A 2.2-million-gallon unlined concrete reservoir in southern California, constructed in 1981, was experiencing problems with cracks in the floor and shell, and associated water leakage. Underwater epoxy patches applied to the cracks on the interior of the reservoir did not hold for more than a few years. A high-performance elastomeric urethane lining system, used in conjunction with a concentrated moisture barrier and epoxy primer, was recommended for this project.