Aquifers of the Roswell groundwater basin are unconfined and confined types in Permian San Andres Formation and Artesia Group carbonates and evaporites, and the shallow unconfined Quaternary sedimentary and alluvial aquifer. The carbonate-evaporite aquifers were developed from solution by meteoric water, groundwater, the Pecos River, and its tributaries. The structural geology of the region includes Cenozoic folding and wrench faulting. Regional dextral strike-slip faults, <30 Ma to as young as 0.5 Ma, dominate the hydrogeologic framework of the groundwater basin. The faults created major lithologic and structural boundaries for the groundwater systems developed between them. The Roswell groundwater "basin" is actually a series of en echelon structural blocks with aquifers developed in erosion-beveled, fault-displaced Permian carbonates and evaporites partly covered by Quaternary sedimentary rocks and alluvium. The confined portions of the carbonate aquifers are in the San Andres Formation, the Artesia Group, or a solutional-karstic melange of the two. The Permian aquifers developed within each structural block exhibit different hydrochemical and hydrologic properties. The rock groups produce distinctive bulk element water chemistry signatures which are readily visible on ternary plots, Piper diagrams, and Fingerprint diagrams. San Andres Formation waters have high HCO₃⁻, intermediate SO₄²⁻ , and low Cl⁻ that demonstrate a preponderance of carbonates with some evaporites. Waters hosted by the Artesia Group are characterized by low HCO₃⁻, high SO₄²⁻ , and high Cl⁻that reflect evaporites with some carbonates. Quaternary alluvial aquifer waters show low Ca²⁺, low HCO₃⁻, with moderately high SO₄²⁻- and Cl⁻. Normative mineral reconstructions identify the lithologic combinations through which the waters flowed to acquire their present chemical characteristics. Plotted as charts the normative mineral reconstructions can be correlated as are electric well logs. Mineral stability diagrams support exchange by sodium liberation and calcium replacement in Na-smectite marine clays for altering the Ca⁺ - Na⁴ groundwater chemistry. Hydrochemical plots provide a robust means of identifying aquifer sources of groundwater and delineating their structural and stratigraphic boundaries. The work should be expanded to include more water analyses from each group, and as a means to identify unknows, such as the sources of water to the Pecos River.

Three aquifers of diverse lithology and hydraulic character form the ground-water reservoir in the Roswell basin. The main aquifer, the carbonate aquifer, is developed in carbonate rocks of Permian age. It is the source of about two-thirds of the ground water pumped in the basin and receives more than 90 percent of the recharge to the basin. The second most important aquifer is the shallow aquifer which lies near the Pecos River in beds of sand and gravel of both Permian and Holocene age. About one-third of the ground water pumped in the basin comes from the shallow aquifer. Prior to pumping, the main source of recharge to the shallow aquifer was probably ground water leaking upward from the carbonate aquifer. Since pumping began, the main source of recharge to the shallow aquifer is probably return flow of irrigation water pumped from the carbonate aquifer. Natural discharge of ground water from the shallow aquifer into the Pecos River causes a gain in th.e base flow of the Pecos River in the Roswell basin. The third aquifer, the shallow-artesian aquifer, is in red beds and evaporite beds of Permian age. This aquifer overlies the carbonate aquifer and underlies the shallow aquifer. The shallow-artesian aquifer acts as a minor aquifer and also as a semi-permeable unit which partly confines ground water in the carbonate aquifer. Vertical permeability of the shallow-artesian aquifer is variable and depends on the lithology and thickness of the aquifer. Hydraulic head in the shallow-artesian aquifer also varies quite widely depending on the depth to which a well penetrates the aquifer. All ground-water aquifers in the basin transect formational boundaries and are therefore not closely related to the named geologic formations. A flow net analysis of the carbonate aquifer and of the shallow aquifer imply that geologic structure is important in the movement of ground water in the basin by limiting the transmissivity of the shallow and carbonate aquifers, by forming the present pattern of surface water drainage, and possibly by the contamination of fresh ground water by highly saline ground water. The flow net analysis also shows areas of recharge to the shallow and carbonate aquifers, and areas where the carbonate aquifer looses water to the shallow-artesian aquifer and to the shallow aquifer.