Water supply and sewage systems in Mexico D.F.: challenges and solutions for a Megacity

Report presented for the course "Water supply systems" of the Tropical Hydrogeology and Environmental Engineering Master course. Technical  University of Darmstadt.

1.     Introduction

The Metropolitan Area of the Valley of Mexico (MAVM) is one of the five largest agglomerations in the American continent and the biggest in the western hemisphere. This territory comprises the Mexico City (16 boroughs) and 60 adjacent conurbated municipalities of the states of Mexico and Hidalgo. It accounts with about 21 million people settled down in the Federal District and the surrounding areas and is projected to grow around 24 million inhabitants by 2030 (UN, 2014) (Figure 1). This region accounts for the country´s higher concentration of economic activity with at least 33% of Mexico GDP´s (Rojas et al, 2008). Its high demand of water for different uses such as domestic consumption, industrial purposes, irrigation, among others, have influenced the water supply system and the waste water system which joined to other natural factors (altitude above the sea level, local climate conditions, topography of the valley, watershed sources, and social-economic aspects) model the characteristics of the water systems, defining it such as a very complex one. Additionally, this megacity is facing different challenges due to climate change, depletion of groundwater, fails in the provision of drinking water, natural hazards adaptation, and population growth; which make it an interesting case of study in order to learn how to solve different problems that a world with growing urban population will have undoubtedly to tackle.

Figure 1: Ranking of the 5 largest urban agglomerations in the world. The MAVM denominated as Mexico City has been placing the 4^th place since the 90´s. Source: World Urbanization Prospects, 2014.

2. Characteristics of water services

The MAVM is located at 2250 m.a.s.l. and has a subtropical highland climate (according to the Koppen Climate Classification) and Temperate semi-humid, Arid & Semi-arid; Temperate humid with an average temperature of 16° C. The annual rainfall mean is 50 mm in the Arid region and 100 mm in the Temperate humid one (SMA, 2005). The city is settled down on a wide high plain surrounded totally by mountains, where the lowest and central area is drier and less humid than the mountainous areas, in this way the flows of the precipitation converge in the inner part of the metropolitan district.

a.     Drinking water supply

According to the Census (2010), 91% of the population in this area has access to drinking water but there is a considerable part of it which is not receiving the services continuously. The MAVM manages the water supply and sanitation system independently for each municipality, where each one is responsible on the provision and works decentralized, giving thus a number of 23 different water suppliers (Banco Mundial, 2013).

The territory has suffered negative transformations in the way to obtain water, it has gone from the self-sufficiency to high-dependence. Nowadays, current water use in MAVM is approximately 63 m³/s and is provided by three main sources: groundwater, local surface water and transfer water from other basins. Out of the totality, 66% is extracted from 7 aquifers, classified by the CONAGUA (National Water Commission) as overexploited resource.

According to the data, the extracted volume from the aquifer for the Water Supply System is 6.18 m³/s  while the volume extracted for other purposes is close to the  19.78 m³/s. The water from these aquifers possess low quality levels since the wells must go deeper and closer to igneous rocks. The clandestine wells are also a problem as these don’t follow specifications to keep the balance in the groundwater hydraulic characteristics, are not technically constructed, do not pay taxes and present high likelihood of leakages. The depth of the wells reached between 100 and 150 m in the 60´s and 70´s, nowadays they reach between depth between 330 and 450 meters.

Surface water does not play a transcendent role on the provision in terms of quantity, the total area of dams is 15,6 km² which flow is kept by some few rivers, on the other hand, there are several springs that are used for industrial, agricultural and human-consumption use. These surface sources are the most polluted because of the discharge of wastewaters flows to them. The remainder quantity is imported from the Lerma (6 m³/s) and Cutzamala basins (13 m³/s) (Ezcurra et al, 1999 in Major et al, 2011) (Figure 2). The Cutzamala system is one of the biggest in the world due to the quantity of water that provides and the topographic difference it must overcome through pumping infrastructures, around 483 million m³ annually and 1100 m respectively. Therefore, the spend of electricity used for the pumping, reached in 2008 the 0,56% of the electricity generated by the country (Banco Mundial, 2013). This system was started to build in 1976 due the exhaustion of the water in the Lerma Basin and the constant subsidence of the city because of the overexploitation of the aquifers. The civil work consisted in the building of 8 dams in the upper watershed of the Lerma river and the decreasing in electricity generation that as one of the main uses for that river. 

