Available online at www.sciencedirect.com ScienceDirect Procedia Earth and Planetary Science 17 (2017) 822 – 825 15th Water-Rock Interaction International Symposium, WRI-15 Geochemistry and hydrothermal contamination of the AtemajacToluquilla groundwater system (Guadalajara, Mexico) Arturo Hernández-Antonioa,1, Jürgen Mahlknechta, Abrahan Moraa, Juan Antonio TorresMartíneza, Aldo Ramírez-Orozcoa a Centro del Agua para América Latina y el Caribe, Tecnológico de Monterrey, Av Eugenio Garza Sada Sur 2501, 64849 Monterrey, Mexico Abstract Chemical and isotopic data were used to explain the geochemistry and the hydrothermal influence in a groundwater system in Guadalajara, México According to the measured parameters and the characteristics of the groundwater, four water groups were identified: i) thermal waters with high temperatures and high concentrations of Cl, Na, HCO3 and the presence of Li, Mn, F and Mg, apparently of old age with a long residence time; ii) mixed waters of Na-HCO3 to HCO3 water type, which represent a mixture of cold water and thermal water; iii) polluted water characterized by high concentrations of nitrate and sulfate derived from urban and agricultural return flow; iv) cold water characterized by low temperature values, low concentration of Cl, Na and salinity, of Na-HCO3 type This last group represents modern water located in the recharge zone Finally, major proportions of hydrothermal fluids are largest in the well waters of the first group varying from 1.9 to 12.5 % The other groups show hydrothermal fractions below 1.7% Groundwater with elevated hydrothermal proportions is located to the southeastern area of the Toluquilla aquifers This study identifies some processes that govern the groundwater and hydrothermal mixing © 2017 TheAuthors Authors.Published Published Elsevier 2017 The by by Elsevier B.V.B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Hydrothermal water, mixign groundwater, groundwater age Introduction This study was carried out in the Atemajac–Toluquilla aquifer system (ATAS), which is located in a complex neotectonic active volcanic system in the Tepic–Zacoalco Rift and underlies the metropolitan area of Guadalajara, the second most populated city of Mexico (4.6 million inhabitants) Adjacent to this aquifer system is the “La Primavera” caldera Several survey wells have been drilled up to km deep in La Primavera in order to explore * Corresponding author Tel./Fax: +52 8181552573 E-mail address: heran@itesm.mx 1878-5220 © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of WRI-15 doi:10.1016/j.proeps.2017.01.051 Arturo Hernández-Antonio et al / Procedia Earth and Planetary Science 17 (2017) 822 – 825 the potential for geothermal energy Temperatures between 80 and 300 °C have been registered in these wells1, and temperatures higher than 40 °C have been measured in adjacent springs The hydrothermal fluids and springs are characterized by high concentrations of Na, Cl, SiO2, HCO3-, B, F, and TDS A mixture of hydrothermal fluids and meteoric derived water has been identified in the springs of La Primavera The aim of this study is to evaluate the application of environmental isotopes and geochemical methodologies to find answers to hydrogeological questions such as: (i) What are the hydrochemical characteristics of the aquifer systems? (ii) What is the apparent groundwater age? (iii) What is the nature of the mixture of hydrothermal water with cold water? Study area The study area (1368 km2) is located in the central portion of the state of Jalisco in the western portion of the Mexican Volcanic Belt (MVB) The National Water Commission reports an average annual temperature of 20.9 °C and an average annual precipitation of 904 mm, occurring mostly between May and October The mean annual evapotranspiration is 712 mm This area is a complex and active neotectonic structure that controls and regulates the development of the rift floor, limited by normal faults The Atemajac and Toluquilla valleys are located in the lower Tepic–Zacoalco Rift and are bordered by hills, volcanic cones (El Cuatro, San Martín), plateaus (Tonalá) and volcanic calderas (La Primavera), among other features2 The Atemajac and Toluquilla valleys consist of a relatively thin cover of Quaternary lacustrine deposits overlying a thick section of Neogene volcanic rocks, including silicic domes, lava and cinder cones, lithic tuffs, basalts, ignimbrites, pyroclastic rocks, andesites and volcanic breccia, with a basement of Oligocene granite3,4 (Fig 2) Fig Study area Fig Surface geology Sampling and analytical procedures 3.1 Field and laboratory Water samples were collected from 40 production wells using standard protocols in March 2011 The samples were analyzed for major and minor ions, trace elements and tritium Field parameters such as temperature, pH, electrical conductivity (EC) and dissolved oxygen (DO) were measured using portable meters (Thermo, Orion) Alkalinity was determined in the field by volumetric titration At each sampling site, new and pre-rinsed LDPE bottles were filled with filtered (0.45 μm) water samples Water samples for cation and silica determinations were acidified with ultrapure HCl to pH < All samples were stored in the laboratory at °C Dissolved cations and anions were determined by ICP-MS and ion chromatography, respectively Duplicates of selected samples were analyzed using ICP-OES and ion chromatography, following standard methods 823 824 Arturo Hernández-Antonio et al / Procedia Earth and Planetary Science 17 (2017) 822 – 825 3.2 Interpretation The description of the hydrogeochemical characteristics and the identification of possible processes of groundwater were performed using hierarchical cluster analysis with Minitab 17.1 The proportions of the hydrothermal fluids were determined by the M3 model, this model determines the composition of groundwater assuming that the chemistry of groundwater is the result of mixing of reference waters and waters having undergone