A NEW ENVIRONMENTAL PERSPECTIVE RESULTING FROM A WIDE-SYSTEM VIEW OF GROUNDWATER RESOURCES IN MEXICO CITY

The city of Tenochtitlan built on a fresh water lake housed by 1519 some 80,000 inhabitants in a surface area of »1.9 km². The site has become part of the Metropolitan Zone of the Valley of Mexico (MZVM) with >22 million inhabitants settled in »1,200 km² of dry soil. The urbanization process severely modified both water regime and natural environment increasing the complexity of environmental risks.

Before any sustainable groundwater management may be discussed, an understanding of the relation among groundwater processes (recharge, physico-chemical characteristic of water, discharge), and biophysical surface indicators (geomorphic properties, topography, soil type, lithology, type of vegetation) is required to propose any control measures to observed environmental impacts.

Previous studies considered groundwater in the city area as a quasi-stagnant reservoir, which had no practical hydraulic continuity beyond the borehole field limits and the depth of boreholes (»400 m). Boreholes are found over most of the city area. The only officially accepted recharge zones in the basin (»9,000 km²) are located to its SW and E limit (sierras Chichinautzin-Las Cruces and Nevada).

The Mexico Basin lays in the Mexican Trans-volcanic Belt with >2.5 km thick Plio-Quaternary geological units forming a continuous body of »50 to 250 km wide and »900 km in length. These units overlay fractured limestone of Cretaceous in age. Hydrogeological evidence suggests these units form an aquifer as groundwater is tapped from them to supply other major cities and related productive activities beyond the basin of Mexico. The hydraulic continuity of the basin of Mexico with adjacent basins is supported by: (i) geological framework, (ii) chemistry and age (>6,000 years) of abstracted groundwater in the centre of the ZMVM, (iii) a lower topographic altitude of the basin of Mexico than that of surrounding hills and planes, (iv) hydrogeological modelling results that reproduce initial conditions (discharge areas). The initial conditions scenario was achieved using the flow systems theory where superficial manifestations of groundwater were incorporated to the hydrogeological frame; a further insight to the identification of potential recharge areas outside the basin limits and a regional extensive groundwater functioning was established.