MODEL CALIBRATION AND SIMULATED RESPONSES TO PUMPING OF THE ALLUVIAL AND SPARTA AQUIFERS OF THE MISSISSIPPI EMBAYMENT IN ARKANSAS AND LOUISIANA

Since the early 1900s, the shallow, unconfined Mississippi River Valley alluvial aquifer and deeper, confined Sparta aquifer of the Mississippi Embayment Aquifer System have been used to supply large volumes of water. Continued, large withdrawals from these aquifers for agricultural, municipal, and industrial water supplies have caused cones of depression (more than 100 feet deep in some locations) to form in several Arkansas counties and Louisiana parishes. The effects of current and future ground-water withdrawals on water availability and water quality are major concerns as yields decline and water quality degrades. Long-term, sustainable yields from these aquifers are the management goal for State water planners responsible for ground-water resources. MODFLOW models of both aquifer systems were developed and calibrated to help achieve this goal. These models accurately simulate observed hydraulic heads from predevelopment to 1998. Automated parameter estimation of model hydraulic conductivity and storativity values reduced model error, and resulted in models sufficiently accurate to simulate the aquifers. The root-mean-square errors for the alluvial and Sparta models are 6.5 and 18.0 feet, respectively, based on 1,229 hydraulic-head observations for the alluvial aquifer and 795 observations for the Sparta aquifer from 1970 to 1998. Once calibrated, the numerical flow models were used to predict ground-water levels in both transient and steady-state simulations based on several potential future water-use scenarios. Pumping scenarios included (1) recent pumping rates extended through time, (2) increasing pumping rates based on county water use trends, (3) recent pumping rates extended indefinitely, and (4) decreased pumping in selected areas. Preliminary results verify that water-levels will continue to decline over much of the modeled areas in both aquifers if pumping rates remain unchanged or increase. For the alluvial aquifer, a decrease in pumping by 10 percent through 2049 shows no recovery, but only decreased rates of water-level decline. Conservation initiatives in the Sparta aquifer to reduce or remove withdrawals by the largest industrial users from two Arkansas counties through 2027 allow recovery of water levels in the two major cones of depression formed in those counties.