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Recent environmental in the basin

Like many other Latin American countries, Mexico entered the twentieth century under a paternalistic, semi-feudal regime that was determined to incorporate the industrial revolution to the country, while maintaining the privileges of the ruling elite. Under the prolonged dictatorship of Porfirio Díaz from 1884 to 1911, factories and railroads were built, and Mexico City was modernized for the benefit of a small, centralist, and very powerful bourgeoisie whose aim was to transform the wealthier quarters of Mexico, copying the plan of contemporary European cities. The newly laid railroads brought peasants looking for employment in the new industries, and some of the smaller towns nearer the old city, like Tacuba, Tacubaya, and Azcapotzalco, were engulfed by the urban perimeter.

Between 1910 and 1920, the Mexican revolution brought a decade of ruthless confrontation between the old Porfirian bourgeoisie, which defended its privileges, and other social sectors demanding more participation in the distribution of the national wealth. This was one of the first social revolutions of modern times, and it marked the beginning of a series of large-scale social uprisings that shook the world in the twentieth century. Mexico City had at that time approximately 700,000 inhabitants and, unexpectedly, suffered little damage. The revolution was mostly a rural movement, and the city became a haven for middle-class provincial families, who flocked into the Basin of Mexico searching for protection under the new bureaucracy and the rising industries.

During the post-revolutionary period (1920-1940), and particularly after World War II, the industrial growth of Mexico developed at an increasing speed. Mexico City became an industrial complex, and a massive migration started from the country into the urban area. In less than 80 years the population of the urban conglomerate jumped from less than 1 million to nearly 18 million in 1987. Former peripheral towns, like Coyoacán, Tlalpan, and Xochimilco, were incorporated within the urban perimeter. A deep drainage system was built to remove the torrential surface runoff from the urban portion of the basin, while most of the old lake-beds dried up. The overexploitation of the aquifer system and the contraction of the expansive clays of the former lake-bed on which the modern city sits above the former Aztec capital of Tenochtitlán lowered the centre of the city by approximately 9 m between 1910 and 1987. The extremely low wind speeds in the high-altitude basin, together with intense industrial activity and the emissions of 3 million vehicles, have degraded the quality of the atmosphere in the basin to levels that are dangerous to human health. These and other changes are explored in detail below.

Vegetation and surface water

Several geographical characteristics influence the development of a diverse flora in the basin: its high altitude and inter-tropical location, the high mountain ridges that surround and isolate the valley bottom, and the fact that the Central Volcanic Axis of Mexico constitutes a boundary area between the Nearctic and the Neotropical biogeographic regions. Additionally, internal geological discontinuities create different patches originally covered with distinct vegetation types. Rzedowski (1986) remarked that in the 7,500 km² of the basin more species could be found than in several European countries. This vegetational diversity, however, is now heavily disrupted by human use and urban growth, continuing a process that began with the emergence of large populations in the basin over 2,000 years ago (Sanders 1972).

The different vegetation types in relation to the environment, described by Sanders (1976) and by Sanders, Parsons, and Santley (1979), were summarized in the introduction to this chapter. Several detailed classifications have been made of them. The most important ones are the studies of Miranda and Hernández Xolocotzl (1963) and Rzedowski (1975). Table 7.3 summarizes Rzedowski's ecological zones, their main characteristics, and the vegetation associated with them.

During the long years of human occupation, the environment of the Basin of Mexico has changed drastically (Niederberger 1987), and probably irreversibly in many aspects. Changes range from transformations of the natural systems, without altering their main ecological structure, to radical elimination of whole ecosystems, with the concomitant extinction of species and lateral effects on the surroundings. One of the earlier and important changes in the landscape of the basin was associated with agricultural activities in its shallow lakes and wetlands. Chinampas, an extremely efficient form of wetland cultivation based on the construction of canals on the wetland and lake floors and platforms of earthen and vegetal materials between the canals, created an intricate network of water and fields (raised) on the surface. The effect was to lower the water table (via the canals) relative to the raised planting surface while maintaining subsurface irrigation at all times. The planting beds were held together with the roots of a willow (Salix bomplandiana, locally known as ahuejotes) planted along their periphery, the slender silhouette of which is characteristic of lakes Chalco and Xochimilco where this highly efficient agriculture developed and still persists.

