Geochemical and hydrodynamic characterization of the groundwater system of Puebla Valley, Mexico

Includes bibliographical references (pages [198]-221).This project examined major and trace elements, isotopic measurements of ¹⁸O, ²H, ¹³C, ³⁴S, and ³He/⁴He ratios that assisted in the geochemical and hydrodynamic characterization of the aquifer system of Puebla Valley, and the adjacent Atlixco-Izucar de Matamoros area. The study area covers approximately 4000 km² within a semi-arid region in the south central part of the Trans Mexican Volcanic Belt. Puebla City, the major urban center within the study area, is threatened by volcanic activity and challenged in its development by relatively low availability of groundwater. Water samples were collected from production wells, including artesian wells, springs, and intermittent streams; these represent at least two hydro-stratigraphic units with distinctive chemical composition in the study area. A noteworthy contribution of the project was the preliminary examination of the chemical evolution of groundwater of the Puebla confined aquifer unit, whose chemical characteristics have previously been attributed to the influx of magmatic volatiles from Mt Popocatepetl. The confined aquifer is rich in sulfur species and further characterized by high dissolved CO₂ and high alkalinity; ratios of major ions, trace elements, analysis for sulfur reducing bacteria (SRB), and δ³⁴S, indicate that evaporites, probably of Middle Cretaceous age are an important rock source responsible for its high sulfate content. Molecular analyses and SRB tests indicate that there are at least two bacteria species present in the system: Desulfovibrio- Desulfomicrobiunv, these are probably partially responsible for the production of H₂S. Within the upper aquifer is a group of wells, circumscribed within an area of approximately 10 km², and labeled for this project as the Cholula-Atlixcayotl subgroup. Groundwater from these wells is similar to the water in the confined aquifer, with respect to its high CO₂ and alkalinity content; however, it differs from it by lacking sulfide. δ¹³C and ³He/⁴He ratios in water of this subgroup indicate a contribution from a mantle-derived gas emanation. This information, together with tectonic reports about the area, suggests that gas of magmatic/mantle origin is emerging to the surface through faults. Recharge elevations were estimated based on δ¹⁸O and δD analyses of groundwater. These results produced four groups of samples associated with distinctive elevation and geochemical characteristics.Ph.D. (Doctor of Philosophy