A dead horse floats with sewage down a primary canal in the Mezquital Valley, Hidalgo, where farmers have used Mexico City’s raw sewage to irrigate crops for over a century. At various points along the 60-kilometer trajectory people further pollute the filthy waters by tossing solid waste and even dead animals into the canals that carry water to farmers’ fields.

Built up detergents in wastewater form immense mounds of foam as Mexico City’s black water (sewage) flows through a small irrigation canal in Mezquital Valley, where farmers use the untreated wastewater to irrigate crops. Such sites are common in the valley and illustrate the high levels of contamination in the region.

Leonides Cortéz García quides black water, or untreated sewage, from Mexico City into a small field, in the neighboring state of Hidalgo, where the sewage is used to irrigate calabaza fields. The arrangement appears to be an excellent solution for both the mega-city and the countryside, as irrigation with waste water can improve soil structure and enhance fertility by adding organic matter. However, this practice raises serious environmental and health concerns both for the eventual consumers of the agricultural products, and for the local communities that grow the produce.

Florentina Aguilar, 74 (L) and Malania García, 49, graze sheep in Hidalgo’s primary irrigation district. Throughout the region livestock survive predominantly on alfalfa, all of which is grown using black water, or untreated sewage. Some farmers have witnessed pregnant animals aborting after being exposed to the undiluted sewage that arrives during the dry season. Human consumption of livestock exposed to black water through fodder carries potential risk, though few studies have been undertaken to measure health effects.

Jesús Aldana Angeles, 74, tends his alfalfa fields in Mexico's Hidalgo state. The community leader passionately defends his belief that black waters are much better for plant health and yields than cleaner treated water. Several years ago Mexican authorities launched the construction of a colossal water treatment plant next to the Mezquital Valley to provide treated water alternatives. The proposed 2012 opening is now estimated for 2015. Many farmers in the region are against the treatment plant. Aldana hopes the cleaner water might finally provide opportunities for farmers to export their crops.

José Luis Galindo plants cauliflower barefoot while black waters flow into the fields where he is planting in Mexico's Hidalgo state. Throughout the Mezquital Valley, residents and farm workers have long complained of gastrointestinal, respiratory and skin disorders, which they connect with exposure to the contaminated waters. Although cauliflower is understood to be a prohibited crop, a number of farmers produce it anyway. They are confronted with a serious trade-off. They want access to the water to irrigate their crops, but they must risk the adverse health consequences of using untreated wastewater.

Valenciano Cortéz Villa Hernán shares a moment with his granddaughter, on his farm in El Salto, near Tula, in Hidalgo State. He yearns for her to be able to enjoy a clean environment, as he did as a child, before black waters began to contaminate his region known as the Mezquital Valley.The cumulative effects of many decades of the use of wastewater sewage to irrigate the Valley has created conflict between farmers who are dependent on this nutrient rich source of water and inhabitants of the valley’s towns whose health is negatively affected by the contamination.

Sabino Guerrero Guerrero, 15, glances down at the Endho Dam Reservoir, next to which he and his family live. Guerrero is angry. He is among many residents near the reservoir who suffer from skin rashes and respiratory difficulty from breathing noxious gases produced by the raw sewage. The lake attracted tourists and provided residents with abundant fish up until 1973, when authorities converted it into a holding tank for Mexico City’s sewage en route to farms in Hidalgo.

Fog blankets San Salvador, a farming village located deep in the Mezquital Valley. Some residents can remember when the Valley was a semi-arid region with only cacti and mesquite trees for vegetation, before black waters arrived to help farmers produce year-round. Most recognize that the black waters have helped transform the valley into a more prosperous region.

A concrete settlement envelops a hillside in northern Mexico City, illustrating the uncontrolled growth that has made the Mexico City Metropolitan Zone (MCMZ) one of the most densely populated and unsustainable urban areas in the world. With an estimated population of more than 20 million, the MCMZ faces tremendous water challenges, including overexploitation of groundwater, poor water quality, subsidence, flooding, inadequate wastewater treatment, and health concerns about the reuse of wastewater in agriculture. Most of the city’s rivers are dried up or full of sewage and trash. Continued uncontrolled urbanization exacerbates flood risks and prevents the region’s abundant rains from being reabsorbed into the city’s aquifers.

Pumping stations propel untreated human waste into the Rio de La Compañia, one of Mexico City's largest sewage canals. Residents living next to the canal continue to suffer from frequent floods of toxic waters that provoke wide scale epidemiological alerts in many parts of the Mexico City Metropolitan Zone.

Rosario Martinez, 37, attempts to remove water from her apartment, flooded after torrential rains caused an open drainage canal to break and propel raw sewage into streets and homes in Colonia Ampliación Emiliano Zapata. The low-lying area is one of the most flood-prone areas of the Mexico City Metropolitan Zone (MCMZ).

