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Origin of the Salinity in the Coastal Aquifer of La Paz, Mexico
Available online at www.sciencedirect.com
ScienceDirect Procedia Earth and Planetary Science 17 (2017) 520 – 523
15th Water-Rock Interaction International Symposium, WRI-15
Origin of the salinity in the coastal aquifer of La Paz, Mexico Juan Antonio Torres-Martíneza, Jürgen Mahlknechta,1, Arturo Hernández-Antonioa, Abrahan Moraa a
Tecnológico de Monterrey, Eugenio Garza Sada 2501, 64849 Monterrey, Mexico
Abstract Coastal areas are attractive places for the establishment of urban settlements owing to the many benefits in terms of quality of life. An example of these areas is La Paz in Southern Baja California, Mexico, where demographic growth and agricultural activities under dry climatic conditions are threatening the availability of water resources. In recent years, an increasing salinity of groundwater has been identified, however, the origin of the salinity is not clear. Near the coastline, salt water intrusion is considered to be the main factor, while others play a role for increased salinities in the middle and upper aquifer zone. The study, using a hydrochemical tool analysis shows that the salinity of the coastline is the result of over-exploitation of wells, which increases saline intrusion resulting in a reverse cation exchange. It also indicates that the salinization of water in the recharge area is product of carbon dioxide dissolution and weathering of rock-forming silicate minerals, and in the central part result from cycling and use of fertilizer in agriculture. © 2017 2017The TheAuthors. Authors. Published by Elsevier Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of WRI-15. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Water-rock interaction; groundwater; salinity; Mexico
1. Introduction Human settlements in coastal areas provide many benefits for quality of life. However, under arid climate conditions, groundwater is usually the main or the only source of water supply, so it is very common that the availability of water resources in these regions is relatively low1,2. This situation usually results in water balance deficits, inducing an overexploitation of aquifers and thus the hydraulic gradient leads sea water intrusion. Mexico has at least fifteen coastal aquifer zones under arid conditions affected by saltwater intrusion; most of them located in the northwestern region of the country3. The capital and area of La Paz (~260,000 habitants), located 1
Corresponding author. Tel.: +52 (81) 8358 2000 Ext. 5561 E-mail address: [email protected]
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.2016.12.131
Juan Antonio Torres-Martínez et al. / Procedia Earth and Planetary Science 17 (2017) 520 – 523
in Southern Baja California, is a clear example of a touristic coastal area (Fig.1a) with important agricultural activities nearby. There, groundwater is practically the only one available local water resource. b)
a)
Fig. 1. (a) Location of La Paz aquifer; (b) Geological map and water sampling sites.
The aquifer of La Paz (1,275 km²) is located in the Valley of La Paz (Fig 1a). According to the Köppen classification, the climate of the region is hot desert (BWh), with an average annual temperature and precipitation of 22.1 °C and 257 mm, respectively, and no perennial surface runoff. One-hundred-fifty-five production wells extract a total of 32 million m3/year. The well depths vary from 3 to 200 m4. Aquifer mining caused a gradual decrease of the water table of up to 10 m that led to the reversal of the hydraulic gradient near the coastline of La Paz, resulting in an increased groundwater salinity of up to 4,800 mg l-1 (TDS) from salt water intrusion in the coastal plain5. In 2005 the increase of salinity reached ~6 km inland from the coastline, however, at the year 2013 this fringe has extended to ~13 km6. The geology of La Paz area consists of a complex of metamorphic shales, phyllites, gneiss and sillimanite from Mesozoic period, covered by intrusive rocks (gabbro, granite and granodiorite) from Cretaceous period, sedimentary rocks (Miocene sandstones, conglomerates, fractured volcanic rocks), and alluvial material (sand, silt and clay) (Fig. 1b)4. This study concentrates on the upper geological units considering sedimentary rocks and alluvial material. The aim of this study was to perform an investigation on groundwater chemical processes that cause salinity in the coastal area of La Paz, by using different hydrochemical tools. 