An underground water-storage system that includes a sump having a base and sidewalls that cooperate to define an open-topped box that defines a reservoir, a plurality of plastic water matrices each defining a void space for receiving water, and a lid. The sump and lid are formed of a reinforced geomembrane material and include welds and are prefabricated in a location remote from an installation site of the underground water-storage vault more than two days prior to installation.