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The Valley of Mexico is located in the Trans-Mexican Volcanic Belt located in the high plateaus of central Mexico.¹² It has a minimum altitude of 2,200 meters above sea level and is surrounded by mountains and volcanoes that reach elevations of over 5,000 meters.³ It is an enclosed valley with no natural outlet for water to flow and a small gap to the north. The valley extends for most of the Mexico City Metropolitan Area, as well as parts of the State of Mexico, Hidalgo, Tlaxcala and Puebla. Geologically, it consists of three parts, the old lakebed in the southern part of the valley, the piedmont which rises up to the mountainsides themselves.¹ Seismic activity is frequent here.⁴

The Valley of Mexico can be subdivided into four basins, but the largest and best-studied is the area which contains Mexico City itself. This section of the valley is also popularly called the Valley of Mexico.⁵ This area used to contain five lakes, Zumpango, Xaltoca, Xochimilco, Chalco, and the largest, Texcoco covering about 1,500 km2 of the valley floor.²

The valley has been inhabited for at least 12,000 years, attracting humans with its mild climate (average temperatures between 12-15 C), abundant game and ability to support large-scale agriculture.⁶⁷ Civilizations that have arisen in this area include the Teotihuacan (800 BCE to 800 AD), the Toltec Empire (10th to 13th century) and the Aztec Empire (1325-1521).⁶ When the Spaniards arrived the Valley of Mexico, it had one of the highest population concentrations in the world with about one million people.² After the Conquest, the Spaniards rebuilt the largest and most dominant city here, Tenochtitlan, renaming it Mexico City. They continued to expand the city and began to drain the lakes' waters to expand the city and control flooding.⁶ Although, violence and disease significantly lowered population in the valley after the Conquest, by 1900 it was again over one million people.⁸ The 20th and 21st centuries have seen an explosion of population in the valley along with the growth of industry. Since 1900, the population has doubled every fifteen years. Today, around 21 million people live in the Mexico City Metropolitan Area with extends throughout almost all of the valley into the states of Mexico and Hidalgo.²

The growth of a major urban, industrial center in an enclosed basin has created significant air and water quality issues for the valley. Wind patterns and thermal inversions trap contaminants in the valley. Over-extraction of ground water has caused new flooding problems for the city as it sinks below the historic lake floor. This causes stress on the valley’s drainage system, requiring new tunnels and canals to be built.⁴⁹

Contents 1 Geography of the valley 2 Climate 3 Hydrology 4 The old lake system 5 History of human habitation 5.1 First human habitation 5.2 Pre-Teotihuacan 5.3 Teotihuacan and the Toltecs 5.4 The rise of the city-state in the valley 5.5 The Aztec empire 5.6 Mexico City metropolitan area 6 Air pollution 7 Draining the basin and water issues 7.1 History of water control in the valley 7.2 Drinking water and sinking 8 References

Geography of the valley

The Valley of Mexico, also called the Basin of Mexico, is located within the Trans-Mexican Volcanic Belt that runs through central Mexico.¹ The region it is located in is called the ^altiplano, or high plateau of the country.² Seismic activity is frequent here, and the valley is popularly considered to be an earthquake zone.⁴ It is the highest valley in the Volcanic Belt with an altitude of 2,200 meters above sea level³ and a surface area of 9611.4km2.⁵ It is surrounded by mountains that reach elevations of over 5,000 meters above sea level.³ While there is an opening to the north (at 19°3' north latitude and 99°1' west longitude⁴, it has no natural drainage outlet and therefore is called an endorheic basin.¹ The valley was artificially opened to drain lake and river waters beginning in the early 1600’s.³ Politically, the valley includes all of Mexico City proper, almost all of the Federal District and 96 municipalities located in the states of Mexico, Hidalgo, Tlaxcala and Puebla.⁵ The geology of valley consists of three distinct parts : a flat plain where rain, river flow and snow melt accumulate, a piedmont zone where the land begins to elevate towards the mountains and the mountainsides themselves.¹

