Dirty Water: Pollution Problems Persist

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.)

Trends in phosphorous concentrations in the United States showed greater improvement, with five times more states showing downward trends than upward trends. Decreases were more likely to be found in the East, Midwest, and the Great Lakes regions, while the majority of increases occurred in the Southeast (Smith et al. 1994:124).

The decreased concentrations of phosphorous in streams and rivers in the United States is attributable to reduced phosphorous in laundry detergents and improved controls in wastewater treatment plants. The increased number of sewage treatment plants has also reduced the amount of nitrogen in the form of ammonium, which is toxic to fish. However, the sewage treatment process converts ammonium to nitrates that are still released into waterways. Thus, the greater number of sewage treatment facilities has not necessarily decreased the total amount of nitrogen flowing into waterways (Mueller and Helsel 1996).

Groundwater Contamination
Surface waters like streams and lakes are not the only water sources that suffer from pollution. Groundwater aquifers, which are critical sources of both drinking water and irrigation water, are also affected. The major causes of groundwater pollution are leaching of pollutants from agriculture, industry, and untreated sewage, as well as saltwater intrusion caused by overpumping.

Once pollutants enter a groundwater aquifer, the environmental damage can be severe and long lasting, partly because of the very long time needed to flush pollutants out of the aquifer (UNEP 1996:14). Because groundwater is primarily used for drinking water, pollution from untreated sewage, intensive agriculture, solid waste disposal, and industry can cause serious human health problems (Shiklomanov 1997:42).

Global data on the quality of groundwater resources are lacking. Even where available, data usually are not comparable because of the different measures and standards used, which vary by country (Shiklomanov 1997:42; Scheidleder et al. 1999:11; S. Foster, personal communication, 2000). However, there is evidence that groundwater contamination from fertilizers, pesticides, industrial effluents, sewage, and hydrocarbons is occurring in many parts of the world.

As with surface waters, nitrate pollution is one of groundwater's most serious threats. In general, the risk of nitrate pollution for groundwater supplies is directly related to the amount of fertilizers or other nitrogen inputs to the land, as well as the permeability of the soil. For example, half the groundwater samples in a heavily fertilized region of northern China contain nitrate levels above the safe limit for drinking water (Zhang et al. 1996:224). In the United States, where groundwater supplies drinking water for more than half the population, a preliminary analysis of nitrate contamination found that high nitrate concentrations are widespread in shallow groundwater aquifers in agricultural areas (USGS 1999:41). Groundwater pollution in Europe is similarly widespread (Scheidleder et al.1999).

Conclusion
Surface water quality has improved in most developed countries during the past 20 years, but nitrate and pesticide contamination remain persistent problems. Data on water quality in other regions of the world are sparse, but water quality appears to be degraded in almost all regions with intensive agriculture and rapid urbanization. Unfortunately, little information is available to evaluate the extent to which chemical contamination has impaired the health of freshwater ecosystems. However, incidents of algal blooms and eutrophication are widespread in freshwater systems all over the world—an indicator that these systems are profoundly affected by water pollution. In addition, the massive loss of wetlands at a global level has greatly impaired the capacity of freshwater systems to filter and purify water. Groundwater quality suffers from many of the same pollution problems as surface waters and faces the additional challenge of being very difficult to restore once the underlying aquifer is contaminated.