Thickness of the upper and lower confining units of the Mississippi River alluvial aquifer in northwestern Mississippi

The thickness of the upper and lower confining units of the Mississippi River alluvial aquifer in north- western Mississippi (the Delta) has a significant influence on the vertical recharge and contamination susceptibility of the aquifer. The upper confining unit is thicker in the southern part of the Delta, the upper confining unit is less than 10 feet thick in small areas in Bolivar, Coahoma, Sunflower, Quit- man, and Tallahatchie Counties. The lower confining units are characterized by the geologic units directly underlying the alluvial aquifer. The geologic units directly underlying the aquifer, from youngest to oldest, are the Jackson Group, Cockfield Formation, Cook Mountain Formation, Sparta Sand, and Zilpha Clay. The lower confining unit thickness is greatest (greater than 250 feet) in southern Issaquena and northwestern Warren Counties. The largest area where the lower confining unit is less than 10 feet thick is Quitman, southern Coahoma, western Panola, southern Tunica, and the northern one-half of Sunflower Counties.