OPTIMIZATION MODELS: TOOLS FOR DEVELOPING SUSTAINABLE YIELD MANAGEMENT STRATEGIES FOR THE MISSISSIPPI RIVER VALLEY ALLUVIAL AQUIFER IN ARKANSAS

Conjunctive use optimization modeling was conducted with the objective of maximizing pumpage of ground water from wells within the Mississippi River Valley alluvial aquifer without violating hydraulic-head, drawdown, or stream-discharge constraints. In 1995, more than 5 billion gallons of water per day were pumped from the aquifer by more than 45,000 wells primarily for irrigation and for fish farming. Several large cones of depression over 100 feet deep have formed in the potentiometric surface, resulting in lower well yields and degraded water quality in some areas.

MODFLOW-based ground-water flow models were developed for the north and south portions of the aquifer. MODMAN optimization models based on these flow models showed that continued pumping at 1997 rates are unsustainable for the next 50 years without violating head or drawdown constraints imposed as a part of Arkansas' Critical Ground-Water Area criteria. These criteria state that if water levels drop below half the original saturated thickness of the formation and/or decline at a rate greater than 1 foot per year over 5 years, then a "critical ground-water area" may be designated. In addition, stream-discharge constraints were established based on minimum flow requirements for maintaining water quality and fish habitat. Linear programming was done using these models to optimize pumpage within specified limits, without violating head, drawdown, or stream-discharge constraints.

Preliminary results show that continuous pumping at 1997 rates (636 million cubic feet per day (mcfd) in the north model; 78 mcfd in the south model) for a 50-year period resulted in water levels dropping below the half-thickness constraint, making these rates unsustainable. Optimized pumping for the same period was 596 mcfd for the north model and 62 mcfd for the south model, resulting in an unmet demand of 40 mcfd and 16 mcfd, respectively. In both models, water levels were maintained at or above the half-thickness constraint. Results will assist Arkansas water managers in quantifying water shortfalls and to manage the aquifer in a sustainable manner.