Alan V. Morgan, Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, N2L 3G1
Mexico City is the world's third largest city, with an estimated population of about 18 million (Ezcurra and Maziri-Hiriat 1996). Increasing population -- the annual growth rate is about 2 percent -- coupled with the accelerating socially-derived problems, such as potable water supplies, waste water removal, and garbage disposal, mean that there are going to be severe challenges to the community in the near future. Couple these problems with the natural hazards inherent in the positioning of Mexico City and the importance of the geosciences in planning becomes very obvious. In this short note I shall describe some of the geological problems which beset Mexico City. These problems are not unique to large cities, but this megalopolis exemplifies some of the more serious aspects of burgeoning human numbers when they come face to face with the geological realities of location.
Water supply
In order to understand the problems of water supply in Mexico City and environs it is important to comprehend the recent geological history of the region. The Mexico Basin is a geological depression covering some 7,500 km2 within the late Tertiary Trans-Mexican Volcanic Belt. The Basin is at an elevation of about 2,250 m, and it is floored with a series of isolated saline to brackish lacustrine areas, bounded by the high volcanic peaks of Popocatapetl and Ixtaccihuatl in the southeast, and lesser volcanoes and volcanic products which form the southwestern, western, northern and eastern highlands.
When Cortes first viewed the great city of the Aztecs it was located in a large lake connected to the mainland by a series of causeways. Water supplies were derived from seepages around the margins and from artesian springs on islands in the lake. The indigenous population was self-sufficient in terms of water supplies. In fact the lack of water was not a serious problem until about 30 years ago when population numbers reached about 6,500,000. Until that time the populace was provided with potable water from the huge aquifer which underlies the lacustrine clays of the Mexico Basin. Today the situation is different with about 63m3/second of water being needed to support the populace for drinking water and agricultural irrigation. The problem is that the annual rainfall catchment in the Mexico Basin amounts only to 24m3/s. The main aquifer is being pumped at a rate of 55.5m3/s, but is only being replaced at 28m3/s, leaving a shortfall of 27.5m3/s. This shortfall is supplemented by about 1.5m3/s from small surface water sources. The remaining 26m3/s has to come from supplies diverted into the Mexico Basin from two catchments west of Mexico City. These are the Lerma and Cutzamala basins, which provide 6m3/s of groundwater and 13.5m3/s of surface waters respectively. The net negative balance in the the Mexico Basin is about 6.5m3/s.
The transfers of waters from the aquifers and from the surface waters outside the Mexico Basin has tremendous costs. These may be described as energy loss (the costs of pumping 19.5m3/s into the Mexico Basin) and indirect costs within and outside the Basin caused by pumping and over-extraction of water. The former cost is considerable since the water has to be brought uphill over 1 km to Mexico City from the Cutzamala and Lerma basins. In general terms it is equivalent to an 800 Mw reactor running permanently. The indirect costs are much more difficult to quantify since they involve the real and hidden costs of subsidence in the Mexico Basin and the effects of water reduction to rivers and lakes outside the Basin. The costs to humans and to biological diversity in the affected catchments are largely unknown.
Subsidence
Groundwater over-extraction appeared as a problem in the early years of the 20th century. In 1954 the overexploitation was sufficiently serious that pumping was banned in the city centre and wells were moved to the north and south of the basin. Although subsidence has greatly diminished in the core area (now stable at about 6 cm/yr), it did not prevent some areas from sinking up to nine metres. This has left some spectacular results near the Monument of the Revolution where a standpipe from an old well rises over 6m from the present ground surface in a small park. The effects of a deflating ground surface can also be seen in tilted churches and cracked cathedrals, a subsiding Palace of the Arts, and monuments which have to have new stairs added periodically to keep pace with the lowering street level! Add to this the problems of keeping rainwater from flooding areas where the natural drainage has been reversed, and keeping wastewater from backflooding into artificially lowered areas, you can begin to see the magnitude of the problem. To illustrate this one can visit the barrio areas on the urban fringes of the southeast side of Mexico City where frantic pumping in the Chalco Basin is pulling the ground surface down by as much as 1mm per day. The barrio areas which were located on low lying ground now face the prospects of drowning an an artificial lake which is being created by massive ground subsidence.
The wastewater canals
There is an old adage that says "What goes in must come out" and this is equally true in the case of water in Mexico City. It comes in from the sources described above, passes through the citizens, animals, factories and sewage farms and passes out of the Mexico Basin to the Tula Basin to the north. A series of "black water" canals drain sewage and chemical-contaminated water from the various parts of the city. About 7 percent of the water (4.3m3/s) is passed to 27 different treatment plants where it is treated, and this joins the remaining 44m3/s of untreated water to make its way into the Tula catchment and from there to the Gulf of Mexico. Since the sewage lagoons and reservoirs, sewage plants, landfills, garbage tips and the black water canals are all situated on the aquitard which overlies the Mexico City aquifer, there is ample cause for concern that eventually there could be cross contamination of bacteria and chemicals from leachate and the contaminated surface waters into the underlying potable water supplies. This is particularly true when many pumping wells drawing drinking water from the aquifer lie immediately alongside the black water canals!
