STRATIGRAPHIC AND TRACER EVIDENCE FOR COMPARTMENTALIZATION OF GROUNDWATER FLOW IN THE EOCENE MEMPHIS AQUIFER IN EASTERN ARKANSAS AND WESTERN TENNESSEE

The Memphis aquifer is a 244-m thick, mainly sand aquifer in the northern Mississippi embayment and is a regional water supply for approximately 1.5 million people. A recent stratigraphic study of Tertiary aquifers in the region indicates that the Eocene Memphis Sand, which is the lithostratigraphic equivalent of the Memphis aquifer, is divided into three informal members: upper, middle and lower Memphis Sand. The upper Memphis Sand is primarily sand with interbedded discontinuous clay, has a mean thickness of 59 m and is correlative to the Sparta Sand in southeastern Arkansas and Kosciusko Sand in central Mississippi. The middle Memphis Sand comprises variably interstratified sand, silt and clay with a prominent capping clay interval, has a mean thickness of 126 m and is correlative to the Cane River Formation in southeastern Arkansas and the Tallahatta Formation, Winona Sand, and Zilpha Shale in central Mississippi. The lower Memphis Sand is primarily sand, has a mean thickness of 59 m and is correlative to the Carrizo Sand in southeastern Arkansas and Meridian Sand in central Mississippi. Tritium values from production wells screened in the upper Memphis Sand near surface recharge sources commonly range from 0 to 2.5 T.U., whereas values from wells screened in the middle or lower Memphis aquifer are generally < 0.3 T.U. The tritium data and allied water quality data suggest compartmentalized groundwater flow in the upper Memphis aquifer, likely aided by partial confinement due to a stratigraphic equivalent to the Zilpha Shale. These results require that the water-resource potential of the Memphis aquifer and its vulnerability to contamination need to be re-assessed.