What are the floaties in my water bottle?

I have nalgene water bottles. I fill them about 1/4 full and freeze that, then add more water so my water stays cold. The water I use is spring water from a big 5 gal jug which is on a water cooler. When I go to drink it I see little white things floating around in my water which grosses me out. I rinse my bottle after every use and I notice these before I even have the first drink, so its not backwash. Anyone know what this is? Is it some wierd reaction from freezing? Should I be grossed out and toss the water out or not? Thanks!