Corn, beans, squash, chile, and flowers, among other species cultivated on these extremely rich soils, faced no water shortage. Their growth was not rainfall dependent, and aquatic muck from the canals provided a regular source of plant and soil nutrients. When the top soil was exhausted in one chinampa, it was thrown back into the lake or canals, and a new layer was taken out. During the apex period of chinampa use (Late Horizon), the chinampa system was protected from and expanded into the saline waters of Lake Texcoco by the use of hydraulic controls (Palerm 1973).

The arrival of the Spaniards and the succeeding conquest of the Aztec empire were turning points in the use of natural resources in the basin. Tenochtitlán was a lacustrine city, constructed in the western portion of Lake Texcoco. Most transportation to and within the city was by shallow-bottom canoes (chalupas) that moved across the lakes and through the network of chinampa and city canals. As power changed hands, a new view of the world was imposed, complete with a new version of landscape configuration and use. The introduction of Spanish preferences and European biota and technologies included a demand for wheat and livestock, the use of the plough, and the use of large animals for transport and ploughing (Whitmore and Turner 1992). These changes required open, dry land, not a lacustrine system. Tenochtitlán now called Mexico, was appropriately transformed by converting the canal networks into roads and draining water away from the centre of the city. The alluvial plains and piedmonts were also transformed, first by the loss of Amerindians to keep up the ancient terrace systems, and secondly by Hispanic-controlled uses. Although the degree of change in these areas is unclear, apparently considerable deforestation on the upper slopes took place to construct the new, Hispanic Mexico City and to provide wood fuel (as much as 25,000 trees per year). Erosion on slope lands, perhaps well under way during the Late Horizon (Williams 1989), was exacerbated in some cases by overgrazing livestock but diminished in others owing to decreased land pressures attendant on the dramatic depopulation of the Amerindians and to their improved ability to control their own lands as formal colonial policy was better enforced (Butzer 1991; Butzer and Butzer 1995).

Table 7.3 Vegetation zones in the Basin of Mexico

Sources: Sanders (1972); Sanders, Parsons, and Santley (1979); Rzedowski (1975).

Major environmental changes took place in Mexico owing to the conquest, but the degrading nature of all or most of these may have been overstated. Major afforestation occurred initially because of the reduced native population. And, according to the Butzers (1995), the Spaniards and Amerindians readily adapted their livestock and other land-use practices to local environmental conditions. With the exception of the destruction of the lacustrine system in the Basin of Mexico, major environmental degradation, as opposed to change, perhaps waited for the nineteenth century.

Desiccation of the lakes and deforestation of the upper slopes of the basin continued through the colonial and independence period of Mexico and into the beginning of the present century. This included the construction of the canal to drain the lakes. In the 1930s, formal action was taken to protect the forests and the mountain catchment areas around the basin. Lázaro Cárdenas, President from 1934 to 1940, changed the official attitude towards the use of natural resources in the basin. Most of the basin's 20 national parks were established between 1936 and 1939. Only one national park, Desierto de los Leones, had been established previously, in 1917. Among the most recently created protected areas is a small part of the Pedregal de San Angel, a unique plant community that developed on a basalt outcrop in the southern part of the basin, mostly on the grounds of the National University.

Now 80,164 ha of land within the basin receive formal protection, but most of it suffers severe disruption by erosion and deforestation. Some of this land has completely lost its plant cover and is little more than part of the urbanized area of the basin. The deterioration of national parks set in shortly after their creation. For example, during 1946-1952, while Miguel Alemán was the President of Mexico, the Cumbres del Ajusco National Park was given in concession to one of Mexico's most important paper factories.

