Which body of water, supplying much of the irrigation and drinking water to many of the Great Plains states, is a pivotal concern in a current environmental battle?

The picture above is simply a rough map of the Great Plains.

The Ogallala Aquifer is a shallow water table aquifer underlying 8 states of the Great Plains. 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.