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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 BOX3.1. Water: the Challenge for Mexico City
Mexico City's struggle to secure enough water is a good example of how urban growth can quickly outstrip the natural resources of a region and lead to environmental degradation. Sprawling over 3,773 square kilometers (1), the city is home to more than 15.6 million people (2). The city's location--in a high, naturally closed basin--uniquely challenges water provision. The absence of an adequate nearby surface water source means that the city must depend largely on the local groundwater source, or import water from several hundred kilometers away. However, the high elevation of the valley makes water importation an expensive alternative (3). In addition, continued urban growth and poor system financing have limited the government's ability to expand service coverage, repair leaks, and provide wastewater treatment (4).
The largest problem, however, is the depletion of Mexico City's aquifer. Today, almost 72 percent of the city's water supply comes from the aquifer that underlies the metropolitan area (5). The groundwater level is sinking by about 1 meter each year (6) (7). Although overdrafting of the aquifer has been occurring at least since the early 1900s, the problem has intensified recently. From 1986 to 1992, the water level of the aquifer showed a net lowering of 6 to 10 meters in the heavily pumped zones (8).
Because of this overextraction, Mexico City is suffering from severe land subsidence. (See Figure 1.) In part, the city's location is to blame, because the clay soils in the region are especially susceptible to dewatering and compaction (9). Over the past 100 years, the central area of the Mexico City Metropolitan Area (MCMA) has fallen by an average of 7.5 meters. Neighborhood children mark their height on well casings to see whether they are growing faster than the ground is sinking (10). The result has been extensive damage to the city's infrastructure, including building foundations and the sewer system (11). The city is also especially vulnerable to flooding. In 1900, the bottom of Texcoco Lake was 3 meters lower than the average level of the city center. By 1974, the lake bottom was 2 meters higher than the city (12). Expensive drainage channels have been built, but flooding remains a problem during heavy rainfall (13).
The aquifer is also at risk from contamination and faces expensive and difficult cleanup. Currently, 90 percent of the municipal and industrial liquid wastes from MCMA are discharged untreated into the sewer systems (14). Industries generate an estimated 3 million metric tons of hazardous wastes per year, of which more than 95 percent are process effluents or treated effluents discharged directly into the municipal sewage system (15). In many areas, this wastewater travels in unlined drainage canals (16). There is the potential that pollutants may leak into the underlying soil and leach through fractures (from land subsidence) into the aquifer, contaminating the water supply (17). Other identified threats to the groundwater include hazardous wastes illegally dumped in landfills, pesticides, and saline intrusion (18) (19).
Demand for water in the region continues to grow. Overall, 94 percent of MCMA's residents are serviced by either a piped water connection or a standpipe (20), but coverage varies widely. In Tlalpan in 1990, for example, 14 percent of homes did not have access to any form of public water supply (21). Mexico City's urban periphery is growing quickly, and providing adequate supplies of water to these residents poses a further challenge. Average per capita water use is still far below that of developed countries, indicating the potential for increased demand. The Federal District uses 364 liters per capita per day (22), compared with New York City, which uses 680 liters per capita per day (23).
Mexico is actively pursuing new solutions to meet these demands and to protect the environment. Aggressive efforts are under way to protect the aquifer recharge areas from urban encroachment (24). Officials are attempting to institute new pricing systems that would ensure that the full cost of urban water use includes the cost of developing sewage systems and wastewater treatment facilities. Currently, only $0.10 is collected per cubic meter of water, even though the marginal cost of supplying water to MCMA is estimated at about $1.00 per cubic meter (25). In 1991, the MCMA began a new rate structure that charges more per cubic meter as consumption levels increase. The goal is to provide metered industries with the incentive to conserve water, eventually leading to full cost recovery--an ambitious goal. Now only 53 percent of the users are metered, and not all meters function properly. To achieve full metering, several million additional meters would have to be installed at a cost of roughly $100 each (26).
Some more modest conservation efforts are already showing success. Water utilities are making routine repairs part of their overall strategy, and more than 3,800 leaks in the MCMA distribution system are fixed each month. In 1989, the Federal District initiated a program for retrofitting large office and apartment buildings with low-flow toilets that use only 6 liters of water per flush; older models use 16 liters (27). By 1996, this program alone is expected to reduce water consumption by 4.3 cubic meters per second within the Federal District (28). (Total water consumption in the MCMA is approximately 60 cubic meters per second (29).) The State of Mexico recently began a similar program.
Despite these efforts, the financial and environmental costs of supplying water to Mexico City are expected to increase as demand continues to outstrip supplies in the near term (30).
References and Notes
1. National Research Council, Academia de la Investigacion Cientifica, A.C., and Academia Nacional de Ingenieria, A.C., Mexico City's Water Supply: Improving the Outlook for Sustainability (National Academy Press, Washington, D.C., 1995), p. 6.
2. United Nations (U.N.)Population Division, World Urbanization Prospects: The 1994 Revision (U.N., New York, 1994), Table 1, p. 4.
3. Op. cit. 1, p. 7.
4. Op. cit. 1, p. 1.
5. Op. cit. 1, p. 1.
6. Op. cit. 1, p. 17.
7. For a detailed discussion of these calculations, see I. Herrera-Revilla et al., "Diagnostico del Estado Present de las Aguas Subteraneas de la Ciudad de Mexico y Determinacion de sus Condiciones Futuras," and AIC-ANIAC, "El Agua y la Ciudad de Mexico," both of which are cited in National Research Council, Academia de la Investigacion Cientifica, A.C., and Academia Nacional de Ingenieria, A.C., Mexico City's Water Supply: Improving the Outlook for Sustainability (National Academy Press, Washington, D.C., 1995).
8. Op. cit. 1, pp. 12-13.
9. Op. cit. 1, pp. 6-7.
10. Op. cit. 1, pp. 13-14.
11. Op. cit. 1, p. 14.
12. Op. cit. 1, p. 14.
13. Op. cit. 1, p. 14.
14. Op. cit. 1, p. 40.
15. Op. cit. 1, p. 41.
16. M. Mazari and M.D. Mackay, "Potential Groundwater Contamination by Organic Compounds in the Mexico City Metropolitan Area," Environment, Science, and Technology, Vol. 27, No. 5 (1993), as cited in National Research Council, Academia de la Investigacion Cientifica, A.C., and Academia Nacional de Ingenieria, A.C., Mexico City's Water Supply: Improving the Outlook for Sustainability (National Academy Press, Washington, D.C., 1995), p. 39.
17. Op. cit. 1, p. 40.
18. Op. cit. 1, p. 41.
19. Op. cit. 1, p. 44.
20. Op. cit. 1, p. 20.
21. Op. cit. 1, p. 58.
22. Op. cit. 1, p. 20.
23. World Resources Institute, The 1994 Information Please Environmental Almanac (Houghton Mifflin Company, Boston, 1994), p. 209.
24. Op. cit. 1, p. 53.
25. Op. cit. 1, p. 55.
26. Op. cit. 1, pp. 55, 65.
27. Op. cit. 1, pp. 63-64.
28. Op. cit. 1, p. 64.
29. Op. cit. 1, p. 21.
30. Op. cit. 1, p. 70.