Like you stated, the water cycle is a closed sytem.Therefore, it's never ending it will keep on going, and repating it's self forever.So we need to keep it clean because, we use water for food, dishes, etc which is a pro.The con of trying to keep water is fresh is the fact we bathe, and clean with water. Even though we bathe, and stuff with water we can try to keep water clean.Even though water goes through a process and people say the water system cleans water doesn't mean it's true.The best ting we can do to keep free from population is use less water, and try keeping large, small bodies of water clean.