Mexico City: Opportunities and Challenges for Sustainable Management of Urban Water Resources
December, 2004
Wastewater disposal and reuse
The unique physical characteristics of the basin that pose such a challenge for sourcing water for the huge population of Mexico City, present a similar problem for its disposal.
Mexico City generates vast quantities of wastewater, and the question of how to dispose of it and where to put it has become an increasingly costly and contentious issue. Mexico City is running out of sinks. This problem is compounded by the inescapable ecological fact that where it hopes to source its water in the future is also where much of its wastewater is sent.
Currently, only 10 per cent of Mexico City's wastewater is treated. There are 27 different treatment plants, but many are operating under capacity because there is not enough storage available for the wastewater (Morgan, 1996). The remaining 90 per cent of the municipal wastewater is untreated and diverted out of the Basin of Mexico through Mexico City's extensive network of drainage. In the Federal District alone, the drainage system has a primary network of 1,217 km and a secondary network of 12, 299 km (Perez de Leon and Biswas, 1997). This network includes an enormous pumping system because subsidence has caused the gran canal, the open drainage canal in use since the early 1900s, to sink below the lowest point of the basin floor. Wastewater that used to be gravity-controlled must first be pumped to Texcoco Lake and then to the gran canal where it passes out of the Basin of Mexico. Wastewater also leaves the city via the deep drainage system (drenaje profundo) constructed in the 1970s. This drainage system, however, is not available for the 25 per cent of the city's population that is located around the city limits. In these areas, wastewater is discharged into surface water, ditches, or even streets, where it subsequently contaminates the aquifer.
The question of wastewater disposal poses serious problems for Mexico City's planners and engineers, who are once again confronted by the constraints of a closed hydrological system. The answer to this dilemma, as in many other urban areas, is to export the city's waste. This strategy, however, is becoming less viable due to increasing challenges from the communities who are the intended recipients of wastewater. Once again, this highlights the tensions between the urban centre and its relationships to surrounding communities and rural areas. Two examples illustrate most clearly the tensions between competing users of water - the Mezquital Valley, 100 km to the north of Mexico City; and Xochimilco, in southern outskirts of the city.
Mezquital Valley
The primary destination of Mexico City's untreated wastewater is the semi-arid Mezquital Valley in the neighbouring state of Hidalgo. Once it leaves the valley, wastewater is eventually discharged to the Tula-Moctezuma-Pánuco River system that empties into the Gulf of Mexico. The Mezquital Valley is an important agricultural zone and the largest area in the world irrigated with wastewater. Sewage mixed with surface water collected in reservoirs within the basin has enabled farmers in the valley to provide agricultural produce for the capital city since the late nineteenth century (Perez de Leon and Biswas, 1997). The use of wastewater has allowed farmers to reclaim land that was formerly barren due to the overgrazing of sheep from colonial times.
At first glance, it may appear that this arrangement is an excellent example of water recycling, as irrigation with wastewater improves soil structure and enhances fertility because of its added organic matter. However, it also raises serious environmental and health concerns both for the eventual consumers of agricultural products, and the local community. The extended use of wastewater causes the build-up of pathogens, salts and heavy metals that are detrimental to the crops, soils, groundwater quality and human health (Perez de Leon and Biswas, 1997). The protection of health from these contaminants is managed through crop restrictions prohibiting the cultivation of produce that can be eaten raw. Alfalfa and maize, therefore, are the major products covering between 60-80 per cent of the area (Perez de Leon and Biswas, 1997). In this way, public health is safeguarded for the consumers of the produce in the city, but not for local people. The Mezquital Valley is an extremely poor area with little safe access to potable water, as much that is available is contaminated with leachate or residue from wastewater. Farmers in the Mezquital Valley are thus confronted by a serious trade-off - they want access to water to irrigate their crops, but they must risk the adverse long-term health consequences of using wastewater. Farmers working directly with the wastewater complain about loss of work due to diseases such as stomach problems and skin infections. The use of untreated wastewater affects the entire community; it is estimated that 400,000 inhabitants of the valley are vulnerable to disease, including incidences of cholera and other illnesses from the bacteria and pathogens transferred by the wastewater.
