Here is a link to the Wikipedia article. Summarizing, this is one of the world's largest aquifers overlying 174,000 square miles in the Great Plains. It supplies 82% of the drinking water of those living within it's boundaries. "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." And finally, the aquifer has a recharge rate of 6000 years. In other words i