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Volcano, Tinaco, Sink & Toilet, Leaky pipes, Pipa, Pozo
Mexico City
Chelsea Kilburn
The Wastewater Aquifer in Mexico City
For a city of nearly 26 million inhabitants, Mexico City has a perilous relationship to its water supply and distribution systems. The Basin of Mexico sits within the Trans-Mexican Volcanic Belt, a region of hills and mountains roughly 9,000 square kilometers in area. Much of Mexico City’s fresh water is imported from this surrounding region, where snowmelt and runoff from higher elevations create subsurface flows. Prior to the European conquest of Mexico City, the basin supported several lakes, among them Xochimilco, Chalco, and Texcoco, bodies of water whose presence is now felt only through what they left behind.
Figure 1: Outline of lacustrine areas and overlay on present-day Mexico City from History of the Present: Mexico City
The deposits left from the lakes take two forms: the shallow capa duras (hard layer) composed mostly of sand and silt that sits typically conceals the aquifer 10-40 meters below the ground surface, and the clayey lens sits a bit further down at roughly 100 meters below the former lake bed. This clay impedes the movement of water and confines the aquifer below, making the underground water more difficult to access. In Mexico City’s Pre-Colonial existence, a series of highly-engineered canals and aqueducts were used to transport water through the Basin. These canals were also utilized in flood management strategies, but the realization that there was clean water under foot led to a proliferation of groundwater wells, and infrastructure developed in favor of extracting water already in the Basin.
Subsurface conditions have also been affected by lava flows from the surrounding active mountains, with the areas closest to the Belt composed of volcanic basalt. These deposits from nearly 1700 years ago are both highly permeable and stable. Their porosity allows for water to percolate back into the aquifer without major disruption. In a piece for the New York Times, journalist Michael Kimmelman equates the volcanic soils of the basin to a bucket of marbles, stating “You can pour water into the bucket, and the marbles will hardly move. Stick a straw into the bucket to extract the water, and the marbles still won’t move. For centuries, before the population exploded, volcanic soil guaranteed that the city had water underground.” The ability of these basalt deposits to recharge the aquifer, particularly the lower strata of its multi-layered composition, is becoming increasingly valuable in the face of a growing reliance on groundwater secured through extraction via deep wells.
In conjunction with water supplied via the National Water Commission (CONAGUA) by the inter-basin water transfer from the Cutzamala and Lerma Rivers, Mexico City’s fresh water comes mainly from the underlying aquifer. The Mexico City Aquifer is a key resource yet it has a precarious relationship to the ever-expanding urban population as continued over-extraction leads to overdraft and the need to seek out water at deeper elevations. As Mexico City continues to grow, so too does the amount of impermeable surfaces. Without spaces in which water can move back down into the aquifer, water accumulates in the City, exacerbating flooding and spreading pollution via runoff. Like the hardening of the city, groundwater extraction has also hardened the aquifer. When extracted, groundwater will leave the lacustrine clay, compacting its molecules and resulting in subsidence.
Subsidence is also contributing to flooding, creating pockets of lowered elevation in the city. Although the effects of subsidence are not always immediately apparent, the slow sinking of Mexico City is not exactly slow. Parts of the city have subsided more than 9 meters in the past century, and some continue to move downward at a pace of nearly 15 cm per year. This shift spells disaster not only for the larger urban fabric but particularly for water infrastructure. When the Grand Canal del Desague was constructed at the turn of the twentieth century to transport wastewater through and out of the basin, the canal worked via gravity. With the ground sinking below it, however, the canal was quickly rendered inefficient as it’s slope was reversed, and by the 1950s the wastewater could no longer flow. To combat flooding and to continue moving the water, the city constructed the Emisor Central, a project that was followed by several other feats of engineering including the Emisor Oriente.
Today Mexico City continues to struggle with its relationship to the aquifer. Past studies have indicated that the water table has dropped nearly a meter each year since groundwater extraction has intensified. With deeper wells needed to reach deeper pockets of the aquifer, groundwater has also been exposed to more pollutants that can leach into the soil as runoff and permeate below the now compacted clay that used to protect the main aquifer.
