Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, sr and b isotopes in groundwater

Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isotopes in groundwater Science of the Total Environment xxx (2017) xxx–xx[.]

STOTEN-22074; No of Pages 14 Science of the Total Environment xxx (2017) xxx–xxx Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isotopes in groundwater J Mahlknecht a,⁎, D Merchán a, M Rosner b, A Meixner c, R Ledesma-Ruiz a a b c Centro del Agua para América Latina y el Caribe, Tecnológico de Monterrey, Mexico IsoAnalysis UG, Berlin, Germany Faculty of Geosciences and MARUM-Center for Marine Environmental Sciences, University of Bremen, Germany H I G H L I G H T S G R A P H I C A L A B S T R A C T • Seawater intrusion has increased salinity in La Paz aquifer • Significant exchange processes affected major cations • Strontium increased mainly as a consequence of cationic exchange • Boron probably increased as a consequence of desorption processes • One third of the samples showed significant seawater contribution a r t i c l e i n f o Article history: Received December 2016 Received in revised form 16 February 2017 Accepted 16 February 2017 Available online xxxx Editor: D Barcelo Keywords: La Paz aquifer Overexploitation Salinization Cationic exchange pool 87 Sr/86Sr δ11B a b s t r a c t The La Paz aquifer system (Baja California Sur, Mexico) is under severe anthropogenic pressure because of high groundwater abstraction for urban supply (city of La Paz, around 222,000 inhabitants) and irrigated agriculture (1900 ha) In consequence, seawater has infiltrated the aquifer, forcing the abandonment of wells with increased salinity The objective of this study was to assess seawater intrusion, understand the hydrogeochemical processes involved and estimate the contribution of seawater in the wells tested The aquifer comprises mainly the alluvial filling and marine sediments of a tectonic graben oriented north-south, in contact with the Gulf of California Groundwater samples were collected in 47 locations and analyzed for major constituents A subset of 23 samples was analyzed for strontium and boron concentrations and isotopic signatures (87Sr/86Sr and δ11B) Results were interpreted using standard hydrochemical plots along with ad hoc plots including isotopic data Seawater intrusion was confirmed by several hydrogeochemical indicators, such as the high salinity in areas of intense pumping or the Na+-Ca2+ exchange occurring in sediments that were previously in chemical equilibrium with fresh water However, seawater contribution was not sufficient to explain the observed concentrations and isotopic signatures of Sr and B According to the isotopic data, desorption processes triggered by a modification in chemical equilibrium and an increase in ionic strength by seawater intrusion significantly increased Sr and probably B concentrations in groundwater From a calculation of seawater contribution to the wells, it was estimated that one-third of the sampled abstraction wells were significantly affected by seawater intrusion, reaching concentrations that would limit their use for human supply or even irrigated agriculture In addition, significant agricultural ⁎ Corresponding author E-mail address: jurgen@itesm.mx (J Mahlknecht) http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 0048-9697/© 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx pollution (nitrates) was detected Planned management of the aquifer and corrective measures are needed in order to invert the salinization process before it severely affects water resources in the long term © 2017 The Authors Published by Elsevier B.V This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) Introduction Mexico, with 9330 km of coastline, has experienced an important growth in population and agricultural surface in coastal areas over the past 30 years, which has led to the overexploitation of several aquifer systems (SEMARNAT and CONAGUA, 2014) The problems of seawater intrusion are most notable west of Sonora and in the Baja California Peninsula, with minor problems in the Yucatan Peninsula, and