SAVING THE VALLEY OF MEXICO FROM DESASTER: THE NEED AND THE METHOD

Many urban areas where underground aquifers are the main source of clean water are subject to extraction of the liquid at a rate higher than natural recharge. Such a water over-draw induces ground settlement and consequent tensile stress and cracks on the ground surface, which damage existing buildings and infrastructure; the problem worsens if some of the affected strata within the subsoil are highly compressible, like those in the Valley of Mexico. A process of that type started in Mexico City soon after the rate of population growth increased steadily during the early XX Century. In a prior publication in Spanish by three of the present authors, dated May 2017, the cause-effect relationships of the observed phenomenon were explained in detail and a proposal was advanced in order to deal with the detrimental consequences of subsidence. Land subsidence caused by water extraction is not rare: similar processes are occurring in many other urban areas around the world, while new cases often appear. Yet the Mexico City case is outstanding because the consolidation process has persisted during nearly one century, from which cumulative settlement has reached impressive magnitudes: 10 to 14 m up to now in extensive urban zones resting on thick, compressible soil deposits of lacustrine origin. Furthermore, the process has extended beyond the City, now encompassing in addition a big portion of the surrounding Valley of Mexico. All of this has motivated international interest. The present publication continues and expands the precedent one in the purpose of exploring engineering and scientific phenomena involved in the case, including environmental effects and sustainability. After a brief history of the local and regional subsidence, this paper proceeds with a survey of the international literature on the subject matter published between the date of our former paper and the current days. Then a discussion is made of the survey findings and their applicability to the Valley of Mexico case. The conclusion is that, given the large dimensions of the affected territory, the complexity of its compressible subsoil, and the extremely high subsidence occurred, a drastic reduction of water withdrawal will be necessarily. Any possible solution will require an approach different from the one-step optimizing algorithm applied elsewhere for similar cases. Hence, the inevitable approach to solve the Valley of Mexico problem should start by recognizing the need to reduce progressively water extraction down to the limits of natural recharge by progressively reducing underground-water over-extraction. Toward that objective, the following actions are desirable: 1) preserve and stimulate natural recharge in regional aquifers; 2) reduce losses in the water-pipe network; 3) for agricultural purposes, use recycled, properly treated water instead of fresh water; 4) optimize the uses of water in domestic and industrial purposes and 5) use predetermined blocks of imported water to cover any short-term deficit of underground-water, rather than allowing inertial increments of local extraction. The above-referred equilibrium between ground-water extraction and natural recharge is exactly the general condition necessary to minimize subsidence.