2. Methodology 2.1. Field and laboratory work Water samples from forty-six production wells from the basin-fill sediments and one spring were collected and analyzed for their chemical and isotopic composition in August 2013 (Fig 1b). Samples were collected in HDPE plastic bottles and temperature, pH, electrical conductivity (EC), total dissolved solids (TDS) and dissolved oxygen (DO) were measured in the field using portable meters. Alkalinity was determined in the field by volumetric titration (0.02N H2SO4), using filtered water samples (0.45 µm). Cation samples were acidified with ultrapure HCl to pH < 2; all samples were stored at constant temperature of 4°C. Cations (K +, Na+, Ca2+, Mg2+, Sr2+) were determined by inductive-coupled plasma mass spectrometry (ICP/MS) using a Perkin Elmer ELAN 9000 ICP/MS, and an inductive-coupled plasma optical emission (ICP-OES) using a Perkin Elmer Optima 3000 ICP OES when limits were exceeded. Anions (Cl-, NO2-, NO3-, Br-, SO42-, PO43- and SiO2) were determined by ion chromatography with a DIONEX DX-120.
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Juan Antonio Torres-Martínez et al. / Procedia Earth and Planetary Science 17 (2017) 520 – 523
2.2. Interpretation Water samples were grouped by a hierarchical cluster analysis using 15 variables (pH, temperature, electrical conductivity, TDS, DO, alkalinity, Na+, SiO2, K+, Ca2+, Sr2+, Cl-, Br-, NO3-, SO4 2- and the distance of the sample location from the coastline). Ward's linkage rule was applied to link nearby clusters with a similarity matrix and an analysis of variance (ANOVA) developed to assess the distance between the clusters7. From this analysis, water samples were classified into three groups: group 1 (8 samples in the upper area of the aquifer away from the coast), group 2 (16 samples located in the urban area of the aquifer), and group 3 (23 samples in the central area of the study area, with agricultural activities8. 3. Results and discussion The study area shows a wide variety of salinity. Group 1 has salinities ranging from 479 to 924 μS cm-1 (freshwater) and chloride concentrations from 1.54 to 3.33 mol·m-3; salinity of group 2 ranges from 683 to 1,658 μS cm-1 and chloride concentrations from 2.70 to 10.86 mol·m-3; and group 3 (brackish water) with conductivity between 1,777 and 8,920 μS cm-1 and chloride concentrations between 11.23 and 83.49 mol·m-3, indicating a likely increase of salinity along the flow path as water moves closer to the sea. The general dominance of cations is Na > Ca = Mg > K, while the dominance of anions was Cl > HCO3 >> SO4. Group 1 is derived from rainwater, so the hydrochemistry facies evolve from Na-HCO3 and Ca-HCO3 close recharge areas to Na-Cl and Ca-Cl (group 3) near the coast (non-recharge areas) (Fig. 2a). According to Gibbs’ diagram (Fig. 2b) groups 1 and 2 are dominated by processes of weathering of rocks containing Na+ and K+ ions (Fig 2c), while group 3 shows a trend to evaporation processes, typical for warm climates. As expected, samples acquire their salinity by silicate weathering and dissolution of evaporites (Fig. 2c)9; the dissolution of carbonate minerals plays no significant role. According to Fig 2d, groups 1 and 2 follow a trend below the 1:1 line indicating silicate weathering, while group 3 shows an excess of earth alkali ions in relation to HCO3- + SO42- for almost all samples, suggesting reverse cation exchange processes rather than carbonate rock dissolution. In the reverse cation exchange process, Na+ is adsorbed and Ca2+ released, suggesting that the Na+ from seawater replaces Ca2+ adsorbed on clay surface which results in a reduction of Na+ in groundwater solution. This process occurs under conditions of salt water intrusion which leads to the change of water facies from water Na-Cl to Ca-Cl. a)
