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The U.S. Geological Survey's participation in a study of the water resources in eastern Arkansas involved the development and calibration of a digital ground-water flow model of the Mississippi River Valley alluvial aquifer and the use of that model to evaluate the effects of future pumping on the saturated thickness of the aquifer. The study was prompted by the growing concern about significant water-level declines in the alluvial aquifer north of the Arkansas River. Thickness of alluvial sediments in the study area ranges from 125 to 200 feet and averages about 150 feet. The most permeable material lies in the lower part of the sediments and averages about 100 feet in thickness. Hydraulic conductivity of the aquifer ranges from about 120 to about 390 feet/day. Recharge to the aquifer occurs mainly by percolation through the overlying confining layer in interstream areas and by seepage through riverbeds. A small amount of recharge also comes from the sediments underlying the alluvium. Recharge rates in interstream areas range from less than 1 to as much as 10 inches in areas where recharge from underlying sediments is substantial. Digital model simulations were made to represent flow in the aquifer and to quantify flow into and out of the system. The dynamic ground-water system was simulated by using seven stress periods between 1917 and 1987. Pumpage used in the simulation ranged from 86,950,000 to 420,970,000 cubic feet per day. Pumpage was distributed uniformly over each county after eliminating areas where the predominant land use precluded ground-water withdrawals. -4 Storage coefficient values of 0.3 and 1x10 for unconfined and confined aquifer conditions, respectively, were used in the simulations. The effects of increased future pumpage on saturated thickness of the aquifer are shown for each decade from 2000 to 2050. The pumpage projections are based on projected water needs as determined by the U.S. Soil Conservation Service assuming two scenarios, one without and one with conservation measures in place. They assumed that conservation measures in 1990 would decrease pumpage by about 9 percent, and that the percentage decrease would be about 30 percent by the year 2040. There were no critical areas where saturated thickness of the alluvial aquifer was 20 feet or less in 1980. However, model simulation indicated that projected pumpage for 2050 would result in less than 20 feet of saturated thickness in 422 of the 1,595 active cells (2,430,720 acres) if conservation measures were not in place and 258 (1,486,080 acres) of the active cells with conservation measures in place. Simulation indicated that these critical areas would be in the Grand Prairie; west of Crowleys Ridge, in all or part of St. Francis, Cross, Poinsett, and Craighead Counties; and east of Crowleys Ridge in all or part of Crittenden, St. Francis, Cross, Poinsett, and Mississippi Counties.

Simulation of ground-water flow in the Mississippi River Valley alluvial aquifer in eastern Arkansas