water-rock interaction5 In this study, the following compositions of samples were used as references: (a) those from three deep wells located in the La Primavera geothermic field, representing hydrothermal water 6,7, (b) those from well 37, representing local groundwater with low temperature and salinity; (c) those from well 12 with low temperature and high salinity, which has been referenced as polluted water The water from the three deep wells in La Primavera caldera are probably the best appropriate selection of hydrothermal reference water, because their temperatures are in the range of geothermic temperatures1 In this paper only summary data is presented in Table Table Average values of concentration of chemicals8, Data are given in mg L-1, except where otherwise indicated EC T DO Na K Ca Mg HCO3 Si F (°C) (PS cm-1) 6.9 33.8 1575 5.4 130.3 41.0 54.4 79.5 852.7 45.9 1.11 7.2 30.2 300 4.5 31.2 9.2 10.9 5.9 6.8 23.4 556 4.7 52.2 10.9 22.2 7.0 25.0 295 5.9 32.5 6.2 Group pH 8.8 NO3-N SO4 Cl Li Ba Mn H (TU) 0.17 11.0 87.2 0.28 0.28 0.33 0.73 128.0 33.6 0.67 0.97 3.9 0.01 0.04 0.01 0.66 8.7 69.9 0.3 12.4 70.6 38.9 0.00 0.08 0.01 2.1 3.4 74.8 38.9 0.98 9.58 23.0 10.6 0.02 0.06 0.01 1.58 41 3.5 Results and discussion 4.1 Hydrochemistry of the aquifer system Table shows the chemical composition of the four groups defined by the hierarchical cluster analysis Group (n=6) had the highest temperature values, high salinity, high electrolyte concentration and low concentrations of NO3-N The water from these wells had concentrations of Li, Mn, Ba and Mg associated with thermal influence These wells were located in the aquifer Toluquilla The water that moves from the recharge zone at Primavera caldera southeast towards the central part of Toluquilla Valley attains a Mg-HCO3 and mixed HCO3-type (Fig 3) In thermal fluids, the Mg concentration is generally low because the incorporation of modified minerals derived from ion exchange reactions, which results in low levels of Mg However, in the thermal water group, the high Mg concentrations indicate mixing between ascending hot water and cold water9 On the other hand, this process indicates the dilution of hot water with cold water10 The Cl/SO4 ratio is generally high in deep geothermal water, in this respect water of Group had high Cl/SO4 ratios (average 12.4), whereas in the other groups the ratios vary from 0.6 to 1.7, which suggest that the water from these wells come from a geothermal system In Group of waters (n=12), the temperature decreases slightly However, electrolyte concentrations decreased significantly with little variation from the group of lower temperature These wells were located in different sites of the valley aquifer, mainly in the east and south of the study area This group represents the Na-HCO3 to mixed HCO3 water type (Fig 3) Also, the water of this group is a mixture of hot water and cold water, receiving a contribution from modern water Group (n=3) represents groundwater in the discharge area in central Guadalajara This group evolves to a Na-SO4 to mixed-HCO3 water type (Fig 3), with higher concentrations of several elements, indicating an important impact from anthropogenic pollution, i.e., SO4, NO3-N, Na and Cl Groundwater that moves toward the northern and eastern direction attains a Na-HCO3 to mixed HCO3 water type (group 1), with higher temperatures, but similar low salinities, indicating water-rock interaction Group (n=19) is a Na-HCO3 water type (Fig 3) located in recharge zones in the western portion and reflects a short (local) groundwater flow path with poor circulation It shows low temperatures and salinity However, elevated NO3-N concentrations were found, possibly derived from agricultural practices Arturo Hernández-Antonio et al / Procedia Earth and Planetary Science 17 (2017) 822 – 825 4.2 Groundwater age Tritium results indicate that groundwater within the study area includes both pre-modern (pre-1950s) and modern recharge The values range from 0.3 to 3.0 TU, which suggests a contribution from modern water in most sampled sites Waters of Group and Group show less tritium activity in comparison with the cold groundwater (group 4) and polluted groundwater (group 3) (Table 2) Cold groundwater (Group 4) with elevated 3H values (> 1.5 TU) was located mostly in the La Primavera volcanic system, represents young waters or recent recharge with little mixing of path lines Waters with 3H values < 1.5 TU illustrate that these wells may represent mixing of flow paths with modern and pre-modern groundwater residence times These waters are found mostly in Toluquilla, referred to as hydrothermal groundwater (Group 1), corresponding to elevated EC and Cl 4.3 Mixing of groundwater The estimated mixing proportions results indicate that the proportions of hydrothermal fluids are largest in the wells of group 1, varying from 1.9 to 12.5 % The other groups show hydrothermal fractions below 1.7%, according to M3 modeling Geographically, groundwater with elevated hydrothermal proportions is located in the southeastern area of Toluquilla aquifers (Fig 4) Fig Piper diagram of water samples Fig Distribution of hydrothermal water Acknowledgements This work was financially supported by Fundación FEMSA and the Chair for Water Science and Technology (Tecnológico de Monterrey) References Verma SP, Arredondo-Parra UC, Andaverde J, Gómez-Arias E, Guerrero-Martínez FJ.: Three-dimensional temperatura 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(ii) What is the apparent groundwater age? (iii) What is the nature of the mixture of hydrothermal water with cold... determines the composition of groundwater assuming that the chemistry of groundwater is the result of mixing of reference waters and waters having undergone water-rock interaction5 In this study, the. .. concentration and low concentrations of NO3-N The water from these wells had concentrations of Li, Mn, Ba and Mg associated with thermal influence These wells were located in the aquifer Toluquilla The