User:Spacewanderer7/Ogallala Aquifer

(EDITING NOTE: The striked out text was from the original article, with no editing.)

The Ogallala Aquifer (oh-guh-LAH-lah) is a shallow water table aquifer surrounded by sand, silt, clay, and gravel located beneath the Great Plains in the United States. One of the world's largest aquifers, it underlies an area of approximately 174,000 sq mi (450,000 km²) in portions of eight states (South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas). It was named in 1898 by geologist N. H. Darton from its type locality near the town of Ogallala, Nebraska. The aquifer is part of the High Plains Aquifer System, and rests on the Ogallala Formation, which is the principal geologic unit underlying 80% of the High Plains.

Large scale extraction for agricultural purposes started after World War II due partially to center pivot irrigation and to the adaptation of automotive engines for groundwater wells. Today about 27% of the irrigated land in the entire United States lies over the aquifer, which yields about 30% of the ground water used for irrigation in the United States. The aquifer is at risk for over-extraction and pollution. Since 1950, agricultural irrigation has reduced the saturated volume of the aquifer by an estimated 9%. Once depleted, the aquifer will take over 6,000 years to replenish naturally through rainfall.

The aquifer system supplies drinking water to 82% of the 2.3 million people (1990 census) who live within the boundaries of the High Plains study area. The aquifer's geochemistry varies regionally as a result of changes in the bedrock geology and surface water systems, such as rivers, alluvial fans, and water table depths. Since 2008, the Ogallala Aquifer has been the subject of a political, scientific, and public safety-driven environmental controversy concerning the proposal of an extension to the Keystone XL Pipeline.

High Plains Geochemistry (to be added as a subheading in the General Characteristics section)

The water in the aquifer has total dissolved solids (TDS) values that vary from 148 parts per million (ppm) to nearly 4000 ppm. Total dissolved solids is the term used to describe the dissolved inorganic and organic chemicals in water. The United States guidelines for drinking water recommend TDS concentrations be less than 500 ppm to maintain consumer happiness. Approximately 85% of the water has a concentration of less than 500 ppm TDS. TDS values in drinking water that are below 30 ppm and above 1200 ppm can create an undesirable taste. Water surpassing 1000 ppm TDS comprises up only 3% of the water in the aquifer.

Water from the Northern High Plains aquifer in Nebraska has low TDS content, at approximately 148 ppm, but TDS content increases in Texas to the maximum value of approximately 3,970 ppm. An outlier to this trend is found in eastern Colorado, north of the Arkansas River in the middle latitudes of the aquifer, where TDS can be approximately 2,140 ppm.

These TDS values can be traced back to the bedrock geology of the region. Areas of the aquifer with higher TDS content have rocks that contain extensive, very soluble evaporite or salt deposits. Areas of the aquifer with lower TDS content are surrounded by sandy soil and sand dunes and have higher precipitation rates that drive recharge that dilutes the TDS in the aquifer.

The major water facies found in the aquifer are aligned from northwest to southeast along depositional systems associated with rivers. Regions with differing TDS content are dominated by differing hydrochemical facies, which is a term used to describe water type that is dominated by different major ions:

• Water with <250 ppm TDS in the Northern High Plains and western portion of the Southern High Plains is dominated by the Ca-HCO₃ facies
• Water with 250 ppm to 500 ppm TDS in the central and northern latitudes of the aquifer is dominated by the Na-SO₄ facies.
• Water in the southern portion of the aquifer is dominated by mixed facies, with the most prevalent ions being calcium, sodium, sulfate, and chloride

The structures within the aquifer also impact aquifer geochemistry. There are three major alluvial fan systems in the region. Alluvial fans are accumulations of sediment deposited when a river or stream loses energy. The alluvial fans are thick, permeable, and porous. The water in the areas of the aquifer associated with the alluvial fans is mostly the Ca-HCO₃ facies with a saturated thickness larger than 90 ft and consistent groundwater conditions due to preferential flow paths. Between the fans, the percentage of gravel and sand decreases and the aquifer thins. The hydrochemical facies in these areas vary and the aquifer has heterogeneous conditions due to inconsistent flow patterns. The water table in the regions between fans can be extremely shallow at times, which can lead to increased arsenic concentrations, nitrate concentrations, and TDS, as well as salinity, because the clay above the water table is unable to absorb these contaminants fast enough during recharge. The shallowest water table measurements are from less than 5 feet deep, which allows for direct evaporation from the aquifer and formation of saline lakes on the land surface. The waters near saline lakes have anomalous hydrochemical facies, being Na-Cl and Na-SO₄.