Numerical Model for Mexico Valley Aquifer

A numerical model for the Mexico Valley Aquifer is presented which is intended to be a tool in planning the curtailment of excessive pumping from the aquifer that has caused serious and expensive damage to city infrastructure as a result of associated 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. 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. The model accounts for the hysteresis effect of pressure recovery. Model boundary conditions are only of Neumann type representing the aquifer recharge at the surrounding impermeable mountain 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 detailed characteristics of underlying geologic formations and hydrogeology. Calibration is made comparing measured against simulated evolutions of piezometric levels, land subsidence and spring discharges.

Results indicate that pumping curtailment raises water levels substantially and immediately, but that land subsidence exhibits inertia and takes a long time before terminating. Demand management, while politically difficult, is a necessary measure to avoid further costly damage to city infrastructure and buildings.