What action should be taken if a UST has had its contents below the threshold for an extended period?

Cap and secure all line entrances is the appropriate action when a UST has had its contents below the threshold for an extended period. This is a crucial step in preventing potential environmental hazards, as it minimizes the risk of contamination that could arise from vapors or leaks due to the empty state of the tank. Securing the line entrances helps in maintaining the integrity of the underground storage system and limits exposure to the atmosphere, which is especially important if there are residual fuels or other hazardous materials.

Taking this action allows for a safe environment while further evaluations or inspections are planned. It keeps the site secure, which is essential for compliance with environmental regulations aimed at protecting soil and groundwater from contamination. Capping and securing prevents any unintended access or drainage, ensuring that no spills or outflows occur.

Other options may not adequately address the risks associated with a UST that has been out of service, such as conducting immediate closure and removal, which might be premature if there's potential for future use after proper evaluation. Leaving vent lines open could lead to vapor releases or environmental hazards, and a thorough inspection could be warranted, but securing the entrances should be the immediate priority to protect against potential leaks and contamination.