Jaime Durazo

Geochemistry of Rare Earth Elements in Groundwaters from a Rhyolite Aquifer, Central México

Rare earth element (REE) concentrations were measured in groundwaters collected from wells finished in a fractured, rhyolitic (Cuatralba Ignimbrite) aquifer from the La Muralla region of the central Mexican State of Guanajuato. The study site is located within the Faja Volcanica Transmexicano (i.e., Trans-Mexican Volcanic Belt), an extensive region of active volcanism within central Mexico. La Muralla groundwaters are relatively warm (32.2 ± 2.7 °C), dilute Na-Ca-HCO3 waters (5.1 mmol/kg ≤ I ≤ 9.5 mmol/kg) of circumneutral pH (7.27 ≤ pH ≤ 8.01). Concentrations of REEs in La Muralla groundwaters are exceedingly low, as demonstrated by Nd values, which range from ∼ 10 pmol/kg to 34 pmol/kg. La Muralla groundwaters exhibit enrichments in the heavy REEs (HREE) over the light REEs (LREE) compared to Average Shale, as well as volcanic rocks from the Trans-Mexican Volcanic Belt, including rhyolitic volcanic rocks similar to those of the Cuatralba Ignimbrite aquifer. Shale-normalized Yb/Nd ratios of La Muralla groundwaters range from 1.85 to 6.55, with a mean (± standard deviation) of 4.2 ± 1.2. Rare earth element concentrations for La Muralla groundwaters are normalized to the average REE values of 27 different calc-alkaline rhyolites (from the literature) from the Trans-Mexican Volcanic Belt. The average Trans-Mexican Volcanic Belt rhyolite-normalized Yb/Nd ratios for La Muralla groundwaters range from 1.57 to 5.55, with a mean (± standard deviation) of 3.52 ± 1. Speciation calculations predict that REEs occur principally as carbonate complexes in La Muralla groundwaters, with LREEs predominantly in the form of positively charged, carbonato complexes (LnCO3 +), and to a lesser extent, free metal ions (Ln3+), and HREEs chiefly in solution as negatively charged, dicarbonato complexes (Ln(CO3)2 −). The speciation model predictions suggest that the HREE enrichment of La Muralla groundwaters originate from solution and surface complexation reactions within the system. Specifically, the preferential complexation of HREEs as negatively charged, dicarbonato complexes acts to stabilize HREE is solution owing to both the strength of these complexes and their low affinity for aquifer surface sites. Because La Muralla groundwaters are of circumneutral pH, surface complexation sites within the Cuatralba Ignimbrite are expected to predominantly be negatively charged. Therefore, because LREEs occur primarily as positively charged, carbonato complexes in La Muralla groundwaters, they are preferentially removed from solution owing to complexation to aquifer surface sites.

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 3 s −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.

Robert N. Farvolden − Pioneering contributions in Mexican hydrogeology

At the end of the 1970s at the height of the petroleum bonanza, Mexico seemed to be growing away from economic underdevelopment. At that time, as in many parts of the world, the country was also beginning to realize the limitations of its water resources and to be sensitive to the related risks of water degradation. The issue of water-resource management in Mexico had already received international attention because of the huge and growing demands of Mexico City. The situation in Mexico, however, became of particular interest to two world-renowned hydrologic scientists and professors from the University of Waterloo in Canada: Drs. Peter Fritz and Robert N. Farvolden. Through an almost prophetic foresight, they recognized that a water crisis in Mexico was looming and that a comprehensive evaluation of the countrys water resources was of paramount importance. Their interest was primarily focused on the groundwater resources. Over a 15-year period, beginning in the early 1980s, Drs. Fritz and Farvolden made significant and lasting contributions to the understanding of the groundwater resources in Mexico. Their two stories are different; here, we focus on that of Professor Farvolden.