Ground-water geochemistry of the Ogallala Aquifer in the Southern High Plains of Texas and New Mexico

Concentrations of calcium, magnesium, potassium, sulfate, and chloride ions in the ground water generally increase from the north to the south in the Ogallala Aquifer of the Southern High Plains of West Texas and eastern New Mexico. In the south, higher concentrations of solutes generally occur in areas which are underlain by Cretaceous rocks and are located beneath large saline lakes. The distribution of bicarbonate and flouride ions and also silica molecules are variable throughout the aquifer. High concentrations of nitrate ions usually occur south of the northern edge of the Cretaceous boundary and along the northeastern edge of the aquifer.

The primary sources of solutes in the ground water north of the Cretaceous subcrop are the weathering and dissolution of potassium feldspar, biotite, plagioclase feldspar, calcite, and dolomite. Minor amounts of sodium, sulfate, and chloride may have been added to the ground water by seepage of oil and gas field brines into the aquifer. It is proposed that ground water south of the Cretaceous subcrop obtains additional sodium, sulfate, and chloride ions from the mixing of saline Cretaceous water with the Ogallala water. The mixing between the two aquifers may result from a difference in head between the two aquifers or by diffusion. In these areas of mixing, the sodium ions from the Cretaceous water may be exchanging for calcium and magnesium ions on clay minerals in the Ogallala Formation. Some of the calcium ions produced from the exchange are probably combining with bicarbonate ions and precipitating calcium carbonate. Amorphous silica and flourite may be precipitating in several areas of the aquifer along with calcite and is probably limiting the concentrations of silica, flourine, calcium, and to a minor degree bicarbonate.