Journal of Earth Science, Vol. 31, No. 5, p. 1007–1015, October 2020 ISSN 1674-487X Printed in China https://doi.org/10.1007/s12583-020-1294-x Moran-Ramírez, J., Morales-Arredondo, J. I., Armienta-Hernández, M. A., et al., 2020. Quantification of the Mixture of Hydrothermal and Fresh Water in Tectonic Valleys. Journal of Earth Science, 31(5): 1007–1015. https://doi.org/10.1007/s12583-020-1294-x. http://en.earth- science.net Quantification of the Mixture of Hydrothermal and Fresh Water in Tectonic Valleys Janete Moran-Ramírez 1 , José Iván Morales-Arredondo * 2 , Maria Aurora Armienta-Hernández 2 , José Alfredo Ramos-Leal 3 1. National Council of Science and Technology, National Autonomous University of Mexico, Institute of Geophysics, Scientific Research Circuit, University City, Coyoacán 04150, Mexico City, Mexico 2. National Autonomous University of Mexico, Institute of Geophysics, University Campus, Mexico City, Mexico 3. Potosino Institute for Scientific and Technological Research, 78216 San Luis Potosí, S. L. P., Mexico Janete Moran-Ramírez: https://orcid.org/0000-0002-3209-0069; José Iván Morales-Arredondo: https://orcid.org/0000-0002-7529-992X ABSTRACT: This study was conducted to identify the origin, hydrogeochemical processes and evolution of groundwater in a tectonic valley. This study was carried out with the aim of quantifying the proportions of groundwater flows contributing to the water chemistry abstracted in a zone of convergence favored by the presence of active faults. The study area is located in the Trans-Mexican Volcanic Belt. End members meth- odology was applied to identify the mixing of hydrothermal with fresh groundwater, where changes in the aquifer geology result in distinct groundwater chemical signatures. Ternary mixing was quantified using con- servative elements. Moreover, other evolutionary processes, such as ion exchange and silicate weathering oc- cur due to changes in the geology of the area. In ternary mixing, each of the end members is associated with the lithology through which it circulates. The local flow contributes 70% of the water to the system, the inter- mediate flow contributes 14%, and the regional flow contributes 16%. Three types of water are produced: Na-HCO3, due to the interaction of water with volcanic rocks of rhyolitic composition, Na-Mg-HCO3, due to the interaction of water with volcanic rocks of basaltic-andesite composition, and Ca-HCO3, due to the inter- action of water with sedimentary calcareous rocks. KEY WORDS: hydrogeochemistry, ternary mixing, end members, conservative elements, local flow, Mexico. 0 INTRODUCTION Volcano-sedimentary aquifers in tectonic valleys are com- plex due to the interdigitation of sedimentary and volcanic ma- terials as well as the presence of faults. The hydraulic conduc- tivity is influenced by this heterogeneity; moreover, the infiltra- tion rates through fractures may be extremely high (Cook et al., 2003). In general, these changes in geological conditions are reflected in the chemical characteristics of groundwater. Differ- ent studies have been investigated to understand these processes in tectonic valleys, some with the presence of active faults. These hydrogeochemical processes facilitate the mixing of thermal and cold groundwaters, as reported in a study to identify geothermal fields in the Xinzhou Basin, Shanxi Province, China (Han et al., 2010). Hydrogeochemical studies allowed to define the mixing processes of water with distinct temperatures at Pocheon, South Korea (Chae et al., 2006). In Romania, aquifers strongly influ- enced by hydro-geothermal phenomena acting within two main geological structures were identified (Povară et al., 2008). Stud- ies in the valleys of central Italy showed that recent or active *Corresponding author: ivanma@igeofisica.unam.mx © China University of Geosciences (Wuhan) and Springer-Verlag GmbH Germany, Part of Springer Nature 2020 Manuscript received December 12, 2019. Manuscript accepted March 10, 2020. tectonics favor the mixing of recharged groundwater with upwelling hydrothermal fluids that influence the hydrogeochem- istry of groundwater (Barbieri et al., 2017). In the Leon Valley, Guanajuato State, Mexico, hydrochemical analysis showed that the abstracted groundwater is the product of a sequential process of binary mixing involving three end members (Ramos-Leal et al., 2007). Mix models allow identifying the contribution of distinct groundwater sources to the aquifer of a certain zone. There are different mix models, such as M3 (Multivariate Mixing and Mass balance calculations) that uses principal component analy- sis code, which approaches the modeling of mixtures and mass balance from a purely geometric perspective (Laaksoharju et al., 1999). The Mix model that involves computing the ratios by which two or more end-members are mixed in a sample. The MIX software, a maximum likelihood method to estimate mix- ing ratios, allows recognizing the uncertainty in the concentra- tions of the final members (Tubau et al., 2014; Carrera et al., 2004). In the present work we applied the methodology, of relat- ing the concentrations of conservative elements as proposed by Douglas et al. (2000). The investigated area is located in the State of Guana- juato, Mexico, which is recognized worldwide for its indus- trial, mining, and agricultural activities, where the use of groundwater is of paramount importance (CEAG, 2000). Se- vere problems associated with water quality and availability