Post-Wilcox Group geologic units important to this study are, from youngest to oldest, the alluvium and loess of Quaternary age; the fluvial deposits of Quaternary and Tertiary(?) age; and the Cockfield and Cook Mountain Formations and Memphis Sand of Tertiary age (table 1). Kingsbury and Parks (1993) show normal faults in the Memphis area with displacements of the Memphis Sand ranging from about 50 to 150 feet. Displacements along the faults decrease upward (Kingsbury and Parks, 1993). Geologic sections (figs. 4a and b) by Carmichael and others (1997) illustrate the geologic units identified in the shallow subsurface at NSA Memphis. Geologic sections A-A' and B-B' (fig. 4b), oriented north-south, show a prominent step-up in the Cockfield Formation which is interpreted as an erosional scarp. The strata are relatively flatlying above and below this scarp (fig. 4b). Faults displacing the Cockfield and Cook Mountain Formations and the Memphis Sand also are illustrated (fig. 4b).
The shallow aquifers in the NSA Memphis area were recently described by Carmichael and others (1997) and are, in descending order, the alluvial-fluvial deposits aquifer and the Cockfield aquifer. Silt and clay in the upper alluvium and the loess overlie and confine the alluvial-fluvial deposits aquifer which is separated from the Cockfield aquifer by strata of low permeability in the upper part of the preserved section (table 1) of the Cockfield Formation. Silt and clay of the Cook Mountain Formation comprise a confining unit and separate the Cockfield aquifer from the underlying Memphis aquifer. The Memphis aquifer is the principal aquifer used for water supply by NSA Memphis and the city of Memphis.
A map of the altitude of the base of the sand and gravel in the lower alluvium or fluvial deposits at NSA Memphis was prepared by Carmichael and others (1997) (fig. 7). Beneath the NSA Memphis Southside, the basal altitude of the sand and gravel deposits is about 220 feet above sea level (fig. 7), with lower altitudes indicated in areas where Big Creek and its tributaries flowed before being channelized. The basal altitude of the fluvial deposits in the northern part of NSA Memphis is about 300 feet above sea level. The sand in the fluvial deposits is described as fine to very coarse and generally poorly sorted (Carmichael and others, 1997). The thickness of the sand and gravel in the lower alluvium or fluvial deposits is irregular and varies greatly over short distances (fig. 8), with thicker deposits indicated generally southwest of the erosional scarp (30 to 70 feet) and particularly in the flood plains of Big Creek Drainage Canal and its tributaries. Thickness of the fluvial deposits that overlie the Cockfield Formation north of the erosional scarp ranges from about 10 to 20 feet (fig. 8).
The fluvial deposits south of the erosional scarp generally are saturated, and the ground water is confined (Carmichael and others, 1997). The fluvial deposits north of the scarp generally are dry or contain only a few feet of saturated thickness. The fluvial deposits on either side of the scarp may be hydraulically connected along the scarp boundary (Carmichael and others, 1997). Potentiometric-surface maps (figs. 9 and 10) were prepared for the alluvial-fluvial deposits aquifer by Carmichael and others (1997). These potentiometric maps show a ground-water mound centered over the NSA Memphis Southside, with lower water levels centered over Casper Creek and the original drainage area of Big Creek before channelization. Ground-water levels also decrease to the west towards the channelized drainages of North Fork Creek and Royster Creek. An area of lower ground-water levels, oriented northwest-southeast, is indicated in the area of the erosional scarp and the northeasternmost of the two northwest trending faults (fig. 4b).
The hydraulic properties of the sand and gravel in the lower alluvium and the fluvial deposits have been estimated using analyses of core samples, an aquifer test, and well specific-capacity tests. The vertical hydraulic conductivity of three samples of the lower alluvium ranged from about 5.1 x 10-1 to 2.4 x 100 ft/d, and the total porosity ranged from about 22 to 34 percent (table 2). The vertical hydraulic conductivity of 13 samples of the fluvial deposits ranged from about 1.1 x 10-3 to 7.4 x 10-1 ft/d, and the total porosity of the samples ranged from about 26 to 39 percent (table 2). Estimates of the horizontal hydraulic conductivity within the fluvial deposits, determined from nine specific capacity tests (table 3), ranged from about 8 to 150 ft/d. A constant-withdrawal aquifer test was conducted to determine the hydraulic properties of the alluvial-fluvial deposits aquifer at the location of water-level observation wells Sh:U-100, Sh:U-101, Sh:U-102, and Sh:U-103 (fig. 2). The aquifer was tested over a 3-day period beginning August 22, 1995. The results calculated from the test came from calibrating VS2DT, a variably saturated, radial-flow model (Lappala and others, 1987; Healy, 1990) to the measured drawdowns in the observation wells during the test. Horizontal hydraulic conductivity for the alluvial-fluvial deposits aquifer was estimated to be about 5 ft/d (table 3).
