Greater Mexico City (Zona Metropolitana del Valle de México), a metropolitan area with more than 19 million inhabitants including Mexico's capital (Ciudad de México, or CDMX) with about 9 million inhabitants, faces tremendous water challenges. These include groundwater overexploitation, land subsidence, the risk of major flooding, the impacts of increasing urbanization, poor water quality, inefficient water use, a low share of wastewater treatment, health concerns about the reuse of wastewater in agriculture, and limited cost recovery. Overcoming these challenges is complicated by fragmented responsibilities for water management in Greater Mexico City: The Federal government is in charge of regulating the use of water resources, contributing to the financing of investments and supplying bulk water from other basins through the National Water Commission (CONAGUA); The State of Mexico provides bulk water, treats wastewater and assists municipalities in providing water and sanitation services in its part of Greater Mexico City; 59 municipal governments in the part of Greater Mexico City located in the State of Mexico and one municipality in Hidalgo State are in charge of water distribution and sanitation for their constituents; the government of Federal District provides water supply and sanitation services to its constituents through its water department; and two irrigation districts in Hidalgo state are in charge of irrigation with wastewater from Greater Mexico City.
Given the size and political importance of Greater Mexico City, a major flood or a major water supply interruption would be a national political crisis potentially threatening the stability of the federal government. The security of water supply and the functioning of the storm water drainage of the metropolitan area thus are major concerns for the local, state, district and federal governments. In response to the challenges outlined above, the Federal Government, the State of Mexico and the Federal District initiated a US$2.8 billion Water Sustainability Program in 2007.
In parallel, the government of the Federal District launched a Green Plan which includes water conservation as an important element. Investments envisaged under both plans include an increase in wastewater treatment, the import of groundwater from irrigated areas North of the city where the groundwater table increased due to irrigation with wastewater, the construction of a major new storm water drainage tunnel, increased water imports from an expansion of the energy-intensive Cutzamala system that pumps water up over more than 1000 meters, and the reduction of non-revenue water from 36% to 25%.
Geography and climate
The climate of the Valley of Mexico ranges from a semi-arid belt in the north to a tropical one in the south. The valley receives about of annual rainfall, which is concentrated from June through to September/October with little or no precipitation during the remainder of the year. There are hardly any permanent rivers today. Groundwater is the main water resource in the valley.
The valley has no natural drainage outlet for the waters that flow from the mountainsides, making the city vulnerable to flooding. It was artificially opened through the use of canals and tunnels starting in the 17th century, entirely draining what used to be Lake Texcoco. The rural southern portion of the Federal District and of the Valley of Mexico, in particular the Sierra Chichinautzin is the most important natural recharge zone for the Mexico City Aquifer due to relatively high levels of precipitation and the high permeability of its basalt rock.Mexico City's Water Supply:Improving the Outlook for Sustainability, National Research Council, Academia Nacional de la Investigación Científica, A.C., Academia Nacional de Ingeniería, A.C., NATIONAL ACADEMY PRESS, Washington, D.C. 1995, Chapter 4
Sector responsibilities
Water resources management. The National Water Commission (Conagua) is responsible for water resources management in Mexico, including granting of water abstraction and wastewater discharge permits.
Water supply and sanitation. The National Water Commission also supplies bulk water to the Federal District and to parts of Mexico state through the Cutzamala and Lerma systems.
In Mexico State, the State Water Commission buys bulk water from Conagua, transmits it through its own bulk water infrastructure and sells it on to 57 municipalities with 4.1 million inhabitants. The State Water Commission also monitors water quality, provides technical assistance to municipalities in water disinfection and sewer cleaning, operates wastewater pumping stations and five wastewater treatment plants, empties septic tanks and provides water in tankers in emergency situations. It also provides training and assists municipalities in the establishment of municipal utilities (organismos operadores). In the 59 municipalities of Mexico State and one municipality of the state of Hidalgo that are part of Greater Mexico City, each municipality is in charge of water supply and sanitation.
The municipal water utility of Mexico City, Sistema de Aguas de la Ciudad de México (SACM), is responsible for water supply and sanitation in the Federal District. Its head is appointed by the government of the District.
Infrastructure
The water infrastructure in Greater Mexico City consists of infrastructure for bulk water supply and water distribution (water supply), wastewater collection, storm water collection and wastewater treatment (sanitation), and for irrigation using mainly wastewater.
Water supply
The Federal District and State of Mexico combined had 1,089 registered wells at depths of 70 to 200 meters in 1994. This does not include wells operated by the National Water Commission, which are deeper. There are also a large number of non-registered wells, many of which are located in the State of Mexico. Wells are generally located in four different well fields. These are labeled as South (or Xochimilco), Metropolitan, East (or Texcoco region) and North well fields.