The earthquakes
The Pacific coast lies some 350 km south and west of Mexico City. This section of the coastline is close to the Acapulco Trench, a zone of demarcation where the Cocos Plate dives beneath the North American Plate. Almost due west is the Rivera Plate also moving beneath the North American Plate and separated from the nearby Pacific Plate by a transform fault. Not surprisingly this zone running parallel with the coast has a number of earthquakes caused by subduction. Although relatively minor damage has been caused to coastal settlements much greater damage has been caused in Mexico City hundreds of kilometres to the north and east. The reason for the damage is the geological structure of the Mexico Basin and the stratigraphy within the Basin. Earthquake waves propagated from the Trench travel through the Trans-Mexican Volcanic Belt and eventually reach the Basin where they are reflected around within the confining outline of the Basin, rather akin to ripples in a small bowl: - the interference patterns which are set up then impact on structures built within the Basin. The stratigraphy of the lacustrine sediments has also contributed to building failures. This occurs because smaller structures are built on the surficial aquitard clay. In some areas the substrate has liquified allowing foundations to shift or sink. More expensive public buildings are anchored to relatively firm volcanic tuff bands deep in the lacustrine sediments or have been designed to "float" during earthquake oscillations.
On September 25, 1985 a magnitude 8.1 earthquake originated in the Acapulco Trench. Damage in Mexico City was severe as the earthquake waves bounced around the Basin for almost four minutes. Buildings in the core area between 6 and 15 storeys were severely damaged. Smaller and larger buildings than this remained relatively unscathed. The low-rise buildings (like the older Spanish style structures) some of which dated back to the 1700's survived with minimal damage, although some subsided and tilted. The high-rise government and commercial buildings built to more exacting earthquake standards also stood up well. The intermediate buildings suffered major damage because the harmonics generated by the earthquake waves interferred with the integrity of the foundations, or caused damage to higher floors. Damage to the upper stories was caused by the top floors of smaller buildings immediately adjacent to larger buildings "hammering" the sides of the larger structures thus causing structural failure in the upper floors of the taller buildings. In many cases only the top (or bottom) stories pancaked; in other cases the mid stories collapsed leaving lower levels and top levels damaged but intact. In more tragic situations the entire building compacted leaving few survivors. Unfortunately several larger public buildings were in this category, including several hospitals.
Volcanic hazards
Anyone who has visited Mexico City on a rare clear day cannot fail to be impressed by the mountains which outline the Basin. These range from the gigantic snow-capped peaks of Ixtaccihuatl and Popocatapetl rising to over elevations of well over 5,000 m in the southeast, through the numerous smaller shield volcanoes and cinder cones along the southwestern and eastern margins, to the Sierra de Cruces which defines the western portion of the Basin. Any geologist would be impressed with the freshness of many of these features. These range from the basaltic lava flows which cascaded from the Xitle volcano and upon which the campus of the National University of Mexico and the Olympic Stadium have been built. This lava flow extends 13 km into the urban environs, and almost one quarter of Mexico City would be covered if the flow had taken place today. A sobering reminder of the youthfulness of this flow can be seen at the edge of the university campus where an early circular pyramid has almost 6 metres of basalt lapping up the sides. Slightly further to the south and east smaller cinder cones, now partially excavated for aggregates and fill, reveal fresh volcanic bombs and serve as reminders that there have been a number of volcanic outbursts within the recent past, probably during the past 2,000 to 4,000 years. If the signs of basaltic volcanism were not enough to be worried about, the huge dacitic volcano, Popocatapetl, has recently exhibited signs of activity. This is one volcano which could certainly cause tremendous devastation in Mexico City. We know that eruptions from "Popo" and Ixtaccihuatl, the sister volcano to the north, have occurred in Quaternary time; there is ample evidence in the tuff horizons buried within the lacustrine sequences. Why should we be concerned about the potental activity from "Popo" and what kinds of hazards might the volcano pose?
Popocatapetl belongs to a category of explosive volcanoes like Mt. St. Helens and the former Mt. Mazama, huge strato-volcanoes which have the potential of venting large quantities of ash in paroxysmic outbursts, and, under rare circumstances of self-destructing in gigantic explosions which can reduce the volcano to a fraction of its original size. Should Popocatapetl continue to heat up, (and geothermal springs and ground water measurements indicate that heating is taking place), then the glacier ice on the summit will melt. If this happens there is the potential of lahar flows, similar to those that caused 20,000 deaths at Nevada del Ruiz in Colombia in 1989. Some of these will impact the edge of the Mexico Basin. A more serious eruption could allow considerable amounts of ash to drift into Mexico City, and almost any ash emissions will affect air traffic in and out of the Mexico City Airport. Moving up in seriousness the potential of an "ash-blast" eruption similar to Mt. St. Helens might be considered. Although Popocatapetl is far enough from the major metropolitan area that the majority of people would probably escape, a blast toward Mexico City would cause chaos and panic and it could cause serious damage in closer communites such as Amecameca. Any truly catastrophic eruption would liberate vast quantities of ash which could seriously impact drainage and water supplies in the basin. Even if none of this comes to pass, but the seismic monitoring indicates increasing volcanic activity, the Mexican authorities will have difficult decisions to make on whether to leave the populace relatively uninformed, or to keep them informed but raise fears about the potential for disaster. Should the level of alert become sufficiently high to warrant evacuation, then even harder decisions would have to be made about who goes? who stays? and where to put 2 million, 5 million or, perhaps in a worst case scenario, 20 million, people?
Mexico City is a fascinating place; a city where the problems of population growth and survival come face to face with the geological ramifications of living on an active planet. It is a textbook case which illustrates that every citizen should be aware of the earth sciences. These range from the problems of getting enough of that peculiar mineral, water, to ways of safely transporting it and disposing of it, to the just as deadly hazards of earthquakes and potential volcanism in the region. We may live in interesting times, but the inhabitants of Cuidad Mexico also live in a very interesting place!
References
Ezcurra, E. and Maziri-Hiriat, M. 1996. Are Megacities Viable? - A Cautionary Tale from Mexico City. Environment 38: 26-31.