Surface-water drainages at NSA Memphis may not be major discharge areas for the alluvial-fluvial deposits aquifer. A comparison of streambed altitudes of the major drainages in the NSA Memphis area (U.S. Army Corps of Engineers, 1989a, b) to the altitude of the potentiometric surface of the alluvial-fluvial deposits aquifer indicates that the potentiometric surface of the aquifer is lower than most streambed altitudes, except for limited reaches of Big Creek Drainage Canal, Casper Creek, and North Fork Creek along the southern boundary of NSA Memphis and near SWMU 2 (fig. 2). The alluvial-fluvial deposits aquifer rests unconformably upon the Cockfield Formation in these areas.
Clay and silt lenses in the Cockfield Formation slow downward movement of ground water from the alluvial-fluvial deposits aquifer (Carmichael and others, 1997) and form the Cockfield confining unit. Vertical hydraulic conductivities of five clay samples from the Cockfield Formation ranged from about 4.5 x 10-5 to 2.5 x 10-3 ft/d, and the total porosity ranged from about 41 to 55 percent (table 2).
At NSA Memphis, the Memphis aquifer is present at depths ranging from about 150 to greater than 220 feet below land surface (Carmichael and others, 1997). Thickness ranges from about 865 to 880 feet. Wells Sh:V-4 (fig. 5) and Sh:V-20, located within the NSA Memphis Northside, are screened in the Memphis aquifer. Analyses of water samples collected from these wells showed concentrations of tritium less than detectable limits, indicating that near the wells leakage of water from the shallower aquifers was not a major source of recharge to the Memphis aquifer (Carmichael and others, 1997).
Carmichael and others (1997) mapped faults in the Memphis Sand and the Cook Mountain and Cockfield Formations, but found no evidence for faulting of sediments younger than the Cockfield Formation. This finding agrees with the estimated time of last movement on faults in the Memphis area (Kingsbury and Parks, 1993). An overlay of the locations of faults mapped at NSA Memphis onto the potentiometric map of the alluvial-fluvial deposits aquifer is illustrated in figure 14. No relation is evident between the occurrence of ground water in the alluvial-fluvial deposits aquifer and two of the faults; but a potentiometric low is centered over the northeasternmost of the two northwest trending faults. One possible explanation for the potentiometric low is that the fault has created a zone of increased hydraulic connection between the alluvial-fluvial deposits aquifer and the Cockfield and Memphis aquifers, which have lower potentiometric heads. Comparison of the altitude of the potentiometric surface in the alluvial-fluvial deposits aquifer in the depression (less than 255 feet) to the altitude of the potentiometric surface in the Memphis aquifer in the NSA Memphis area (figure 13) indicates a vertical head difference of about 40 feet downward between the two units. This condition would allow water to flow vertically downgradient towards the deeper aquifer(s) creating a potentiometric low in the alluvial-fluvial aquifer.
Carmichael and others (1997) show the potentiometric maps for the alluvial-fluvial deposits aquifer for April and October of 1996 (figs. 9 and 10) with lower ground-water levels centered over the inferred valleys of Big Creek Drainage Canal and its major tributaries. Possible explanations for the shape of the potentiometric surface include preferential flow of ground water along the axis of buried river valleys through thick alluvial deposits, and increased hydraulic connection between the alluvial-fluvial deposits aquifer and the Cockfield and Memphis aquifers, with lower potentiometric heads resulting from erosional windows in the confining unit in the areas of the river valleys. Geologic section C-C´ in the area of well Sh:V-9 (fig. 4b) may illustrate the second situation. The natural gamma-ray log of well Sh:V-9 can be interpreted to indicate no clay confining unit between the alluvial-fluvial deposits aquifer and a sand lens in the Cockfield Formation.
Low water levels in two wells completed in the alluvial-fluvial deposits aquifer, located west of NSA Memphis near Royster Creek (figs. 9 and 10), are only about 5 feet higher than the potentiometric surface of the Memphis aquifer for the same area (fig. 13). These low water levels may result from a window in the upper confining unit of the Memphis aquifer. The confining unit is known to be absent or thin locally in the Memphis area (Parks, 1990; Kingsbury and Parks, 1993).