Some rural areas and watercourses have been contaminated by excessive use of chemical pesticides and defoliants. Agricultural workers have suffered health problems related to these chemicals.

Sewage, industrial and agricultural wastes contaminate most Mexican rivers, and have turned some into real health hazards. The Río Pánuco, flowing into the Gulf of Mexico at Tampico, carries some 2000 tons of untreated sewage a day, mainly from Mexico City, which expels it via a 50 km tunnel.

Mexico City, despite extracting groundwater at a rate that causes the earth to sink all over the city, has to pump about 1/2 of its water from outside the Valle de México. One of the rivers from which the capital and other cities take water is the Lerma, which receives raw sewage and industrial effluent from 95 towns on its way into poor Lago de Chapala, Mexico's biggest natural lake, near Guadalajara. Chapala itself is shrinking because Guadalajara takes more water out of it than its feeder rivers now bring in.

Along the US border, about 45 million liters of raw sewage enter the Río Tijuana daily and flow into the Pacific Ocean off San Diego. The Río Grande receives more than 370 million liters of saw sewage, pesticides and heavy metals a day.

The New River, entering California from Mexicali carries about 100 toxic substances and more than 1 billion liters of sewage and industrial waste daily.

The growth of towns near the US border has brought environmental problems in its wake. In some towns, only half the population has running water and only slightly more have sewerage. So much water is extracted from Río Grande that it dries up before reaching the sea.

The underground aquifer supplying water to Ciudad Juárez and its US neighbour, El Paso, is expected to dry up by 2025.