al poor family must get its water from distant standpipe which may cost it 15 percent of its total income.
             Many cities are also worried about the availability of future water sources. In Bangkok, the capital of Thailand, six million inhabitanats draw their water from a natural underground storage area known as an aquifer. The results is land subsidence ( a downwards settling of the land) and a potential shorage of water. Mexico city, which is residents to about 20 million people, has sank by more than 7 meters between 1940 and 1985. This sinkage was in large part due to the excessive pumping of water from the hundreds of wells located just below the surface of the city. The city consumes at least 60,000 liters of water per second; 80% of this supply is from groundwater. Another problem for cities is water contamination originating from industrial or agricultural wastes as well as human excrement. In Karachi, Pakistan, only 70 percent of necessary water is provided by the city. Those who cannot afford are force to drink from untreated supplies often contaminated with diseas!
             es. The provision of water and wastewater service for the growing population in the cities presents a formidable challenege.
             Air pollution is a serious problem which occurs in almost all of the world's largest cities. Regulations to control emissions from automobiles and industries are, in general, weak or in some cases absent. The valley of Mexico City is located at 7,400 feet above sea level, surrounded by volcanoes and mountains that block adequate drainage and make the valley a huge swamp. The surrounding peaks and temperature inverions prevent industrial smog and vehicle emissions from escaping. More than 3.6 million vehicles clog the roads and combined with 32,000 industrial plants, spew more than 12,000 tons of pollutants into the air every day. Figure 1 belows shows the air quality index for Mexico City spanning 7 years.
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