The Vamoosa-Ada aquifer, which underlies an area of about 2,320 mi2, consists principally of the Vamoosa Formation and the overlying Ada Group of Pennsylvanian age. Rocks comprising the aquifer were deposited in a nearshore environment ranging from marine on the west to nonmarine on the east. Because of changes in depositional environments with time and from place to place, the aquifer is a complex sequence of fine- to very fine-grained sandstone, siltstone, shale, and conglomerate, with interbedded very thin limestone. The aggregate thickness of water-bearing sandstones is greatest south of the Cimarron River, where it reaches a maximum of 550 ft in the vicinity of Seminole. North of the Cimarron River, the average aggregate thickness of the sandstones is about 100 ft, but locally it may be as much as 200 ft. Transmissivity values derived from seven aquifer tests made for this study range from 70 to 490 ft2 per day; values decrease from south to north with decreasing sandstone thickness. Hydraulic-conductivity values range from 2 to 4 ft per day. Storage coefficients for the confined part of the aquifer, as determined from four aquifer tests made during 1944, have an average value of 0.0002. The average storage coefficient for the unconfined part of the aquifer is estimated at 0.12, based on an analysis of geophysical logs and grain-size data. The specific capacity of wells tested is generally less than 1 gallon per minute per foot of drawdown. An approximate hydrologic budget for the aquifer for 1975 gives values, in acre-feet per year, of 93,000 for recharge, 233,000 for runoff, and 2,003,000 for evapotranspiration. The total of these values is almost equal to the average annual precipitation of 2,330,000 acre-ft per year. The estimated amount of water containing a maximum of 1,500 milligrams per liter of dissolved solids stored in the aquifer is estimated at 60 million acre-ft. Of this amount, an estimated 36 million acre-ft is available for use. The quality of water in the Vamoosa-Ada aquifer generally is suitable for municipal, domestic, and stock use. Of 55 water samples analyzed in the laboratory, about 75 percent were of the sodium bicarbonate or sodium calcium bicarbonate type; the remainder were of the sodium sulfate, calcium sulfate, sodium chloride, or indeterminate types. Laboratory and on-site chemical-quality data indicate that mineralization of both ground and surface waters is greater than normal in some areas. Water samples from 7 wells and 12 stream sites had concentrations of bromide exceeding 1 milligram per liter; the only known source of bromide in the area is brine associated with petroleum production.