The shallow aquifers in the NSA Memphis area 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 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.
The vertical hydraulic conductivity of core samples of the alluvium-loess confining unit ranged from 8.5 x 10-5 to 1.6 x 10-2 ft/d. Total porosity of the samples ranged from 35 to 48 percent. The results of an aquifer test were used to estimate a horizontal hydraulic conductivity of about 5 ft/d for the alluvial-fluvial deposits aquifer. The total porosity of core samples of the alluvial-fluvial deposits aquifer ranged from about 22 to 39 percent. The vertical hydraulic conductivity of core samples of the Cockfield confining unit 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. Well specific-capacity tests indicate that the horizontal hydraulic conductivity of sand units that comprise the Cockfield aquifer range from about 0.5 to 3 ft/d. The vertical hydraulic conductivity of core samples of the Cook Mountain confining unit ranged from about 5.0 x 10-6 to 9.9 x 10-4 ft/d, and the total porosity ranged from about 30 to 42 percent.
A conceptual hydrogeologic model of the ground-water-flow system was formulated based on the results of stratigraphic and structural correlations. In the conceptual hydrogeologic model, the shallow aquifer system is composed of five hydrogeologic units: (1) the alluvium-loess confining unit; (2) the A1 aquifer including the entire alluvial and fluvial deposits aquifer and sand lenses in the upper part of the Cockfield aquifer; (3) the Cockfield confining unit; (4) the Cockfield aquifer comprising sand lenses within the lower part of the Cockfield aquifer; and (5) a confining unit formed by sediments of low permeability within the Cook Mountain confining unit. Surface-water drainages at NSA Memphis may not be major discharge areas for the ground-water system. A comparison of streambed elevations of the major drainages in the NSA Memphis area to the potentiometric surface of the A1 aquifer indicates that the potentiometric surface of the aquifer is lower than streambed elevations, 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. Structural and depositional features that affect the occurrence of ground water in the shallow aquifer system include faulting, an erosional scarp, and windows in the confining units. The Memphis aquifer underlies the shallow aquifer system.
A three-layer, quasi-three-dimensional, steady-state numerical model of the shallow aquifer system was constructed and calibrated to the potentiometric surface of the A1 aquifer using MODFLOW. Model calibration was facilitated using a parameter-estimation program. Values for model input parameters were based on the results of sediment core analyses, an aquifer test, well specific-capacity tests, and a parameter-estimation program. Results of numerical modeling support the proposed conceptual hydrogeologic model of the shallow aquifer system. The particle-tracking program MODPATH was used to simulate ground-water-flow direction and time-of-travel in the shallow aquifer system. An effective porosity of 25 percent produced typical ground-water-flow velocities on the order of 15 to 25 ft/yr in layer 1, which represented the A1 aquifer in the model. The average residence time of particles seeded in layer 1 was about 800 years.
Ground-water-flow paths and time-of-travel within the A1 aquifer was simulated at three sites: (1) the former N-6 hangar area; (2) the grassy area near SWMU 7; and (3) at SWMU 2. The close agreement between the estimated extent of migration of the contaminant plumes at the former N-6 hangar area and the grassy area near SWMU 7 and the 40-year travel distance predicted by the particle-tracking analyses suggests that the estimates of effective porosity and hydraulic conductivity for the A1 aquifer in these areas are reasonably accurate. Based on the results of particle-tracking analyses, the potential for contaminants to reach the Memphis aquifer in the next 100 years is negligible. Particle-tracking analysis of the SWMU 2 area indicates that ground-water time-of-travel to Big Creek Drainage Canal from SWMU 2 is generally less than 30 years; however, at present, there is no map of the extent of contaminant migration at SWMU 2 to compare to particle-tracking simulations. The calibrated flow model and the MODPATH program were not used to evaluate remedial designs at NSA Memphis. The results of the calibrated model and MODPATH analyses may simulate the expected contaminant migration if no remedial actions are undertaken.