Water pollution is the introduction of "foreign" substances to a body of water. The foreign substances can take the form of chemical substances (e.g. petroleum products from oil spills/leaks), biological substances (e.g. sewage leaks/spills) or physical substances (e.g. debris, demolition wastes). The substances alter the quality of the body of water they entered and thus prevent the water from been useful for certain or in most cases, all purposes; the quality of water required for different purposes varies. For instance the quality of water required for drinking is different from that required for agricultural purposes.

Precipitation (as Rain water or snow) is the purest source of water. As precipitation interacts with the atmosphere (e.g. air, land), it gets polluted by the substances in the air and the land. Some natural and human activities also facilitate the pollution of the pristine precipitation. Some of the main causes of water pollution are presented below:

Sewage and Wastewater

Sewage and wastewater are sometimes used interchangeably. More precisely, sewage is used to refer to used water from toilets while wastewater refers to used water from other locations in the house. Whichever way you define it, used water is dirty and contains toxic substances (human faeces, detergents, wash soaps etc) that should not mix with clean water. Whenever this mixture takes place, the clean water is said to be contaminated or polluted. There have been cases of water contaminated by faecal coliforms in this way.

Instances of water pollution through sewage/wastewater occur when there are sewer leaks or damages releasing sewage/waste water that makes its way to underground water or a surface lake/stream.

Industrial waste

Wastewaters are also generated in the industry in significantly high amounts. Besides, the industries generate processed water that contains tons of wastes. Examples are tailing water (from mining/milling operations) which contains lots of metals. Fresh water bodies coming into contact with industrial wastes will be polluted.

Marine Dumping

Disposal of wastes (industrial and domestic) by dumping in large bodies of water was widely practiced in the past and it is still being practiced today in some places. Marine dumping of wastes (sewage and chemical wastes) is widely practiced because it takes the wastes out of sight and provide temporary relief. Our knowledge of the self-purification potential of the water bodies makes many homes and industries to continue to use marine waste dumping. It is assumed that wastes dumped into water bodies (lake, river ocean etc) will eventually "assimilate" into the environment.

Unfortunately, wastes dumped in a water body never disappears. It can be carried away by water flow, but it will reappear at the down stream in one form or the other - as the waste itself, dispersed / diluted in the water or transformed chemically into other forms. Furthermore, the self purification potential of water bodies has got a limit. With too much biological load on the water body, it reaches a point that it cannot self-purify anymore - a state of Eutrophication.

Physical debris or solid waste in water also constitute navigational challenges.

Radioactive Wastes

There is yet no good approach for radioactive or nuclear waste disposal but disposal in water. Spent nuclear rods and other wastes from nuclear power plants are buried in ocean depths to isolate them from human contacts.

Oil Spills

Oil spills, during operations in offshore locations or transportation over water from offshore oil fields are common occurrences. Globally, tremendous number of oil spills into water bodies occur per annum. Only few highly catastrophic spills that occur in some civilized part of the world, make it to the news headlines / breaking news. Oil exploration or extraction in oil producing developing countries such as Gabon, Nigeria, Venezuela etc do not follow due diligence and lead to oil spills and the messing up of drinking water bodies, which may go unreported.

Oil spills pollute water bodies and causes the water bodies unusable for drinking and several other purposes. Marine animals are also killed or rendered sick.

Some examples of previous major spills that caused the pollution of large bodies of water are:

• Exxon Valdez Spill ~10.8 m gallons
• Gulf Oil War Spill ~380-520 m gallons
• Gulf of Mexico Spill ~200-250 m gallons

Ogoniland, Delta Region Nigeria - 25-30 years clean up & cost billions of Dollars

Spills can be an unavoidable /unexpected occurrence or due to human error or both. Spills occur, most frequently during fuel transfer or machinery operation. Consequently, standard operating procedures, emergency response plans and other similar tools ought to be developed prior to the beginning of operations.

Storage Tanks (underground and above ground) and pipeline leakages

Leaky fuel tanks or pipelines containing petroleum products release substances to the land surface or underground (in cases when the tank or the pipelines are buried underground). These substances can be carried by surface runoff to surface water bodies or they can flow through the underground regime into the water bearing aquifers underground, thus contaminating the surface and underground water.

Leaks from tanks and pipes could occur due to damages on the tank/pipe due to corrosion/rusting, improperly constructed tanks/pipes etc.

Leaks or spills occur at manufacturing facilities, gas stations, railway stations etc.

Atmospheric Deposition

Atmospheric deposition occurs when substances emitted to the air (e.g. vehicular emissions) settled back to the land surface or to a body of water. These susbtances pollute water and introduce chemicals that are dangerous to aquatic organisms in the water. Substances that could be deposited in water bodies through atmospheric deposition include: Toxins (e.g. dioxins, furans, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons).

Other Sources of Water Pollution

These are some of the causes of water pollution. There are several other causes. Some others that have not been covered specifically above are:

• leachates/runoffs from landfills or solid waste dumps;
• leachate from Septic Tanks;
• leachates from tailing ponds / sludge ponds of refineries;
• contamination from land spreading of sewage or sewage sludge.