Abstract: An underground storage tank provided with a manway is equipped with a riser extending from the storage tank, about the manway, to a point just below the access way provided in the ground level of the installation. The riser is provided with a water-tight cover which is released through operation of a cam. The water-tight riser excludes water from the interior of the riser and the manway, ensuring access to the manway, operation of the fittings provided in the manway, an additional containment of fluid passing through the manway and the area of the tank adjacent thereto. Because it is water-tight, an alarm sensitive to liquid may be placed in the interior of the riser to alert the operator to the possible loss of containment, or loss of water-tight sealing between the cover and the riser.