Mexico: not enough water in a huge metropolis

Mexico: a thirsty city

 Danilo Antón

There are few cases in the world in which the physical environment has been so completely transfigured by urban development as it was in Mexico City. The valley of Mexico is a 9 600 square kilometre closed basin that is more than 2 200 metres above sea level, in the heart of the Mexican neovolcanic belt. Before the arrival of Europeans in 1521, the valley was a depression in whose bottom several large lakes had developed because of volcanic obstruction of their outlets about 700 thousand years ago. The lakes covered a total area of about 2 thousand square kilometres and were partly connected, especially during periods of high water. Three of the lakes contained fresh water — Chalco, Mexico, and Xochimilco — and the other three, brackish water — Ecatepec, Texcoco, the largest at 800 square kilometres, and Zumpango.

The area was, and to a certain extent still is, subhumid. Rainfall was probably slightly more than the current amount, which ranges from 600 millimetres per year at the bottom of the valley to 1 200 millimetres per year in the nearby mountains. The average temperature was relatively cool for the subtropic latitude at which the city is located, ranging from 8° to 15°C depending on the altitude. Soils were deep, highly fertile, and easy to work.

The land was completely covered by thick forests, particularly on the slopes of the mountains and highland areas. The plains in the valley, which were originally also covered by forests, were soon allocated for agriculture, and parts of the forest were cleared to make way for farms. In addition to the freshwater lakes, a large number of springs around the lakes and in the foothills of the nearby mountains provided considerable volumes of good-quality water.

Because of its abundant resources, the valley was occupied early by a number of indigenous peoples, who based their economy on locally domesticated crops and farm animals: corn, tomatoes, chili peppers, cacao, turkeys, dogs, honey bees, and fish. Because these people did not have draft animals or use the wheel, most trade was carried out by boat (or walking).

Several peoples successively inhabited and established political control over the lacustrine area during the few centuries before the arrival of the Europeans. The last group was the Aztecs, who arrived from the legendary land of Aztlán (probably in the northern arid territories) during the 14th century.

The Aztecs probably maintained a livelihood by fishing and trading with neighbouring groups. Gradually, they managed to build an island

in the centre of the Lake of Mexico on which a town developed: Tenochtitlán. Through alliances and wars, the Aztecs built an empire, and Tenochtitlán became a thriving city of several hundred thousand people. A bridge was built to connect the island with the mainland, and large boats transported people and merchandise. The Aztecs also built earth dikes to control flooding and to separate the brackish lakes from the fresh water. Aqueducts carried fresh water from springs to the city through the lake and along the dikes.

It is difficult to comprehend the extent of the changes that took place in the few centuries after the Spanish conquest. Today, the proud Tenochtitlán has disappeared, and only scattered archaeological remnants can be found. In its place stands the highly urbanized downtown area of Mexico City.

The Lake of Mexico is gone. In its place are several hundred square kilometres of urban neighbourhoods built on what used to be the lake bottom. A few canals and small lakes are the only remnants of Chalco and Xochimilco lakes. Like the southern lakes, the three northern lakes were gradually drained (beginning in 1786), and the former Texcoco Lake has become a vast flat plain on which little vegetation grows because of the highly alkaline soil (pH is over 10). An intricate maze of wells and pipes pump brine from the lacustrine sediments for sodium carbonate and sodium chloride extraction.

The old springs that provided water to the riverine populations are also gone. Now over 5 thousand wells draw more than 50 cubic metres of water per second from an average depth of 100 metres, causing the level of water in the aquifers to subside by as much as 1 metre per year. As a result of this overpumping and the compaction of the upper layers of sediments, widespread subsidence is occurring. The surface has dropped 6 metres in several places and, because of differential rates of subsidence, many structures have been weakened. This phenomenon has been exacerbated by frequent seismic activity, of which the most recent destructive example was the earthquake of September 1985.

The forests that used to cover the adjacent hills have practically disappeared, and widespread soil erosion occurs. Most former agricultural land has been covered by pavement, houses, and other urban

constructions. Quarries, which supplied construction materials, can be found throughout the region. Some have become garbage dumps, into which some of the annual 10 million tonnes of garbage is thrown. A significant portion of the garbage is dumped on the “shores” of the former Texcoco Lake, particularly in the south. Ciudad Netzahuatcoyotl, in that area, is a neighbourhood of 3 million people. Although recently established, this urban area is extremely degraded; developed areas alternate with garbage dumps and slums.

