A method is provided for preventing water from entering a containment sump of an underground storage tank. After accessing an interior of the sump and interrupting the flow paths of piping, a form is positioned in the sump. An annular open space formed between the sump's sidewall and the form is substantially filled with a settable material. An inside wall of the form next has applied to it a polymeric material to seal off an source of water seepage. The piping flow paths are reestablished to create a waterproof containment sump.