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World Resources 1996-97 (A joint publication by The World Resource Institute, The United Nations Environment Programme, The United Nations Development Programme, and the World Bank) (Data edited by Dr. Róbinson Rojas)
3. Urban Impacts on Natural Resources URBAN WASTES
The increased levels of consumption characteristic of the populations of urban areas lead to the generation of copious quantities of wastes. The impacts of this pollution are experienced both locally and at great distances from the source. Domestic and industrial discharges contaminate air, land, and water with nutrients and toxics. In turn, degraded air, land, and water harm flora and fauna.
Much of this pollution stems from economic growth and industrialization rather than urbanization per se. Cities, however, concentrate these wastes in one place, straining the ability of local ecosystems to assimilate them. Wetlands, for example, can render small quantities of domestic sewage harmless, yet they are no match for vast quantities of urban sewage. Urbanization itself reduces the assimilative capacity of the environment by removing vegetation, slowing the flows of air and water, generating heat, and reducing the infiltration capacity of the land (102).
This section looks at only a few aspects of pollution: air pollution, especially the formation of urban ozone; municipal solid waste; and water pollution, in particular, the problem of urban sewage.
Urban Air Pollution
Despite the potential for energy efficiency in cities, urban energy demand and fossil fuel consumption continue to grow. Already, the concentrations of airborne pollutants in and around cities far exceed those in rural areas. In addition to their toll on human health, these air pollutants can damage terrestrial and aquatic ecosystems. Not all of this damage can be attributed to urban activities. Nonetheless, sources of emissions are concentrated in or around urban areas--especially in developing countries, where industries still tend to be located in cities. In addition, combustion of the fossil fuels used for urban transportation is playing an ever greater role in air pollution problems. (See Box 3.2.)
Air quality standards are typically set with human health in mind, but some forms of ecosystem damage can occur at far lower levels. Table 3.2 compares health-based standards with the levels at which ecosystem damage has been documented. In most northern European cities, for example, sulfur dioxide concentrations rarely exceed World Health Organization guidelines, yet sulfur deposition still exceeds the levels at which ecosystem damage can occur. In fact, some efforts to reduce the health effects of urban air pollution in developed countries have actually increased damage to ecosystems. The tall smokestacks built to disperse pollutants in cities such as New York, Philadelphia, and Pittsburgh in the United States ultimately contributed to the acidification of lakes in the Adirondack Mountains (103). In most developed countries, however, stringent pollution control laws and new technologies have reduced sulfur emissions dramatically since the 1950s.
In China, where urban use of high-sulfur coal for cooking and domestic heating is common, urban emissions of sulfur dioxide--a precursor to acid rain-- may double or even triple over the next two or three decades (104). Already, damage to aquatic and terrestrial ecosystems has been documented downwind from most Chinese cities. In the Wanxian District downwind from the city of Chongqing, 26 percent of a 65,000-hectare pine forest has died, at least in part because of air pollution. Extensive soil acidification in the region has caused damage to farm produce and a drop in harvest yields (105).
Whereas in most regions of the world problems related to acid deposition stem more from industrialization than urbanization, ground-level ozone--which damages both human health and vegetation--is a distinctly urban problem. The combination of cars, pollutants, and meteorological conditions unique to cities is key to ozone formation. Ozone levels seem certain to increase as the number of cars (a primary source of the pollutants that produce ozone)in cities continues to climb.
Urban ozone is a particularly difficult problem to address because no one polluter actually emits it. Ozone is produced when nitrogen oxides, carbon monoxide, and hydrocarbons react with sunlight, a process that takes 8 to 10 hours. In addition to cars, other sources include the production and use of organic chemicals, the use of natural gas, municipal waste disposal, and wastewater treatment plants.
The greatest damage to ecosystems from urban ozone often occurs many kilometers from the city itself, although inversion layers can trap ozone within city limits and cause health problems there as well. Plumes downwind of large North American cities can have ozone concentrations of between 70 and 200 parts per billion, often over distances of several hundred kilometers (106) (107). Ozone concentrations as low as 40 parts per billion can injure plant leaves, whereas exposure to concentrations of 60 to 100 parts per billion for several hours is sufficient to cause significant plant, tree, and crop damage (108). Once injured by ozone, plants are more susceptible to insect attack, root rot, and other diseases.
In the United States, ozone is responsible for most of the crop yield losses from air pollutants (109). In addition, ozone has been implicated in the declines in the numbers of ponderosa and Jeffrey pines in the San Bernardino National Forest east of Los Angeles, where daytime average ozone concentrations of 100 parts per billion are typical during the summer months, and in the white pine in the eastern United States, downwind from the urban industrial centers in New York and New Jersey (110).
The decline in the numbers of trees near Los Angeles sends an important message to other expanding cities. Without policy interventions, ozone will become a problem for virtually all mid- latitude cities where motor vehicle traffic is increasing. Forest damage associated with ozone is already apparent around Santiago, Chile, and Mexico City (111) (112). In Asia and the Pacific, ozone damage is likely to be occurring in forests downwind from Tokyo and Osaka in Japan; Beijing, China; Seoul, Republic of Korea; Taipei, Taiwan; Delhi, India; and Karachi, Pakistan (113) (114).
