‘Mexico’s most important river dying a slow death’

The Lerma Santiago river, the second largest in Mexico, is under threat from pollution and decreased water flow. Though the government pumped in millions of dollars to clean up the river in the early 1990s, the action plan lacked a holistic approach.

Mindahi Crescencio Bastida Muñoz, professor and researcher of water management in the Metropoliton Autonomous University, Mexico, talks about the issue

Why do you think there is an urgent need to save Mexico’s water?

There has been a drastic reduction in the availability of water for the country in the past 50 years. Mexico has witnessed a reduction of water from 11,000 cubic metres/hectare/year to 4,500 cu m/ha/year. There has also been a decrease in precipitation by 15 to 25 per cent across the country. This has sent the alarm bells ringing to save the sustainable sources of water in the country.

The Lerma Santiago river is the second largest river in Mexico. It used to be a clean source of drinking water for the capital as well as the central parts of the country earlier. Unfortunately, the river is dying a slow death today and there is an urgent need to save it.

What are the major threats to the river?

Lerma Santiago (also known as Rio Lerma) serves as a source of electricity and drinking water for Mexico City. The upper river basin, located in Toluca valley (the central part of the country), was once considered the most fertile area.

Here, the river is fed by lakes and an aquifer. However, since the mid-1950s, more and more water was pumped out from the lakes to cater to the increased population. The groundwater level also reduced as people began extracting water. Disappearance of several lakes in the valley region also led to the river’s shrinking.

Dumping of municipal, industrial and agricultural waste also pollutes the river.

How deep is the impact of groundwater extraction from Toluca valley?

The aquifer lying under Toluca valley is about 2,738 sq km in area, which is about twice the size of Mexico City. Annually, 425 cu m of water is extracted while there is a natural recharge of only 383 cu m. Thus, there is an annual deficit of 42 cu m water which needs to be replenished.

In the past 50 years, the water table has dropped by 90 metres. Overextraction of groundwater has led to the subsidence of land in the central part of Mexico. A receding groundwater table has reduced the flow of the river and has increased the amount of pollutants in it.

What are the consequences of extracting water from the upper reaches of Lerma Santigo?

There have been several impacts like the disappearance of springs, loss of flora and fauna and frequent occurrence of floods. Apart from the environmental impact, there were social impacts also. As water crisis developed, conflicts between communities as well as between communities and the government started. Real estate and industries started using more groundwater as they are more powerful than farmers.

What is the level of pollution in the river?

The river has become the receptacle for every kind of waste. During the dry season, the river has dissolved oxygen (DO) level close to zero, except at a few points. These low DO points also show very high levels of Biological Oxygen Demand (BOD) above 100 mg/l, which is several times higher than the acceptable value. High levels of nitrates and phosphates present in the water also make it highly toxic. The National Water Commission of Mexico, which monitors the water quality, has declared the water quality of Upper Lerma Santiago river unfit for human consumption.

What are the immediate actions required to save the river and its feeders?

There has to be a management plan for the Toluca valley aquifer. The local communities should be involved in the management plan. There should also be a strategic plan to control the flow of wastewater into the river.

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