USGS

Potential for leakage among principal aquifers in the Memphis area, Tennessee

U.S. Geological Survey, Water-Resources Investigations Report 85-4295

by D.D. Graham and W.S. Parks

This report is available as a pdf below


Abstract

The principal aquifers in the Memphis area consist primarily of sand or sand and gravel, and the confining beds consist of clay, silt, sand, and lignite. Water-table aquifers are the alluvium and the fluvial deposits of Quaternary age; artesian aquifers are the Memphis Sand and the Fort Pillow Sand of Tertiary age. The Jackson Formation and upper part of the Claiborne Group serve as the confining bed separating the water-table aquifers from the Memphis Sand, and the Flour Island Formation separates the Memphis Sand from the Fort Pillow Sand.

Thickness of the Jackson-upper Claiborne confining bed ranges from 0 to 360 feet, and aggregate thickness of clay beds thicker than 10 feet in this confining bed ranges from 0 to 250 feet. The Jackson-upper Claiborne confining bed is thin or absent in, at least, four areas in the Memphis urban area--in the eastern part along Wolf River, in the southeastern part along Nonconnah Creek, in the south-central part along Nonconnah and Johns Creeks, and in the western part along the Mississippi River. Thickness of the Flour Island confining bed ranges from 160 to 310 feet, and aggregate thickness of clay beds thicker than 10 feet ranges from 70 to 240 feet.

Differences in total hydraulic head among the principal aquifers in the Memphis urban area result in vertical hydraulic gradients which create a potential for interaquifer exchange of water. Throughout this area, the gradient is downward from the water-table aquifers to the Memphis Sand. In the central part of the Memphis urban area, the vertical hydraulic gradient is upward from the Fort Pillow Sand to the Memphis Sand, and in the eastern and western parts, it is downward from the Memphis Sand to the Fort Pillow Sand.

The vertical distribution of carbon-14 data for water from the fluvial deposits, Memphis Sand, and Fort Pillow Sand shows an increase in the relative age of the water with depth. The areal distribution of carbon-l 4 data for water from the upper part of the Memphis Sand indicates that relatively recent water has been brought into the major cone of depression in the potentiometric surface of the Memphis Sand, either by horizontal movement or from downward vertical leakage. Carbon-14 and tritium isotope data indicate that some water derived from relatively recent precip itation has entered the Memphis Sand in the vicinity of the southern part of the Sheahan well field.

The normal, near-surface geothermal gradient in the Memphis area was determined to be 0.6 degrees Celsius per 100 feet. Deviations from the normal geothermal gradient, in areas affected by intense pumping from the Memphis Sand, indicate that downward vertical leakage occurs from the water-table aquifers through the Jackson-upper Claiborne confining bed to the Memphis Sand. The areal distribution of the depth to the coolest water in the Memphis urban area indicates that most of this leakage occurs within the major cone of depression in the potentiometric surface of the Memphis Sand.

The velocity of downward vertical leakage of water from the Memphis Sand through the Flour Island confining bed to the Fort Pillow Sand was determined to be 6.6 X 10-4 feet per day by analysis of borehole temperature data from an observation well in the northeastern part of the Memphis area. From this velocity and the head difference between the Memphis Sand and the Fort Pillow Sand at this locality, the hydraulic conductivity of the Flour Island confining bed was determined to be 1.14 X 1 OB2 feet per day.

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