Miguel Álvarez Gómez protects his younger brother, Edgar Álvarez Gómez, by carrying him through the streets after a drainage break propelled raw sewage into streets and homes in El Molino, a settlement located next to one of Mexico City's largest open sewage canals. Each time the city experiences torrential rains, thousands of residents suffer when mixtures of rainwater and sewage inundate neighborhoods since urban drainage infrastructure simply can't cope.

Isaac Montalvo Meza 27 (L), Nolberto Rivas Molina 26 (M), and Juventino Gonzalez Canon, 38, strive to unclog excavation machinery underground at the Rio de La Compania Tunnel Project in Mexico City. The tunnel resembles another important project, the Tunél Emisor Oriente (TEO), a 62 kilometer (7 meter wide) tunnel being built to push Mexico City's wastewater to a colossal new water treatment in the neighboring state of Hidalgo. The proposed 2012 opening of the plant is now estimated for 2019.

Workers excavate inside one of 24 shafts (ranging from 26 to 150 meters deep), that form part of the Tunél Emisor Oriente (TEO), one of Mexico's most dramatic water infrastructure projects in 30 years. The shafts (lumbreras) are designed to provide ventilation and to allow the introduction of machinery during and after the project's completion. The seven meter wide, 62 kilometer tunnel was constructed to duplicate drainage capacity in the Valley of Mexico and prevent serious floods that occur throughout the Mexico City Metropolitan Zone (MCMZ).

Evidence of severe subsidence due to overexploitation of Mexico City's aquifers can be seen underneath Mexico City's Metro (line 4) where the sidewalk and road have buckled up to resemble a roller coaster. Imaginably, the undulations make for an interesting metro ride. Such examples of severe subsidence can be seen throughout Mexico City, making walking on city sidewalks a challenging part of daily life.

Water passes through filters in the Los Berros water purification plant, part of the Cutzamala System. The grand infrastructure project, constructed in 1980, transfers surface water to Mexico City from a distance of 120 kilometers and over elevations of 1,200 meters, illustrating the mega-city's immense environmental footprint on surrounding areas. The Cutzamala System provides around 15 percent of the city's potable water. Unfortunately, much of the pristine water is lost through leaks in Mexico City's aging municipal systems before it reaches consumers.

Valentín Pérez Hernandez, 24, keeps plants healthy in Jardines de Pedregal, a quiet enclave of multi-million dollar houses in Mexico City. As in many wealthier districts of the city, gardeners can be seen early in the morning watering plants as if the city's water supply were infinite. In the tiny apartment where Pérez lives, just eight miles away, water is scarce and often unavailable. Graciously aware of this irony, Pérez notes that while he must survive with little water, at least the lush plants he keeps alive in his job create much needed green spaces in a city devastated by environmental destruction.

Valentín Pérez Hernandez, 24, keeps plants healthy in Jardines de Pedregal, a quiet enclave of multi-million dollar houses in Mexico City. As in many wealthier districts of the city, gardeners can be seen early in the morning watering plants as if the city's water supply were infinite. In the tiny apartment where Pérez lives, just eight miles away, water is scarce and often unavailable. Graciously aware of this irony, Pérez notes that while he must survive with

A city water truck delivers water to residents of Campamento 26 de Julio, a marginal area in Iztapalapa, one of Mexico City’s largest and most crowded delegations. Residents depend on the sporadic water deliveries and must store the water in tanks and barrels. The trucked water is not clean enough to drink, and residents are forced to buy water separately in large plastic jugs called garrafones.

Despite the water’s yellowish color, Mari Carla Ramírez and her brother, Carlos Jovan Rodríguez Reyes, marvel at the scarce resource being delivered to their house by a city water truck. The children live in Campamento 26 de Julio, a marginal area on the outskirts of Iztapalapa, one of Mexico City’s largest and most crowded boroughs. In Iztapalapa residents complain that they receive tamarind-colored water from the trucks. Residents of Campamenteo 25 are among thousands in Iztapalapa who battle with water shortages and poor water quality. Drinking water must be bought separately, in large plastic jugs called garrafones.

Various potable water vendors and suppliers circulate in front of a micro-purification plant in Iztapalapa, where some of the most damaging effects from Mexico City’s water crisis are felt. Such micro plants have proliferated throughout the city in recent years. They can offer much cheaper water to residents; yet many wonder how and if the plants are being monitored for quality.

Fetid wastewater and sewage accumulates in an abandoned local treatment plant near Amecameca, one hour outside of Mexico City. Not only do residents have to deal with a lack of quality drinking water in Mexico City, but they also must live amid many open sewage canals prone to flooding and sites like this one, where local municipal officials built a plant but soon abandoned it due to high costs.