And when groundwater is extracted from the aquifer, it is rare that it reaches its intended source due to a system of aging pipe infrastructure throughout the city. It has been estimated that nearly 40% of the water put into Mexico City’s drinking water system is lost through leakage. The porosity of this system has lead to an unreliable supply of fresh water, and although approximately 98% of households have access to water, many people still seek out water delivery via trucks known as pipas. This service is regulated and provided by the state’s water utility, Sistema de Aguas de la Ciudad de México, also known as SACM. Waiting for water can be time-consuming, and many reports how women in particular have been affected by the need to wait for hours day and night to receive clean water, inhibiting them from maintaining regular employment at the risk of not having water for a week or longer. When secured, fresh water is also often stored on site by many households, held in tanks above their homes in large containers known as tinacos.
Water consumption in the city by such a large population taxes the aquifer at a rate that exceeds recharge and it also produces an amount of wastewater that cannot be easily filtered and cleaned. In 2011, it was estimated that only 7.9% of the city’s wastewater was treated, a statistic well-below other areas of Mexico according to a CONAGUA report. The movement of water out of Mexico City has long held implications for the Mezquital region, the area receiving the aguas negras, or wastewater. This area has been receiving the city’s wastewater since 1789, and the water has been used for agricultural irrigation for nearly 130 years, cementing a relationship between Mexico City’s outpt and the Valley’s production, one that has been recently interrupted by the implementation of new water treatment facilities.
National Research Council. 1995. Mexico City's Water Supply: Improving the Outlook for Sustainability. Washington, DC: The National Academies Press. https://doi.org/10.17226/4937.
Image from Brook, Daniel. “Mexico City: History of the Present.” Places Journal, February 1, 2017. https://placesjournal.org/article/history-of-the-present-mexico-city/?cn-reloaded=1.
Schlanger, Zoë “Solving Mexico City's Cataclysmic Cycle of Drowning, Drying, and Sinking.” Quartz. Quartz, May 28, 2018. https://qz.com/1281506/solving-mexico-citys-cataclysmic-cycle-of-drowning-drying-and-sinking/.
https://www.nytimes.com/interactive/2017/02/17/world/americas/mexico-city-sinking.html
Tortajada, Cecilia, and Enrique Castelán. “Water Management for a Megacity: Mexico City Metropolitan Area.” AMBIO: A Journal of the Human Environment 32, no. 2 (March 2003): 124–29. https://doi.org/10.1579/0044-7447-32.2.124.
Herrera cited in Mexico City's Water Supply: Improving the Outlook for Sustainability, pg 17.
Engel, Katalina, Dorothee Jokiel, Andrea Kraljevic, Martin Geiger, and Kevin Smith. “Big Cities. Big Water. Big Challenges. Water in an Urbanizing World.” World Wildlife Foundation, August 2011. https://wwwwwfse.cdn.triggerfish.cloud/uploads/2019/01/big-cities_big-water_big-challenges_2011.pdf.
Jimenez, Blanca. “Chapter 23: The Unintentional and Intentional Recharge of Aquifers in the Tula and the Mexico Valleys: The Megalopolis Needs Mega Solutions.” In Water for the Americas: Challenges and Opportunities, edited by Mordechai Shechter and Alberto Garrido, 1st ed., 414–33. Routledge Press, 2014.
Sand / Silt / Clay + Tuff, Fishbone, Sluice Gate, Shovel, Poop, Toilet Paper, Plastic
Mezquital
Isaac Stein
Wastewater Basin
The Mezquital Valley is the world's largest and oldest agricultural area irrigated by wastewater. Since 1896 wastewater, aguas negras, from Mexico City has been rerouted to the valley and used for agricultural irrigation. Since this time, infrastructure has been added to transport sewage and wastewater from the ever growing Mexico City. As Mexico City has grown to become one of the largest metropolises in the world with a population of 21 million, it’s discharge of wastewater has grown hand in hand. It is now estimated that the City discharges 60 cubic meters of water a second to the Mezquital Valley. 1,370,000,000 gallons of wastewater a day, equivalent to 7 square miles in a foot of wastewater. The Mezquital valley infiltrates 89% of this wastewater, providing the water and nutrients to transform an arid desert into one of the nation's breadbaskets.
