Hydrogeology and geochemistry of the Ogallala aquifer, Southern High Plains

Abstract

The Ogallala aquifer, which underlies the Southern High Plains, consists of saturated sediments of the Ogallala Formation (Neogene) that are deposited mainly as a set of humid-type alluvial fans derived from the Rocky Mountains. The Ogallala aquifer, the main source of water for the High Plains of Texas and New Mexico, has been severely depleted by extensive pumpage. The hydrology and geochemistry of the aquifer are controlled by the surface topography of the underlying formations and by the thickness and permeability of the alluvial-fan deposits. Axes of three major alluvial fans trend from northwest to southeast; the fans have increased thickness and permeability compared with interfan areas. Flow lines follow the orientation of the alluvial fans. Along these major axes, the saturated section is thick and the chemical composition is relatively constant (CaHCO 3 to mixed-cationHCO 3 water, depleted in δ 18O, δD and tritium). External effects on the chemical composition are less pronounced. The aquifer is thinner and less permeable in areas between major fan axes and along the Caprock Escarpment. In these areas, groundwater can discharge from aquifers in the Cretaceous, Triassic and Permian formations into the Ogallala Formation. Cross-formational movement of water and low permeabilities in the Ogallala Formation result in varying hydrochemical facies and in isotopic compositions that differ from those of the major depositional areas. Areas where upward cross-formational flow occurs are evident by the presence of permeable contacts between the Ogallala and the underlying formations, water-level head differences and chemical and isotopic similarities. Secondary factors that locally affect the chemical composition of Ogallala Formation groundwater include contamination from evaporating saline lakes, agricultural chemicals and fertilizers and oil field brines. Impact of chemicals and brines may increase in the future because much of these contaminants may still be moving through the unsaturated zone toward the water table.