I’ve just loaded a couple of maps of the Ogallala/ High Plains Aquifer. This second one shows the counties.
The Ogallala Aquifer, also known as the High Plains Aquifer, is a vast yet shallow underground water table aquifer located beneath the Great Plains in the United States. It is one of the world's largest aquifers and covers an area in portions of eight states, which include Colorado, South Dakota, Nebraska, Wyoming, Kansas, Oklahoma, New Mexico, and Texas.
Noticing Nebraska almost consumes the state except closer to the Missouri River of which is not necessary (had to comment since that is where I grew up)!