Between 1940 and 1985, downtown Mexico City, built on the ruins of the ancient city of Tenochtitlan, sank by more than 7 meters. 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. During this forty five years the groundwater fell by 32 meters.
Not only has this phenemenon caused incredible concern about the future of the water supply, it's caused some of the greatest subsidience of any city in the world. This alarming rate at which groundwater is being pumped has caused urban planners to question the survival of the city itself.
The Capital consumes at least 60,000 liters of water per second; 80% of this supply is from groundwater. By the year 2000, the increase in population is predicted to push water consumption to 100, 000 liters per second, aggravating an already grave situation. Presently, water is being drawn from sources at twice the natural recharge rate.
During the time of the Aztec Empire, Mexico City was built atop a number of lakes at the basin of volcanoes. When the Spanish came, they saw the water as an obstacle to their expansion and drained the lakes. As the lakes were drained and the groundwater extracted, the clay that had formed on the basin lost its water content and began to dry out--causing it to shrink. Because of this the ground level began to sink and the city along with it.
Although the water is now extracted from deep wells, the clay is still affected. Throughout the metropolitan area of Mexico City, the ground level is sinking by between 5 and 40 centimeters a year.
This subsidience has caused serious sewage disposal problems. The Gran Canal de Desague is the sewage canal for the city, but as the city shrank so did the Canal. Today it takes 11 pumping stations which run 24 hours a day all year round to keep the sewage flowing and keep the summer rains from washing it back to the city. Since the 1970's the city has sank so much that the water authorities could no longer count on gravity to carry the sewage away and Mexico City has expanded the sewage treatment and water recycling facilities. Six treatment plants have been built since 1987. There has also been new water legislation due to the fact that only half of all industries comply with the existing waste water discharge regulations.
As far as urban planners and environmentalists are concerned, the only thing that can save Mexico City now is increased awareness of the water supply problem. For the first time in Mexico City's history, water from the surface is being allowed to drain into groundwater reserves located deep underground. If the surface water is contaminated, the groundwater may also become polluted and any clean-up efforts may be more than the city can afford.