I checked all biomes with F1, F2, etc. and confirmed there was no aquifer, and yet I still ended up with a light aquifer (or something that matches what behavior I would expect from one) in this region. Pure vanilla, I did not use DFhack or any mods.

The aquifer is located to the north-west, near but not directly under the river. It has been smoothed over slightly, but there are a few tiles remaining. Wahh, fix!