Water, which used to flow into the lakes, is channeled out of the basin, together with urban wastewater, through a system of canals and tunnels into the Gulf of Mexico hydrographic system. A number of pumping wells used to supply the city are located next to the canal (the Chalco Canal). Risks of contamination are obvious and, in fact, some wells had to be closed because of the presence of nitrates in the water.

The atmosphere of the valley has also changed. Emissions from 4 million vehicles and 25 thousand industrial establishments in a poorly oxygenated environment (because of the altitude) have transformed the air in Mexico City into one of the most unhealthy urban environments for human life, particularly near the downtown core.

Mexico City contains 21 million people, making it the largest urban centre in the world. Every year, its population increases by 750 thousand people, including both births and migration from the rest of the country. By the year 2000, the city will hold 29 million people (surpassing the population of Canada) and, by 2010, 38 million. If corrective measures are not taken, the city’s problems will continue to grow, and the ancient paradise may become one of the worst environmental nightmares of the 21st century.

The aquifer underlying the valley of Mexico is one of the key natural elements in Mexico’s environment. It provides the bulk of the water that makes the existence of the city possible. Although some water is brought in from the Lerma–Cutzamala basin, the volume is less than one-fifth of total requirements.

Any other option for bringing water from outside the valley is becoming impractical or too expensive. The Lerma–Cutzamala resources are almost exhausted, but using other basins (such as the

Balsas basin or the Amacuzac subbasin) may mean pumping water 1 200 to 1 500 metres upward and constructing long pipelines, storage reservoirs, and other expensive engineering works. Bringing this water into Mexico City will also deprive a number of communities that now depend on it for irrigation and other uses.

Mexico’s aquifer is contained in a number of Tertiary and Quaternary units with a thickness ranging from a few hundred metres to nearly 2 thousand metres. These units comprise a wide range of sedimentary materials. Continued volcanic activity produced huge volumes of pyroclastic material, which has been more or less reworked by fluvial action, and intercalated lava flows. During periods of volcanic activity, tuffs, breccias, ashes, and lava formed; at other times, alluvial and lacustrine action was more important. The main water-bearing layers are the Tarango formation and associated alluvia and the Cenozoic sequence of fractured pyroclastic and lava flows. These are covered by younger lacustrine sediments, confining the main aquifer.

The whole sequence can be up to 2 000 metres thick, but the lower 1 500 metres are more consolidated and less porous. The upper few dozen metres of the aquifer are too close to the upper lacustrine clays and continued pumping might produce dewatering and consolidation of these clays, causing subsidence. Therefore, the usable portion of the aquifer is generally between 100 and 500 metres underground.

The aquifer is recharged mainly in the mountain region (Sierra Chichinautzin in the south, Sierra Las Cruces in the west, and Sierra Nevada to the east). The total available recharge volume has been estimated to be 25 to 50% of precipitation: 25% in Sierra Las Cruces, 35% in Sierra Nevada, and 50% in Sierra Chichinautzin. Of these volumes, about half flows toward the valley of Mexico and the rest outward to other basins. An accurate figure for inflow to the aquifer itself is difficult to estimate (probably 30 to 40 cubic metres per second). However, it is certainly below 50 cubic metres per second — the amount being pumped out — because the water level is sinking.

Additional lowering of water levels will increase inflow from the Sierras because of an increase in gradient. This will not compensate for the deficit, however, particularly if pumping is increased. Precise forecasting of the aquifer’s reaction to prolonged extraction requires accurate modeling. Only recently has adequate information on the geometry and hydraulic properties of the reservoir been available. Modeling of the aquifer has been carried out at the Instituto de Geofísica, and it is expected to allow prediction of the actual potential of the groundwater resources of the valley.

It has recently become clear that the groundwater resources of the valley of Mexico are limited and that additional water will have to come from external sources. Such external sources are all found at elevations lower that that of the city. Therefore, tapping this water will not only require enormous energy consumption but will also deprive downstream communities of this vital resource. The bottom line is that the urban model of Mexico City is unsustainable. It has become too large for its territorial base. The city has not only run out of water, but also its air is heavily polluted, the local ecosystems have been destroyed or critically damaged, and the surrounding soils are under severe strain as a result of heavy urbanization. To check this continuous destruction of resources, radical policy shifts are essential. The window of opportunity to save Mexico City is rapidly closing.

From "Diversity, Globalization and the Ways of Nature", Danilo Anton, published by IDRC Books, Ottawa.