Groundwater flow processes and mixing in active volcanic systems : the case of Guadalajara ( Mexico )

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Introduction
Active volcanic systems are frequently accompanied by an intense hydrothermal circulation, which is controlled by the exchange of mass and energy between groundwater systems, magmatic fluids and hot rock (Di Napoli et al., 2009;Goff and Janik, 2000).The characterization of such hydrothermal systems helps on the one hand to quantify its geothermal energy potential and, on the other hand, to assess volcanic-related risks (Di Napoli et al., 2011).Hot springs, mud deposits, fumaroles, vaporization and degassing soils give initial clues about subsurface hydrothermal conditions (Hockstein and Browne, 2000;Navarro et al., 2011).The chemical characterization of fluids and groundwater has been used as an indicator of the subsurface structure and the origin of released fluids when hydrogeological information is scarce (Appelo and Postma, 2005;Henley and Ellis, 1983).Hydrochemical data, such as high electrical conductivity (EC), high temperatures and elevated concentrations of As, B, Br, Cl, Cs, F, Fe, Ge, I, Li, Mn, Mo, Na, Rb, Sb, Ta, U and W denote the presence of hydrothermal fluids in groundwater (Aksoy et al., 2009;Dogdu and Bayari, 2005;Reimann et al., 2003).
However, hydrothermal volcanic systems are sometimes difficult to analyse due to the fact that groundwater is a mixture of fluids from various sources, sometimes consisting of shallow meteoric waters from recent infiltration, seawater and hydrothermal water rising from deep hydrothermal reservoirs (Chiodini et al., 2001;Di Napoli et al., 2009;Evans et al., 2002).

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Full and residence times, especially in fractured rock aquifers.Radioactive tracers like tritium ( 3 H) are relatively inexpensive methods to estimate groundwater ages and characterize groundwater flow systems.Relatively few studies attempt to quantify mixing between different hydrothermal and cold fluids (Forrest et al., 2013).This study was carried out in the Atemajac-Toluquilla aquifer system (ATAS) which underlies the metropolitan area of Guadalajara (∼ 4.6 million inhabitants) and is located in a complex neotectonic active volcanic system in the Tepic-Zacoalco Rift.Adjacent to this aquifer system is the "La Primavera" caldera.Several survey wells have been drilled up to 3 km deep at La Primavera to explore the potential for geothermal energy (CFE, 2000).Temperatures between 80 and 300 • C have been registered in these wells (Verma et al., 2012), and temperatures higher than 40 • C have been measured in adjacent springs (Sánchez-Díaz, 2007).The hydrothermal fluids and springs are characterized by high concentrations of Na, Cl, SiO 2 , HCO 3 , B, F, and TDS.A mixture of hydrothermal fluids and meteoric-derived water has been identified in the springs of "La Primavera" (Sánchez-Díaz, 2007).While it is assumed that this caldera influences the aquifer system below the metropolitan area, the proportion of hydrothermal fluids and cold water is not clear.The diversity of the chemical results from previous studies has contributed to the difficulty in clearly evaluating the relationship between the fluids (see Results and Discussion section).This study aims to understand the flow dynamics of groundwater by using the combination of statistical and geochemical methods.Water groups and factors that control the groundwater chemical processes were identified using a cluster and principal component analysis.Environmental tracers were used to assess chemical evolution.Mixing proportions of selected fluids in public wells were quantified by means of a multivariate mixing calculation.This study is the first of its kind to report a comprehensive understanding of groundwater flow processes below the Guadalajara metropolitan area.This information is strategic to decision makers from local water authorities regarding water resources management.Introduction

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Study area
The study area is situated in the central portion of the state of Jalisco (Fig. 1).It belongs to the Lerma-Santiago river system, which drains into the Pacific Ocean.The climate in the study area is semi-warm to sub-humid.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 potential evaporation is on the order of 2000 mm (CONAGUA, 2010).

Hydrogeological settings
The study area is located in the western portion of the Mexican Volcanic Belt (MVB), a 1000 km-long volcanic arc that crosses central Mexico in E-W direction from the Pacific to the Atlantic Ocean.The MVB originated in the Late Miocene in response to the subduction of the Cocos and Rivera plates below the North American plate along the Middle America Trench.The belt has a composition of intermediate to silicic rocks (Alva-Valdivia et al., 2000).The western end of the MVB defines the faultbounded crustal Jalisco Block (Ferrari et al., 2007;Valencia et al., 2013).The northern and eastern boundaries of this block consist of asymmetric continental rifts formed by tilted blocks with escarpments between 800 and 1000 m (Zárate-del Valle and Simoneit, 2005); the Tepic-Zacoalco Rift to the north runs in an NW-SE direction, and the Colima Rift to the east runs in an N-S direction; these rifts join the E-W oriented Citala or Chapala Rift in what is known as the Jalisco Triple Junction located 60 km SSW of the city of Guadalajara (Fig. 1).This area is a complex and active neotectonic structure that controls and regulates the development of the rift-floor, limited by normal faults (Michaud et al., 2000;Zárate-del Valle and Simoneit, 2005).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 features (Sánchez-Diaz, 2007).Introduction

