Water pollution is one of the most serious environmental problems. It occurs when water is contaminated by such substances as human and animal wastes, toxic industrial chemicals, agricultural residues, oil and heat. Most of our water bodies— rivers, lakes, seas, oceans, estuaries and underground water sources are gradually becoming polluted. In the course "Covered so far, you have seen how deforestation, urbanization, intensive agriculture and industrialization have caused pollution of water bodies. You are familiar with various aspects of air pollution, including harmful effects of acid rain. In this unit we will summaries various types of water pollution caused by different pollution sources, and also explain some important concepts such as point and non-point sources, biological oxygen demand self-purification capacity of natural waters, infiltration of pollutants to groundwater, thermal pollution and pollution caused by accidents in oil tanker ships(oil spills). In the next unit you will read about the degradation of land resources and terrestrial features of earth. Polluted water may look clean or dirty, but it invariably contains germs, chemicals or other materials that can cause inconvenience, illness or death.