Water pollution is thought to be caused only by man-made chemicals from manufacturing plants. The fact is that there are several causes of water pollution, some of which are natural causes. The major categories of water pollution sources are: direct source and indirect source.
Direct sources of water pollution come from sewerage systems, factories, treatment plants, cargo ships and boats where the pollutant flows directly to the body of water. Indirect sources of water pollution come from the atmosphere that becomes rainwater, or groundwater systems. For example, when chemical fertilizers and pesticides are used in farms, some of these seep in the soil, and some evaporate in the atmosphere. When it rains, the chemicals are already part of it, and it becomes part of the run off from the soil. Pollution from ships also directly gets into contact with the sea. Cargo ships that travel for months spill some wastewater in the sea. Oil spills also pollute the sea and greatly affect the ecosystem since oil floats on water, and it blocks oxygen and sunlight. It also contains heavy metals that poison aquatic animals.
The causes of water pollution, whether transmitted to bodies of water directly or indirectly, are categorized as: pathogens, contaminants, and thermal pollution.
Pathogens are bacteria and other microorganisms that thrive and multiply in water systems, some of which are harmful when they multiply excessively. Some pathogens do not cause disease, but they prevent other organisms to thrive, especially in bodies of water where there is an imbalance of growth of algae. They affect the water ecosystem resulting in water pollution. Sewerage systems that release untreated water because of rains and flooding also contain a lot of pathogens.
Contaminants are organic or inorganic substances that are not naturally found in a water system, but become part of it because of man made activities such as run offs from homes, factories, by-products from treatment plants, including water treatment by-products. Organic water pollutants include products such as detergents, insecticides, gasoline and oil. Other organic pollutants include plant waste and tree debris. Inorganic water pollutants include chemical waste, heavy metals and sediments from mining and quarrying sites and riverbanks. Water run offs from agricultural land also add to water pollution, especially because rain washes off pesticides from trees and crops. There are also large items that pollute the water such as trash, plastics and wreckages.
Thermal pollution is caused by the drastic change of water temperature because of human practices. This is mainly caused by factories and power plant facilities that use the body of water adjacent to them as a coolant for their machines. The water becomes warmer than usual, which drives fishes and other water animals away, and encourages the abnormal growth of certain water plants. A sudden drop in water temperature, on the other hand, may be caused by the release of water from dams to warmer rivers. These practices, however, cannot be avoided.
Sewerage and wastewater treatments are being carried out to lessen the impact on water pollution.