b)
c)
d)
Fig. 2. (a) Piper’s diagram; (b) Gibbs’ diagram; (c) Mixing plots as molar ratios of HCO 3-/Na vs Ca/Na for groundwater; (d) Scatter plot of Ca+Mg vs HCO3+SO4
Juan Antonio Torres-Martínez et al. / Procedia Earth and Planetary Science 17 (2017) 520 – 523
Fig. 3a confirms the hypothesis of occurrence of reverse cation exchange. This process is observed with greatest effects in group 3, which lies closest to the coastline. On the other hand, Fig. 3b shows that for groups 1 and 2 HCO3-/SO42- concentrations are relatively low, and for group 3 the values are increasing and follow two trends: a portion of the samples is having an evolution towards sulfated waters causing an increase in salinity under agricultural areas, and an enrichment of HCO3- due to silicate weathering. High sulfate concentrations probably indicate an impact from application of fertilizers in agricultural plots combined with cycling from irrigation. a)
b)
Fig. 3. Scatter plots of: (a) (Na+K)-Cl vs (Ca+Mg)-(HCO3-SO4); (b) SO4 vs HCO3
4. Conclusions The increase of salinity in the coastal aquifer of La Paz is not only due to seawater intrusion, but also result of several other chemical processes. In the upper part (recharge conditions) the chemistry is mainly the result of CO 2 dissolution in the unsaturated zone and silicate weathering in the saturated zone, adding HCO3- and alkali ions; in the central part (discharge zone with agricultural areas) the salinity increases mostly due to the use of fertilizers and evaporation from cycling in agricultural plots, adding SO 42- , NO3- and Cl- among other ions, while in the urban part close to the coastline the water is clearly affected by saltwater intrusion and reverse cation exchange, adding Ca2+ among other ions. Acknowledgements This study was co-financed by Fundación FEMSA and the Water Science and Technology chair of Tecnólogico de Monterrey. Fundación FEMSA had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. References 1. Post VEA. Fresh and saline groundwater interaction in coastal aquifers: Is our technology ready for the problems ahead? Hydrogeol. J. 2005;13:120-123 2. Unsal B, Yagbasan H, Yazicigil H. Assessing the impacts of the climate change on sustainable management of coastal aquifers. Environ. Earth Sci. 2014;72:2183-2193 3. Comisión Nacional del Agua (CONAGUA). Estadísticas del agua en México. Secretaría de Medio Ambiente y Recursos Naturales. México, D.F.; 2015. 4. Monzalvo M. Simulación hidrodinámica del acuífero de La Paz y su aprovechamiento como fuente de desalación. MSc Thesis. Universidad Nacional Autónoma de México; 2010: 196 p. 5. Comisión Nacional del Agua (CONAGUA). Situación actual y posibles escenarios de intrusión salina en el acuífero de La Paz, Baja California Sur y su aprovechamiento como fuente de desalación para abastecimiento de agua potable. Instituto de Ingeniería UNAM. 2010: 463p. 6. Gaona-Zanella P. Análisis de calidad de las aguas subterráneas en La Paz, Baja California Sur, como contribución a la sostenibilidad hídrica de la región, MSc Thesis, Tecnológico Monterrey; 2014: 112p. 7. Ward JH. Hierarchical grouping to optimize an objcetive function. J. Amer. Statist. Assoc. 1963; 58:236-244 8. Tamez-Meléndez C, Hernández-Antonio A, Gaona-Zanella P, Ornelas-Soto E, Mahlknecht J. Environmental isotope signatures and hydrochemistry as tools in assessing groundwater occurrence and dynamics in a coastal arid aquifer. Environ. Earth Sci. 2016;75:830 9. Zhang L, Song X, Xia J, Yuan R, Zhang Y, Liu X. Major element chemistry of the Huai River basin, China. Appl. Geochemistry. 2011;26:293300