Mexico City inherited one of Tenochtitlán's main traits: the high density of its human population (see Whitmore et al. 1990). Since 1940, however, the population has increased constantly at rates of 4-5 per cent per year, so peripheral towns like Coyoacán, San Angel, Atizapan, and Tlatelolco have become an integrated part of the megalopolis. The agricultural lands that separated them have completely disappeared. Only a small proportion of the lakes has survived. Lake Chalco was drained in 1954, and only Xochimilco and Tlahuac subsist as chinampa areas. The remaining chinampas are withering because of the deteriorating quality of the water.

This urban and industrial growth has had enormous impacts on many aquatic, subaquatic, and halophyllous species, many of which have become extinct. Some vegetation types on the surrounding slopes are also on the verge of extinction, as is the case of some forests on the upper slopes of the southern mountains. The arboreal communities have been deeply damaged. More than 9,000 ha of trees have disappeared in only the past 10 years, and insect pests are fiercely attacking some of the protected zones, such as the Desierto de los Leones and the famous Chapultepec Park. Many introduced tree species have been used to reforest, resulting in a loss of the animal species that were associated with the native vegetation. In particular, eucalyptus was planted on a large-scale basis throughout the basin because its demand for water helped to dry up the lake system. Overall, monospecific communities have replaced the species-rich flora of the Basin of Mexico.

In addition to these changes, the plants that remain are affected significantly by air pollution. Little is known about the effects on plants of pollutants such as ozone and lead, both of which are a problem in the basin (see below). Hernández Tejeda, Bauer, and Krupa (1985) and Hernández Tejeda and Bauer (1986) have detected effects of specific pollutants on the growth and photosynthetic rates of trees.

Urbanization

The population of the metropolitan area of Mexico City has accelerated at a particularly rapid rate since the end of the Mexican revolution. From 1950 to 1986, the average annual growth rate was 4.8 per cent (table 7.4). The population has grown more quickly in the industrial area of the neighbouring State of Mexico, north of the Federal District, where the average rate of increase has been 13.6 per cent, compared with 3.3 per cent in the Federal District. Much of the high growth rate of the city is due to the continuous arrival of migrants from the economically depressed rural areas (Goldani 1977; Stern 1977; Unikel 1974). Between 1970 and 1980, for example, 3,248,000 immigrants arrived in Mexico City (Calderón and Hernández 1987). The intrinsic annual growth rate of the city, therefore, can be calculated as approximately 1.8 per cent, considerably lower than the national average, which was around 3.0 per cent for the same period. In short, it is immigration more than reproductive growth that maintains the high rate of increase of the population of Mexico City.

Applying the 1980 growth rates to the 1987 population of 18 million, one can calculate that every day around 900 babies are born and 1,500 new immigrants arrive in the Basin of Mexico. Since many of the newborn are those of immigrants, migration further increases the growth rate of the population by elevating the natural birth rates (Goldani 1977).1

The growth rate of the city in spatial extent, estimated from the urban areas measured on aerial photographs, is close to that of the population (5.2 per cent, see fig. 7.5 and table 7.5). In 1953, the urban area covered 240 km² (8 per cent of the basin), whereas by 1980 it had increased to 980 km² (33 per cent of the basin). The expansion, however, has not kept the old style of urbanization. In many parts of the basin, especially in the poorer areas, the new developments are more dense and less planned and generally include fewer open spaces.

Table 7.4 Population in Mexico City, 1519-1986 (millions)

Sources: DDF (1987); and projections by the authors.
a. Pre-1950 dates have been chosen as approximate indicators, and correspond with the important historical dates indicated in parentheses.
b. Projected value.
c. Preliminary values of the 1990 population census.

Many developments that are now built on hill slopes generate a considerable amount of soil erosion and a significant increase in flash floods after rainstorms (Galindo and Morales 1987).

Table 7.5 Total urban area estimated from aerial photographs, 1953

Sources: DDF (1986); and projections by the authors.