The flat plain covers most of the valley floor and it is broken only by a number of small volcanic cones such as Mount Chimalhuacan, Mount Peñon de los Baños, Mount Peñon del Marques and Chapultepec. This floor used to be covered by five lakes with small mountains as islands. Much of the piedmont area used to be lakeshore.¹⁰ The valley is enclosed by a number of mountain ranges, the best-known of which is the Sierra Nevada, home of the volcanoes Iztaccíhuatl and Popocatepetl.¹ Other ranges include the Sierra del Chichinautzin to the southwest, Sierra de Las Cruces, the Sierra de Monte Bajo and Sierra de Monte Alta to the west. The Sierra del Rio Frio and the Sierra de Calpulalpan are to the east and the Mesa Grande, the Sierra de Tepozán and the Sierra de Pachuca are located to the north.¹ The Sierra Chichinautzin is the most recent mountain range, geologically, erupting approximately 600,000 years ago, and blocked what was the valley’s natural drainage.³ Many of the mountains surrounding the valley, such as Popocatepetl, are older than the valley itself.¹

The entire valley of Mexico can be sub-divided into subbasins. The basin in which Mexico City itself is found is the largest and it itself is also called the Valley of Mexico. There are three other sub-basins called Avenidas de Pachuca, Tecocomulco and Apan which aggregate to the northeast of the main valley.⁵³

The highest parts of the mountains and volcanoes surrounding the valley are still largely forested and act as the “lungs of the city” below,² and capture precipitation to restore the area’s aquifers and springs.² Most of these green areas for the city are located in the southern part of the Federal District, in the boroughs of Milpa Alta, Tlalpan and Xochimilco.⁴ Deforestation of these mountainside green areas has become a serious concern as its degradation is having effects on Mexico City’s air quality and ability to recharge is aquifer.² A number of these mountainous zones have been set aside as national parks. These include La Malinche National Park to the northeast of Mexico City in the states of Tlaxcala and Puebla, Iztaccíhuatl Popocatépetl National Park to the east of the city in the states of Mexico, Puebla and Morelos and Lagunas de Zempoala National Park to the southwest in the states of Mexico and Morelos. The last two are important to capture rainfall and recharge the Mexico City Metro Area aquifers.¹¹

Climate

The climate of the area ranges from a semi-arid belt in the far north of the valley to a tropical one in the far south.⁴ The average annual temperature is 12-15 degrees centigrade. The valley receives about 700 millimeters of annual rainfall, which is concentrated from June through September/October with little or no precipitation the remainder of the year.³ The area has two main seasons. The rainy season runs from June to October when winds bring in tropical moisture from the sea. The dry season runs from November to May, when the air is relatively drier. This dry season subdivides into a cold period from November to February when polar air masses pushing down from the north keep the air fairly dry and a warm period from March to May when tropical winds again dominate but they do not yet carry enough moisture for rain. Annual water evaporation from the basin is 1,640mm, most of which occurs in the warm months from March to May. Warmer temperatures occur in the lower, flat parts of the valley while areas in the piedmont and mountainsides, especially in the north of the Federal District and the eastern section Mexico State experience significantly colder temperatures.⁵

Hydrology

The Valley of Mexico is a closed basin which geologically divides into three hydrologic zones, the plain, which is essentially the lakebeds of the now-extinct lakes, the piedmont area which transitions to the last zone, the mountains themselves. The old lakebeds correspond to the lowest elevations of the valley in the south and are almost entirely covered by the Mexico City Metropolitan Area. This area is mostly clay with a high water content. In the piedmont area, these clays become mixed with silts and sands, and in some areas close to the mountains, the piedmont is largely composed of basalt from old lava flows. This basalt is highly-water-permeable with good storage capacity and considered an important component of the aquifer system. Most of the groundwater is derived from infiltrated precipitation from rain and melting snow from the higher elevations. This groundwater flow produces a number of springs in the foothills and upwellings in the valley floor.³ This underground flow is the source of the five aquifers that provide much of the drinking water to Mexico City located in Soltepec, Apan, Texcoco, Chalco-Amecameca and underneath Mexico City itself.⁵

