Groundwater Resources and Land Subsidence investigations in the Toluca Valley, Mexico

The sustained growth in population in the Toluca Valley and neighboring Mexico City has primarily depended on the continuous development of both local and regional water resources for industrial, agricultural and domestic uses. The Toluca Valley Basin, covering an area of approximately 2000 Km2, is the focus of this study. Currently, there is a significant net loss of water within the basin primarily due to groundwater pumping, and the loss is increasing with time. These stresses on the aquifer have caused significant changes on the water flow patterns, a reversal in the direction of hydraulic gradients, the disappearance of artesian springs and wetlands and noticeable land subsidence within the basin. Neighboring Mexico City's land subsidence problems have been well documented, however, no comprehensive studies exist for the Toluca Basin. This study is divided into two parts: 1) investigation of groundwater depletion in the Toluca Valley; and 2) assessment of land subsidence in the Toluca Valley. We examine various changes in regional flow patterns, and groundwater levels decline throughout the valley and 3D numerical flow simulations are run to predict the ever decreasing level of the piezometric surface. Currently there is a net loss (recharge - extraction) of 142 Mm3 per year of groundwater within the Toluca Basin aquifers. We have documented a decrease in groundwater levels with a rate of up to 1.4 m/year between 1970 and 2006 in the central part of the valley. At the current rate of consumption, groundwater resources will not be sustainable for the population of the valley. Directly related to the drawdown in groundwater levels is the occurrence of land subsidence throughout the valley. Neighboring Mexico City, where total subsidence of up to 9 meters has been observed, has a similar geology as the one in the Toluca valley. We have documented several sites in the Toluca Valley where land subsidence is occurring. Ongoing work includes the mapping of regional land subsidence with the use of InSAR (Synthetic Aperture Radar Interferometry) images obtained from the European Space agency's ERS-1, ERS-2 and Envisat Satellites and the Canadian Space Agency's RADARSAT satellite. Data from years 1995 to 2007 are used to produce subsidence maps of the Toluca Valley. Our findings are verified with in-situ extensometers installed around the city of Toluca and the industrial corridor where the most significant decline in groundwater levels are found. Future land subsidence occurrences will be predicted with the use of a numerical model calibrated with remote-sensing images and on-site surveys.