Wastewater Infrastructure
The Mezquital Valley flows from the south (at an elevation of 2100m) to the north (1700m). Despite its elevation, the region is still several hundred meters beneath Mexico City. Due to this elevation difference, the Mezquital Valley has been an attractive region for centuries for Mexico City to drain it's wastewater to. In the past century, wastewater infrastructure has grown to include three main conveyance infrastructures of wastewater as shown in the figure below. The original Gran Canal still transports 30% of the wastewater to Mezquital, and the new Central Interceptor conveys 55% of the wastewater. These channels outfall to the Tula, El Salado and El Salto rivers, all located in the Mezquital Valley.” The figure below diagrams the large wastewater infrastructure channels and accompanying infrastructure (tunnels, dams, rivers and secondary channels) that drain wastewater to the Mezquital Valley.
Figure 1:Sewerage conduits and disposal sites for Mexico City wastewater.
Wastewater Urbansim
Along the journey from Mexico City to the Mezquital Valley aquifer, wastewater flows through an ad-hoc assemblage irrigation network. Once wastewater crisscrosses through the infrastructure highlighted in the figure above, wastewater reaches an expanse of several hundred square miles of flood irrigated fields, with thousands of kilometers of individually managed channels to bring aguas negras from field to field. Some of these, irrigation fields and channels originate from the “14th century, when the Aztecs settled in the valley and engineered the first network of dikes and dams to control floodwaters.” Today, with an everflowing supply of water and organic material in the form of wastewater, individually managed channels and infrastructures are continually created, repurposed, and assembled to convey wastewater to specific parcels.
Figure 2: Google Street View showing the ad-hoc assemblage of wastewater management.
This image above, highlights some of the complexity and ingenuity of wastewater management that exists in the valleys. Parcels have roodgolber-like labyrinths of sluice gates, channels, bridges, tunnels and pipes to flood their agricultural fields. The axonometric above attempts to show this complexity. Once wastewater leaves Mexico City, it’s journey does not end when the water enters the Central Interceptor, but instead it is just beginning a long journey of being digested and filtered through a landscape assemblage of federally and independently managed wastewater channels en route to flood agricultural fields, only to slowly seep into Mezquital Valley’s aquifers.
Wastewater Aquifer
In the Mezquital Valley, aguas negras is vital to its creation and existence. Prior to wastewater being routed to the valley, the Mezquital was an arid landscape, with evapotranspiration rates more than 3 times the rate of precipitation. Today, the Mezquital has been transformed to an agricultural region with an abundance of water and an aquifer that has risen 30 meters in a fifty year time span.
Through the measures highlighted in the above text and images, the aquifers of the Mezquital Valley have been artificially recharged at a rate of 25.4 meters a second, 13.3 times the natural recharge rate. Due to the years of infiltrating at this rate, groundwater is rising throughout the valley and the appearance of new springs has popped up throughout the valley, including the area in which the axonometric depicts.
Figure 3: Farmers use sluice gates to flood irrigate their fields with Aguas Negras. Photo by Janet Jarman.
The story of wastewater in the Mexico City region has come full circle. There are now plans to redirect and pump excess groundwater overflowing from the Mezquital valley back to Mexico City as drinking water.” In the end if this wastewater is pumped back to Mexico City as potable water, there is no delineation between what is waste and resource.
K. Lüneberg, et al. Water Flow Paths are Hotspots for the Dissemination of Antibiotic Resistance in Soil, Chemosphere, Volume 193, p1198-1206, 2018. https://doi.org/10.1016/j.chemosphere.2017.11.143
Cervantes-Medel, A. and Armienta, M.A., Influence of faulting on groundwater quality in Valle del Mezquital, Mexico. Geofísica Internacional, Vol. 43, Num. 3, pp. 477-493. 2004.
TJiménez, Blanca. The Unintentional and Intentional Recharge of Aquifers in the Tula and the Mexico Valleys: The Megalopolis needs Mega solutions. 413-433. 2014.
Malkin, Elizabethm Fears that a Lush Land May Lose a Foul Fertilizer. New York Times. May 4, 2010. https://www.nytimes.com/2010/05/05/world/americas/05mexico.html?pagewanted=all&_r=1&
Lesser, Luis E. et al. Survey of 218 organic contaminants in groundwater derived from the world's largest untreated wastewater Irrigation system: Mezquital Valley, Mexico. Chemosphere, Volume 198, Pages 510-521. 2018. https://doi.org/10.1016/j.chemosphere.2018.01.154