The old lake system

Before the 20th century, the Mexico City portion of the valley contained a series of lakes, with freshwater lakes to the north near the town of Texcoco and saline ones to the south.³ The five lakes, Zumpango, Xaltoca, Xochimilco, Chalco, and the largest, Texcoco used to cover about 1,500 km2 of the basin floor.² The Sierra de Guadalupe and Mount Chiconaultla separated Lake Texcoco from Lake Zumpango and Lake Xaltoca while Mount de la Estrella, the Sierra de Santa Catarina and Mount del Pino separated it from Lake Xochimilco and Lake Chalco.¹⁰ All the other lakes flowed toward the lower Lake Texcoco, which was saline due to evaporation.² The lakes were fed by a number of rivers such as the San Joaquin, San Antonio Abad, Tacubaya, Becerra, Mixcoac and Magdalena Contreras, carrying runoff and snowmelt from the mountains. Lake Texcoco also took on overflow from the other lakes during the rainy season from June to October.¹⁰

Long before the arrival of the Spanish, the lake system had been shrinking due to the change in climate. Warmer temperatures had increased evaporation and reduced rainfall in the area so that the lakes’ waters were shallow at about five meters deep as early as the Tlapacoya culture, around 10,000 BC.¹² In the dry season during the Aztec empire (1325-1521), the northern lakes were inaccessible by canoe during the dry season from October to May.⁸

The arrival of the Spanish and subsequent efforts to drain the area for flood control, along with the over-pumping of groundwater has hastened the disappearance of the lakes. The old lakebeds are almost all paved² except for some canals preserved in Xochimilco, mostly for the benefit of visitors who tour them on brightly-painted trajineros, boats similar to gondolas.¹³

History of human habitation

First human habitation

The Valley of Mexico began to be a magnet for human habitation early as it had abundant game and was a region capable of growing substantial crops.⁶ Giant mammoths roamed the area and other parts of Mexico, with their bones still being found occasionally in fields today. This valley contains the most and the best-studied mammoth kill sites, most of which are located on what were the shores of Lake Texcoco in the north of the Federal District and the adjacent municipalities of Mexico State such as in Santa Isabel Ixtapan, Los Reyes Acozac, Tepexpan and Tlanepantla. Mammoth remains have been discovered in many parts of the Federal District itself including in Metro lines under construction and the neighborhoods of Del Valle in the center, Lindavista to the center-north and Coyoacan in the south of the city. The symbol for Line 4 of the Mexico City Metro is a mammoth, due to the fact that so many bones were uncovered during its construction. But perhaps the richest site for Pleistocene epoch animal remains is called Tocuila, a 45 hectare site located near the town of Texcoco in Mexico State.¹⁴ There is little evidence of what life was like for humans around this time. There is some evidence around the old lakeshores the first populations here survived by hunting, gathering and possibly by scavenging.⁷

Generally speaking, humans in Mesoamerica, including central Mexico, began to leave a hunter-gatherer existence in favor of agriculture sometime between the end of the Pleistocene era and the beginning of the Holocene. The oldest known human settlement in the Valley of Mexico is located in Tlapacoya, located on what was the edge of Lake Chalco in the southeast corner of the valley in contemporary Mexico State. There is reliable archeological evidence for the site for as far back as 12,000 BC; however, after 10,000 BC the number of artifacts increases significantly. There are also other very early sites such as those in Tepexpan, Los Reyes Acozac, San Bartolo Atepehuacan, Chimalhuacán and Los Reyes La Paz but they remain undated. These sites contain mammoth kills and the last two contain mammal bones that have been modified, which were also found in Tocuila.⁷ Human remains and artifacts such as obsidian blades have been found at the Tlapacoya site that have been dated as far back as 20,000BC, when the valley was semi-arid and contained species like camels, bison and horses that could be hunted by man.¹² However, the dating of these artifacts has been disputed.⁷

