HYDROLOGY OF THE SURFICIAL AQUIFER IN THE FLOYD RIVER BASIN, IOWA K.D. Wahl, M.J. Meyer, and R.A. Karsten Iowa Department of Natural Resources, Geological Survey Bureau, Water Supply Bulletin Number 12, 1982, 53 p. ABSTRACT The Floyd River basin was studied to provide water-resources information for a typical surficial aquifer in northwest Iowa. Data collection included test drilling, water-level measurements, and chemical analyses of surface and groundwater. The Floyd River basin drains 961 square miles of highly dissected to gently rolling topography. Major streams generally are flanked by flood plains underlain by uncemented sand and gravel deposits. Most of the basin is directly underlain by glacial drift of Pleistocene age which is in turn underlain by rocks of Cretaceous age. Sand-and-gravel deposits underlying the major flood plains and in buried bedrock channels within the drift comprise the surficial aquifer. The surficial aquifer ranges from 10 to 40 feet in thickness and averages about 20 feet thick. Both unconfined and confined conditions occur in the aquifer and water levels range from 2 to 55 feet below land surface. An aquifer test conducted in the surficial aquifer where it is about 25 feet thick and is confined by an overlying low-permeability bed indicated an average hydraulic conductivity of 383 feet per day and a storage coefficient of 0.0001. The well was pumped at 650 gallons per minute for 43 hours. An observation well about 70 feet from the pumping well had a maximum drawdown of about 10 feet. Water use in the basin is largely for public supply and rural domestic or rural-livestock use although irrigation use has been increasing in recent years. The groundwater generally is of suitable chemical quality for most uses although it is hard and has excessive concentrations of sulfate in some areas.