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Full These 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 (Tala tuff), basalts, ignimbrites and other pyroclastic rocks, andesites and volcanic breccia, and a basement consisting of Oligocene granite (Campos-Enríquez et al., 2005;Gutiérrez-Negrín, 1988;Urrutia et al., 2000).The principal aquifer, termed the Upper Aquifer unit, consists of the alluvium and underlying Neogene rocks.It is up to 700 m thick, and water is extracted from depths of up to 300 m below the surface (Sánchez-Diaz, 2007).
The water in the alluvial deposits and volcanic rocks in the upper aquifer unit is unconfined to semi-confined and moves under a natural hydraulic gradient that reflects the topography.In the Atemajac Valley, groundwater recharges in the normal fault NE from the La Primavera caldera from rainfall infiltration and flows in an SW-NE direction, while the flow in Toluquilla Valley starts in El Colli and moves in NW-SE direction (Figs. 1 and 2).

Hydrothermal system
The La Primavera caldera, with a diameter of ∼ 10 km, borders the study area to the west.It is a very young (Late Pleistocene) volcanic complex underlain by a magma chamber whose top reaches a depth of 4 km (Verma et al., 2012).Drilling has revealed that the oldest units consist of granitic and granodioritic rocks found mainly at a depth of approximately 3000 m.These rocks are mainly overlain by andesitic rocks approximately 1150 m thick.The third lithologic unit, which is approximately 100 m thick, consists of rhyolites.The uppermost unit is a sequence of lithic tuffs and minor andesite with an average thickness of approximately 750 and 1000 m, respectively (Campos-Enríquez et al., 2005;Urrutia et al., 2000;Verma et al., 2012).The system is characterized by an asymmetric structure with NW-SE regional basalt lineaments that belong to the Tepic Zacoalco Rift and local NE-SW fractures in the upper units extending beneath Guadalajara (Alatorre-Zamora and Campos-Enríquez, 1991; Campos-Enríquez and Alatorre-Zamora, 1998).The temperatures, which were Introduction

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Field and laboratory
Water samples were collected from 40 production wells in March 2011 using standard protocols.The samples were analyzed for major and minor ions, trace elements and isotopes (δ 2 H, δ 18 O, 3 H).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 (0. Water isotopes were analyzed at EIL, University of Waterloo, Canada.To conduct deuterium ( 2 H) analyses, sample water was reduced on hot manganese (512 • C) and the released hydrogen was analyzed by GC-MS.

Interpretation
A preliminary description of water chemistry and identification of possible processes was performed using a correlation analysis.A hierarchical cluster analysis (HCA) organized samples into classified groups which were evaluated according to their geographic correspondence.A principal component analysis (PCA) elucidated the main controls on groundwater chemistry.All of the statistical calculations were performed using Minitab version 17.1 (Minitab, 2013).
The multivariate mixing and mass balance model, or M3 (Laaksoharju et al., 2008), was used to help to understand groundwater composition.The main aim of M3 is to differentiate between what is due to mixing and what is due to water-rock reactions.The M3 method compares the measured groundwater composition of each sample to the selected reference water and reports the changes in terms of mixing and reactions.A PCA is used to summarize the groundwater data by using the majority of the dissolved groundwater constituents Ca, Na, Mg, K, Cl, SO 4 and HCO 3 in combination with the isotopes δ 2 H, δ 18 O and 3 H.The outcome of the analyses can be visualized as a scatter plot (PCA plot) for the first two principal components.The observations inside the polygon of the PCA plot are compared to the previously chosen reference water compositions.The mixing calculations create ideal mixing models that use linear distances of the samples from the selected reference waters in the PCA plot.In this Figures

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Full study, the following sample compositions were used as reference waters: AT5 -highest temperature and salinity as the reference for hydrothermal fluids; AT12 -low temperature and elevated salinity as the reference for polluted or anthropogenically impacted water; and AT37 -low temperature and salinity as the reference for local groundwater.
Although none of these selected reference waters are considered end members, they were selected as most representative for the purpose of the present study to determine the amounts of each mixture.