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ScienceDirect Procedia Earth and Planetary Science 17 (2017) 520 – 523
15th Water-Rock Interaction International Symposium, WRI-15
Origin of the salinity in the coastal aquifer of La Paz, Mexico Juan Antonio Torres-Martíneza, Jürgen Mahlknechta,1, Arturo Hernández-Antonioa, Abrahan Moraa a
Tecnológico de Monterrey, Eugenio Garza Sada 2501, 64849 Monterrey, Mexico
Abstract Coastal areas are attractive places for the establishment of urban settlements owing to the many benefits in terms of quality of life. An example of these areas is La Paz in Southern Baja California, Mexico, where demographic growth and agricultural activities under dry climatic conditions are threatening the availability of water resources. In recent years, an increasing salinity of groundwater has been identified, however, the origin of the salinity is not clear. Near the coastline, salt water intrusion is considered to be the main factor, while others play a role for increased salinities in the middle and upper aquifer zone. The study, using a hydrochemical tool analysis shows that the salinity of the coastline is the result of over-exploitation of wells, which increases saline intrusion resulting in a reverse cation exchange. It also indicates that the salinization of water in the recharge area is product of carbon dioxide dissolution and weathering of rock-forming silicate minerals, and in the central part result from cycling and use of fertilizer in agriculture. © 2017 2017The TheAuthors. Authors. Published by Elsevier Published by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of WRI-15. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of WRI-15 Keywords: Water-rock interaction; groundwater; salinity; Mexico
1. Introduction Human settlements in coastal areas provide many benefits for quality of life. However, under arid climate conditions, groundwater is usually the main or the only source of water supply, so it is very common that the availability of water resources in these regions is relatively low1,2. This situation usually results in water balance deficits, inducing an overexploitation of aquifers and thus the hydraulic gradient leads sea water intrusion. Mexico has at least fifteen coastal aquifer zones under arid conditions affected by saltwater intrusion; most of them located in the northwestern region of the country3. The capital and area of La Paz (~260,000 habitants), located 1
Corresponding author. Tel.: +52 (81) 8358 2000 Ext. 5561 E-mail address: [email protected]
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.2016.12.131
Juan Antonio Torres-Martínez et al. / Procedia Earth and Planetary Science 17 (2017) 520 – 523
in Southern Baja California, is a clear example of a touristic coastal area (Fig.1a) with important agricultural activities nearby. There, groundwater is practically the only one available local water resource. b)
a)
Fig. 1. (a) Location of La Paz aquifer; (b) Geological map and water sampling sites.
The aquifer of La Paz (1,275 km²) is located in the Valley of La Paz (Fig 1a). According to the Köppen classification, the climate of the region is hot desert (BWh), with an average annual temperature and precipitation of 22.1 °C and 257 mm, respectively, and no perennial surface runoff. One-hundred-fifty-five production wells extract a total of 32 million m3/year. The well depths vary from 3 to 200 m4. Aquifer mining caused a gradual decrease of the water table of up to 10 m that led to the reversal of the hydraulic gradient near the coastline of La Paz, resulting in an increased groundwater salinity of up to 4,800 mg l-1 (TDS) from salt water intrusion in the coastal plain5. In 2005 the increase of salinity reached ~6 km inland from the coastline, however, at the year 2013 this fringe has extended to ~13 km6. The geology of La Paz area consists of a complex of metamorphic shales, phyllites, gneiss and sillimanite from Mesozoic period, covered by intrusive rocks (gabbro, granite and granodiorite) from Cretaceous period, sedimentary rocks (Miocene sandstones, conglomerates, fractured volcanic rocks), and alluvial material (sand, silt and clay) (Fig. 1b)4. This study concentrates on the upper geological units considering sedimentary rocks and alluvial material. The aim of this study was to perform an investigation on groundwater chemical processes that cause salinity in the coastal area of La Paz, by using different hydrochemical tools. 