Fig. 7.5 Growth of the urban area of Mexico City, 1910-1990 (Source: Macgregor, GonzálezSánchez, and Cervantes, 1989)

Table 7.6 Rate of change of green areas within the Metropolitan Area of Mexico City, 1950-1980 (estimated from aerial photograph samples)

Source: Lavín (1983).

In 1950, the urban area included a large proportion of agro-pastoral fields, together with numerous empty lots, parks, and public spaces. The relative frequency of these and other open spaces within the city has decreased considerably (table 7.6), but at different rates. Agropastoral fields, previously very important within the city as dairy farms, and domestic maize fields (or milpas) have been disappearing at an annual rate of 7.4 per cent and are now practically absent within the city. Most of these areas are occupied by industrial and housing developments. Parks, private gardens, and public spaces have been somewhat better conserved, disappearing from the city at an average rate of 1.5 per cent. New roads have taken up much of the park and public spaces. Overall, vegetated areas have been decreasing at an annual rate of 3.7 per cent (Guevara and Moreno 1987).

Lavín (1983) conducted an analysis of the rate of change of vegetated ("green") areas within different sectors of Mexico City from 1950 to 1980. She found that the total rate of change of green areas varied considerably from one sector of the city to another. Among her key findings were:

• The east of Mexico City (in particular Ciudad Netzahualcoyotl, with some 2.5 million inhabitants), where the larger proletarian settlements lie, was changing most quickly. Nearly 6 per cent of its open space disappeared each year over the 30-year period (table 7.7).

• Open spaces were disappearing most slowly in the old centre of the city (- 1.0 per cent).

• The rate of change within urbanized areas depends on the social status of their inhabitants and on the time of their establishment. In the poorer and more recently established areas, vacant land was quickly transformed into new houses, leaving less green area per person.

• Although some quarters have more than 10 m² of green land per per son, others have much less. Azcapotzalco, an industrial quarter with a population of some 700,000, has at present 0.9 m² of green area per inhabitant (Barradas and J-Seres 1987; Calvillo-Ortega 1978), whereas the United Nations Environmental Programme (UNEP) suggests a minimum of 9 m² per inhabitant.

The worst problem associated with urbanization in the Basin of Mexico is not so much the size of the city and the demands it generates, both of which create problems enough, but the phenomenal rates of urban growth. These rates generate enormous pressure on the capacities of the social and environmental systems. As the government struggles to meet existing demands, new demands are surfacing at an ever-increasing pace. The projection of present trends (understanding that this is an exercise only) shows that, by the year 2000, Mexico City will occupy around 2,700 km² with a population approaching 30 million (Ezcurra 1990a). Houses and roads will take up most (92 per cent) of the city surface, whereas parks, private gardens, and public spaces will cover a mere 6 per cent. The people living in the basin will enjoy only 5 m² of green area per capita. In the poorer parts of this future city, the inhabitants will have less than 1 m² of open public spaces for recreational use. Mexico City will have been transformed into pavements and roofs (Ezcurra 1990b; Ezcurra and Sarukhan 1990).

Water resources

Water supply

Water resources have always been an important component in the environmental-use history of the basin (Lara 1988; Serra Puche

Area of Mexico City, 1950-1980 (estimated from acrid photograph samples) 1991). Before massive lake drainage and urbanization, runoff from the surrounding mountains filled the lower part of the basin during the rainy season, creating the basin's lacustrine system. During the dry season, evaporation and use conspired in the spatial fragmentation of this system (Bribiesca 1960). As noted above, this system has been destroyed, and the natural bodies of water have almost disappeared. At present, all that remains is a small section of Lake Texcoco and some of the old chinampa canals in Xochimilco; Lake Zumpango, which dried in the 1970s, has now been dredged and refilled.

Table 7.7 Rate of change of green areas within different sectors of the Metropolitan

Source: Lavín (1983).

Runoff flows through seasonal rivers (all small and localized) only during the rainy season. The aquifers that underlie the city, mainly in the northern and southern parts of the old lacustrine zone, had artesian pressure until the nineteenth century and even until 1920 in some areas. Today, pumping has reversed the gradients; natural flow in springs has ceased. Water tends to move downwards through the ancient lake sediments, from which it is heavily pumped (Marsal and Mazari 1969; Mazari and Alberro 1991; Ortega 1988).