Figure 2: Watersheed division in the surrounding areas of the MVMA where the Lerma and Cutzamala watersheeds are shown. Source:  WWF (2011).

The mean annual rainwater input into the MAVM is about 23 m³/s and only the half reaches to recharge the aquifers through different means. This imbalance has caused subsidence (because of the high rate of extraction compared with the recharge rate) up to 9 m in some areas of the city besides the pollution of groundwater (Major et al, 2011) and the presence of bacteria in some wells (Mazari et al, 2000 in Major et al, 2011) due to the contact of groundwater wells with sewage waters.

According to the UNESCO data, the water resources come mainly from local sources (67%) where the superficial water presents a secondary role with a 7%, while the groundwater accounts with 60%; the percentage of external resource shows that at least 21% of the water in the MAVM is brought from other basins (Figure 3). However these data find different contradictors and is common to see that the percentages of water sources vary widely depending on the author and the purpose of the study.

Figure 3: Distribution of the water resources by source in the MAVM, where is evident the high dependence on groundwater systems. (Source: UNESCO (2016).

b.     Water sewage system

The sanitation service in the MAVM only covers the 92% of the population and has a capacity of 57 m³/s. Despite it has the biggest number of Wastewater Treatment Plants -WWTPs- (118) compared with the others megacities assessed by UNESCO, its WWTPs system has a low capacity to treat water and carry out an advanced treatment, therefore, only 6% of the waste water is treated, while the rest of the water presents preliminary treatment (Figure 4).  

However, the low percentage of wastewater treated is not the only concern around this issue, due to the overexploitation of the groundwater in the city, many areas have presented subsidence, affecting the slope of the wastewater channel and the underground pipes and collector reversing the flow and in many cases forcing the government to invest big amounts of money in pumps and civil works to evacuate the stagnant waters.

Figure 4: Sanitation service in 5 megacities. Despite the high number of WWTPs (Wastewater Treatment Plants - 118) in Mexico City, their capacity and technology do not allow the city to cover neither the totality of the waste water nor its complete treatment process. (Source: UNESCO (2016).

The wastewater collection system works due to a complex network of surface and underground pipes which lead the waste and storm waters out of the basin. This system called Deep Drainage System (Translated from the original language: Sistema de Drenaje Profundo) accounts with 153,3 km of tunnels in depths between 10 m to 217 m below the surface of the city.

Finally, an important proportion of untreated wastewater in the MAVM, about 11m³/s which consist of a mix of rainwater and urban wastewater is reused for different purposes such as irrigation, electricity production and public parks maintenance.

c.     Commercial and physical losses

The physical losses of water are associated to leaks and overflows in storage tanks and visible and invisible leaks in distribution and service pipelines. This amount of water can reach up to 17 m³/s (Banco Mundial, 2013) or according to other authors it is more than 40% (Tortajada, 2006). The commercial losses describe the used and consumed water which due to either fails or absence in the measuring system or unauthorized uses is not retributed economically to the water supplier manager. In the MAVN, the coverage of flowmeters is scarce, reason why the collection of incomes present an inequitable flow of taxes (Banco Mundial, 2013).

3. Challenges

The MAVN presents a situation of extreme hydric stress. The water sources are decreasing in terms of quantity due to the depletion of groundwater levels, pollution of surface water, change in climatic conditions and natural circumstances such as the shape of the watershed (endorheic and closed) and the low precipitations. These factors joined together with the elevated social and economic costs to transport water from other watersheds pose a complex and challenging situation for the city.

Additionally, the metropolitan area is located inside the convergence of different streams, but there is not an efficient effluent that carries the input water to outside. This factor is given especially by the geological evolution, since Mexico City is settled down on an old cordilleran lake formed about 1 million years ago, and that was desiccated in several stages after the invasion of America (1492). The geographical configuration of the metropolitan area widely influences the challenges and the problems that the city is confronting around its water supply and disposal such as the high vulnerability to flooding. A failure (breaking or unbalance) in the wastewater channels caused by the increasing subsidence together with rainy seasons could lead the MAVM to a massive flooding and in consequence a social-economic collapse.

Some of the studies of possible climate change scenarios for the MAVM shows an increase of temperature and precipitation by 2025 and 2050 (Gay et al, 2007 in Major et al, 2011) which exerts a significant variable on the management of the water resources, putting the city in risk of shortage or collapse in the drinking water and sewage system.

