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the Oklahoma Water Resources Board the scientific information needed to determine the volume of water that could be withdrawn while protecting springs and streams.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 cooperation 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 confined beneath younger geologic units. The depth of fresh groundwater in the outcrop area is not known, but the few deep wells drilled in the outcrop area do not produce saline water. A test well drilled to a depth of 1,820 feet as part of this study produced freshwater.

The primary source of groundwater in the Arbuckle-Simpson aquifer is diffuse recharge from precipitation in the outcrop area. Groundwater discharge from the aquifer is predominantly to streams and springs. Groundwater discharge from the Arbuckle-Simpson aquifer maintains base flow to perennial streams overlying the aquifer, including Blue River, Pennington Creek, Mill Creek, Travertine Creek, Delaware Creek, and Honey Creek. Many springs discharge from the aquifer, including Byrds Mill Spring, the primary water supply for the City of Ada, and springs in Chickasaw National Recreation Area. A small amount of groundwater discharge is to groundwater withdrawal wells. Most groundwater withdrawals are in the eastern part of the aquifer, where average annual reported groundwater use from 1964 through 2008 was 4,299 acre-feet. Sixty-three percent of the groundwater withdrawal was for public water-supply systems.

Hydraulic properties were calculated by using several methods, including an aquifer test and several regional methods. Transmissivity from the analytical solution analysis of the aquifer test was 12,000 feet squared per day. Storage coefficients calculated by regional methods ranged from 0.00211 to 0.07475.

A MODFLOW groundwater-flow model was developed to simulate discharge to streams and springs in the eastern Arbuckle-Simpson aquifer. Horizontal discretization in the model was 200 meters (656 feet) by 200 meters (656 feet), and six layers were used to represent the aquifer over a model area of 1,002 square kilometers (387.1 square miles). The digital three-dimensional geologic framework model provided the geologic data for the MODFLOW groundwater-flow model. The recharge rate, calculated from streamflow data, was distributed over four recharge zones based on the geology at the land surface. Groundwater discharge to streams was simulated as drains. The model was calibrated by using the parameter-estimation process in MODFLOW to steady-state conditions by using a set of synoptic head and flow observations from August 1995. Increasing model vertical hydraulic conductivity beneath Travertine Creek and other creeks at the boundary between freshwater and saline water flow systems resulted in an improved fit between observed and simulated streamflows.

Transient simulations, using daily recharge and water-use data, were accomplished by running the model with 1-day stress periods for the 5-water-year time period starting October 1, 2003, through September 30, 2008, corresponding to water years 2004 through 2008. Area-weighted annual recharge rates applied to the model domain averaged 5.58 inches (142 millimeters) per year for water years 2004–8, and ranged from 2.57 inches (65.3 millimeters) in water year 2006 to 11.61 inches (295 millimeters) in water year 2007. A specific storage of 0.000008 per meter was used for all transient simulations, which corresponds to a regional storage coefficient of 0.008. Because of concerns that large-scale withdrawals of water from the Arbuckle-Simpson aquifer would cause decreased streamflow in rivers and springs, the model was optimized to simulate the effects on Blue River and Pennington Creek, the streams with the largest flows in the eastern aquifer. Total gaged flow for the 5-water-year period for Blue River near Connerville was 336,712 acre-feet, and the total simulated flow was 336,946 acre-feet. Total gaged flow for the 5-water-year period for Pennington Creek near Reagan was 155,720 acre-feet, and the total simulated flow was 154,699 acre-feet.

Steady-state and transient models were used to evaluate changes in stream and spring flows associated with increased water demands and development. Three simulations of distributed withdrawals were tested, allocating groundwater withdrawals as equal proportionate shares of 0.125, 0.250, and 0.392 (acre-feet/acre)/year (0.392 (acre-feet/acre)/year is equivalent to the average recharge rate of 4.7 inches per year for the Arbuckle-Simpson aquifer reported in a previous study). Simulated depletion of average streamflow for water years 2004–8 at the Blue River near Connerville, Oklahoma, streamgage was 17.62 percent for a distributed withdrawal of 0.125 (acre-feet/acre)/year, 35.63 percent for a distributed withdrawal of 0.250 (acre-feet/acre)/year, and 53.49 percent for a distributed withdrawal of 0.392 (acre-feet/acre)/year. Simulated depletion of average streamflow for water years 2004–8 at the Pennington Creek near Reagan, Oklahoma, streamgage was 18.18 percent for a distributed withdrawal of 0.125 (acre-feet/acre)/year, 36.22 percent for a distributed withdrawal of 0.250 (acre-feet/acre)/year, and 56.59 percent for a distributed withdrawal of 0.392 (acre-feet/acre)/year. The eastern Arbuckle-Simpson aquifer groundwater-flow model was used to estimate the current (2011) streamflow depletion caused by the existing groundwater withdrawals for water years 2004–8. The model-simulated average base flow was 4.1 percent greater when groundwater withdrawals were removed from the simulation. The model also showed that increasing withdrawal of groundwater from the aquifer will result in fewer locations where groundwater is discharging to streams and springs.

First posted September 16, 2011