I believe there is a law and some cert/licensing requirements within a certain zone, but the run-off can come from a lot farther away than just the property right on the lake. But it is more likely a combination of several things and that it doesn't ''flush out'' of the lake that easy.
What they are noting is the phosphorous levels are higher than the natural background.
I believe the alum treatment is to lock that phosphorous into the lake bottom. I think it combines and settles out.
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