The challenges could be summed up in the following list:

-        Sustainable groundwater overexploitation

The aquifers where the water is extracted from are suffering a high rate of groundwater extraction due to the high demand caused by the population increase, agricultural irrigation and industrial growth. Furthermore, they present a low rate of recharge because of the impermeabilization of soils during urbanization and low rainfall levels. This overexploitation has caused subsidence and could originate the reduction in the storage capacity owing to the porosity decrease in the aquifer units.

-        Inter-basin transfers

Supply based on the transfer from the Cutzamala and Lerma watersheds has caused social and environmental conflicts with the populations settled down in those basins since the exploitation of its resources are not being compensated and their economic activities have been affected by the decreased availability of surface water.

-        Surface water pollution

Low percentages of wastewater treated and informal settlements which are not adequately connected to the sewage system have increased the pollutants in surface water. A considerable amount of this water is used for irrigation purposes which could lead to additional health and environmental impacts.

-        Water Governance and Management

Currently, legal instruments are lacking to protect areas of recharge, and a series of adaptations to the National Water Law, General Law of Ecological Equilibrium, and General Law of Human Rights would be necessary (Burns, 2009).

-        Land subsidence and Flooding

The city was built on a filled former lake, dried and disappeared gradually just since just few centuries ago. The composition of the geological substratum, mainly clays have compacted over the years due to the weight of the urbanization. In the MAVM case, flooding has its causes deeply rooted in the way in which the water bodies are used. Over-exploitation of the aquifers to ensure drinking water are causing subsidence in the soil, reaching up to 10 m, breaking the leakages in the sewage and water supply system and reversing flow in streams and rivers. Furthermore, the impermeabilization of the soil and geographical geometry of the watershed make the valley very susceptible to flooding and water systems collapse.

-        Inefficient water use and leakages

A high percentage of the water pumped into the system is not given to the final consumer (40%) due to leakages in the distribution system. Other considerable percentage is not quantified because of a lacking measuring system; and other remarkable percentage is not billed and therefore not revenue for several reasons where the most important is the illegal connections.

-        Water quality and continuous service

This a special concern both in the source and at the point of use. The subsidence is causing contamination of aquifers when the pipelines breaks and the impermeable rock layers which protect the aquifers are broken. On the other hand, the storage system in the houses roofs promote the proliferation of bacteria when the residual chlorine has disappeared causing health problems in the users. The city has undergone several episodes of shortage, specially due to meteorological reasons which however have generated affectation to at least 5 million people. Intermittent supply is common in different parts of the city where the pressure is not sufficient.

-        Social conflicts

The poorest inhabitants in the region have less access, less coverage and worse water quality than the middle and high social classes inhabitants. This issue has been a constant in the MAVM where has been commonly shown that the rationing and differentiated prices are affecting mostly the poor population, who in many cases have to buy water from informal vendors at high prices.

4. Solutions, technologies and responses

The national government and the local authorities have advanced to face these problems through 2 main ambitious plans carried out since 2007: the “Water Sustainability Programme” and the “Green Plan 2007 - 2012”.

Historically, the firsts steps to response to these challenges were adapting the policies and the legal framework to the required needs. It began stablishing fixed tariffs through private sector participation in different stages of production, distribution and sale of water (Tortajada, 2006). One example is the National Water Law, where it is stated that the water issues must be coordinated between the three levels of government, users, and societal organizations. Furthermore, auxiliary institutions conform the Basin Commission operating at the sub-basin level, the Basin Committee operating at the micro-basin scale, and the Groundwater Technical Committee operating at the aquifer level (WWF, 2011).

The city plans to increase its water resources by drawing supplies from an adjacent basin 14 km away, which will require water to be pumped along a drop of nearly 1,850 m. Over the long term, this particularly complex project will enable the transport of 30 m³/s of additional resources, if resistance from local populations and authorities to sharing the basin resources does not block the project. Mexico is also exploring the potential exploitation of the valley’s deepest aquifers, with a well recently drilled to the depth of 2000 m (UNESCO, 2016). On the other hand, Mexico City began artificially recharging its aquifer with treated wastewater and rainwater in 1992 to combat subsidence. This practice is limited however as rainwater and wastewater are extracted in one shared pipe, and the associated cost of treating this larger volume of water is too high (Sosa-Rodrigurez, 2010).

