The Equus Beds aquifer, named for the Pleistocene horse fossils in the aquifer sediments, is the easternmost extension of the larger High Plains aquifer system (fig. 1). The Equus Beds aquifer consists of alluvial deposits of sand and gravel inter-bedded with clay or silt (Williams and Lohman, 1949). The general direction of groundwater movement within the study area is to the east (Aucott and others, 1998) except where the hydraulic gradient is altered by pumping wells and near a low-head dam on the Little Arkansas River at Halstead (fig. 1).