Hydrogeology and Hydrochemistry of the Ogallala Aquifer, Southern High Plains, Texas Panhandle and Eastern New Mexico

The Ogallala aquifer, which underlies the Southern High Plains, consists of the saturated sediments of the Neogene Ogallala Formation. The aquifer is the main source of water for the High Plains of Texas and New Mexico and is being severely depleted by extensive pumpage for irrigation. Contamination from evaporating saline lakes, agricultural chemicals and fertilizers, and oil field brines is locally affecting the chemical composition of Ogallala ground water. The impact of chemicals and brines may increase in the future because many of these contaminants may still be moving downward through the unsaturated zone toward the water table. Furthermore, the aquifer overlies Permian evaporites that were being considered as a potential repository for the isolation of high-level nuclear wastes. All of these concerns necessitated the characterization of the hydrogeology and hydrochemistry of the Ogallala aquifer, which are controlled primarily by the subjacent paleotopography and by the thickness, permeability, and mineralogy of the Ogallala Formation.Two different hydrogeologic provinces were observed in the study area. Increased thickness of formation and saturated section, as well as higher porosity and hydraulic conductivity values, characterize the first province, which is located along paleovalleys filled with coarse fluvial sediments. These paleovalleys trend from northwest to southeast, and ground-water flow lines follow their orientations. Within this first province the hydrochemical composition is relatively constant (Ca-HCO3 to mixed-cation-HCO3 water, depleted in δ18O, δD, and tritium). In the second hydrogeologic province the formation is thinner and less permeable and sediments are mainly fine-grained eolian clastics. In this province ground water discharges into the overlying Ogallala Formation from the Cretaceous, Triassic, and Permian aquifers. In the second province cross-formational movement of water and low permeability in the Ogallala Formation result in variable hydrochemical facies and isotopic composition that differ from those of the first hydrogeologic province. Superimposed permeable strata in the Ogallala and underlying formations and hydraulic-head differences locally allow cross-formational flow, as documented by chemical and isotopic similarities of ground water in the study area.On the basis of both the rapid recharge rates calculated using tritium as a tracer and the slightly enriched values of δ18O and δD in ground water with respect to precipitation, it is assumed that the most likely method of ground-water recharge is focused percolation of partly evaporated playa-lake water.