(7) Water

Stop Water Pollution! Save Our Mother Earth!

International Water Association (IWA)


How Can We Best Deal with Water Pollution

CONCEPT 11-5A Streams can cleanse themselves of many pollutants if we do not overload them.

CONCEPT 11-5B Preventing water pollution usually works better and costs less than trying to clean it up.

CONCEPT 11-5C Reducing water pollution requires preventing it, working with nature in treating sewage, cutting

Water Pollution Comes from Point and Nonpoint Sources

Water pollution is any chemical, biological, or physical change in water quality that harms living organisms or makes water unsuitable for desired uses. Water pollution can come from single, or point sources, or from larger and dispersed nonpoint sources. Point sources discharge pollutants at specific locations through drain pipes, ditches, or sewer lines into bodies of water. Examples include factories, sewage treatment plants (which remove some but not all pollutants), underground mines, and oil tankers.

Because point sources are located at specific places, they are fairly easy to identify, monitor, and regulate.

Most developed countries have laws that help control point-source discharges of harmful chemicals into aquatic systems. In most developing countries, there is little control of such discharges.

Nonpoint sources are scattered and diffuse and cannot be traced to any single site of discharge. Examples include runoff of chemicals and sediments into surface water from cropland, livestock feedlots, logged forests, urban streets, lawns, and golf courses. We have made little progress in controlling water pollution from nonpoint sources because of the difficulty and expense of identifying and controlling discharges from so many diffuse sources.

Agricultural activities are by far the leading cause of water pollution. Sediment eroded from agricultural lands is the largest source. Other major agricultural pollutants include fertilizers and pesticides, bacteria from livestock and food processing wastes, and excess salt from soils of irrigated cropland. Industrial facilities are another source of water pollution; they emit a variety of harmful inorganic and organic chemicals. Mining is the third biggest source. Surface mining creates major erosion of sediments and runoff of toxic chemicals.

Climate change from global warming can also affect water pollution. In a warmer world, some areas will get more precipitation and other areas will get less. Intense downpours will flush more harmful chemicals, plant nutrients, and microorganisms into waterways. Prolonged drought will reduce river flows that dilute wastes.

Major Water Pollutants Have Harmful Effects

The WHO estimates that 3.2 million people-most of them children younger than age 5-die prematurely every year by contracting infectious diseases spread by contaminated water or by having too little water for adequate hygiene. Each year, diarrhea alone kills about 1.9 million people-about 90% of them children under age 5-in developing countries. This means that diarrhea caused mostly by exposure to polluted water kills a young child every 17 seconds.

Streams Can Cleanse Themselves If We Do Not Overload Them

Rivers and streams can recover rapidly from pollution caused by moderate levels of degradable, oxygen demanding wastes and excess heat. They do so through a combination of dilution, biodegradation, and the presence of bacteria that break down the waste. But this natural recovery process does not work when streams become overloaded with pollutants or when drought, damming, or water diversion reduce their flows (Concept 11-5A). Likewise, these processes do not eliminate slowly degradable or nondegradable pollutants.

In a flowing stream, the breakdown of degradable wastes by bacteria depletes dissolved oxygen and creates an oxygen sag curve. This reduces or eliminates populations of organisms with high oxygen requirements until the stream is cleansed of wastes. Similar oxygen sag curves can be plotted when heated water from industrial and power plants is discharged into streams.

Water pollution control laws enacted in the 1970s have greatly increased the number and quality of wastewater treatment plants in the United States andmost other developed countries. Such laws also require industries to reduce or eliminate their point-source discharges of harmful chemicals into surface waters. This has enabled the United States to hold the line against increased pollution by disease-causing agents and oxygen-demanding wastes in most of its streams. It is an impressive accomplishment given the country’s increased economic activity, resource consumption, and population growth since passage of these laws.

But large fish kills and drinking water contamination still occasionally occur in parts of the United States and other developed countries. One cause of such problems is accidental or deliberate releases of toxic inorganic and organic chemicals by industries or mines.

Another is malfunctioning sewage treatment plants. A third cause is nonpoint runoff of pesticides and excess plant nutrients from cropland and animal feedlots. In most developing countries, stream pollution from discharges of untreated sewage and industrial wastes is a serious and growing problem. According to a 2003 report by the World Commission on Water in the 21st Century, half of the world’s 500 rivers are heavily polluted, most of them running through developing countries. Most of these countries cannot afford to build waste treatment plants and do not have, or do not enforce, laws for controlling water pollution.

Industrial wastes and sewage pollute more than two-thirds of India’s water resources and 54 of the 78 rivers and streams monitored in China. Only about 10% of the sewage produced in Chinese cities is treated and 300 million Chinese-an amount equal to the entire U.S. population-do not have access to drinkable water. In Latin America and Africa, most streams passing through urban or industrial areas suffer from severe pollution.

