The water distribution system of Dallas Water Utilities (DWU) is one of the largest in the United States. As of June 2003, DWU provided retail water service to just over 1.2 million people within the Dallas city limits. The major distribution system facilities include 28 pump stations (including the high service pump stations at the three water treatment plants), 11 ground storage reservoirs, 9 elevated tanks, and 78 vault structures separate from the major facilities, in addition to over of distribution and transmission main. The distribution system is divided into 17 pressure zones to maintain adequate water pressures throughout the system. There are four major pressure zones (Central Low,