tz666,

I'd suggest planting some rye or other grass to reduce erosion. I doubt 150 pounds of alum will make a noticable improvement. You will most likely need 400-500 pounds. Unless over-treated, I've seen no evidence that alum is cummulative in the water. I have had several people tell me that they added multiple "less-than-needed" applications (all with little or no clearing happening) that totaled more than the originally required application would have been. Once the larger applications were made, clearing occured.