in the states of Veracruz, Sinaloa and Nayarit (Cardoso, 1993; Marín, 2002) The coastal aquifer system of La Paz (Baja California Sur) represents the sole water resource of La Paz city and the surrounding towns, an urban area of over 222,000 inhabitants (INEGI, 2010) and around 1900 of irrigated agriculture (CONAGUA, 2010a) Following demographic and economic growth, 155 wells have been drilled in the alluvial sediment unit to address an increasing demand for water for urban use, tourism and agriculture over the past decades (CONAGUA, 2010b) Eventually, a lack of planned water resource management has led to overexploitation of the underlying aquifer with the consequent reversal of the groundwater hydraulic gradient near the coastline and marine intrusion (CONAGUA, 2001, 2010b; Tamez-Meléndez et al., 2016) Moreover, the arid climate and frequent periods of drought adversely affect the water reserves To meet increasing demand, La Paz city has started searching for new water sources The local and federal governments have proposed the construction of a desalinization plant which would produce additional 200 L s−1 of freshwater (CONAGUA, 2010b) while controlling seawater intrusion However, civil organizations have voiced a preference for management of the underlying groundwater resources and/or managed aquifer recharge projects (Pronatura, 2010) Previous research on this coastal aquifer system has dealt with hydrogeological characterization (CONAGUA, 2001; Cruz-Falcón, 2007; Cruz-Falcón et al., 2010), assessment of marine intrusion using geophysical methods and models of flow and transport (CONAGUA, 2001, 2010a; Escolero and Torres-Onofre, 2007; Monzalvo, 2010), and evaluation of groundwater quality (CONAGUA, 2002, 2010b) One published study (Tamez-Meléndez et al., 2016) evaluated groundwater flow using multivariate statistical analyses, hydrogeochemical indicators, water stable isotope methods (δD and δ18O in water molecules) and groundwater dating methods (radiocarbon and CFC's) The study concluded that groundwater in that area was mainly of meteoric origin, with minor regional groundwater flow influence, and that agriculture development had provoked an ever-growing quality problem in groundwaters related to anthropogenic pollutants and seawater intrusion In this sense, conventional geochemical methods such as the use of chemical concentrations of major and trace elements or stable isotope ratios may help to elucidate the mixing of end members (e.g., Hernández-Antonio et al., 2015) Nevertheless, the use of these tools can be limited because there are no clear mixing end members, as their signature is altered by surface and near-surface physical, chemical and biological alteration processes, such as evaporation or transpiration (Kendall and Caldwell, 1998) In this context, the use of strontium and boron isotopes can improve our knowledge of hydrogeochemical processes The 87Sr/86Sr ratio is not affected by fractionation processes and its value depends on source waters or rocks (e.g., Faure and Mensing, 2005) In consequence, strontium isotopes have been widely used to identify source waters in groundwater studies (e.g., Petelet-Giraud et al., 2003; Bakari et al., 2013; Paces and Wurster, 2014) Similarly, boron stable isotopes signature can be used in groundwater because of its natural occurrence in most waters, and the wide range of expected values possible from potential intense fractionation processes as a consequence of the high relative mass difference of 10B and 11B (Vengosh and Spivack, 2000; Venturi et al., 2015) Thus, a combination of the classical hydrogeochemical tools and these complementary tools may provide better understanding of the geochemical processes present in aquifers affected by seawater intrusion (e.g., Pennisi et al., 2006; Langman and Ellis, 2010; Meredith et al., 2013; Cary et al., 2015; Petelet-Giraud et al., 2016) The use of multi-isotope studies provides complementary information from different sources, allowing a comprehensive comparison of natural and human-affected processes (e.g., Puig et al., 2016) Therefore, the present study aimed to