The potentiometric surface map of the Floridan aquifer system in the northwest Florida Water Management District depicts the level to which water rose in tightly cased wells that tap the Floridan aquifer system in May 1985. The surface was mapped by determining the altitude of water levels in a network of wells and is represented on maps by contours that connect points of equal altitude. The northwest Florida Water Management District consists of about 12,000 sq mi, including all or part of 16 counties in northwest Florida. The Florida aquifer system is the principal source of freshwater for north Florida.