Hydrogeochemical characterization of thermal waters in the northern part of the Argentinean Andes (Northern part)

Abstract

This thesis focuses on the hydrogeochemical characterization of thermal waters in the northern part of the Andes Mountains in Argentina, covering the provinces of Jujuy, Salta, Catamarca, and San Juan. This geographical area, located in a zone of high tectonic activity, is home to a significant number of thermal springs and mineral sources, which are situated within a fault system between the Andes Mountains and the Pre-Andes. The chemical composition of the groundwater in these springs provides valuable information about the hydrogeological and geochemical processes that regulate their origin, evolution, and characteristics. These processes include the deep circulation of fluids, hydrothermal weathering, mineral dissolution, and, in particular, the possible ascension of sodium chloride fluids from the subducted Nazca Plate beneath the South American Plate. The main objective of this research is to analyze and understand the complex relationships between the deep-water circulation systems, geological faults, groundwater characteristics, and thermal springs on the eastern slope of the northern part of the Andes. In this context, the relationship between the presence and distribution of sodium chloride in the different geological provinces of the region has been explored, hypothesizing that the sodium chloride in the groundwater of the Andes may originate from oceanic sediments deposited in the Pacific Ocean. These sediments, when subducted along with the Nazca Plate, ascend through the tectonic faults of the Andes, reaching higher altitudes. Subsequently, these sediments dissolve in thermal waters and mix with the groundwater, which may explain the high concentration of sodium chloride in thermal springs. To carry out the hydrogeochemical characterization of thermal waters, the PhreeqC software was used to model the chemical compositions obtained from a total of 388 thermal water samples, of which 282 were selected for detailed analysis. The chemical data were processed and analyzed using tools such as Piper diagrams, Excel, and scatter plots, which allowed for the identification of significant trends and relationships. The geographical coordinates of the samples were correlated with a permafrost zoning map to locate the samples within thermally active zones, consistent with the focus on working with thermal waters. The results of the research show that the groundwater in the region is generally neutral to slightly alkaline or acidic. According to the Piper diagrams, thermal waters exhibit significant compositional variability, with a prevalence of calcium sulfate and/or chloride waters (Ca²⁺, SO₄²⁻), calcium-magnesium bicarbonate (Ca²⁺, Mg²⁺, HCO₃⁻), or sodium bicarbonate (Na⁺, HCO₃⁻), and sodium chloride or sulfate waters (Na⁺, Cl⁻, SO₄²⁻). The constant presence of a chloride-sodium association, regardless of altitude, suggests that the origin of sodium chloride is linked to a common geological process affecting both the Andes and Pre-Andes. The water-rock interaction, combined with the processes of evaporation and mineral dissolution, are key factors that explain the chemical composition of thermal waters, with these processes being especially influenced by temperature and local geology. A key finding of this research is the heterogeneity in the chemical data of thermal waters, which shows a nonlinear trend in their behavior over time, highlighting the need to establish a more robust baseline for future studies. The data obtained provide a preliminary view of the chemistry of the waters, so it is proposed to continue monitoring to complete this baseline and more accurately assess the long-term trends in the chemical composition of thermal waters. The hydrogeochemical analysis performed has significant implications for the sustainable development of water resources in the region. The classification of groundwater based on its geochemical potential provides crucial information for understanding the associated thermodynamic systems, which are influenced by variations in temperature and pressure related to altitude differences. This analysis is not only essential for evaluating the hydric potential of the region but also for guiding the responsible and sustainable exploitation of these resources, ensuring their long-term conservation within the framework of balanced geothermal development that respects the natural environment.