Natural water infiltration has decreased in the basin as urbanization has advanced over what previously were water-recharge areas. The almost 1,000 km² of urbanized land generate massive runoff during the rainy season, but almost all this water is lost through the drainage system and does not infiltrate the soil. Various authors have estimated the recharge of the aquifer system to be some 23-27 m³ per second, less than 50 per cent of the volume (50-52 m³ per second) that is currently extracted (Ezcurra 1990a; Garza 1987; Murillo 1990).

Excessive groundwater extraction from the basin has significantly influenced the lacustrine clays and, in the lower strata of the basin sediments, particularly the soil mechanics involved. For the last century, the base level of the city has sunk continuously, and the progression of this subsidence has paralleled that of water extraction. From the beginning of the century until 1938, the rate of subsidence was 4.6 cm per year; in the early forties it increased to 16 cm per year. The greatest subsidence was registered during 1948-1956, a period in which the city sunk at a rate of 30 cm per year. This problem forced the closing of several extraction wells and their relocation north and south of the city in 1954. During the late 1950s, subsidence decreased to 7.5 cm per year, and in the eighties dropped to 4.5 cm per year (Mazari, Marsal, and Alberro 1984; Mazari and Alberro 1991).

A considerable volume of the water consumed in the Basin of Mexico is transported, at high expense, from other basins where it is also needed (table 7.8). In 1976, the city used 1.3 billion m³ of water at an average rate of 41 m³ per second; 30 per cent of this (12 m³ per second) came from the Lerma basin (DDF 1977). At present, the city uses 5764 m³ per second (Alvarez 1985; DDF 1989; Herrera and Cortés 1989; Murillo 1990), of which 18 m³ per second (about 570 million m³ per year) are pumped from the Lerma and Cutzamala basins in the neighbouring states of Mexico, Michoacán, and Guerrero (DDF 1989). Increasing dependency on external sources of water is affecting the very basins from which the water is extracted.

The average daily supply of water in Mexico City is around 300 litres per person, more than in many European cities (Alvarez 1985). Even so, many parts of the city suffer from chronic water shortages, especially during the dry season. Industrial use is very inefficient, wastewater recycling is only about 6 per cent of the total used, and nearly 15 per cent of the water supply is lost through deficient piping systems (table 7.9). Pipe breakage in the muddy subsoil of the old lakebed also represents a potential health hazard, since the migration of micro-organisms and chemicals from the sewer system could contaminate drinking water.

Table 7.8 Water supply systems for the Federal District, 1988

Sources: DGCOH (1989); Guerrero, Moreno, and Garduño (1982); SARH (1985).
a. Water systems in the State of Mexico, also supplying the urban area, are reported globally.

Table 7.9 Distribution anal consumption of water in the Federal District

Source: SARH (1985).
a. Water systems in the State of Mexico, also supplying the urban area, are not reported.

Water quality is also a controversial and poorly understood problem. For decades attention has focused on the bacteriological aspects of water quality. Infectious and parasitic diseases, partly related to poor-quality drinking water, continue to rank among the five most common causes of death in the country, especially for infants (CAE 1990; Martínez-Palomo and Sepúlveda 1990; WHO 1988). Inorganic compounds degrading water in the basin have been studied and are well defined in the drinking-water regulations. Mexico, like many other Latin American countries, is still out-dated in the analysis and regulation of organic chemicals in water. Organic pollutants, mostly in the form of synthetic products used by industry, represent an impending problem since untreated residues are dumped into the drainage system.

Drainage system

Mexico City uses a combined system that carries stormwater and untreated water through sewers, rivers, open canals, reservoirs, lagoons, pumping stations, and a deep drainage system (Guerrero, Moreno, and Garduño 1982). At present the wastewater-treatment capacity through 24 plants is 3.5 m³ per second, which accounts for approximately 6 per cent of the water used in the basin (Murillo 1990; see table 7.10). Untreated wastewater flows to the north into the Tula River basin, before it flows to the Moctezuma-Pánuco river system, passing through the Zimapán Hydroelectric Project (currently under construction), and on to its final destination, the Gulf of Mexico.

