Increasingly, international development agencies are urging that developing countries devote more attention to protecting and improving water quality (165, 198). The developed world also must spend more and do more to clean up degraded waterways, or economic development will stall and the quality of life will fall (63, 85).

Agriculture is the biggest polluter, even more so than industries and municipalities. In virtually every country where agricultural fertilizers and pesticides are used, they have contaminated groundwater aquifers and surface waters. Animal wastes are another source of persistent pollution in some areas. The water that goes back into rivers and streams after being used for irrigation is often severely degraded by excess nutrients, salinity, pathogens, and sediments that often render it unfit for any further use, unless cleaned typically at great expense—by water purification plants (102).

In the US, agricultural chemicals, eroded sediment, and animal wastes have fouled over 173,000 miles of waterways. Farming is said to be responsible for 70% of current water pollution in the US (123). In India, which depends on irrigated agriculture for food supplies, more than 4 million hectares of high-quality land have been abandoned because of salinization and waterlogging caused by too much irrigation (164).

The world's tremendous output of pollutants challenges the capacity of waterways to assimilate or flush away pollution. A saying among water engineers is "the solution to pollution is dilution." This truism is taking on frightening dimensions. Each year roughly 450 cubic kilometers of waste water are discharged into rivers, streams, and lakes. To dilute and transport this dirty water before it can be used again, another 6,000 cubic kilometers of clean water are needed—an amount equal to about two-thirds of the world's total annual useable fresh water runoff (221). If current trends were to continue, the world's entire stable river flow would be needed just for pollutant transport and dilution by the middle of the next century, according to an estimate by the UN Food and Agriculture Organization (187).

Industrialized countries. Europe and North America confront enormous water pollution problems. Over 90% of Europe's rivers have high nitrate concentrations, mostly from agrochemicals, and 5% of them have concentrations at least 200 times greater than nitrate levels naturally occurring in unpolluted rivers (203). In Poland three-quarters of the country's river water is too polluted even for industrial use.

Over half of Europe's lakes are eutrophied from a glut of agricultural and municipal nutrients (202). Eutrophication is a process that occurs when excess nutrients stimulate the growth of algae, which, when they die and decay, rob the water of oxygen. In Europe eutrophication has become one of the most serious problems affecting freshwater and near-shore marine environments (1).

Groundwater pollution in Europe is worsening. Within 50 years some 60,000 square kilometers of groundwater aquifers in western and central Europe are likely to be contaminated with pesticides and fertilizers (125). Of Hungary's 1,600 well fields tapping groundwater, 600 of them are already contaminated, mostly with agricultural chemicals (83). In the Czech Republic 70% of all surface waters are heavily polluted, mostly with municipal and industrial wastes. Some 30% of the country's rivers are so fouled with pollutants that no fish survive (121). In the US, 40% of all surface waters are unfit for bathing or fishing, and 48% of all lakes are eutrophied (50, 202).

Developing countries. Pollution is a vexing problem in countries where the population is growing rapidly, development demands are great, and governments have other investment priorities. In developing countries, on average, 90% to 95% of all domestic sewage and 75% of all industrial waste are discharged into surface waters without any treatment whatsoever (6, 25). Consider these examples:

• All of India's 14 major rivers are badly polluted. Together they transport 50 million cubic meters of untreated sewage into India's coastal waters every year. The city of New Delhi dumps 200 million liters of raw sewage and 20 million liters of industrial wastes into the Yamuna River every day as the river passes through the city on its way to the Ganges (81).
• In Thailand and Malaysia water pollution is so heavy that rivers often contain 30 to 100 times more pathogens, heavy metals, and poisons from industry and agriculture than is permitted by government health standards (125).
• Over three-quarters of China's 50,000 kilometers of major rivers are so filled with pollution and sediment that they no longer support fish life (1). In 1992 China's industries discharged 36 billion metric tons of untreated or partially treated effluents into rivers, streams, and coastal waters (208). In 1986, along sections of the Liao River, which flows through a heavily industrialized part of northern China, almost every aquatic organism within 100 kilometers was killed when over 1 billion tons of industrial wastes were dumped into the river in a period of three months (90).
• In greater São Paulo, Brazil, 300 metric tons of untreated effluents from 1,200 industries are dumped into the Tiete River every day as it flows through the city. As a result, the river contains high concentrations of lead, cadmium, and other heavy metals. The city also dumps some 1,000 metric tons of sewage into the river each day, of which only 12% gets any treatment whatsoever (203).
• Karachi, Pakistan's largest city, has completely overwhelmed the capacity of its outdated sewage treatment plants. Because of frequent breakdowns and clogged sewage pipes, these plants often operate at no more than 15% of capacity. The great majority of all sewage water leaks out into the surrounding soil, contaminating the wells used by city residents for drinking water (151).

Furthermore, pollutants such as sulfur dioxide and oxides of nitrogen, which combine in the atmosphere to form acid rain, have had pervasive effects on both freshwater and land ecosystems. Acid rain lowers the pH of rivers and streams. Unless buffered by calcium (as contained in limestone), acidified waters kill many acid-sensitive fish, including salmon and trout. In the soil, acids can release heavy metals, such as lead, mercury, and cadmium, that then percolate into waterways (88).

Some of the worst pollutants are synthetic chemicals. Some 70,000 different chemical substances are in regular use throughout the world (147). Every year an estimated 1,000 new compounds are introduced (207). Many of them find their way into rivers, lakes, and groundwater aquifers. In the US alone, more than 700 chemicals have been detected in drinking water, 129 of them considered highly toxic (112).

A number of synthetic chemicals, particularly the group known as persistent organic pollutants (POPs), which includes halogenated hydrocarbons, dioxins, and organochlorines such as DDT and PCBs, are long-lived and highly toxic in the environment (198). They do not break down easily under natural processes and thus tend to accumulate up the biological food chain, until they pose risks to human health. For example, Beluga whales swimming in the highly polluted St. Lawrence River, which connects the Atlantic Ocean to North America's Great Lakes, have such high levels of PCBs in their blubber that, under Canadian law, they now qualify as "toxic waste dumps" (147). Indigenous communities that once hunted these whales no longer are permitted to take any because of the health risks.