Analysis of over-exploitation of the Texcoco aquifer, Mexico

The Texcoco aquifer is the main source of water for 12 municipalities in the eastern part of the Valley of Mexico, with a population of over 1.5 million inhabitants. The socio-economic development of this zone strongly depends on the possibility to achieve sustainable management of this aquifer, which at the present time is heavily over-exploited, although the real degree of over-exploitation is unknown. Since accurately knowing the extent of over-exploitation is indispensable in order to develop and implement a sustainable management plan for the aquifer, the aim of this work was to analyze the sources of uncertainties and errors in the estimation of over-exploitation. For this purpose, two methods were applied: a) the hydrologic balance method, consisting of measuring or calculating the different water inputs and outputs in the aquifer area, and b) the method to monitor the hydraulic head drawdown and knowledge of specific storage (in confined aquifers) or specific yield (in unconfined aquifers), determined by aquifer tests. As a result of the application of these methods, over-exploitation of 62 hm(3)/year was estimated using the first method and 67.6 hm(3)/year using the second. The similarity of the two values was a mere coincidence for several reasons: there are errors in the calculation of real evapotranspiration that should be subtracted from the precipitation volume in order to calculate vertical recharge; surface runoff to and from the aquifer area is not measured at the sites where it is needed; the hydrogeological aquifer characteristics are not measured in situ, but rather are taken from neighboring aquifers; there are not enough hydraulic head measurements to correctly estimate the gradient and the groundwater inflows and outflows. Even the volume of water extracted by the pumping wells is poorly determined, because of an unknown number of clandestine wells and the information corresponding to the registered wells is incomplete and not updated.