Pre-Teotihuacan

Tlatilco was a large pre-Columbian village and culture in the Valley of Mexico situated near the modern-day town of the same name in the Mexican Federal District. It was one of the first chiefdom centers to arise in the valley, flourishing on the western shore of Lake Texcoco during the Middle Pre-Classic period, between the years of 1200 BCE and 200 BCE. It was originally classified as a necropolis when it was first excavated, but it was determined that the many burials here were really under houses of which nothing remain. It was then classified as a major chiefdom center. The Tlatilcans were an agricultural people growing beans, amaranth, squash and chili peppers, reaching its peak from 1000-700 BCE.¹⁵ The next oldest confirmed civilization is in the far south of the valley and is called Cuicuilco. The archeological site is located where Avenida Insurgentes Sur crosses the Anillo Periférico in the Tlalpan borough of the city. The old settlement used to extend far beyond the current site but it is buried under lava from one of the volcanic eruptions that led to the culture’s demise as well as the modern city itself. The settlement was located where an old river delta used to form in the valley with waters from Mount Zecatépetl and the now Tlalpan Forest. Cuicuilco was probably a city by 1200 BC and began to decline around 100 BC-150 AD due to heightened volcanic activity in the area. However, even though the ceremonial pyramid was abandoned, the site remained a location to leave offerings up to 400 AD, despite the fact that lava from the nearby Xitle volcano covered it.¹⁶

Teotihuacan and the Toltecs

Around 2,000 years ago, the Valley of Mexico became one of the world’s most densely populated areas and has remained so since.² After the decline of Cuiculco, the population concentration shifted north, to the city of Teotihuacan and later to Tula, both outside of the lakes portion of the valley.⁸ Teotihuacan became an organized village around 800 BC but it was around 200 BC that it began to reach its height. When it did, the city had approximately 7,000 inhabitants and covered six km2 of territory. It was dedicated primarily to the obsidian trade and at its peak was an important religious center for the valley, receiving pilgrims.¹⁷ In the early eighth century, Teotihuacan ceased to be a major urban unit and the population shifted to Tollan or Tula just on the northern edge of the Valley of Mexico with the rise of the Toltec empire. In the 13th century, this empire fell as well.⁸

The rise of the city-state in the valley

After the end of the Toltec empire, the population shifted once again, this time to the lakes region of the valley. With this migration came the concept of a city-state based on the Toltec model. By the end of the 13th century, some fifty small urban units, semi-autonomous and with their own religious centers, had sprung up around the lakeshores of the valley. These remained intact with population of about 10,000 each under Aztec domination and survived into the colonial period. All of these city-states, including the largest and most powerful, Tenochtitlan, with more than 150,000 inhabitants, claimed descent from the Toltecs. None of these cities was completely self-sufficient which spurred a complex agricultural system and a conflictive political situation in the valley.⁸ These city-states had similar governmental structures based on the need to control flooding and store water for irrigating crops. Many of the institutions created by these hydraulic societies, such as the building and maintenance of chinampas, aqueducts and dikes, were later co-opted by the Spanish during the colonial period.¹⁸

