I was going to suggest throwing some of that sand/dirt in the bottom of your pool. There has to be something in the dirt, everytime we dig a pond it gets hammered with tracks. My friend has property in W WI with some beautiful springs coming right out of the rocks, there has never been any sign the deer use them. He has a 3/4 mile easement to the back with some potholes, after a rain there is tracks all around them, they prefer drinking that dirty muddy water. I've also seen a doe drink out of a 5 gallon bucket that I was using a seat for turkey hunting in spring. I cant believe it was drinking that nasty water that was green from all the algae. I guess she preferred that over the clean water in my pond a 150 yards away.