Besides these well fields, the bulk water supply infrastructure of Greater Mexico city consists of two systems: Lerma and Cutzamala. Both systems are operated by the National Water Commission.
The Lerma system, built in the 1940s, transfers 4.8 m3/s of water (6% of total water supply to Greater Mexico City) from well fields in the upper basin of the Lerma River in the west to Mexico City.
The Cutzamala System was built in stages from the late 1970s to the late 1990s. It can transfer 14.9 m3/s (19% of total supply) of water from the Cutzamala River in the Balsas River basin in the Southwest to Greater Mexico City for use as drinking water, lifting it over more than 1000 meters.Cecilia Tortajada and Enrique Castelán:Water Management for a Megacity: Mexico City Metropolitan Area, Ambio, Volume 32, Issue 2 (March 2003) It utilizes 7 reservoirs, a 127-km long aqueduct with 21 km of tunnels, 7.5 km of canal, and a water treatment plant. Its cost was US$1.3 billion. While providing Mexico City with over 20% of its water, the Cutzamala system currently operates at only 47% of its total capacity.BBC:Millones sin agua en la Ciudad de México, 13 April 2009
The water distribution system in the Federal District included nearly 11,000 km of distribution lines and 243 storage tanks with a capacity of 1.5 million cubic meters in 1994. Water from all the separate sources is added to the common distribution system. The Federal District also operates a water transmission line (the Acueducto Periférico) that transports water from the Cutzamala System - entering the distribution system from the west - to the southern and eastern part of the district. The State of Mexico system has nearly 800 km of distribution lines and 32 storage tanks with a capacity of 440,000 cubic meters. As of 2000, there were 2.5 million water connections with 67% of these being domestic and this only accounts for legal connections. It is estimated that there could be another 900,000 illegal connections.
The State of Mexico operates the 49-km water transmission line (the Macrocircuito) to transport water entering from the west side of the service area (including the imported water from the Cutzamala-Lerma System) to the east side. This transmission line is being upgraded to increase the volume of water taken from the Cutzamala-Lerma system to 7.3 m3, and to provide service to the eastern service area. The Macrocircuito is operated by the State Water Commission.
It is expected that the dependency of Mexico City on external sources of water will increase. Additionally, the absence of economic compensation mechanisms for those communities from which water is extracted has created conflicts among users and sometimes limits water transferred to the city. One example of this is Mexico City's high use of bottled water. Those that do not have access to water from pipes, pay private vendors from 6 to 25% of their daily salaries. General distrust of tap water quality has led to much of the population purchasing drinking water; Mexico was ranked the third largest consumer of bottled water in 2009.
Combined sewer system
Greater Mexico City is served by a single combined sewer system, collecting municipal wastewater, industrial wastewater and storm water. It includes of pipes, 68 pumping stations, numerous dams, lagoons, and regulatory tanks for flow control, 111 km of open canals, 42 km of rivers (rio) used primarily for drainage, and 118 km of underground collectors (interceptor and emisor) and tunnels. The three interceptors are:
The Western Interceptor (Interceptor del Poniente), draining into the Nochistongo Channel, which ultimately joins the Emisor Central; The Central Interceptor (Interceptor Central), draining into the Emisor Central (Drenaje Profundo) and then into the Salto River in Hidalgo state near the Requena dam, from where it flows to the Mezquital Valley; and The Eastern Interceptor (Interceptor del Oriente), draining into the Grand Canal, then into the old and new tunnels of Tequixquiac and ultimately to the Salado River.La Administración del Agua en las Zonas Metropolitanas del Estado de México Gobierno del Estado de México, Secretaría del Agua y Obra Pública, Marrakech, October 29, 2006, Slide 16-17
The total dry weather flow for Greater Mexico City, which consists mainly of untreated municipal wastewater, was estimated at 44 m3/s in 1993. During the monsoon season, the region experiences many storms of high intensity and short duration. A single storm can produce up to 70 millimeters (about 3 inches) of rainfall, representing 10 percent of the total annual precipitation. Because of this rainfall pattern, the general drainage system was designed to carry 200 m3/s over a 45-hour period
Until 1910 the Grand Canal functioned purely by gravity, with an inclination of 19 cm per km. Over the next five decades its inclination declined to 10 cm per km due to land subsidence of 7 meters. Several large pumps were installed in an attempt to maintain its capacity. After heavy floods in 1950 and 1951 it became clear that the Grand Canal could not protect the city any more from flooding and a deep drainage system (Drenaje profundo) was proposed for the first time. Studies on the system began in 1959; its construction began in 1967 and it was completed in 1975. It consists of a deep tunnel, the Emisor Central with a length of 68 km and a depth of up to 250 m. It today constitutes by far the most important element of the Mexico City drainage system. It was designed for a flow of 170 m3/s.