CONJUNCTIVE-USE OPTIMIZATION MODELING OF THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER: ASSESSING THE SUSTAINABLE YIELD OF A MAJOR WATER RESOURCE

The Mississippi River Valley alluvial aquifer (the alluvial aquifer) is a water-bearing assemblage of gravels and sands that underlies about 32,000 square miles of Missouri, Kentucky, Tennessee, Mississippi, Louisiana, and Arkansas. In Arkansas, the alluvial aquifer occurs in an area generally 50 to 125 miles wide by about 250 miles long adjacent to the Mississippi River. The alluvial aquifer of Quaternary age overlies older formations that make up the Mississippi Embayment. The alluvial aquifer ranks third among the most productive aquifers in the United States. In 2000, more than 9 billion gallons per day of water were pumped from the alluvial aquifer by more than 45,000 wells, primarily for irrigation and for fish farming. Since the widespread agricultural use of the aquifer began, several large cones of depression have formed in the potentiometric surface, resulting in lower well yields and degraded water quality in some areas.

Conjunctive-use optimization modeling was done to assist water managers and planners by estimating the maximum amount of ground water that hypothetically could be withdrawn from alluvial wells and from hydraulically connected streams without violating hydraulic-head or streamflow constraints. Optimization models showed that continued pumping at 1997 rates are unsustainable without violating head constraints imposed as a part of Arkansas's Critical Ground-Water Area criteria. Streamflow constraints specified within the model were based partly on minimum flow requirements for maintaining either navigation requirements, water quality, or fish habitat. Continuously pumping at 1997 rates resulted in water levels dropping below the hydraulic-head constraints (either half the aquifer thickness or 30 feet of saturated thickness), making those rates unsustainable. Optimized sustainable pumping was obtained such that water levels were maintained at or above the hydraulic-head constraints, and streamflow was maintained at or above minimum flow requirements. Assuming mean-annual streamflow entering the head of each stream represented in the model, optimized sustainable yields from streams were nearly two orders of magnitude greater than for ground water.