The Green Plan pretends reaching an equilibrium in the aquifer, reducing residential water use, reducing network losses, increasing the reuse and the treatment of wastewater, and the creation of parks around the lakes Tláhuac and Xochimilco.

The main objectives of this plan are (Sistema de Aguas de la Ciudad de México, 2013):

1. -        Stop the collapse of the city through control of the overexploitation of the aquifer.
2. -        To progress substantially in the recharge of aquifers and in the recovery and protection of conservation soil.
3. -        Protect the aquifer of possible contamination risks.
4. -        Attack the risk of leaks, detecting them and suppressing them opportunely.
5. -        To progress substantially in the treatment of sewage and the reuse of them.
6. -        Reduce the imbalance between supply in terms of drinking water and demand.
7. -        To obtain forms of metropolitan management in services such as potable water supply, sewerage and sanitation.
8. -        Advanced water infrastructure, drainage and sanitation.
9. -        Improve the distribution of drinking water (leakage control).
10. -        Promote the savings and efficient use of the household.
11. -        Protect conservation areas and strengthen balance of the aquifer.
12. -        Avoid human settlements in areas of risk and improve the drainage infrastructure.
13. -      Increase production and improve the efficiency of plants of wastewater treatment operated by Sacmex and by individuals.
14. -        To encourage fair and timely payment of services.
15. -        Prevent and control the contamination of bodies of water.
16. -        Protect and restore ecosystems in the lacustrine zone.

References

1.     1.       Agua urbana en el Valle de México: ¿un camino verde para mañana?. -- México : Banco Mundial, 2013. 92 p.: il.

2.       Burns, E. (coordinator) (2009). Repensar la cuenca: la Gestión de ciclos del agua en el Valle de méxico. México, DF: Universidad Autónoma Metropolitana: Centro para la Sustentabilidad Incalli Ixcahuicopa, 160p.

3.       Ezcurra , E. , M. Mazari-Hiriart , M. Pisanti , and A. Aguilar ( 1999 ). The Basinof Mexico: Critical Environmental Issues and Sustainability , Tokyo, Japan : United Nations University Press .

4.       Gay , C. , Estrada , F. , and Conde , C. ( 2007 ). Some implications of time series analysis for describing climatologic conditions and for forecasting. An illustrative case: Veracruz, México . Atmósfera , 20 (2), 147–170.

5.       Major, D. C., A. Omojola, M. Dettinger, R. T. Hanson, R. Sanchez-Rodriguez, 2011: Climate change, water, and wastewater in cities. Climate Change and Cities: First Assessment Report of the Urban Climate Change Research Network, C. Rosenzweig, W. D. Solecki, S. A. Hammer, S. Mehrotra, Eds., Cambridge University Press, Cambridge, UK, 113–143.

6.       Mazari , M. , E. Cifuentes , E. Velazquez and J. Calva ( 2000 ). Microbiological groundwater quality and health indicators in Mexico City . Urban Ecosystems, 4 , 91–103 .

7.       Rojas, E., Cuadrado-Roura, J.R., Fernández Güell, J.M., eds (2008). Governing the metropolis : principles and cases. Inter- American Development Bank.

8.       Sistema de Aguas de la Ciudad de México, (2013) “El gran reto del agua en la ciudad de México: pasado, presente y prospectivas de solución para una de las ciudades más complejas del mundo.

9.       Sosa-Rodriguez, F.S. (2010b). Exploring the risks of ineffective water supply and sewage disposal: A case study of Mexico City. Environmental Hazards 9 (2010) 135–146.

10.   SMA (2005). Informe Climatológico Ambiental del Valle de México 2005. Secretaría del Medio Ambiente, Gobierno del Distrito Federal.

11.   Tortajada, C. (2003). Water Management for a Megacity: Mexico City Metropolitan Area. Ambio 32 (2) 124-129

12.   UNESCO & International Hydrological Programme, (2016), “Water, Megacities and Global Change: Portraits of 15 Emblematic Cities of the World”, France.

13.   United Nations, Department of Economic and Social Affairs, Population Division (2014). World Urbanization Prospects: The 2014 Revision, Highlights (ST/ESA/SER.A/352).

14.   WWF Report, (2011), Big cities, Big water, Big challenges: Water in an urbanizing world, Berlin.