When required by the agency, owners and operators of aboveground storage tank (AST) or underground storage tank (UST) systems must follow the procedures in §334.74 of this title (relating to Release Investigation and Confirmation Steps) to determine if the AST or UST system is the source of off-site impacts. These impacts include the discovery of regulated substances (such as the presence of Non-Aqueous Phase Liquids (NAPLs) or vapors in soils, basements, sewer and utility lines, and nearby surface and drinking waters) that have been observed by agency staff or brought to the agency's attention by another party.