Greenhouse Gas Emissions
Although the upswing in fossil fuel consumption is not due solely to urbanization, there is no doubt that major metropolitan areas have the greatest concentrations of population, industry, and energy use, and hence the largest amount of pollution and the highest greenhouse gas emissions. One recent study estimates that almost 40 percent of total carbon dioxide emissions from North America come from 50 metropolitan areas (115). Although this should be considered only a rough approximation, it demonstrates the need for policy interventions to reduce urban output of greenhouse gases. In developing countries especially, the rapid growth in energy demand in urban areas (i.e., from electricity and transportation) is projected to greatly increase greenhouse gas emissions (116) (117) (118). Global warming is predicted to cause a rise in sea level, placing coastal cities at risk (119).
Solid waste generation, both municipal and industrial, continues to increase worldwide in both absolute and per capita terms (120). Wealth is a primary determinant of how much solid waste a city produces. Wealthy cities such as Los Angeles and New York are vast producers of solid waste, whereas per capita solid waste generation is still low in cities such as Calcutta, India, and Accra, Ghana. (See Figure 3.3.) As per capita incomes increase in cities in the developing world, the quantity of solid waste will likely grow in tandem. With increased wealth, the composition of wastes changes from primarily biodegradable organic materials to plastics and other synthetic materials, which take much longer to decompose. When solid waste is not collected and disposed of efficiently and effectively, it pollutes and degrades land and water resources. Managing the volume of solid waste can pose a major challenge for city governments, from ensuring that all waste within city boundaries is collected, to reducing health impacts, to acquiring vacant land sites for landfills.
In developing countries, the environmental impacts of improper solid waste disposal are especially severe. In many cities, only 30 to 50 percent of solid waste is collected; the rest is either burned or dumped in unregulated landfills. Uncontrolled disposal of urban waste into water bodies, open dumps, and poorly designed landfills is a principal cause of surface water and groundwater contamination. In Manila, the biggest solid waste dump is Balut, Tondo, which receives approximately 650 metric tons of solid waste each day. This dump site has reclaimed 34 hectares of Manila Bay and has created an enormous mountain of refuse towering 40 meters above sea level (121).
Many cities dispose of household wastes along with industrial wastes, exacerbating pollution problems. In China, for example, most toxic solid wastes are disposed of in the municipal waste stream without treatment (122), leading to contamination of soils and water bodies with heavy metals such as mercury, chromium, lead, and arsenic. These toxics can threaten or destroy marine life (123) (124).
Disposal of solid waste in legal landfills, as is the norm throughout the United States and Europe, averts many of these problems. If the landfills are not properly managed, however, runoff and leachates can contaminate surface water and groundwater supplies. Landfills are also becoming increasingly expensive, owing to the rising costs of construction and operation (125). Incineration, which can greatly reduce the amount of incoming municipal solid waste, is the second most common method of disposal in developed countries (126). However, incinerator ashes may contain hazardous materials, including heavy metals and organic compounds such as dioxin (127) (128). Recycling plays a large role in solid waste management, especially in cities in developing countries, and should be encouraged not only to reduce the need to dispose of vast amounts of waste but also to protect new raw materials from extraction and use.
Water pollution probably began with the foundation of the first cities 7,000 years ago along the major river systems of the Tigris- Euphrates and Indus (129). Cities have long used rivers, lakes, and coastal waters as receptacles for diluting and dispersing wastes. The natural processes of water flow help to break down wastes and render them harmless. Ever-increasing urban populations and their growing amounts of wastes, however, have overtaxed the natural recycling capabilities of local rivers and lakes. In cities close to coasts, untreated sewage and industrial effluents flow into the sea and damage beaches and inshore waters.
Although there has been significant progress in controlling water pollution in developed nations over the past three decades, pollution has continued to rise in most cities in the developing world and remains high around cities in the transition economies of Russia and Central Europe, posing a threat to human health and to the health of aquatic ecosystems. In some areas, entire estuaries and even international water bodies such as the Mediterranean Sea and the Caribbean are affected.
Urban-generated pollution comes from both localized and dispersed, or point and nonpoint, sources. Major point sources include municipal sewage, industrial outfalls, and air emissions from power plants and heavy industries. Nonpoint sources include silt from earth-moving activities; storm runoff from roads, home gardens, and industrial sites; infiltration from aquifers contaminated with sewage or industrial chemicals; and automobile emissions.
Of the many problems associated with urban effluents, nutrient loading, or eutrophication, of local waters is one of the most serious (130). Nutrients are essential plant foods, but excessive amounts can cause radical plant growth--often massive algal blooms- -that blocks the sunlight that other organisms need. As plants die and decompose, the dissolved oxygen in bottom waters is depleted--a condition that is deadly for fish and other aquatic life (131). Those fish and other mobile species that can survive may nonetheless lose critical habitat, their food supplies may be disrupted, or they may be forced into shallow areas where they are subject to increased predation (132) (133) (134) (135).
