Abstract: An improved containment sump assembly which provides an enclosure for pipes and other tank appurtenances associated with an underground storage tank. The containment sump assembly comprises a generally cylindrical main body section closed at its lowermost end forming a hollow interior for enclosing the pipes and equipment, a generally cylindrical top section having the same diameter as the main body section along its lower portion and tapered to provide an opening of a smaller diameter at its uppermost point. A lid for closing the top opening is also included. The main body section is further provided with a tongue component around its uppermost edge; the top section is provided with a complimentary groove component around its lowermost edge. The tongue and groove components of the respective sections are capable of mating engagement to form a liquid-tight seal when the top section is placed on the main body section.