Well, jar test in a 5 gal bucket revealed I needed ~1/5 tsp alum to clear the water in ~30 minutes. If I have the numbers right... that works out to 1/25 tsp alum/gal water, so for my 250K gal pond I need 10K tsp which I think is in the neighborhood of 130 lbs of alum (using .013 lb/tsp for table salt). This also agrees pretty well with using ~100 lb for a 0.8 acre-ft pond.
So that all seemed good, unfortunately after posting yesterday we had a crazy downpour and washed a whole bunch of new runoff (that I'm working to control) into the pond... in the end I'm back where I started and will need to do this again! Ugh.