The Aztec empire

The largest and most dominant city at the time of the Spanish conquest was Tenochtitlan. It was founded on a small island in the western part Lake Texcoco in 1325 and was extended with the use of chinampas, or floating gardens and fill in the shallow lake covering about 9,000 hectares.⁸ They controlled the lake with a sophisticated system of dikes, canals and sluices. Much of the surrounding land in the valley was terraced and farmed as well, with a network of aqueducts bringing fresh water from springs in the mountainsides into the city itself.² Despite being the dominant power, the need to rely on resources from other parts of the valley led to the Aztec Triple Alliance between Tenochtitlan, Tlatelolco and Tlacopan at the beginning of the empire; however by the time the Spanish arrived, Tenochtitlan had suppressed the other two allies, causing grievances that the Spaniards were able to exploit. Despite Tenochtitlan’s power outside the valley, it never completely controlled all of the valley itself. The Spanish would complete that process in the colonial period.⁸

By 1520, the estimated population of the valley was over 1,000,000 people.²

Mexico City metropolitan area

After the Conquest, the Spanish rebuilt and renamed the city. They started with essentially the same size and layout as the Aztec city but as the centuries progressed, the city grew as the lakes shrank. Disease and violence decreased the population in the valley but by the early 20th century, the population of just Mexico City was over one million people.⁸ A significant population explosion began early in the 20th century. The population of the city itself has doubled about every 15 years since 1900, mostly due to the fact that the federal government has favored development of the metropolitan area over other areas of the country. This has spurred investment in infrastructure for the city, such as electricity, other power sources, as well as water works. These have attracted businesses which in turn have attracted more population. Since the 1950’s, urbanization has spread out from beyond the bounds of the Federal District to the surrounding jurisdictions, especially to the north into the State of Mexico making for the Mexico City Metropolitan area, which fills most of the valley. Today, this metropolitan area accounts for 45 per cent of the country’s industrial activity, 38 per cent of GNP, and 25 per cent of the population. While population growth has slowed in even declined in the city proper, the outer limits of the metropolitan area keep growing. Much of this growth has occurred on the mountainsides of the valley, in the form of illegal settlements in ecologically-sensitive areas. Overall urban settlement in the valley has expanded from about 90 km2 in 1940 to 1,160km2 in 1990.² About 21 million people live in the metropolitan area, and about 6 million cars are in circulation.¹⁹ Much of its industry is concentrated in the northern part of the Federal District and the adjoining cities in the state of Mexico.⁴

Air pollution

The fact that the valley is surrounded by mountains, is at a high altitude, and subject to tropical climate patterns makes Mexico City vulnerable to severe air pollution problems.^{20 94} The basin has only one shallow opening, which is to the north. Growth of the metropolitan area, especially industry, has largely followed this gap into adjoining Mexico State. The high altitude makes for poor combustion of fossil fuels leading to unsafe levels of nitrogen oxides, hydrocarbons, and carbon monoxide.⁴ While pollution from human activities accumulates over four-to-six day cycles, similar to western U.S. cities, the valley’s wind patterns make it very difficult to clear out the resulting smog.⁹ The valley has its own internal wind patterns based on daily warming and cooling from sunlight. Wind patterns vary significantly with elevation and location within the valley itself, resulting in a complex wind pattern with no prevailing wind to push the contaminants in one direction. These work to keep the air circulation of the valley relatively closed.⁴ However, the most severe aspect of the valley’s climate is “thermal inversion,” a phenomenon most prominent during the winter months. Thermal inversion occurs when the cooler air of the valley is trapped by a layer of warmer air above it.⁹ Also keeping the smog in the valley is the prevalent wind pattern outside the valley in the winter. During these months winds push from north to south, moving air into the valley through the gap at the north, where most of the industry is located.⁴ In the summer, wind direction over the country of Mexico flows south to north and the lack of thermal inversion allows contaminants to escape the valley. During the rainy season, precipitation has a cleansing effect on the air.⁴ However, the valley’s southerly latitude and abundance of sunlight during these months allows for the creation of ozone and other dangerous compounds to form in the air and linger.²⁰ While still considered one of the most polluted places on the planet, the valley’s air pollution problems are not as bad as they were several decades ago. One major problem that was brought under control was the lead contamination in the air. This was solved principally by the introduction of unleaded gasoline. Two other contaminants that have been brought under control are carbon monoxide and sulfur dioxide.²⁰ The contamination problems that remain are primarily with ozone and small particles (soot) (between 2.5 micrometers and 10 micrometers).²⁰¹⁹ The metropolis did not meet acceptable air quality standards for ozone 209 days in 2006, although this is better than 1994, when 340 days did not meet standards.¹⁹ Today, ozone levels are still twice as high as the maximum allowed limit for one hour a year and this occurs several hours per day every day.⁴ Average concentration of small particles exceeded EPA limits by as much as three times the standard, with forty to sixty percent of these particles being small enough to lodge in the lungs. Thirty to fifty percent of the time, Mexico City’s levels of small particles of ten micrometers, the most dangerous, exceed levels recommended by the WHO.¹⁹ In the 1940’s, before large-scale burning of fossil fuels in the area, the visibility of the valley was about 100 km, allowing for daily viewing of the mountain ranges that surround the valley, including the snow-capped volcanos of Popocatepetl and Iztacihuatl. Since that time, the average visibility has come down to about 1.5 km. Mountain peaks are now rarely visible from the city itself.⁴ While in the past, reduced visibility in the valley was due to sulfur emissions, it is now due to small particles in the air.⁹ The effects on humans living in such an enclosed, contaminated environment have been documented, especially by Nobel Prize winner Dr Mario J. Molina. The major issue for him is small particle pollution because it damages health by penetrating deeply into the lungs.²⁰ According to him, the city’s residents lose about 2.5 million working days every year due to health problems associated with small particles. Another problem is air pollution’s effects on those with asthma. About nine percent of Mexico City residents have asthma and the city’s concentration of pollen and ozone make it worse.¹⁹