further investigate the origin of groundwater salinity in a coastal aquifer affected by seawater intrusion incorporating the use of the radiogenic strontium isotopes ratio (87Sr/86Sr) and stable boron isotopes (δ11B) for a thoughtful hydrogeochemical evaluation of major constituents The specific objectives include an assessment of water quality in relation to human uses (mainly human consumption and agriculture), to better understand the controlling geochemical processes and to estimate the seawater contribution to the abstracted water Study area 2.1 General setting La Paz aquifer system is located in the southern portion of the Peninsula of Baja California, Mexico (Fig 1A) It is situated within the municipality of La Paz, in the state of Baja California Sur, about 60 km north of the Tropic of Cancer The aquifer is in direct contact with La Paz Ensenada, which is hydraulically connected to the Sea of Cortéz (Gulf of California) The main economic activity in La Paz and its surrounding area is tourism, with a great increase in the number of visitors in recent years, putting the area's water resources under severe pressure The climate in the study area is predominantly warm and dry with an average annual temperature and rainfall of 22.5 °C and 263 mm, respectively (CONAGUA, 2010b) Precipitation is not evenly distributed during the year, with almost 75% of precipitation occurring between July and October Given the high potential evaporation, recharge of this aquifer occurs mainly after rainfalls associated with tropical storms and cyclones at the end of the summer (CONAGUA, 2002) 2.2 Geological background The geomorphological structure of La Paz valley consists of intermountain plains, table mountains and mountain ranges of up to 900 m above sea level Structurally, the valley of La Paz corresponds to a graben (Fig 1B; Hausback and Frizzell, 1984; Cruz-Falcón et al., 2010) in north-south orientation which is limited to the west and east by El Carrizal and La Paz normal faults, respectively (Álvarez et al., 1997; Cruz-Falcón et al., 2010) The presence of structurally controlled wadies and bathymetric data suggest the existence of a series of secondary faults inside the graben (Álvarez et al., 1997; Cruz-Falcón, 2007) La Paz region is located to the west of the boundary between the Pacific and North American plates, a boundary that extends throughout the Gulf of California Tectonically, the region forms part of the Gulf Extensional Province (Munguía et al., 2006) The valley of La Paz is the result of distention tectonics which created the Gulf of California, probably during the Pleistocene, and the subsequent accumulation of marine and continental granular sediments in the lower topography (ArandaGómez and Pérez-Venzor, 1995) The base of the lithological column in the La Paz area is a metamorphic complex which consists of Mesozoic Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx Fig A) Surface lithology of the study site with equipotential lines corresponding to the summer of 2012 B) Hydrostratigraphic cross-section Adapted from SGM (1999) and CONAGUA (2001) Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx shales, phyllites, schists, sillimanite and gneiss (Monzalvo, 2010) This unit is overlain by Cretaceous intrusive rocks (gabbro, granite and granodiorite) in the oriental part of La Paz fault, constituting the Las Cruces and El Novillo mountain ranges (Cruz-Falcón et al., 2010) This group of intrusive rocks is known as Los Cabos block (Fletcher et al., 2000) Towards the western part of the valley, a sequence of outcropping sandstones and conglomerates of the Pleistocene are present A sequence of Miocene volcanic and sedimentary rocks surrounds the city of La Paz The filling materials of La Paz valley consist in a Quaternary sequence of marine sediments (alternating lutites and sandstones) and alluvial deposits (conglomerates, sandstones and unconsolidated sediments of sands, silts and clays; Álvarez et al., 1997; Cruz-Falcón et al., 2010, 2013) to the well owners, in general the screening interval covered the whole well Privately-installed pumps were used to purge