Predicted hydrologic effects of pumping from the Lichterman Well Field in the Memphis Area, Tennessee

Abstract

The Lichterman well field is scheduled to go into operation early in 1965 to supplement the municipal water-supply system for the city of Memphis, Tenn. Although the initial rate of withdrawal from the well field will be about 8 mgd (million gallons per day), the ultimate design capacity of the field is 20 mgd. A study of sand samples, drillers' logs, and geophysical logs collected during preliminary test drilling at the site for the Lichterman well field was used as a basis for defining three zones of sand favorable for the construction of high-capacity (1,000 gallons per minute or more) water wells. The three zones occur in the '500-foot' sand and are here designated (in descending order) as zone A, zone B, and zone C. The depth to the top of these zones below land surface has the following ranges: zone A, 125 to 225 feet; zone B, 200 to 350 feet; and zone C, 700 to 775 feet. Zones A and B range from 0 to 100 feet in thickness, and zone C ranges from 10 to 100 feet in thickness. Within the well field proper these zones are expected to react to the stress of pumping as separate hydrologic units, but outside the well field the three zones are expected to react as a single hydrologic unit. The '500-foot' sand in the Germantown-Collierville area is recharged chiefly by precipitation on the outcrop area of the sand to the east, but the evidence indicates that additional recharge is entering the aquifer from the Wolf River. In spite of this additional recharge, water levels in the '500-foot' sand are declining at an average rate of about two-thirds of a foot per year, owing to municipal and industrial pumpage in the Memphis area. However, this decline is not expected to alter the excellent quality of the water in the '500-foot' sand at the site of the Lichterman well field. Pumping in the Lichterman well field will create a cone of depression in the free-water (piezometric) surface of the '500-foot' sand. The decline in water levels will be directly proportional to the rate of pumping and inversely proportional to the distance from the well field. The resultant changes in hydraulic gradients will alter the direction of ground-water movement in the vicinity of the well field and increase the rate of movement toward the well field from areas of recharge. The lowering of water levels might also accelerate locally the changeover from artesian conditions to semiartesian or water-table conditions in the '500-foot' sand. Within the well field proper, water levels are expected to fluctuate as individual wells are turned on and off to accommodate the demand for water. The presence of clay beds in the aquifer will tend to limit the specific capacity of individual production wells, but could serve to limit interference between wells if adjacent wells are screened in different sections of the aquifer. Interference between wells might also be lessened by pumping those wells having the highest specific capacities for the longest periods of time.