There are many different definitions of water pollution, but perhaps the best explanation is the most basic. Water pollution describes a situation in which there has been such a large accumulation of one or more substances in the water that life in the surrounding environment experiences problems or harm. This description clearly shows that water pollution is first and foremost about quantities: that is, the quantity of a polluting substance versus the volume of water that the substance is contaminating. For example, if a ship spills a small quantity of a toxic chemical into the ocean, the impact will be far less severe than if the same amount of the chemical is introduced into a small lake.
This concept of quantity is important to understand because the fact is that virtually any substance—whether it be chemical or biological—can eventually become a water pollutant if enough of it builds up. Some of the most common causes of water pollution include:
The disposal of sewage waste is a major global problem. Approximately 2.5 billion people, according to estimates from the World Health Organization, do not have access to proper sanitation and hygienic toilet facilities. Sewage waste affects the immediate environments of 40% of the world’s population, leading to huge problems with water-related illnesses such as diarrhea. Even in industrialized countries with developed sewage systems, that waste still has to go somewhere, and the practice of dumping sewage into the sea remains common. In the 1990s, for example, New York City was dumping 5 million tons of sewage by barge every year, while some of the islands in the English Channel were revealed to still be dumping thousands of tons of raw sewage into the sea every day as recently as 2012.
Nutrients such as nitrogen and phosphorus that are found in chemical fertilizers (and, indeed, in sewage) are substances that help plants and animals to grow, but the problem is that they are often released into the water in quantities that are far too great for the natural environment to cope with. An overdose of nutrients in lakes, rivers, or oceans can lead to a huge increase in the growth of plankton or algae (this scenario is referred to as a “harmful algal bloom” or sometimes “red tide”). The problem is that as these life forms grow, they remove oxygen from the water and thus kill off any other species in the environment, leading to the creation of so-called “dead zones.”
Waste water, chemicals that are either washed down drains or discharged from factories, are a huge contributor to ocean pollution. Anywhere from between 5 billion and 10 billion tons of industrial waste is generated globally every year, and a significant percentage of this untreated waste is pumped directly into oceans, rivers, and waterways. And factories are not the only ones to blame. The average household pours all kinds of chemicals down its drains and toilets, ranging from dishwasher detergent to garden pesticides. Highway runoff is yet another wastewater source: chemicals ranging from spilled fuel to brake fluid are washed from highways into drains and rivers whenever it rains.
Oils spills are one of the most graphic examples of water pollution. Most people can call to mind images of the huge slicks of oil covering the surface of the ocean after a tanker accident, for example. However, while such spills are certainly catastrophic for marine environments given the quantity of oil they release all at once, it’s important to remember that tanker accidents are responsible for only about 12% of oil in the oceans. A staggering 70% of ocean oil pollution actually comes just from routine shipping and from oil poured down drains on land.
Plastic is a unique kind of water pollutant in that it doesn’t disperse in water the same way that toxic chemicals do, but it nevertheless poses a major hazard to marine creatures (of the world’s seabird species, for example, about half are known to have consumed plastic or plastic residue). What makes plastic especially problematic is that it builds up in such huge quantities due to its ubiquitous use on land. Furthermore, the fact that plastic is both lightweight and sturdy means that it can travel huge distances across oceans. In addition, because most plastic is not biodegradable, it can survive in a marine environment for potentially hundreds of years.
Sometimes water pollution is caused not by chemical substances, but by biological species that have ended up in places where they do not belong. Invasive species are plants or animals from one region that have been directly or indirectly introduced into a new ecosystem. In this new environment, they have no natural predators, so they are able to multiply at an incredible rate and crowd out the species that were native to the ecosystem in the first place. Examples of invasive marine species include zebra mussels, which were introduced from Europe into the Great Lakes of North America through the flushing of ballast water from ships, and a type of Asian clam called Potamocorbula amurensis, which was introduced by ballast water into the San Francisco Bay.