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The 7,000 square mile Mississippi River alluvial plain in northwestern Mississippi (the Delta) is underlain by a prolific aquifer that currently (1983) yields about 1,100 Mgal/d (million gallons per day) of water to irrigation wells. Commonly about 20 feet of clay underlying the Delta land surface is underlain by about 80 to 180 feet of sand and gravel that forms the Mississippi River alluvial aquifer. This study of the alluvial aquifer was prompted by recent declines of water levels. The study was designed to better define the hydrology of the aquifer and to quantify effects of future withdrawal of water from the aquifer. The Mississippi River is in good hydraulic connection with the alluvial aquifer and freely recharges or drains the aquifer, depending on head differences between the aquifer and the river. Generally smaller streams are less likely to recharge the aquifer. In the interstream areas vertical recharge to the alluvial aquifer from the 52 inches of precipitation is small, especially in the central part of the Delta. A two-dimensional finite-difference computer model of the alluvial aquifer was constructed, calibrated, and verified using water levels observed for five dates within the period April 1981 to September 1983. Results from simulation show that the aquifer had a net loss in storage of about 360 Mgal/d for the 2-year period April 1981 to April 1983. During this period, pumpage was about 1,100 Mgal/d (1,270,000 acre feet per year) and the net inflows from the sources of recharge were: Mississippi River, 390 Mgal/d; recharge along east edge of the Delta, 170 Mgal/d; streams within the Delta, 81 Mgal/d; and areal recharge from infiltration of rainfall, 180 Mgal/d. The effects of several levels of pumpage of ground water 670, 1,100, and 1,900 Mgal/d were projected 20 years into the future. In 2003 the effects of the 1,100-Mgal/d pumping rate, about average for the early 1980's, indicated that 46 percent of the water would be coming from storage, water levels would be lowered more than 20 feet in a large area in the central part of the Delta, and ground-water levels would continue to decline. 1

water-resources investigations report

Summary of results of an investigation to define the geohydrology and simulate the effects of large ground-water withdrawals on the Mississippi River alluvial aquifer in northwestern Mississippi