The shallow aquifers described by Carmichael and others (1997) have been treated as separate units by previous investigators. For this report, however, based on the stratigraphic and structural correlations made using lithologic and geophysical logs, the aquifers were organized into a shallow aquifer system. The structure and thickness maps of the units (Carmichael and others, 1997), along with geophysical and lithologic logs, were used to determine the vertical distribution and thickness of the hydrogeologic units. The results of the aquifer test, well specific-capacity tests, and sediment-core analyses were used as estimates of the hydraulic properties of the hydrogeologic units. The potentiometric-surface maps for the alluvial-fluvial deposits aquifer were used to infer the importance of depositional, erosional, and structural features to the occurrence of ground water and the pattern of ground-water flow at NSA Memphis.

For this study, the shallow aquifer system is subdivided into five hydrogeologic units: (1) the alluvium-loess confining unit; (2) the A1 aquifer that, southwest of the erosional scarp (fig. 4b), includes the entire alluvial-fluvial deposits aquifer (fig. 15a), and northeast of the erosional scarp, also comprises sand lenses in the upper part of the Cockfield aquifer (fig. 15b); (3) the Cockfield confining unit; (4) the sand lenses within the lower part of the Cockfield aquifer; and (5) the Cook Mountain confining unit. The hydrogeologic framework of the shallow aquifer system is shown in table 5 and on section B-B' in figure 16. In the conceptual hydrogeologic model, the alluvial-fluvial deposits aquifer southwest of the erosional scarp is hydraulically connected with shallow sand units in the upper Cockfield Formation that are present northeast of the scarp.

Recharge to the shallow aquifer system occurs as infiltration from precipitation across the alluvium-loess confining unit. Ground-water discharge occurs primarily as leakage across the Cook Mountain confining unit to the Memphis aquifer. The pattern of ground-water flow is heavily influenced by faulting and buried river valleys. The shallow aquifer system appears to have limited interaction with the surface drainage network, except for short segments of Big Creek Drainage Canal, Casper Creek, and North Fork Creek along the southern boundary of NSA Memphis. In general, ground-water flow in the A1 aquifer is away from topographic highs, and radially away from the center of NSA Memphis. 

---