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