Wastewater is discharged into the Tula basin through two systems: an open surface channel (the Gran Canal), which has been in operation since the beginning of the twentieth century, and a closed, deep underground drainage system (the Drenaje Profundo), which was built in the early 1970s. As the city subsided through overexploitation of the aquifer, the Gran Canal lost its slope and stopped functioning. At present, water has to be moved up slope through the channels by means of auxiliary pumping stations. The maximum drainage capacity of this system is 100 m³ per second. The closed drainage system is composed of a network of underground tunnels 30-200 m deep. It operates mostly during the rainy season. Since it was built to account for subsidence, it does not require supplementary pumping. During the rainy season, it can carry a maximum flow of 200 m³ per second (Guerrero, Moreno, and Garduño 1982).

Table 7.10 Sewage treatment elands in the Federal District. 1982

Source: SARH (1985).
a. Since 1982, three new plants have been constructed in the Federal District, and 13 new plants in the State of Mexico, giving a total working capacity of approx. 3,500 litres/s in 1991 (Murillo 1990; Comisión Nacional del Agua, unpublished data).

Waste

Liquid waste

Untreated wastewater is eliminated from the basin by means of both drainage systems. The drainage water is used mostly in the State of Hidalgo to irrigate 580 km² of agricultural fields (Strauss 1988). The untreated wastewater used for irrigation is a significant source of soil and plant contamination in agricultural areas. For example, the mean concentration of surfactants in the water used for irrigation is around 13 mg/litre (Mazari Hiriart 1992). If a field is irrigated with about 2,000 mm per ha per year (i.e. 20,000 m³ha^-1y^-1, a common volume for the area), it will receive around 260 kg of surfactants per hectare each year. Heavy metals as a group can show concentrations as high as 0.75 mg/litre in the wastewater used for irrigation. This means that as much as 16 kg of heavy metals can be incorporated every year into a hectare of irrigated agricultural land. A similar situation obtains with boron, which has mean concentrations of 1.1 mg/litre in the Gran Canal (DDF 1979), representing about 22 kg of boron incorporated into each hectare of agricultural fields every year. One of the main causes of water contamination is the dumping of industrial waste in the sewer system as a means to dispose of toxic substances. The basin urgently needs a waste-processing plant, for it currently has no disposal system for chemical and toxic waste other than the city sewers or transportation to distant landfills.

Irrigation with wastewater in Hidalgo has also generated a severe problem of microbiological contamination. Using most-probable-number (MPN) procedures, total coliform counts have been reported in the ranges of 6 x 10⁶ to 2 x 10⁸ MPN/100 ml in the Gran Canal waters, which are used directly for irrigation at Chiconautla. Very high densities of colibacteria have also been reported in vegetables grown at these sites. Median values of faecal coliform counts are 43 MPN/10 g within plant tissues and 96 MPN/10 g in plant surfaces. Some samples, however, have shown faecal coliform counts as high as 3,000 MPN/10 g (Strauss 1986). Additionally, viable amoebic cysts have been found in the Canal waters and in irrigation ditches (Rivera et al. 1980).

Solid waste

The city produces approximately 12,000 tons of domestic solid waste per day. Almost one-half (48 per cent) of the solid waste produced is of industrial origin, while the remaining 52 per cent is domestic waste (table 7.11). Around 50 per cent of the domestic waste is organic refuse, and the rest is paper (17 per cent), glass (10 per cent), textiles (6 per cent), plastics (6 per cent), metals (3 per cent), and other refuse (9 per cent). In contrast with developed countries, which generate garbage with a low proportion of organic residues, the waste of Mexico City is rich in vegetable and fruit waste (Restrepo and Phillips 1985). Until 1987, the disposal of most of these residues in open yards represented a public health hazard. These dump-yards have now been closed, but many smaller, and very often clandestine, disposal yards exist throughout Mexico City. In 1986, the inauguration of a more modern system of landfills east of the city reflected an attempt to deal with, in part, the tremendous environmental problem posed by garbage disposal.

