank was emptied, cleaned, and the steps taken to ensure that no leaks or contamination occurred during the process.

By maintaining a permanent closure report, facility operators demonstrate compliance with regulatory requirements and provide a traceable history of the closure that is vital for future site assessments or potential environmental remediation tasks. The closure report serves as a critical document for notifying relevant authorities and ensuring that the site does not pose any environmental risks associated with the decommissioned UST.

Other options, such as a closure plan summary or site assessment report, may contain useful information but do not fulfill the specific requirement for documentation of the closure event itself. Similarly, an environmental impact statement is not directly pertinent to the closure process of UST systems and is not a required document in this context. Therefore, the permanent closure report stands out as the key document that must be maintained during this process.