Draining the basin and water issues

History of water control in the valley

For two thousand years, humans have been interfering with and altering the hydraulic conditions of the valley, especially in the lakes region.¹² The Aztecs built dikes for flood control and to separate the fresh water of the northern lakes from the saline water of the southern ones. After the destruction of Tenochtitlan in 1521, the Spaniards rebuilt the Aztec dikes but found they did not offer enough flood protection.²¹

The idea of opening drainage canals first came about after a flood of the colonial city in 1555. The first canal was begun in 1605 to drain the waters of Lake Zumpango north through Huehuetoca which would also divert waters from the Cuautitlan River away from the lakes and toward the Tula River. This project was undertaken by Enrico Martínez and he devoted 25 years of his life to it. He did succeed in building a canal through nearby Nochistongo to the Tula Valley but the drainage was not sufficient to avoid the Great Flood of 1629 in the city. Another canal, which would be dubbed the “Grand Canal” was built parallel to the Nochistongo one ending in Tequisquiac. The Grand Canal consists of one main canal, which measures 6.5 meters in diameter and 50 km long,²²and three secondary canals, with most of it built between 1856 and 1867. Porfirio Díaz completed all of it officially in 1894 although work continued thereafter. Despite the Grand Canal’s drainage capacity, it did not solve the problem of flooding in the city. From the beginning of the 20th century, Mexico City began to sink rapidly and pumps needed to be installed in the Grand Canal, which before had drained the valley purely with gravity.²¹ Along with the pumps, the Grand Canal was expanded with a new tunnel through the low mountains called the Xalpa to take the canal past Tequisquiac.¹⁰ Even so, the city still suffered floods in 1950 and 1951.²¹ Despite its age, the Grand Canal can still carry 2.4 million gallons a minutes out of the valley, but this is significantly less than what it could carry as late as 1975 because continued sinking of the city (as much as seven meters) weakens the system of water collectors and pumps.²³²¹