2. Methodology 2.1. Field and laboratory work Water samples from forty-six production wells from the basin-fill sediments and one spring were collected and analyzed for their chemical and isotopic composition in August 2013 (Fig 1b). Samples were collected in HDPE plastic bottles and temperature, pH, electrical conductivity (EC), total dissolved solids (TDS) and dissolved oxygen (DO) were measured in the field using portable meters. Alkalinity was determined in the field by volumetric titration (0.02N H2SO4), using filtered water samples (0.45 µm). Cation samples were acidified with ultrapure HCl to pH < 2; all samples were stored at constant temperature of 4°C. Cations (K +, Na+, Ca2+, Mg2+, Sr2+) were determined by inductive-coupled plasma mass spectrometry (ICP/MS) using a Perkin Elmer ELAN 9000 ICP/MS, and an inductive-coupled plasma optical emission (ICP-OES) using a Perkin Elmer Optima 3000 ICP OES when limits were exceeded. Anions (Cl-, NO2-, NO3-, Br-, SO42-, PO43- and SiO2) were determined by ion chromatography with a DIONEX DX-120.
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Juan Antonio Torres-Martínez et al. / Procedia Earth and Planetary Science 17 (2017) 520 – 523
2.2. Interpretation Water samples were grouped by a hierarchical cluster analysis using 15 variables (pH, temperature, electrical conductivity, TDS, DO, alkalinity, Na+, SiO2, K+, Ca2+, Sr2+, Cl-, Br-, NO3-, SO4 2- and the distance of the sample location from the coastline). Ward's linkage rule was applied to link nearby clusters with a similarity matrix and an analysis of variance (ANOVA) developed to assess the distance between the clusters7. From this analysis, water samples were classified into three groups: group 1 (8 samples in the upper area of the aquifer away from the coast), group 2 (16 samples located in the urban area of the aquifer), and group 3 (23 samples in the central area of the study area, with agricultural activities8. 3. Results and discussion The study area shows a wide variety of salinity. Group 1 has salinities ranging from 479 to 924 μS cm-1 (freshwater) and chloride concentrations from 1.54 to 3.33 mol·m-3; salinity of group 2 ranges from 683 to 1,658 μS cm-1 and chloride concentrations from 2.70 to 10.86 mol·m-3; and group 3 (brackish water) with conductivity between 1,777 and 8,920 μS cm-1 and chloride concentrations between 11.23 and 83.49 mol·m-3, indicating a likely increase of salinity along the flow path as water moves closer to the sea. The general dominance of cations is Na > Ca = Mg > K, while the dominance of anions was Cl > HCO3 >> SO4. Group 1 is derived from rainwater, so the hydrochemistry facies evolve from Na-HCO3 and Ca-HCO3 close recharge areas to Na-Cl and Ca-Cl (group 3) near the coast (non-recharge areas) (Fig. 2a). According to Gibbs’ diagram (Fig. 2b) groups 1 and 2 are dominated by processes of weathering of rocks containing Na+ and K+ ions (Fig 2c), while group 3 shows a trend to evaporation processes, typical for warm climates. As expected, samples acquire their salinity by silicate weathering and dissolution of evaporites (Fig. 2c)9; the dissolution of carbonate minerals plays no significant role. According to Fig 2d, groups 1 and 2 follow a trend below the 1:1 line indicating silicate weathering, while group 3 shows an excess of earth alkali ions in relation to HCO3- + SO42- for almost all samples, suggesting reverse cation exchange processes rather than carbonate rock dissolution. In the reverse cation exchange process, Na+ is adsorbed and Ca2+ released, suggesting that the Na+ from seawater replaces Ca2+ adsorbed on clay surface which results in a reduction of Na+ in groundwater solution. This process occurs under conditions of salt water intrusion which leads to the change of water facies from water Na-Cl to Ca-Cl. a)
