When waste materials are passed into water, living things have to compete for oxygen with the decaying wastes. Some kind of organism need more oxygen than others. Ecologists and environmentalists have observed that those organisms which need a lot of oxygen are quick to disappear from a polluted body of water.

The addition of nitrates and phosphate also indirectly decreases the amount of oxygen in water. Nitrates and phosphate are generally found in fertilizers. Phosphates are also added to laundry detergents. When these substances are washed from the land into streams and rivers, algae begin to multiply. With an increase in algae population decreases the amount of oxygen in the water. When algae and other organisms die, their decay uses up even more oxygen. Water in this condition will have a foul smell because of the large amount of decay occurring in it. Such water is useless to most living things including man.

Keeping water rich in oxygen is not the only way of keeping it healthy. The water must also be free of poisonous chemicals. Pesticides that do not break down into harmless materials are a source of poisonous chemicals. Heavy metals such as lead and mercury are another.

Water can be healthy for other living things yet it can be unsafe for humans to use. In most cases, when water is treated for humans to use, large particles and deadly micro-organisms are removed. So, let's be careful of things we dispose into our water, because they come back anyways as harmful into the things we eat, and the water we drink. Poisonous chemicals, however, usually pass through water treatment processes as, dissolved salt.