Most of the water in the ogallala Aquifer came from glaciers that melted thousands of years ago. What is the Aquifers primary water source today?

The correct answer is - rainfall.

The Ogallala Aquifer is a very large aquifer. It traces its origins at the end of the last ice age, where the water from the melting glaciers filled it up. After that initial filling of from the water of melting glaciers, the Ogallala Aquifer has been reliant on the water from the rainfall to fill it, and replenish the lost water.

Since the humans started to use it though, this aquifer has been constantly experiencing decrease in its water levels. The reason for that is that the humans use more water from the aquifer than what the rainfall can replenish, and that is especially a big problem because the aquifer is located in an area where there isn't a lot of precipitation.