Compositions including pellets of alum, a smectite mineral material, and optionally sodium or calcium carbonate, that can be delivered to a phosphate impoundment located at or near the bottom of a body of water. The pellets of alum, smectite mineral material and optionally sodium or calcium carbonate are dropped in a body of water, and the alum is released over time, as the pellet descends to the bottom of the body of water.