Table 7.11 Concentration of industries production of industrial waste in the different municipalities within Mexico City

Source: Dirección General de Servicios Urbanos, Direccíon de Desechos Sólidos, Departamento del Distrito Federal, unpublished data, 1990.

Air quality

One of the worst environmental problems associated with the uncontrolled growth of the city (and certainly the one that is most perceived by the population) is the high level of atmospheric pollution in the basin (SAHOP 1978; SMA 1978b,c). This problem is particularly severe during the winter season (December to February), during which the low temperatures stabilize the atmosphere above the basin and the air pollutants accumulate in the stationary mass of air that hovers over the city (SEDUE 1986; Velasco Levy 1983). Studies of the lead (Pb) and bromine (Br) content in the air particulate pollutants in Mexico City have shown that most of the air pollution originates from automobile exhaust (Barfoot et al. 1984; Sigler Andrade, Fuentes Gea, and Vargas Aburto 1982). Government estimates show that motor vehicles are responsible for 85 per cent of all atmospheric pollutants in Mexico City. In some parts of the city, particularly towards the east central area, the concentration of total suspended particles exceeds the Mexican and the international air-quality standards more than 50 per cent of the time (Fuentes Gea and Hernández 1984).

Table 7.12 Number of vehicles in Mexico City, 1978-1989

Sources: Legorreta (1988); and projections by the authors.
a. Projected.

Although air quality during the rainy season has remained more or less constant since the mid-1980s, the total of suspended particles during the dry season is increasing at approximately 6 per cent per year (calculated from Fuentes Gea and Hernández 1984). In agreement with the idea that most of the atmospheric pollution derives from automobile exhaust, the number of cars in the city is also increasing at a 7 per cent annual rate (more than 3 million cars in 1986; see table 7.12). According to these data, the deterioration in the air quality in the Basin of Mexico during the dry season outstrips the rates of population growth and urban expansion. If the trend continues, in a few years atmospheric pollution will exceed the acceptable air-quality standards most of the time, with very serious consequences for human health.

According to Bravo Alvarez's (1987) detailed report, vehicles produce most of the carbon monoxide and hydrocarbon residues in the basin, but fixed sources are responsible for most of the particles, sulphur dioxide, and nitrogen oxides (table 7.13). Particulate pollution is highest towards the east central part of the city, but sulphur dioxide is highest in the north, where most of the industries are located. Until 1986, lead was probably the worst pollutant in the atmosphere of the basin (Salazar, Bravo, and Falcón 1981). Only high-leaded gasoline was sold in Mexico City at that time, and the concentration of lead in the air increased steadily with the number of cars, reaching an average value of 5 m g/m³ 1968 (Halffter and Ezcurra 1983) and around 6 m g/m³ in 1986 (four times the Mexican standard of 1.5 m g/m³ see table 7.14).

Table 7.13 Atmospheric emissions estimated the Metropolitan Area of Mexico City 1983

Source: Bravo Alvarez (1987).

Table 7.14 Average concentration of lead in the atmosphere of Mexico City, compared with several cities in the United States, 1970

Source: Bravo Alvarez (1987).

The problem became so critical that from 1980 on the national oil company (PEMEX) gradually decreased the concentration of lead in the gasolines sold in Mexico City (table 7.15). In September 1986, regular leaded gasoline in the basin was replaced by a low-lead fuel in which synthetic oxidizing additives replaced the action of leaded compounds. The change produced unexpected side-effects. Although the atmospheric concentration of lead indeed decreased, ozone concentrations rose quickly as a result of a reaction between ultraviolet solar radiation, atmospheric oxygen, and the oxidizing effect of the new gasoline additives (Bravo Alvarez et al. 1991). The present mean ozone concentration is, on average' around 0.15 ppm (300 m g/m³ 10 times the normal atmospheric concentration, more than double the maximum limit in the United States and Japan (Avediz Asnavourian 1984), and high enough to damage most of the urban vegetation (Skärby and Selldén 1984). Because of the time-lag involved in the formation of ozone, the highest ozone levels are registered towards the south-west of the city in the direction of the prevailing winds. During the winter of 1987-1988, the ozone levels in this area exceeded the maximum allowable standard (0.11 ppm) more than 50 per cent of the time and generated continuous health complaints from the population.

