The geologic formations that crop out near Naval Air Station Chase Field and Naval Auxiliary Landing Field Goliad military bases consist of fluvial to fluvial-deltaic sediments of Tertiary and Quaternary age. These formations include the Fleming and Goliad Formations of Miocene age, Lissie Formation of Pleistocene age, fluvial terrace deposits of Pleistocene to Holocene age, and alluvium of Holocene age. The lithology of these formations consists of sand, sandstone, silt, and clay, with lesser amounts of gravel and caliche in the outcrops.

The freshwater aquifers underlying the study area are the unconfined Evangeline (watertable) aquifer, comprising the upper sandy parts of the Fleming Formation and Goliad Formation, and confined Fleming aquifers, comprising the thick sandstone beds of the Fleming Formation. Both military bases withdraw potable water from one of the confined aquifers. At Naval Air Station Chase Field, the transmissivity and storativity of the confined aquifer where the base withdraws its public water supply are 1,060 feet squared per day and 1.2x10^-4, respectively, as computed from the results of a 74-hour constant-discharge aquifer test. Selected water-quality field measurements of specific conductance, pH, and temperature indicate that each of the three aquifers at Naval Air Station Chase Field are somewhat insulated from one another by the intervening confining units.

Large vertical hydraulic-head gradients are present between the unconfined Evangeline aquifer and confined Fleming aquifers at Naval Air Station Chase Field and Naval Auxiliary Landing Field Goliad. These gradients, together with the results of the aquifer test at Naval Air Station Chase Field and assumed characteristics of the confining units, indicate that downward flow of ground water probably occurs from the water-table aquifer to the underlying aquifers. The rate of downward flow between the two confined Fleming aquifers (from A-sand to B-sand) can be approximated using an estimate of vertical hydraulic conductivity of the intervening confining unit obtained from assumed storage characteristics and data from the aquifer test. Under the relatively high vertical hydraulic-head gradient induced by the aquifer test, ground-water movement from the A-sand aquifer to the B-sand aquifer could require about 490 years; and about 730 years under the natural gradient. Future increases in ground-water withdrawals from the B-sand aquifer might increase downward flow in the aquifer system of the study area.