A method of controlling overflow in a below-ground transfer pump sump used in the dispensing of liquid fuels. The method comprises providing a sump having a sump bottom below a level of ground immediately adjacent the sump, providing a storage tank having an internal cavity for the storage of liquid fuel, the storage tank having a conduit in communication with the sump and providing an overflow assembly having an inlet disposed within the sump and an outlet communicating with the conduit. The method includes permitting water from the ground adjacent the sump to enter the sump and mix with any liquid fuel in the sump and if the liquid fuel and water mixture in the sump reaches a predetermined level, allowing the liquid mixture to pass through the overflow assembly, into the conduit and into the storage tank containing the liquid fuel.