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Reuse solution for the
Valley of Mexico’s Water
Challenges
by Ernesto Espino de la O
Urban population growth in Mexico City has far outstripped water supply and wastewater treatment
capacity, leading to the overexploitation of aquifer supplies, resulting in subsidence, and pollution and
health risks from the disposal of untreated sewage. ERNESTO ESPINO DE LAO reviews the project
underway to address these issues, which will feature one of the largest wastewater treatment plants in
the world and will provide treated recycled water for irrigation and aquifer recharge.
Mexico City experienced a rapid
growth in population in the
second half of the 20th Century,
increasing from three to nearly 18 million
people. In the same period of time, water
demand grew at an even faster rate. Under
those circumstances, pressing day-to-day
problems left little time for city managers
to ponder the long-term consequences of
emergency measures taken to cover water
demand. To satisfy the water demand of
the growing city it was necessary, among
other things, to drill more wells right in the
Valley of Mexico. Soon afterwards, water
extraction began to exceed natural recharge
of the aquifers, with the resulting imbalance
growing every year.
Another serious consequence of this
practice was that, due to soil conditions
in the Valley of Mexico, the lowering of
the water table caused subsidence of the
ground across large areas (Figure 1). If one
were to look at a typical cross section of the
soils in this area, what will most likely be
found is an upper layer of clay – the bed of
what used to be large lakes that not long
ago covered almost 2000 km 2 of the Valley
of Mexico, and under the clays layers of
granular soils where most of ground- water
is stored. When the water table drops, air
fills the interstices of the granular soil, what
is called the vadose zone, and the previously
existing water pressure equilibrium in the
interface between the clay and granular soil
is irremediably lost. The clays gradually lose
their water and shrink, causing the ground
to sink. The magnitude of the sinking is
proportional to the thickness of the clay
layer and, because that thickness is variable,
22 Environmental Engineer and Scientist: News, Current Events & Careers V 49, N 1 | Winter 2013