Water pollution remains one of the most visible and persistent signs of our impact on the natural world. Cleanup of some older pollutants has been offset by new contaminants that threaten freshwater ecosystems and foul our drinking water.
The sight and smell of grossly polluted waterways provided some of the original impetus to the environmental movement in the 1970s. Nearly a century before that, the dangers of polluted water to human health drove what became known as the "sanitary revolution" in Europe and the United States, emphasizing clean water supplies and sewer systems in cities. Today, despite progress in cleaning up waterways in some areas, water pollution remains a serious global problem, with impacts on the health of freshwater ecosystems and the human communities that rely on them for water supply.
The Changing Pollution Profile
Water pollution spans a wide range of chemical, physical, and microbial factors, but over the years the balance of major pollutants has shifted markedly in most industrialized countries. (See Figure 1 for a summary of major pollution sources and their effects.) One hundred years ago, the main water contamination problems were fecal and organic pollution from untreated human waste and the byproducts of early industries. Through improved treatment and disposal, most industrialized countries have greatly reduced the effects of these pollutants, with consequent improvements in water quality. Pollution laws and pollution control technologies have succeeded especially well in cutting emissions from concentrated "point sources" like factories and sewage treatment plants. For example, from 1972 to 1992 the amount of sewage treated at wastewater treatment plants in the United States increased by 30 percent, yet the organic pollution (measured as the Biological Oxygen Demand) from these plants dropped 36 percent (CEQ 1995:229).
Unfortunately, a new suite of contaminants from intensive agriculture and development activities in watersheds has kept the cleanup from being complete. In general, national water clean-up programs have not been effective in reducing "nonpoint" pollutants such as nutrients, sediments, and toxics that come in runoff from agriculture, urban and suburban stormwater, mining, and oil and gas operations (NRC 1992:47; EEA 1999:178).
Meanwhile, in most developing countries, the problems of traditional pollution sources like sewage and new pollutants like pesticides have combined to heavily degrade water quality, particularly near urban industrial centers and intensive agricultural areas. (Shiklomanov 1997:28; UNEP/GEMS 1995:6). An estimated 90 percent of wastewater in developing countries is still discharged directly to rivers and streams without any waste processing treatment (WMO 1997:11). (See Figure 1.)
Nutrient Pollution: The New Danger
The level of nutrients such as nitrates and phosphorous in freshwater ecosystems is a problem worldwide (Shiklomanov 1997:34-36). In most cases, the major cause of these contaminants is the increased use of manure and manufactured fertilizer in global agriculture. In the United States, for example, agriculture is the single greatest source of pollution degrading the quality of surface waters like rivers and lakes, with croplands alone accounting for nearly 40 percent of the nitrogen pollution and 30 percent of the phosphorous (Faeth 2000:6-7). (See Figure 2.)
Natural waters have very low concentrations of nitrates (a soluble from of nitrogen) and phosphorous, but nutrient levels increase with runoff from farm lands as well as from urban and industrial wastewater. Dissolved nutrients act as fertilizers, stimulating algal blooms and the eutrophication of many inland waters. This can rob the water column of dissolved oxygen, kill aquatic organisms, and degrade water quality. Dissolved nitrates in drinking water can also harm human health.
Data on nutrient trends in global waters are spotty and only...