the well (three-well volumes) and collect the samples The spring sample was collected manually Field parameters were measured in situ, and recorded after the variables had stabilized In-situ measurements included electrical conductivity corrected to 25 °C (EC), temperature (T), pH, dissolved oxygen (DO) and alkalinity A pre-calibrated portable multi-meter (Orion Star A329, Thermo Fisher Scientific, Waltham, MA, USA) was used for EC, T, pH and DO Alkalinity was determined in the field by titration with 0.02 N H2SO4 Samples were filtered using 0.45 μm membrane filters, preserved at °C and sent to laboratory for analysis within a week 2.3 Hydrogeology All the chemical analyses were performed in Activation Laboratories Ltd., Ancaster, Ontario, Canada The concentrations of dissolved cations (Na+, K+, Ca2 +, Mg2 +), silica (SiO2) and Sr2 + and B were measured by inductive-coupled plasma mass spectrometry (ICP-MS) using a Perkin Elmer SCIEX ELAN 6000 ICP/MS equipment (Waltham, MA, USA) During the trace element determinations, a blank and two water standards were run at the beginning and end of the analyses Measurement uncertainties for all parameters were evaluated and controlled using regular laboratory duplicates of samples and verifying the precision/calibration of the instruments through regular runs of various primary standard solutions The international geostandard SRM-1640 (trace elements in natural water – certified by the National Institute of Standards and Technology NIST) was measured during the analyses of groundwater samples to check the accuracy of the ICP-OES and ICPMS methods The accuracy of these analyses (defined as the systematic difference between the reference values and the measurements of the geostandard SRM-1640) was lower than 9% for major ions and trace elements If upper limits of major cations were exceeded, they were measured by inductive-coupled plasma optical emission spectrometry (ICPOES) using an Agilent Axial ICP Optical Emission Spectrometer Major − anions (Cl−, SO2− , NO3 ) were determined and quantified by ion chromatography in un-acidified samples using a Dionex DX-120 equipment (Sunnyvale, CA, USA) Samples with high Cl− (N75 mg L) and SO24 − (N375 mg L) concentrations were diluted in order to avoid oversaturation The charge balance was cross-checked as follows: In the valley of La Paz it is possible to identify two aquifers The first is an intergranular-porosity aquifer (CONAGUA, 1997, 2001, 2002; Cruz-Falcón, 2007; Escolero and Torres-Onofre, 2007) It is an unconfined aquifer, but it behaves semiconfined locally where different levels of clays are present The porous medium is developed in clastic materials mainly of fluvial origin, which are widely distributed in the valley It presents mainly continental facies, which are made up of sand and gravel sometimes embedded in an argillaceous matrix These materials reach a maximum thickness of 400 m in the center of the valley (CONAGUA, 2010a) Below the intergranular aquifer, a fractured aquifer system is located within the sequence of igneous and metamorphic rocks, associated with the crystalline complex that forms mainly the eastern part of the valley (CONAGUA, 1997, 2010a) There are no conclusive data regarding the hydraulic parameters of this deep aquifer Natural recharge originates mostly from precipitation captured in the mountainous elevations located east and southeast (Las Cruces and El Novillo mountain ranges) Runoff infiltrates when it reaches the flat areas at the bottom of the valley In addition, rainwater infiltrates through the cracks or fractures of the igneous and weathered metamorphic materials, providing minor lateral and vertical recharge to the intergranular aquifer (Cruz-Falcón et al., 2011) Recharge also occurs, although to a lesser extent, by direct infiltration of rainfall in the valley (CONAGUA, 2010a) The depth to groundwater level in this aquifer varies from around m near the coastline to N70 m south of La Paz city (Cruz-Falcón et al., 2010; Monzalvo, 2010), presenting under natural conditions a rather low hydraulic gradient in respect to the sea Recharge in the