Nutrients come from several sources, including runoff from upstream agricultural and urban areas, particularly silt, and air emissions. Atmospheric deposition is thought to be responsible for about one third of the nitrogen in the Chesapeake Bay, which is surrounded by several large urban populations (136). The biggest single source of nutrient loading in urban waters, however, is human waste. Even after conventional wastewater treatment to remove much of the organic material and pathogens, human waste still contains copious amounts of nitrogen and phosphorus--the primary ingredients in fertilizers.
Nutrient enrichment problems are especially severe in urban estuaries, where water flushing is minimal and inputs, often from numerous cities, are large (137) (138). The Baltic Sea, for instance, receives the effluents of more than 70 million people and related industries in dozens of cities. Since 1980, it has manifested increasing symptoms of eutrophication, with a lengthening list of biological effects, from declining lobster and cod catches to increasing numbers of nuisance algal blooms (139) (140) (141). Without a major restructuring of how urban wastewaters are handled, nutrient loads in waters seem certain to rise as urban populations increase and agricultural production expands to feed urban residents (142) (143).
Given its sheer volume, sewage is a major threat to local urban waters, as well as one of the most vexing problems for urban managers charged with its safe disposal. Not only is sewage the major source of nutrients in urban waters but it also poses a significant risk to health from such sewage-borne pathogens as the cholera bacterium, hepatitis viruses, salmonellae, and shigellas (144) (145) (146).
Most of the world's sewage is still disposed of untreated. In developing countries, 90 percent or more is released without treatment of any kind--usually to a water body, whether a river, a lake, or an ocean (147) (148). Even in many developed countries, only a portion of the sewage receives conventional treatment (149).
In countries where a higher percentage of sewage is treated, building the infrastructure to collect and treat wastewater has required a concerted and costly national effort, and pollution episodes still occur (150). Many older cities still have outmoded sewer systems that collect sewage and storm water together, so that when rainfall is heavy, the capacity of the treatment plant is overwhelmed and untreated wastewater is released through overflow drains (151).
Increasingly, fisheries are being damaged or destroyed by the volume of urban sewage (152) (153). Major declines in fish catches have been documented in rivers and estuaries around cities in India, China, Venezuela, and Senegal (154). In Manila, two rivers carry vast quantities of the city's sewage into Manila Bay; fishery yields there declined by 39 percent from 1975 to 1988 (155). In addition, fecal coliform counts in most urban rivers in developing countries far exceed health standards. For the urban population that relies on these rivers as a source of drinking water and food, this poses severe health risks. The Tiete River downstream from Sao Paulo, Brazil, is heavily contaminated by the city's wastes, yet it is still used as drinking water by several rural communities in the interior of Sao Paulo state and as a source of irrigation for nearby vegetable farms (156).
Even the release of treated effluents to waters is not without environmental repercussions, because these effluents are a prime source of nutrients and subsequent eutrophication. The chemicals used in wastewater treatment can also have toxic effects. Chlorine, for example, is both toxic to aquatic organisms in its own right and can also combine with some organic compounds in the effluent to form organochlorines such as chloroform and various chloramines, which may be carcinogenic or directly toxic. Moreover, conventional treatment results in the accumulation of large quantities of sewage sludge, which often contains heavy metals and other contaminants and which can have a variety of toxic effects if it is disposed at sea (157).
Especially in the developing world, industry is concentrated in urban centers, resulting in severe water pollution problems in most large cities. Major sources of water pollution include chemical- intensive industries such as tanneries, metal plating operations, pulp mills, and refineries. Typical contaminants include organochlorines such as polychlorinated biphenyls (PCBs) and dioxins, pesticides, grease and oil from automobiles and shipping traffic, acids and caustics, heavy metals such as cadmium and lead, sewage sludge, and a long list of synthetic organic compounds.
Urban runoff is another source of industrial pollutants. A 1990 study found that a single year's runoff from the Washington, D.C., metropolitan area carried with it 3.8 million to 19 million liters of oil, 180 metric tons of zinc, 29 metric tons of copper, and 10 metric tons of lead (158). For some pollutants, urban runoff rivals or exceeds the output from industrial sources and sewage treatment plants and is often much more difficult to track and control (159).
Industrial releases of toxics have declined in many cities thanks to stringent pollution control measures (160) (161). On a global basis, however, toxic effluents are still a major threat to urban waters, particularly in many developing countries where industrial growth is rapid (162) (163). In Jakarta Bay in Indonesia, where untreated industrial wastes are discharged by some 30,000 small industries such as batik factories, heavy metal accumulations are alarmingly high. In fact, shrimp taken from Jakarta Bay have levels of mercury contamination second only to those of shrimp taken from Minamata Bay in Japan (164).
Cleaning up contaminated sediments is extremely difficult and costly. In the United States, where sediment cleanup is being contemplated at a number of harbor sites, costs are estimated at from $143 per kilogram of PCBs removed in easily accessible areas to more than $6,600 per kilogram for more dispersed contamination. Such costs mean that, once contaminated, most sediments are likely to remain so for years, despite the effects on the local environment (165).