Results and discussion
Sánchez-Diaz ( 2007) used groundwater temperature and total dissolved solids as criteria to classify wells in hydrothermal water from Toluquilla (HT), hydrothermal water from springs NE of Guadalajara (HG), non-hydrothermal, local groundwater (LG), and mixed groundwater (MG) with both HT and LG (Fig. 3).Considering different sets of historical and new data, this classification is too subjective, especially in the lower TDS range.Furthermore, some inconsistencies between correlation results from different sampling campaigns show that the interpretation is not straightforward.The Mg concentration, for example, decreases with increasing temperature as expected from hydrothermal fluids (Panichi and Gonfiantini, 1981); on the other hand, an increasing Mg trend at low temperatures is observed indicating saline groundwater.Finally, it was not clear if there are different sources of hydrothermal or saline waters that affect the local groundwater.

Multivariate statistics
Table 1 shows the concentrations of measured groundwater elements, field parameters and isotopic ratios, along with the hydrochemical classification.The classification of waters was performed with HCA using 20 variables (pH, temperature, EC, DO, Na, K, Ca, Mg, Cl, HCO 3 , SO 4 , NO 3 -N, Sr, SiO 2 , Fe, F, Zn, 3 H, 2 H, 18 O).With the help of Ward's linkage rule iteratively neighboring points (samples) were linked through a similarity matrix (Ward, 1963).The squared Euclidian distance was selected as the similarity measurement.The second method was a PCA.For both cluster algorithms, lognormal distributed data were previously log-transformed, and all of the variables standardized (z-scores).The HCA samples were classified into 4 major groups as represented by the dendrogram in Fig. 4 and median values (Table 2).The values for Li, Mn and Ba were not considered in the cluster analysis, because most samples had concentrations below the detection limit.Group 1 (n = 6) had the highest temperatures (average 33.8 • C), high salinity (EC = 1575 µS cm −1 ), and low NO 3 -N (0.17 mg L −1 ) and 3 H (0.73 TU) values, along with traces of Li, Mn and Ba that were possibly due to geothermal influence.These waters were of mixed HCO 3 type, as shown in the Piper diagram (Fig. 5), and located in the central area of Toluquilla.Group 2 (n = 12) had slightly lower temperatures (30.2 • C), with considerably lower salinity (300 µS cm −1 ) and 3 H activity similar to that of group 1. (15.5 mg L −1 ) values.These predominantly Na-HCO 3 to mixed HCO 3 type waters are located in the western portion of the aquifer system in areas of elevated altitude where most of the recharge takes place.Figure 5 shows that groups 3 and 4 are relatively similar compared to the other two groups.
A factor analysis transformed the 20 variables into a reduced number of factors.
The PCA, which loads most of the total variance onto one factor, was used in this study.The factors were extracted through the principal components method.Varimax rotation, where one factor explains mostly one variable, was selected.For fixing the maximum number of factors to be extracted, only factors with eigenvalues higher than one were taken into consideration (Kaiser normalization).
Table 3 shows that 4 factors may explain 77 % of the variance.Factor 1 (42 % of the variance) largely represents high salinity.The correlations of temperature, Na and Cl indicate hydrothermal influence, while HCO 3 , Na and Sr could be connected to mineralization and rock dissolution processes, and cationic exchange.In factor 2 (17 %) the temperature is inversely related with DO, 3 H, and to a lesser degree, NO 3 and SO 4 , suggesting that this factor represents water affected by human activities, either urban or agricultural.In addition, Table 1 shows that waters affected by human activities are most evaporated.Sulfate could be related to contamination due to the infiltration of commonly applied sulfate-based fertilizers during the rainy season.This occurs because all the wells are undersaturated with regard to gypsum, indicating that the water does not move through deposits of this mineral.In factor 3 (11 %) the relationship be-  (2007) indicates that the rhyolitic rocks and ashes of the study area are responsible for releasing F. Comparable trends have been observed in similar volcanic environments in central and northern Mexico (Mahlknecht et al., 2004(Mahlknecht et al., , 2008)).Introduction

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Isotope hydrology
The δ 2 H vs. δ 18 O graph shows that the analyzed groundwater is of meteoric origin with variable evaporation and hydrothermal effects (Fig. 6a and b).Group 1 (hydrothermal influenced) waters, collected in Toluquilla, have a narrow range of δ values ranging from −7.9 to −5.7 ‰, and δ 2 H values from −59.6 to −47.5 ‰, and are stronlgy affected by evaporation (Fig. 6b).The enriched outlier AT12 represents groundwater from a recreational park with lagoons.Group 4 waters, mostly from La Primavera recharge area, are covering a relative wide range of values compared to group 1 and 2. Their δ 18 O signatures vary from −10.3 to −8.4 ‰, and their δ 2 H signatures from −72.2 to −63.9 ‰.The overlapping of group 1, 2 and 4 indicates that aquifer formations are mostly hydraulically interconnected.Although altitude variations are in the order of only 400 m around La Primavera Caldera, this seems to be enough to generate an altitude effect (Fig. 6c).
Tritium results indicate that groundwater within the study area includes both premodern (pre-1950s) and modern recharge.The values range from 0.3 to 3.0 TU which indicates a contribution from modern water in every sampled site (Table 1).The majority of waters with 3 H lower than 1.0 TU are in the southern portion of the aquifer system.

