Author(s): Carlos Cruickshank-Villanueva; Adriana Palma-Nava
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Abstract: A numerical model for the Mexico City Aquifer is presented which is intended to be a tool in planning the suspension of excessive pumping from the aquifer that has caused serious damage to city infrastructure and buildings through the provoked land subsidence. This is within a larger planning effort to provide potable water from other sources, mainly avoiding losses from the water supply network and incorporating water today used in irrigation. The model integrates the differential equation of water motion using a finite volume mesh in space and an implicit finite difference scheme in time. In the case of Mexico City the main aquifer is connected to a phreatic aquifer through a clay layer (aquitard); the interaction between layers is computed with the integration of the vertical flow differential equation in the aquitard whose solution allows the determination of the layer compaction when the pressure is lowered and the hysteresis effect when pressure is regained. Model boundary conditions are only of Neumann type representing the aquifer recharge at the surrounding impermeable mountains piedmonts or at volcanic permeable formations. Interaction with the surface permits the determination of spring discharges. The simulation spans a period of 150 years starting in year 1900. A large effort was made to collect historical information about pumped water volumes, spring discharges, piezometric heads and land subsidence; also about characteristics of underground formations, their extension, depth, thickness, permeability, porosity and compressibility. Calibration is made comparing measured against simulated evolutions of piezometric levels, land subsidence and spring discharges. Results indicate that pumping suspension raises water levels substantially and immediately but that land subsidence exhibits inertia and takes long time before stopping. Still, the measure is necessary to avoid further and heavier damage.
Year: 2009