Nonlinear modeling of groundwater flow and total subsidence of the Mexico City aquifer-aquitard system

Abstract

The water budget in the Mexico city closed basin was in equilibrium until the begining of the thirties, natural recharge, of approximately8 m3 sec -1 for a 1500 km2 surface area, was the main source of some springs to the west and groundwater exploitation only represented less than 1 %. In 1980 the water pressure had diminished under the effect of a heavy groundwater pumping which exceeded nat­ ural recharge by more than 260 %. As a consequence the aquitards in the system have compacted to a total land subsidence of about 6.5 me­ ters in downtown area for that period. In this study a numerical model is presented to simulate groundwater flow and total subsidence of mul- tiaquifer systems. The model accounts for the non-linear compaction and total subsidence on multilayered systems by coupling a simultane­ ous numerical solution of the groundwater flow equation with the one- dimensional consolidation equation of aquitards through the Terzaghi's effective-stress concept. An important issue of this investigation is the model's capabilities to simulate the phenomenon at a regional scale. The application of this model to the Mexico city's basin has proved of great interest,, its aim is to quantitatively analyse: (1) the water bud­ get in the basin, (2) the land subsidence due to overexploitat ion, and (3) the response of the system to artificial recharge. The analysis of the water budget in the basin permits the calibration of,the model in steady-state. Simulations in unsteady-stat e are perforated for the pe­ riod of 1930-1986 using the steady-state results as theiaitial conditions. The calibration process for this period is done through two variables: observed groundwater heads and land subsidence. The simulations are done with the linear and non-linear versions of the model. The ob­ served subsidence is reproduced with a great detail with the non-linear version of the model. The linear version fails to reproduce the observed phenomenon.