Hydrogeology and simulation of groundwater flow in the Arbuckle-Simpson aquifer, south-central Oklahoma

Abstract

The Arbuckle-Simpson aquifer in south-central Oklahoma provides water for public supply, farms, mining, wildlife conservation, recreation, and the scenic beauty of springs, streams, and waterfalls. Proposed development of water supplies from the aquifer led to concerns that large-scale withdrawals of water would cause decreased flow in rivers and springs, which in turn could result in the loss of water supplies, recreational opportunities, and aquatic habitat. The Oklahoma Water Resources Board, in collaboration with the Bureau of Reclamation, the U.S. Geological Survey, Oklahoma State University, and the University of Oklahoma, studied the aquifer to provide the Oklahoma Water Resources Board the scientific information needed to determine the volume of water that could be withdrawn while protecting springs and streams. The U.S. Geological Survey, in coopertion with the Oklahoma Water Resources Board, did a study to describe the hydrogeology and simulation of groundwater flow of the aquifer. The outcrop of the Arbuckle-Simpson aquifer covers an area of about 520 square miles in Carter, Coal, Johnston, Murray, and Pontotoc Counties. Three subdivisions of the aquifer outcrop were designated for this study: the eastern, central, and western Arbuckle-Simpson aquifer. This study emphasized the eastern Arbuckle-Simpson aquifer because it is the largest part of the aquifer by area and volume; most groundwater withdrawals are from the eastern Arbuckle-Simpson aquifer; and the largest (by flow) streams and springs sourced from the aquifer are on the eastern Arbuckle-Simpson aquifer. The aquifer lies in an uplifted area commonly referred to as the Arbuckle Mountains, which is characterized by great thicknesses of mostly carbonate rocks, uplifts, folded structures, and large fault displacements. The Arbuckle-Simpson aquifer is contained in three major rock units of Late Cambrian to Middle Ordovician age: the Timbered Hills, Arbuckle, and Simpson Groups. The aquifer is underlain by low-permeability Cambrian and Proterozoic igneous and metamorphic rocks, and is confined above by younger sedimentary rocks of various ages in areas where the top of the aquifer dips below the surface. The major part of the Arbuckle-Simpson aquifer is the Arbuckle Group, which consists of as much as 6,700 feet of limestone in the western Arbuckle-Simpson aquifer, but which thins to an estimated 3,000 feet of predominantly dolostone in the eastern Arbuckle-Simpson aquifer. Water is obtained from cavities, solution channels, fractures, and intercrystalline porosity in the limestone and dolostone. The overlying Simpson Group, consisting of sandstones, shales, and limestones, is as much as 2,300 feet thick in the western Arbuckle-Simpson aquifer, but generally is less than 1,000 feet thick in the eastern aquifer. Water in the Simpson Group is stored primarily in pore spaces between the sand grains in the sandstones. A digital, three-dimensional geologic framework model was constructed to define the geometric relations of fault blocks and subsurface rock units across complex fault zones of the eastern Arbuckle-Simpson aquifer. Geologic data for the model were obtained from 126 drill holes; stratigraphic contacts and faults defined from a digitized version of the surface geologic map; and fault geometry, stratigraphic thickness, and information compiled from geologic and hydrogeologic reports and maps. Groundwater in the aquifer moves from areas of high head (altitude) to areas of low head along streams and springs. The potentiometric surface in the eastern Arbuckle-Simpson aquifer generally slopes from a topographic high from northwest to the southeast, indicating that regional groundwater flow is predominantly toward the southeast. Freshwater is known to extend beyond the aquifer outcrop near the City of Sulphur, Oklahoma, and Chickasaw National Recreation Area, where groundwater flows west from the outcrop of the eastern Arbuckle-Simpson aquifer and becomes confin