The groundwater regime of the Valley of Mexico from historic evidence and field observations

Abstract

Groundwater is a matter of major importance in the Valley of Mexico because some 20 million people depend on it for most of their water supply. In Mexico, historical accounts, documents and native legends provide additional information of past conditions which relates to hydrogeological conditions. In any analysis of groundwater resources it is important to know the original conditions. The Valley of Mexico is a graben structure, closed hydrologically and covered by a series of lakes at the time of the Conquest. Groundwater recharge occurs in the mountains of volcanic rocks that surround the Valley to form the Basin of Mexico. Where the rocks are visibly permeable, the water-table is deep, for the most part, and runoff is low. Thick lacustrine clays cover the Valley floor and artesian conditions once prevailed. Large springs of potable water were numerous at the edge of the Valley, and where permeable aquifers pinch-out. Thermal mineral springs occur along lineaments thought to be fractures in the rocks below the alluvial fill. The entire Valley floor and the lowest slopes of the mountains were zones of groundwater discharge. All water discharge from the Valley was by evaporation and transpiration, and salts accumulated in the lake-water and in the clays. The main lakes were nonpotable and the Aztecs and later the Spanish colonials depended on groundwater from the springs. Salt production from brines was an important industry in the Aztec society as it is today. The ahuehuete tree, ( taxodium mucronatum), which commonly lives to be many hundreds of years old, is a phreatophyte and an indicator of fresh groundwater discharge in the Valley. It used to be much more abundant. Its occurence where earthquake damage is worst suggests upward migration of fresh groundwater through fractures in the clay tht have been opened by seismic response. The water table and the capillary fringe are near ground surface over a wide zone of lowlands around the edge of the ancient lakes. A small amount of rain produces overland flow almost immediately and flooding has always been a problem to societies that occupy the Valley. Except for one site known to us, groundwater gradients have been reversed everywhere in the Valley by pumping of the aquifers, which began in 1847 and became intense beginning in the 1930s, so that the direction of flow is downward, which allows, for the first time, contaminants from the surface to migrate downward to the aquifers. Heavy pumping has also caused drainage and consolidation of the lacustrine clays, and consequently land subsidence of up to 8 m in the central part of the city. A simple water-balance indicates that groundwater discharge to the Valley floor is about 43 m 3s −1, which is less than is being pumped for municipal supply. Some of the deficiency is made up from compaction of the clays, a nonrenewable source.