1] Potassium metal burns in air on contact with water, making KOH, a very strong base. However, I suspect that you did not intend to ask about disposing of the metal....
2] Potassium is an essential nutrient for plants; fertilizers contain potassium compounds for this reason. Too little soil potassium is of great concern to farmers and gardeners [and anyone who eats, for that matter].
3] Intravenous KCl will stop the heart, so I suspect that adding a lot of potassium salts to a body of water would be bad.
