Hydrogeology and Hydrochemistry of the Ogallala Aquifer, Southern High Plains

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Nativ, Ronit

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The Ogallala aquifer, which underlies the Southern High Plains, consists of the saturated sediments of the Ogallala Formation (Neogene). It serves as the main source of water for the High Plains of Texas and New Mexico but has been severely depleted due to extensive pumpage. The hydrology and hydrochemistry of the aquifer are influenced by the surface topography of the underlying formations and the thickness and permeability of formation deposits.

Two distinct hydrogeologic provinces were observed. The first province, located along paleo valleys filled with coarse fluvial sediments, exhibits increased formation thickness and saturated section, as well as higher porosities and hydraulic conductivities. Ground-water flow lines within this province follow the orientation of the paleo valleys. The hydrochemical composition in this province remains relatively constant, characterized by Ca-HCO3 to mixed-HCO3 water, depleted in 0180, δD, and tritium.

In contrast, the second hydrogeologic province features thinner and less permeable formations, primarily composed of fine-grained eolian elastics. Ground-water discharge from aquifers in the Cretaceous contributes to the hydrochemical facies, resulting in isotopic compositions different from those of the first province. Cross-formational movement of water and low permeability in the Ogallala Formation in these areas lead to varying hydrochemical facies and isotopic compositions.

Secondary factors influencing the chemical composition of Ogallala Formation ground water include contamination from evaporating saline lakes, agricultural chemicals and fertilizers, and oil field brines. The impact of these chemicals may increase in the future as contaminants continue to move through the unsaturated zone toward the water table.


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