The effects of water pollution depend on locations and the types of pollutants present. Industrial wastes account for a large percentage of water pollution, including heavy metals, chemical toxins and petroleum products. Similar types of waste can also reach waterways as runoff, as rainwater washes surface pollutants into groundwater. While sewage contamination is a major problem in developing countries, developed countries like the United States face issues relating to agricultural runoff. Ocean pollution results from oil spills, plastic waste and polluted river flows.
Most living things require clean, fresh water for good health. In humans, polluted water leads to elevated levels of infectious diseases, including bacterial diseases such as typhoid, cholera and dysentery. Viral and protozoal disease vectors also flourish in water contaminated by sewage. According to the World Health Organization (WHO), gastrointestinal infections kill 2.2 million people every year, mostly children in developing countries. Chemical wastes can result in a wide range of health problems for those exposed, including chemical poisoning, reproductive disorders, cancers and damage to the nervous system. Chemical wastes can also bioaccumulate, traveling through the food chain to contaminate human food supplies.
Pollutants leave rivers, lakes and oceans less capable of sustaining life. Organisms vary in terms of their abilities to tolerate pollution. Insects, amphibians, crustaceans, fish, arthropods, aquatic plants and corals can all experience drastic population declines when subjected to a polluted environment. When pollution reaches critical levels, ecosystems break down as individual species become unable to tolerate the introduced substances. Specific ecosystems that harbor considerable biodiversity, such as coral reefs, can be highly sensitive to polluted conditions.
Coastal environments are particularly prone to the negative effects of water pollution, particularly river outlets, deltas and estuaries. The areas where rivers and oceans meet are generally biologically rich. Deltas and estuaries often receive accumulations of river sediments, which can carry concentrations of pollutants. Agricultural runoff and other types of nutrient pollution cause algae blooms, which introduce toxins and deplete the water of oxygen. Depletion of oxygen at the mouths of major rivers can cause sizable dead zones, which are incapable of supporting marine life.