white "plume" when shocking???

[clueless]

ok, this is a first for me. got my water tested today, and unfortunately this am I rilled about 3" from my garden hose. when I got to the store, I found out that there is something bad happening with the city water, and it PUT contaminates (nitrates) into the pool. I now have to shock again to the tune of 22 lbs!!! I added my PH minus, and then my conditioner thru the skimmer, and started adding shock, and all of a sudden, I started getting white white WHITE "plumes" of water rising to the surface. I am a pool newbie, only had this thing about 3 yrs, but I have NEVER seen this before. I'm not even half thru the 22 bags, and I don't know if I should continue. The shock is fine until it falls about halfway down, then all of a sudden, this solid white plume rises up from the shock. Help!!!

[Buggsw]

Are you using a different brand/type of shock?

I vaguely remember a particular brand doing something like this in my pool a few years ago.