aquifer system has been estimated at 28 million m3 (CONAGUA, 2010b) According to official estimates, current annual extractions average 32 million m3, of which 68% is extracted for urban water supply, 27% for agriculture, 3% for livestock and 2% for industry and other services However, these data are uncertain because they not account for illegal extractions This scenario creates a deficit in long-term water availability In consequence, over pumping has caused hydraulic gradient inversion (CONAGUA, 2010b), so that the groundwater flow occurs mainly in a southwest direction from the coast, allowing seawater to enter the aquifer from the Gulf of California According to official estimates, in 2004, the wedge intrusion of seawater was located approximately 5.25 km away from the coastline on the east side, and 6.25 km on the western side of the aquifer, with an estimated speed of advance into the valley between 120 and 200 m year− (CONAGUA, 2010a) A decrease in the quality of groundwater has been reported by the water authority, and had even led to the closure of some abstraction wells Methodology 3.1 Sampling procedures Forty-six production wells and a spring were sampled in August 2013 The well depths varied between and 200 m (Table 1) According 3.2 Analytical procedures Charge balance%ị ẳ 100 meq Cationsmeq Anions meq Cations ỵ meq Anions 1ị Absolute charge balances averaged 3.7% One sample (LP17, C.B = 13%) exceeded the 10% acceptable criteria and was discarded for posterior interpretations A subset of 26 samples was selected for isotopic investigations Strontium isotope ratios (87Sr/86Sr) of water samples were determined by IsoAnalysis UG (Berlin, Germany) using thermal ionization mass spectrometry Prior to mass spectrometry, strontium was separated from the matrix by ion chromatography with Sr-Spec resin The raw data were corrected for interfering Rb and mass fractionation (86Sr/88Sr = 0.1194) and normalized to a NIST SRM 987 87Sr/86Sr ratio of 0.71025 During the course of the analysis, three aliquots of IAPSO seawater yielded an 87Sr/86Sr ratio of 0.709181 with an external reproducibility of 0.000049 (2 SD) An expanded uncertainty of 0.00007 for 87 Sr/86Sr isotope ratios of samples was exemplary calculated for a seawater standard Strontium isotopic composition is expressed as the atomic proportion 87Sr/86Sr Boron isotopic composition of the groundwater samples was analyzed in the Isotope Geochemistry Laboratory at the MARUM-Center for Marine Environmental Sciences, University of Bremen (Germany) The entire analytical procedure follows the procedure for fluid samples in Kutzschbach et al (2016) A cation column separation using Bio-Rad AG 50W-X8 resin and mannitol (Bio-Rad Laboratories, Hercules, CA, USA) was performed to isolate boron from matrix elements Purified Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx Table La Paz aquifer system hydrochemical data collected on August 2013 Sample Units Depth m T °C EC μS cm−1 pH − O2 mg L−1 Na+ K+ mmol L−1 Ca2+ Mg2+ Cl− SO2− HCO− NO− Br− μmol L−1 Sr2+ μmol L−1 87 Sr/86Sr Ratio B μmol L−1 δ11B ‰ Group LP7 LP14 LP15 LP16 LP18 LP19 LP20 LP21 LP22 LP23 LP26 LP27 LP28 LP29 LP30 LP31 LP32 LP33 LP35 LP40 LP41 LP43 150 60 200 30 64 64 60 130 15 26 18 69 15 20 22 30 70 70 70 28 50 60 31.1 29.6 30.6 29.9 31.0 29.6 29.3 33.4 30.7 30.0 29.1 28.2 29.0 29.7 27.9 27.1 29.0 29.3 31.3 28.7 29.4 30.3 2109 3480 2868 2552 5100 5570 3840 2124 7520 2603 3080 2944 6880 3460 5160 4770 2589 1777 2751 8920 6000 4250 7.1 7.1 7.2 7.3 7.0 6.9 7.1 7.1 7.0 7.3 7.2 7.2 7.0 7.1 6.8 7.1 7.2 7.4 7.2 7.2 7.0 7.3 7.7 5.6 6.6 7.9 6.3 6.4 7.2 7.8 7.5 8.3 8.1 4.8 8.6 5.6 3.9 5.2 5.8 8.7 6.1 7.1 5.7 7.4 3.52 9.92 14.22 13.01 17.01 15.40 10.13 5.09 46.98 14.92 17.18 19.27 33.19 12.31 17.18 30.14 5.83 5.31 4.44 25.36 24.45 14.14 0.13 0.13 0.21 0.20 0.23 0.26 0.20 0.14 0.26 0.15 0.10 0.08 0.23 0.15 0.18 0.09 0.09 0.08 0.08 0.36 0.20 0.24 3.87 6.56 3.34 2.59 8.36 12.48 8.66 4.97 8.88 5.11 4.09 3.67 10.50 6.51 10.53 4.72 5.91 3.27 6.36 16.42 8.53 7.76 2.48 3.73 2.09 1.68 7.65 6.67 4.94 2.82 5.39 2.50 2.51 2.34 7.65 4.53 6.46 4.53 2.30 1.59 3.07 14.16 6.54 4.32 