GROUND-WATER MODELS OF THE ALLUVIAL AQUIFER IN ARKANSAS: MANAGEMENT TOOLS FOR QUANTIFYING SUSTAINABLE YIELD

Arkansas is the fourth largest user of ground water in the United States. The Mississippi River Valley alluvial aquifer is a water-bearing assemblage of gravels and sands that underlies most of eastern Arkansas and several adjacent States, and supplies the majority of ground water used in Arkansas. Cones-of-depression have developed in the alluvial aquifer water-level surface, some as much as 100 feet deep. Long-term water-level measurements show an average annual decline of 1 foot per year. Several counties in the Grand Prairie area have been designated Critical Ground-Water Areas by the Arkansas Soil and Water Conservation Commission (ASWCC). The expansion of these cones-of-depression and the consistent water-level declines indicate that ground-water withdrawals are occurring at a rate that is greater than the sustainable yield of the aquifer.

Two ground-water flow models of the alluvial aquifer (divided by the Arkansas River) were developed for eastern Arkansas and parts of southeastern Missouri and northeastern Louisiana. The flow models showed that continued ground-water withdrawals at rates commensurate with those of 1997 could not be sustained indefinitely without causing water levels to decline below half the original saturated thickness of the aquifer. To develop estimates of withdrawal rates that could be sustained relative to the constraints of critical ground-water area designation, conjunctive-use optimization modeling was applied to the flow models. Conjunctive-use optimization modeling is a technique that simulates maximum withdrawal rates from both surface water and ground water while honoring constraints with respect to streamflow and water levels.

Results from the optimization models show that present water use from the alluvial aquifer is unsustainable if pumping at 1997 rates were maintained indefinitely. However, sustainable yield from ground water is possible but at a substantially reduced rate and number of wells relative to 1997 amounts. Furthermore, sustainable yield from rivers represents a potential source of water that could supplement ground water and meet total water demand although its use may be limited by location.