Table 7.15 Concentration of lend tetra-ethyl in Mexican gasolines, 1978-1990 (ml/litre)

Source: Bravo Alvarez, Sosa, and Torres (1991).
a. Last year that Extra gasoline was sold.
b. For pre-1991 vehicles.
c. For 1991 vehicles with catalytic converters.

The distribution of air pollutants within the city is not uniform. Whereas suspended particles and sulphur dioxide tend to concentrate above the industrial sectors of the city (north and north-east) (fig. 7.6), carbon monoxide shows high concentrations near the centre of the city, where automobile emissions are greatest. Ozone, which is a result of photochemical reactions, is produced from pollutant precursors with a certain time-lag. Thus, ozone tends to concentrate towards the south-west, as the dominant winds usually blow from the north-east. It is interesting to note that the south-west of the city is a residential area, with relatively open spaces and lower population densities. In spite of being one of the environmentally most benign areas of the city, it receives the highest concentrations of ozone, the worst pollutant during 1991.

Atmospheric pollution also has a considerable influence on the quality of rainwater. Páramo and colleagues (1987; see also Bravo Alvarez 1987) reported, for the 1983-1986 period, a significant decrease in the pH of incoming rainwater in Mexico City owing to the increasing concentration of sulphur and nitrogen oxides in the air.

Fig. 7.6 Concentration of suspended particles m g/m³ sulphur dioxide (ppm), carbon monoxide (ppm), and ozone (ppm) above Mexico City, 1990 (Source: Jáuregui, 1990)

In the urban parts of the basin the average pH of rainwater is around 5.5, and a few rain events have been registered with pH values as low as 3.0. The effects of air pollution are not restricted to the urban areas; they can also have considerable impact on the surrounding natural ecosystems. Hernández Tejeda, Bauer, and Krupa (1985; see also Bauer, Hernández Tejeda, and Manning 1985; Hernández Tejeda, Bauer, and Ortega Delgado 1985; and Hernández Tejeda and Bauer 1986), for example, have found that the ozone produced above the city and carried by the dominant winds to the Sierra del Ajusco, south-west of the basin, have significantly reduced the chlorophyll content and the growth of Pinus hartwegii, the dominant pine species in the high mountains (c. 3,500 m) around the basin. One of the main functions of these forests is the collection of water for the city. Thus, atmospheric pollution may have a considerable impact on the water balance on the hillslopes of the basin and consequently on the availability and quality of water used for human consumption.

Drainage of the lake-beds, related to a seasonal phenomenon of dust storms between February and May, has also affected air quality. The midday air temperatures during the dry season generate strong advective currents that elevate salt and clay particles from the former lake bottom. These particles are blown into the city by the prevailing easterly winds. The problem of dust storms peaked in the 1970s and has declined slightly since (Jáuregui 1983). The decline (or at least the lack of increase) in soil particles in the atmosphere during the dry season seems to be associated with successful government efforts to vegetate the dry mud-bed of the former lake Texcoco (Jáuregi 1971, 1983), which has now become a pasture of halophyllous grasses and fortes. In spite of this moderate success, faecal contamination from wastewater is still common in the lake-beds, and the dust storms remain a potential source of infection. The concentration of faecal bacteria in the rainwater of Mexico City is 100-150 micro-organisms/litre (Soms García 1986). Gamboa (1983) sampled the micro-organisms suspended in the air of Mexico City and found a significantly high frequency of potential pathogens.

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