Altitude of potentiometric surface, fall 1985, and historic water-level changes in the Memphis aquifer in western Tennessee
Recharge to the Memphis aquifer of Tertiary age is from precipitation on the outcrop, which forms a broad belt across western Tennessee, or by downward infiltration of water from the overlying fluvial deposits of Tertiary(?) and Quaternary age and alluvium of Quaternary age. In the outcrop-recharge belt, where the Memphis aquifer is under water-table conditions, the potentiometic surface is complex and generally conforms to the topography. To the west of the outcrop-recharge belt where the Memphis aquifer is confined, the potentiometric surface gently slopes westward, and water moves slowly in that direction. A major cone of depression in the potentiometric surface in the Memphis area is the result of long-term (1886-present) pumping at municipal and industrial well fields. Data from five observation wells in the Memphis aquifer indicate that water levels have declined at average rates ranging from less than 0.1 to 1.3 feet per year during the period 1928-85. The largest declines have been in the Memphis area, where withdrawals averaged about 191 million gallons per day in 1985. The record from an observation well located near the center of the major cone of depression in the Memphis area indicates that water levels ceased to decline in about 1975 and that the center of the cone essentially has stabilized. The record from another well away from the center of the cone indicates that water levels are still declining at a low rate, and that the cone is still expanding as a result of the effects of pumping. Water levels in large areas of western Tennessee, away from the effects of pumping, have fluctuated only in response to long-term variations in precipitation on the outcrop-recharge belt. Long-term changes in water levels in these areas have been small.