This area has been the site of increasing social tensions and conflict due to growing water scarcities and the high rates of wastewater discharge. The cumulative effects of many decades of wastewater use has generated a conflict between farmers who are dependent on this source of irrigation and its increased yields, and inhabitants of the valley's towns whose health is detrimentally affected by wastewater contamination. The potential for conflict is exacerbated because more people than ever before are living near the irrigated areas and dams that store the wastewaters of Mexico City (Perez de Leon and Biswas, 1997). After repeated high incidences of disease in the valley and continued agitation by its residents, the CNA began a sanitation project in 1996 with a billion dollar loan from the Inter-American Development Bank. These funds are directed to improving sanitation and wastewater treatment capacity for the MCMZ and the Mezquital Valley. This project has contributed to alleviating some tension, but rural people continue to be disadvantaged against the demands of urban users in Mexico City, and many people have left the valley altogether to search for economic opportunities in the United States.
Xochimilco
Delegación Xochimilco is located one hour south of the city centre, at the edge of the Basin of Mexico. Xochimilco is, in many ways, a kind of microcosm of the common pool water challenges that Mexico City faces as a whole. It exemplifies the severity of Mexico City's water problems and past mismanagement, and its residents have witnessed some of the most extreme effects of water scarcity. In Mexico City, Xochimilco is known as a prime tourist attraction, but it is also a working agricultural community and the last remaining area where chinampa cultivation is still practised. Chinampa cultivation is the Aztec system of garden plots surrounded by canals that feed crops with water and nutrients. Today's farmers still provide the city with agricultural products such as flowers, fodder, maize, and a variety of vegetables from these "floating" gardens. Xochimilco's historical importance is recognised both nationally and internationally; in 1987, UNESCO declared it a place of "universal historic and cultural patrimony". Yet despite its cultural, economic and ecological importance, Xochimilco is threatened by the water demands of the city.
Xochimilco is simultaneously a valuable aquifer resource for Mexico City and a sink for its wastewater. Since the 1950s water has been diverted to meet the consumption and industrial needs of the city at the expense its agricultural economy. Deep wells were drilled to supply the city with groundwater, and the springs that fed the chinampa canals began to dry up. To replace the water diverted to the city, the government authorised the channelling of raw and semi-treated sewage into the canals. Drilling for groundwater also led to a sinking of the old lakebed area, which resulted in some of the most severe subsidence in the basin. Sinking land meant that farmers could no longer regulate water effectively in the centuries old canals, as some garden plots would be plunged underwater because of different land heights for the chinampas.
Farmers in Xochimilco also had to contend with a rapid influx of migrants to the area, whose hastily constructed homes multiplied along the ecologically sensitive areas alongside the canals, and in the adjacent forested and hilly areas that are essential to the recharge of the aquifer. The subsequent destruction of vegetation led to conditions of rapid runoff and reduced the quantity of water reabsorbed by the aquifer. As a result of land use changes in the Xochimilco region, it now receives 30 per cent less precipitation and experiences slightly higher temperatures (Wirth, 1997). Even though areas alongside the canals were nominally a protected zone, homes and residences continued to go up, often in conjunction with the bribing of local officials to allow construction (Wirth, 1997). In addition, 20,000 or more of these irregular settlements regularly used the canals for disposal of their sewage. By the late 1980s, as a result of urban encroachment and decades of watering crops with polluted water, the historic chinampa canals were severely degraded and their once fertile soils were salinized and unproductive. The polluted, stagnant canals threatened tourism and many farmers lost their livelihood.
In the early 1990s, Xochimilco became the scene of large-scale protests and community environmental action to save the chinampas. A coalition of groups in Xochimilco, comprised of owners of the chinampas (los chinamperos), ecologists from the Universidad Nacional Autonoma de Mexico, and environmental groups, such as the Movimiento Ecologista Mexicano, demanded that treated water be pumped into the canals, and that flood control measures for the chinampas be implemented by the government (Wirth, 1997). Government officials were concerned that social conflict might erupt between the chinampa farmers in Xochimilco and the federal government (Wirth, 1997), and responded by creating the Parque Ecológico de Xochimilco which protected 160 hectares of the chinampa zone. Nevertheless, many stakeholders in Xochimilco remained unsatisfied. The greater portion of the area is still unprotected, illegal and irregular settlement continues at a high rate, and many chinampa farmers feel excluded from the government's agricultural assistance programmes.
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Key Stakeholders and Issues