How can the use of water by people affect water in the natural environment?

In agriculture, pesticides are often used and are washed off as return flow back into surface water bodies or infiltrate into the ground water. Many of these chemicals are harmful and can affect the growth and survivability or organisms who drink from the ground water ifit resurfaces into streams and ponds. Other the other hand, when the fertilizers used in agriculture washes off into oceans or streams, they cause an increase of growth. These are called algae blooms. It is a large body of growth that covers the surface of the water. It blocks sunlight to things living below it, causing them to die and use up the oxygen in the environment (when things die, they use oxygen and release carbon dioxide when they decompose). The overall phenomenon is called a dead zone.