Mixing patterns with M3
The M3 mixing model compared all the samples to the three reference-water components (hydrothermal water -HW, groundwater -GW, human impacted water -HIW).Introduction

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Full The calculation results indicate that the proportion of hydrothermal fluids within group 1 wells is between 32 and 87 % (Fig. 7, Table 4), while the proportion is lower than 13 % within the other three groups.On the other hand, anthropogenically impacted waters within group 3 show proportions between 59 and 63 %, while all of the other groups are lower than 36 %.These results validate the initial selection of groups based on cluster analysis.Geographically, groundwater with elevated hydrothermal proportions is located in the south to southeastern area (Toluquilla), and elevated proportions of polluted groundwater are located mostly in the urbanized area of Guadalajara.Samples AT7, AT11, AT29, AT35 and AT38 lie outside the polygon in Fig. 7 and have thus been excluded from evaluation.However, note that they represent local groundwaters and indicate that the selected reference waters are only an approximation of unknown end members, so the polygon may not include all of the samples.

Groundwater flow system of Guadalajara
The hydrogeological Atemajac-Toluquilla system is located in the northeastern area of the Tepic-Zacoalco Rift, a complex and active neotectonic structure.Polluted waters infiltrate mainly from urban water cycling and, to a minor degree, from agricultural plots.This waters are characterized by low temperature, variable salinity, high concentrations of nitrate and sulfate and, elevated concentrations of chloride and sodium, and are of Na-SO 4 to mixed HCO 3 type.
The isotopic composition of groundwater indicates interconnectivity between water from deeper and shallow rock materials.Practically all groundwater sampled contains at least a small fraction of modern water.The proportions of hydrothermal fluids in sampled well waters ranged from 13 (cold groundwater) to 87 % (hydrothermal water), while the proportion of polluted water is between 0 and 63 %. Figure 8 shows two schematic flow sections of Atemajac and Toluquilla valley.

Conclusions
This work represents the first time that groundwater flow dynamics of Guadalajara region have been analyzed and characterized by using a suite of statistical and geochemical methods.Geochemical methods have been combined with multivariate statistical analysis and the multivariate mixing and mass-balance model (M3) to determine the mixing patterns of different fluids when considering fresh groundwater, hydrothermal fluids and human impacted waters.
The initial classification of groundwater groups by cluster analysis has been confirmed by water isotopic technologies, and identifying the controlling factors by principle component analysis is consistent with M3 modeling.The origin of groundwater recharge and the mixture of fresh groundwater with hydrothermal fluids and polluted water components was identified.A conceptual flow model was constructed for the Atemajac-Toluquilla aquifer system.The local flow is associated with the infiltration of rainwater that occurs at higher altitudes.Hydrothermal waters are probably related to Introduction

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Full recharge outside the study area and upward vertical flow in the La Primavera caldera and the valley of Toluquilla.TDS, Cl, Na, Mn and Li, are most indicative of hydrothermal fluids.Modern water polluted with SO 4 and NO 3 can be associated with urban development and agricultural practices.The proportion of hydrothermal fluids within older waters is between 32 and 87 %, whereas it is lower than 13 % within other waters.
These outcomes may help water authorities to identify wells with hydrothermal mixture or polluted water and act accordingly.The information regarding the proportions of fresh groundwater, hydrothermal fluids and polluted waters in each well indicates that the contaminants can be attributed to source waters.For example, Li, Mn, Ba, F and As can be associated with hydrothermal fluids, and SO 4 and NO 3 are related to the production or use of fertilizers, dyes, glass, paper, soaps, textiles, fungicides or insecticides.This result helps authorities to decide whether certain wells have to be isolated or closed in order to provide Guadalajara with the required drinking water quality.
M3 is a powerful tool to evaluate the mixing proportions of selected reference waters present in aquifers.We recommend the use of the suite of traditional methods, environmental tracers, statistical analysis and M3 modeling in other aquifers with potentially multiple groundwater origins, especially in active volcanic systems.Introduction

