Alejandra Cortés

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.

Isotope studied of precipitation and groundwater in the sierra de las Cruces, Mexico

Abstract An environmental isotope study was carried out on water samples collected from springs, streams, wells and precipitation events in the upper part of the Sierra de las Cruces mountains in Mexico. These mountains form the western flank of the Valley of Mexico in which Mexico City is located and may be an important area of recharge for the aquifers which supply a large portion of the municipal water supply of the city. A local meteoric water line is constructed from the precipitation data by a linear regression technique and is described by δ D = 7.95 δ 1H 0 + 11.77. An important feature of these data is the wide spread observed in isotope values throughout the sampling season. These values range from - 153 to - 9.0% for deuterium and from - 20.9 to - 3.1% for oxygen-18. This wide isotope range cannot be attributed to an altitude effect because all samples were collected at nearly the same elevation. The history of the air masses that generate each precipitation event and variable climatic conditions at the site could be responsible for this effect. Groundwater samples obtained from springs, streams and wells do not show the same ranges of variation as precipitation; most of them are close to the average precipitation values: δ D- 70% and δ 18 0 - − 10.3%. This value can be cosidered representative of the isotope composition of the water that may recharge the Valley of Mexico aquifer system in the Sierra de las Cruces area.

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.

Interdisciplinary hydrogeological investigations

Traditionally, the responsibility for hydrogeological investigations in developing countries falls on governmental institutions, accustomed more to the management and handling of the resource rather than to its research. At the present time, the characteristics of developing countries restrict the possibility of applying and developing contemporary techniques and methodologies in the field of hydrogeology to universities or high‐level educational institutions, which do not necessarily maintain communication with the authorities in charge of the supply and administration of water. As an example of this research policy the Aquifer Development Project, carried out by the Geophysics Institute of the National University of Mexico (UNAM), is presented.