As a result, another tunnel, called the Emisor Central, was built to carry wastewater. Although it is considered the most important pipe in the country, it has been damaged by overwork and corrosion of its 20-ft diameter walls.²³ Because of lack of maintenance and gradual decrease in this tunnel’s ability to carry water, there is concern that this tunnel will soon fail. It is continuously filled with water, making it impossible to inspect it for problems. If it fails, it would most likely be during the rainy season when it carries the most water, which would cause extensive flooding in the historic center, the airport and the boroughs on the east side.²⁴

Because of this, another new drainage project is planned that will cost $1.3 billion USD. The project includes new pumping stations, a new 30-mile drainage tunnel and repairs to the current 7,400-mile system of pipes and tunnels to clear blockages and patch leaks.^{23 25}

Drinking water and sinking

Historically, Mexico City’s potable water supply came via aqueduct from the mountain springs on the valley sides like that in Chapultepec as most of the water in Lake Texcoco was saline. These were originally built by the Aztecs and were rebuilt by the Spaniards. In the mid-1850s, potable groundwater was found underneath the city itself, which motivated the large-scale drilling of wells. Today 70% of Mexico City’s water still comes from five principle aquifers in the valley. These aquifers are fed by water from natural springs and runoff from precipitation. Only about thirty five years ago did Mexico City have problems supplying water to its population, when such was about six million, forcing it to appropriate water from outside the valley.² Today, Mexico City faces a serious water deficit. Because of increased demand of a growing population and industry as well as ecosystem degradation in the form of deforestation of the surrounding mountains, more water is leaving the system than is entering. It is estimated that 63m3/second of water is needed to support the potable and agricultural irrigation needs of Mexico City's population. The main aquifer is being pumped at a rate of 55.5m3/s, but is only being replaced at 28m3/s, or about half of the extraction rate, leaving a shortfall of 27.5m3/s.² This over-extraction of groundwater from the old clay lakebed has been causing the land upon which the city rests to collapse and sink. This problem began in the early 1900’s as a consequence of the drainage of the valley for flood control. Since the beginning of the 20th century, some areas of Mexico City have sunk nine meters.² In 1900, the bottom of the lake was three meters lower than the median level of the city center. By 1974, the lake bottom was two meters higher than the city.The first signs of dropping ground water levels was the drying up of natural springs in the 1930’s, which coincides with the beginning of intensive exploitation of the aquifer system through wells between 100 and 200 meters deep.³ Today, Mexico City is sinking between five and forty cm per year, and its effects are visible. The “Angel of Independence” statue, located on Paseo de la Reforma was built in 1910, anchored by a foundation deep beneath what was the surface of the street at that time. However, because the street has sunk around it, steps have been added to allow access to the statue’s base.² Subsidence of the valley floor beneath has caused flooding problems as now much of the city has sunk below the natural lake floor. Currently, pumps need to work 24 hours a day all year round to keep control of runoff and wastewater.² Despite this, flooding is still common, especially in the summer rainy season, in lower-lying neighbourhoods such as Iztapalapa. Many residents have built miniature dikes in front of their houses to keep heavily polluted rainwater from entering their homes.²³ Subsidence also causes damage to water and sewer lines, leaving the water distribution system vulnerable to contamination which carries risks to public health.³ Measures other than drainage have been implemented to contain flooding in the city. In 1950, dikes were built to confine storm runoff.³ Rivers that run through the city were encapsulated in 1950 and 1951.²¹ Rivers such as the Consulado River, Churubusco River and the Remedio River are cased in concrete tunnels which take their waters directly to the drainage system to leave the Valley. Two other rivers, the San Javier and the Tlalnepantla, which used to feed the old lake system, are diverted before they reach the city and their waters now flow directly into the Grand Canal. None of water from these rivers is allowed to sink into the ground to recharge the aquifer. While the rivers and streams that flow down from the mountain peaks still begin the way they always have, their passage through the shantytowns that surround Mexico City turns them into sewer lines for these neighborhoods with no city services. Therefore, the goal is to keep this water from contaminating the aquifer.²⁶

References

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