b)
c)
d)
Fig. 2. (a) Piper’s diagram; (b) Gibbs’ diagram; (c) Mixing plots as molar ratios of HCO 3-/Na vs Ca/Na for groundwater; (d) Scatter plot of Ca+Mg vs HCO3+SO4
Juan Antonio Torres-Martínez et al. / Procedia Earth and Planetary Science 17 (2017) 520 – 523
Fig. 3a confirms the hypothesis of occurrence of reverse cation exchange. This process is observed with greatest effects in group 3, which lies closest to the coastline. On the other hand, Fig. 3b shows that for groups 1 and 2 HCO3-/SO42- concentrations are relatively low, and for group 3 the values are increasing and follow two trends: a portion of the samples is having an evolution towards sulfated waters causing an increase in salinity under agricultural areas, and an enrichment of HCO3- due to silicate weathering. High sulfate concentrations probably indicate an impact from application of fertilizers in agricultural plots combined with cycling from irrigation. a)
b)
Fig. 3. Scatter plots of: (a) (Na+K)-Cl vs (Ca+Mg)-(HCO3-SO4); (b) SO4 vs HCO3
4. Conclusions The increase of salinity in the coastal aquifer of La Paz is not only due to seawater intrusion, but also result of several other chemical processes. In the upper part (recharge conditions) the chemistry is mainly the result of CO 2 dissolution in the unsaturated zone and silicate weathering in the saturated zone, adding HCO3- and alkali ions; in the central part (discharge zone with agricultural areas) the salinity increases mostly due to the use of fertilizers and evaporation from cycling in agricultural plots, adding SO 42- , NO3- and Cl- among other ions, while in the urban part close to the coastline the water is clearly affected by saltwater intrusion and reverse cation exchange, adding Ca2+ among other ions. Acknowledgements This study was co-financed by Fundación FEMSA and the Water Science and Technology chair of Tecnólogico de Monterrey. Fundación FEMSA had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. References 1. Post VEA. Fresh and saline groundwater interaction in coastal aquifers: Is our technology ready for the problems ahead? Hydrogeol. J. 2005;13:120-123 2. Unsal B, Yagbasan H, Yazicigil H. Assessing the impacts of the climate change on sustainable management of coastal aquifers. Environ. Earth Sci. 2014;72:2183-2193 3. Comisión Nacional del Agua (CONAGUA). Estadísticas del agua en México. Secretaría de Medio Ambiente y Recursos Naturales. México, D.F.; 2015. 4. Monzalvo M. Simulación hidrodinámica del acuífero de La Paz y su aprovechamiento como fuente de desalación. MSc Thesis. Universidad Nacional Autónoma de México; 2010: 196 p. 5. Comisión Nacional del Agua (CONAGUA). Situación actual y posibles escenarios de intrusión salina en el acuífero de La Paz, Baja California Sur y su aprovechamiento como fuente de desalación para abastecimiento de agua potable. Instituto de Ingeniería UNAM. 2010: 463p. 6. Gaona-Zanella P. Análisis de calidad de las aguas subterráneas en La Paz, Baja California Sur, como contribución a la sostenibilidad hídrica de la región, MSc Thesis, Tecnológico Monterrey; 2014: 112p. 7. Ward JH. Hierarchical grouping to optimize an objcetive function. J. Amer. Statist. Assoc. 1963; 58:236-244 8. Tamez-Meléndez C, Hernández-Antonio A, Gaona-Zanella P, Ornelas-Soto E, Mahlknecht J. Environmental isotope signatures and hydrochemistry as tools in assessing groundwater occurrence and dynamics in a coastal arid aquifer. Environ. Earth Sci. 2016;75:830 9. Zhang L, Song X, Xia J, Yuan R, Zhang Y, Liu X. Major element chemistry of the Huai River basin, China. Appl. Geochemistry. 2011;26:293300
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