Arizona is divided into 67 ground-water areas, and individual areas are selected for intensive data collection once every 6 years. The data collected in the lower Big Chino Valley and Williamson Valley areas are given on maps that show depth to water, well depth, and altitude of the water level, 1975-76; pumpage, 1950-74; and specific conductance, fluoride concentration, and irrigated area, 1974. Scale 1:125,000. (Woodard-USGS)