WBBSE Class 7 Geography Chapter 7 Water Pollution Introduction
197 percent of the water on Earth is saline seawater, 2 percent is trapped in ice caps and only 1 percent remains in the rivers, lakes, underground aquifers, and other fresh water bodies. Water pollution occurs when any chemical or organic substances, toxic substances or germs are mixed in the water due to different indiscriminate human activities and various natural causes and the water becomes unfit for humans and other animals to use.
When the used, warm and contaminated wastewater from thermal power stations, nuclear power plants, and factories mix directly in the ponds or rivers reducing the oxygen level of those waterbodies, thermal pollution occurs. As a result of the uncontrolled extraction of excess groundwater, arsenic compounds react with air to form toxic metallic compounds. These compounds contaminating water, are called arsenic contaminators.
Radioactive substances used in nuclear reactors, medical centers or scientific laboratories are dumped into the sea or river after use. This causes radioactive pollution. Wastewater from agricultural fields rich in nitrates, phosphates, and sulphates are carried to nearby ponds, lakes, rivers, etc.
As a result, the nutrient level of the water bodies increases, which promotes the excessive growth of algae, weeds, and water hyacinths, and subsequently aquatic animals die due to reduced levels of dissolved oxygen. This phenomenon is called eutrophication. In 1932, liquid waste containing methylmercury from a chemical factory off the coast of Minamata Bay in Japan was discharged into the sea. This methyl mercury got deposited in the body of marine fishes.
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Later, the inhabitants of the coast became infected with incurable diseases (deaths and disabilities) as a result of eating these fishes. This mercury poisoning is known as Minamata disease. When high doses of arsenic enter the human body through drinking water and food, it causes black sores on the palms of hands and soles of feet. It is known as Blackfoot disease.
When high doses of cadmium enter the human body through drinking water and food, the bones of the human body become brittle and cause intolerable pain. It is called Itai-Itai disease. Fluoride contamination causes problems in human teeth and bones. It is known as fluorosis disease. It is a disease caused by calcium deficiency. Waterborne infection can be prevented by drinking pure water, maintaining general rules of cleanliness, using lavatories, and disposing of garbage at the proper place.
Water can be purified in various ways, such as boiling water for 10 minutes at 100°C temperature, covering the turbid water for some hours so that most of the filthy things can be deposited at the bottom, mixing adequate amounts of chlorine with water, etc. A severe shortage of water all over the world can be compensated by the prevention of water pollution as well as minimizing excessive use of water and reusing it to the maximum capacity