Major Water Pollutants and Their Sources

Type/Effects - Examples - Major Sources

Infectious agents cause diseases - Bacteria, viruses, parasites - Human and animal wastes Oxygen-demanding wastes deplete dissolved oxygen needed by aquatic species -  Biodegradable animal wastes and plant debris - Sewage, animal feedlots, food processing facilities, pulp mills

Plant nutrients cause excessive growth of algae and other species growth - Nitrates (NO3-) and phosphates (PO4 3-) - Sewage, animal wastes, inorganic  fertilizers.

Organic chemicals add toxins to aquatic system - Oil, gasoline, plastics, pesticides, cleaning solvents - Industry, farms, and households.

Inorganic chemicals add toxins to aquatic system - Acids, salts, metal compounds - Industry, households, surface runoff.

Sediments disrupt photosynthesis, food webs, and other processes - Soil, silt - Land erosion

Thermal pollution makes some species vulnerable to diseases- Heat - Electric power and industrial plants.

Low Water Flow and Too Little Mixing Makes Lakes Vulnerable to Water Pollution

In lakes and reservoirs, dilution of pollutants often is less effective than in streams for two reasons. First, lakes and reservoirs often contain stratified layers that undergo little vertical mixing. Second, they have little flow. The flushing and changing of water in lakes and large artificial reservoirs can take from 1 to 100 years, compared with several days to several weeks for streams.

As a result, lakes and reservoirs are more vulnerable than streams are to contamination by runoff or discharge of sediment, plant nutrients, oil, pesticides, and toxic substances such as lead, mercury, and selenium.

These contaminants can kill bottom life and fish and birds that feed on contaminated aquatic organisms. Many toxic chemicals and acids also enter lakes and reservoirs from the atmosphere.

Eutrophication is the name given to the natural nutrient enrichment of a shallow lake, estuary, or slow-moving stream, mostly from runoff of plant nutrients such as nitrates and phosphates from surrounding land. An oligotrophic lake is low in nutrients and its water is clear. Over time, some lakes become more eutrophic as nutrients are added from natural and human sources in the surrounding watersheds.

Near urban or agricultural areas, human activities can greatly accelerate the input of plant nutrients to a lake-a process called cultural eutrophication. It is mostly nitrate- and phosphate-containing effluents from various sources that cause this change. These sources include runoff from farmland, animal feedlots, urban areas, chemically fertilized suburban yards, and mining sites, and from the discharge of treated and untreated municipal sewage. Some nitrogen also reaches lakes by deposition from the atmosphere.

During hot weather or drought, this nutrient overload produces dense growths or “blooms” of organisms such as algae and cyanobacteria and thick growths of water hyacinth, duckweed, and other aquatic plants. These dense colonies of plant life can reduce lake productivity and fish growth by decreasing the input of solar energy needed for photosynthesis by the phytoplankton that support fish.

When the algae die, they are decomposed by swelling populations of aerobic bacteria, which deplete dissolved oxygen in the surface layer of water near the shore and in the bottom layer. This can kill fish and other aerobic aquatic animals. If excess nutrients continue to flow into a lake, anaerobic bacteria take over and produce gaseous products such as smelly, highly toxic hydrogen sulfide and flammable methane.

According to the U.S. EPA, about one-third of the 100,000 medium to large lakes and 85% of the large lakes near major population centers in the United States have some degree of cultural eutrophication. According to the International Water Association, more than half of the lakes in China suffer from cultural eutrophication. There are several ways to prevent or reduce cultural eutrophication. We can use advanced (but expensive) waste treatment to remove nitrates and phosphates before wastewater enters lakes. We can also use a preventive approach by banning or limiting the use of phosphates in household detergents and other cleaning agents and employing soil conservation and land-use control to reduce nutrient runoff.

There are also several ways to clean up lakes suffering from cultural eutrophication. We can mechanically remove excess weeds, control undesirable plant growth with herbicides and algicides, and pump air through lakes and reservoirs to prevent oxygen depletion, all of which are expensive and energy-intensive methods.

As usual, pollution prevention is more effective and usually cheaper in the long run than cleanup (Concept 11-5B). The good news is that if excessive inputs of plant nutrients stop, a lake usually can recover from cultural eutrophication.

Water in many of central China’s rivers is greenish-black from uncontrolled pollution by thousands of factories. Water in some rivers is too toxic to touch, much less drink. The cleanup of some modernizing Chinese cities such as Beijing and Shanghai is forcing polluting refineries and factories to move to rural areas where two-thirds of China’s population resides. Liver and stomach cancer, linked in some cases to water pollution, are among the leading causes of death in the countryside. Farmers too poor to buy bottled water must often drink polluted well water.

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