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Full  Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 02 N H 2 SO 4 ) of filtered water samples to pH 4.3.At each sampling site, new and pre-rinsed low density polyethylene bottles were filled with filtered (0.45 µm) sample water.Cation and silica samples were acidified with ultrapure HCl to pH < 2, and all of the samples were stored in the laboratory at a constant temperature of 4 • C. Dissolved cations and anions were determined by inductive-coupled plasma mass spectrometry (ICP-MS) and ion chromatography, respectively.Duplicates of selected samples were analyzed using inductive-coupled plasma optical emission spectrometry (ICP-OES) and ion chromatography, following standard methods (APHA, 2012).Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | These complications motivated us to use multivariate techniques instead of commonly used scatterplots and criteria to divide samples into groups and interpret for potential factors/sources.Because the measured parameters varied considerably from study to study, only data from this study were considered for the multivariate techniques.Discussion Paper | Discussion Paper | Discussion Paper | Chloride concentrations are low (3.5 mg L −1 ) compared to the other groups.Wells from group 2 were found mainly in the eastern and southern region of the study area.These waters were of Na-HCO 3 to mixed HCO 3 type.The smallest group 3 (n = 3) represents the coldest waters (23.4 • C), that had variable salinity (EC = 556 µS cm −1 ), and the highest SO 4 (70.6 mg L −1 ), NO 3 -N (12.4 mg L −1 ), 3 H (2.1 TU) values, and relatively elevated Na (52.2 mg L −1 ) and Cl (38.9 mg L −1 ) concentrations.These waters were a mixture of Na-SO 4 to mixed HCO 3 type, located in urban and agricultural areas, possibly indicating anthropogenic pollution.Finally, group 4 (n = 19) was cold (25.3 • C), similar to group 3, with low salinity (254 µS cm −1 ), and elevated NO 3 -N (9.1 mg L −1 ) and SO 4 Discussion Paper | Discussion Paper | Discussion Paper | the existence of recharge water.This factor is generated almost entirely by the linear relationship between O and H isotopes.The relation with temperature indicates the recharge conditions at different recharge sites.Factor 4 (7 %) may be indicative of dissolution of minerals that contain F. The study of Sánchez-Díaz Discussion Paper | Discussion Paper | Discussion Paper | to −68 ‰) values.They tend to fall slighly below and parallel to the RMWL, possibly indicating different climate conditions during recharge.The increased Cl concentrations compared to other groups evidences mixture with hydrothermal fluids (Fig. 6b).Group 2 waters, collected in the eastern and southern part of the ATAS, have δ 18 O values ranging from −9.6 to −8.6 ‰, and δ 2 H values from −63 to −71 ‰.These waters fall along the regional meteoric water line.Group 3 waters (influenced by anthropogenic pollution) are quite different from the rest; they have δ 18 O Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Local groundwater recharge for Atemajac-Toluquilla Valley originates from rainfall mainly over the La Primavera caldera in the central western portion of the study unit.It flows in the upper alluvial sediments towards the valley floor and Santiago River.It is characterized by low temperature, and SDT, Cl and Na concentrations, and is of predominantly Na-HCO 3 type.This kind of groundwater predominates in the western Atemajac Valley.Underground heat flow suggests the existence of a magma chamber below the La Primavera caldera, which provides hydrothermal fluids observed on surface expressions such as the La Soledad solfatara and the Cerritos Colorados geothermal field.These emerging fluids circulate towards the deeper Atemajac-Toluquilla aquifer unit specifically below Santa Anita and Toluquilla.Calcedonia geothermometers indicate that these fluids are related to basaltic and andesitic rocks (San Cristobal group) (Sánchez-Díaz, 2007).They are characterized by elevated temperatures, salinity, Cl, Na and HCO 3 values, and contain Li, Mn, B and F, which indicate thermal influence, cir-Discussion Paper | Discussion Paper | Discussion Paper | culation through fault zones and an active volcanic center, and water-rock interactions.These waters are of mixed HCO 3 type.They are representative of the southwestern portion (Toluquilla Valley).

Figure 1 .Figure 2 .Figure 5 .
Figure 1.Location of study area (black area) in Mexico and tectonic structures of western Central Mexico.
To conduct oxygen-18 ( 18 O) analyses, water was equilibrated with CO 2 .Preparation and extraction took place on a fully automated system vessel attached to a VG MM 903 mass spectrometer.The

Table 1 .
Concentrations of measured field parameters, groundwater elements, stable isotopic ratios, and hydrochemical classification.Data are given in mg L −1 , except otherwise indicated.Note: T = temperature, DO = dissolved oxygen, EC = electrical conductivity.