14.70 26.40 19.75 15.99 39.49 47.67 31.59 15.51 63.75 20.99 20.54 16.64 57.26 26.35 44.85 32.16 20.62 11.23 22.37 83.50 51.90 32.16 0.80 1.42 1.73 1.76 2.03 3.02 1.54 1.11 5.10 1.58 1.67 3.29 4.59 1.43 1.49 2.37 0.52 0.72 0.67 2.53 1.83 3.04 1.90 3.70 4.01 3.94 5.77 3.72 4.31 4.05 4.97 5.11 7.23 4.41 6.33 4.75 5.67 12.88 3.25 3.40 2.76 9.83 6.55 5.27 0.50 0.68 0.84 0.74 1.99 0.43 0.29 0.46 0.89 0.34 0.80 29.21 1.21 0.67 0.71 3.49 0.76 0.45 0.58 0.40 0.02 0.63 16.6 44.7 23.9 23.3 42.4 62.7 36.8 17.5 84.9 23.9 28.4 6.3 80.9 25.5 49.6 29.0 37.0 15.5 32.0 89.6 64.8 36.7 20.3 46.2 16.5 9.9 33.5 51.2 34.5 20.3 58.3 28.3 22.9 23.7 76.4 45.9 74.4 35.2 35.5 23.7 29.0 92.8 52.5 31.0 0.705764 0.705888 n.d 0.705908 0.706010 n.d n.d n.d 0.706114 0.706010 n.d n.d 0.705962 n.d 0.705723 n.d 0.705730 n.d 0.705509 n.d 0.705913 n.d 8.7 80.3 75.0 79.4 80.5 93.2 76.7 33.3 299.0 65.0 56.8 99.9 219.1 93.4 82.9 80.3 18.2 27.6 21.0 118.4 134.3 84.8 37.1 54.9 n.d 42.0 52.0 n.d n.d n.d 46.6 50.8 n.d n.d 47.6 n.d 41.0 n.d 44.1 n.d 50.3 n.d 38.3 n.d Group LP1 LP2 LP3 LP4 LP8 LP11 LP12 LP13 LP24 LP25 LP34 LP36 LP37 LP38 LP39 LP42 n.a n.a n.a 76 57 80 80 60 35 38 75 35 60 24 n.a 50 28.9 30.1 29.2 31.5 29.9 25.0 25.0 31.8 31.0 32.7 29.7 31.0 29.4 29.5 29.9 30.2 893 702 1537 1566 1142 998 1150 1155 1295 1292 1023 1207 1658 799 683 1182 6.8 7.1 6.9 7.2 7.8 7.4 7.4 7.6 7.5 7.4 7.6 7.4 7.3 7.8 7.6 7.4 8.3 8.1 7.8 8.6 8.2 7.9 6.1 7.1 4.5 7.7 8.4 7.1 5.9 7.3 7.0 7.3 2.94 2.31 4.35 5.96 4.21 4.57 9.70 6.13 8.13 6.26 3.48 3.07 6.00 2.60 2.54 2.58 0.10 0.10 0.14 0.11 0.10 0.10 0.14 0.14 0.07 0.11 0.07 0.07 0.08 0.05 0.06 0.09 1.27 1.02 2.57 2.06 1.30 1.03 0.97 1.09 1.65 2.04 1.60 2.27 2.64 1.11 0.83 1.98 0.68 0.58 1.39 1.63 0.95 0.70 0.43 0.42 0.76 0.97 0.87 0.95 1.10 0.85 0.63 1.63 4.49 3.02 10.86 9.65 5.81 4.74 8.01 5.30 7.45 7.50 5.16 6.91 10.72 4.63 2.70 8.15 0.39 0.29 0.36 0.46 0.34 0.38 0.78 0.56 0.44 0.39 0.44 0.30 0.69 0.12 0.15 0.20 1.66 1.75 1.85 3.05 2.10 2.32 3.25 2.50 4.25 4.05 2.50 2.36 2.60 1.86 2.18 2.06 0.72 0.63 0.35 0.27 0.26 0.63 0.40 0.35 0.28 0.15 0.14 0.23 0.17 0.11 0.13 0.20 4.8 2.1 13.6 12.6 7.5 5.4 10.5 6.6 10.8 10.5 6.3 7.6 14.1 5.8 3.8 11.3 9.5 7.3 16.7 8.2 5.2 6.3 5.1 4.8 10.7 13.5 12.4 11.3 17.7 6.0 5.3 11.8 0.705692 n.d 0.705797 0.706034 0.706005 0.706171 n.d 0.706225 n.d 0.706047 n.d n.d 0.705638 0.705807 n.d 0.705974 11.7 8.7 11.1 21.9 16.8 14.9 51.6 34.2 29.8 22.8 16.8 13.3 25.5 10.2 20.6 12.8 41.5 n.d 37.2 33.6 33.3 36.8 n.d 35.7 n.d 31.4 n.d n.d 43.0 27.8 n.d 29.2 Group LP5 LP6 LP9 LP10 LP44 LP45 LP46 LP47 201 201 201 156 80 50 21 0a 32.5 32.5 30.5 33.0 30.2 28.5 28.1 27.4 642 701 865 561 630 610 924 479 7.1 7.4 7.4 7.7 7.2 7.2 7.1 8.3 7.8 7.7 6.7 7.3 7.1 7.8 6.3 14.7 1.77 1.94 3.55 2.92 2.18 1.60 3.52 1.71 0.06 0.04 0.07 0.10 0.04 0.05 0.06 0.11 1.12 1.17 1.31 0.85 0.84 0.98 1.23 0.69 0.65 0.60 0.82 0.47 0.64 0.89 1.14 0.38 2.28 3.13 3.33 1.96 2.53 1.75 3.02 1.54 0.15 0.11 0.49 0.10 0.15 0.12 0.15 0.08 2.15 1.75 2.80 2.54 1.98 2.75 3.97 1.82 0.21 0.21 0.10 0.11 0.58 0.21 0.17 0.13 3.1 4.6 4.1 2.1 4.1 2.0 3.8 1.6 4.5 5.1 7.7 4.2 4.0 4.3 6.5 3.2 0.705565 0.705397 0.705764 n.d 0.705517 n.d n.d n.d 6.5 7.6 13.4 9.5 9.8 8.8 17.3 8.3 34.8 33.1 36.0 n.d 36.9 n.d n.d n.d Discard LP17 50 29.5 3320 7.1 6.8 28.62 0.25 4.54 2.62 23.50 1.87 5.39 0.92 24.5 20.3 0.705799 81.7 43.9 n.a.: Not available n.d.: Not determined E.C.: Electrical conductivity a LP47 is a spring, the sample was collected at the surface samples and processed standards were dissolved in 2% HNO3 and adjusted to 100 ppb B concentrations All standards and samples were analyzed repeatedly on a MC-ICP MS Neptune Plus (Thermo Fisher Scientific) The analyses were performed in the standard sample bracketing mode using a 100 ppb SRN NIST 951 reference solution A 2% HNO3 solution was analyzed before and after each standard and sample The average of these two measurements represents the analytical baseline and was used for baseline correction Boron isotopes are expressed in the per-mil notation relatively to untreated boric acid (NIST SRM 951) During the study, the processed SRM NIST 951 showed δ11B values of −0.04 ± 0.06‰ (2 SD, N = 4) and indicates no fractionation during the chemical separation Several reference materials were analyzed in parallel with samples δ11B of analyzed seawater B1 (39.8 ± 0.1‰ [2 SD, N = 3]), groundwater B2 (14.7‰), groundwater B3 (−21.2 ± 0.4‰ [2 SD, N = 2]) and the internal laboratory seawater standard BSW Susu Knolls (39.7 ± 0.4‰ [2 SD, N = 4]) demonstrate the accuracy of the analytical procedure 3.3 Results interpretation In a previous study using major constituents and water stable isotopes from the same sampling campaign, Tamez-Meléndez et al (2016) classified the obtained water samples into three groups according to their similitude (cluster aggregates) Group (23 samples, crosses in figures) was sampled closer to the seashore, in areas where higher volumes were abstracted for irrigation and presented the highest salinities in the aquifer (brackish groundwater) Group (16 samples, triangles in figures) corresponds to wells in urban areas and some wells in proximity to the seashore; and the group corresponds to fresh to brackish groundwater Group (8 samples, circles in figures) corresponds to Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx fresh groundwater and was collected in the southern study area, further from the shore and with no significant anthropogenic influence We followed this classification in our study Group was considered as representative of natural groundwater Mean levels of major ions and trace elements found in samples were within the range typically reported for unpolluted groundwater (Appelo and Postma, 2005) with the exception of Na+ and Cl−, which slightly exceeded the reference value of mmol L−1 However, the location of the study area, between the Pacific Ocean and the Sea of Cortez, justifies a higher concentration of these parameters in relation to marine aerosols deposition In the presented plots (Results and discussion section), the fresh groundwater composition was used as one end-member The other end-member was a theoretical mixture of seawater composition (Stumm and Morgan, 1996) From hydrogeochemical data, the saturation indices (SI) were calculated for selected mineral phases using PHREEQC 3.3.7 (Parkhurst and Appelo, 2013) and its database phreeqc.dat PHREEQC is a U.S Geological Survey computer program for simulating chemical reactions and transport processes in natural or polluted water, and allows the calculation of a mineral phase SI, i.e., the thermodynamic feasibility of a given water composition to dissolve or precipitate specific minerals Although variable with different minerals, in general a SI N 0.2 is considered oversaturation (with a tendency to precipitate), while a SI b − 0.2 indicates undersaturation (with a tendency to dissolve) (Merkel and PlanerFriedrich, 2008) Mineral phases used in the calculations included calcite, dolomite, gypsum and halite In addition, CO2 (g) partial pressures in equilibrium with the analyzed water were computed based on the alkalinity and pH of the samples (Kehew, 2001) Finally, for each groundwater constituent or computed variable (such as saturation indices), the nonparametric Kruskal-Wallis test (Helsel and Hirsch, 2002) was used to infer whether significant differences existed between Groups 1, and If the null hypothesis (H0 = there is no differences between groups) was rejected, the nonparametric version of Tukey's test (Helsel and Hirsch, 2002) was used to detect differences between specific groups Results and discussion 4.1 Water quality for urban supply and agriculture The chemical composition of the collected samples in the La Paz aquifer system is displayed in Table Groundwater presented a wide range of salinity, from fresh water (479 μS cm− 1) to brackish water (8920 μS cm− 1), with a median value of 1658 μS cm−1 Median EC's were significantly different, with 3470 μS cm− 1, 1153 μS cm− and 636 μS cm−1 for Groups 1, and 3, respectively pH ranged from 6.8 to 8.3, although this last value belonged to the spring water collected (LP47) Maximum groundwater pH was 7.8, being neutral to slightly basic Group had a significantly lower pH than Groups and DO ranged from 3.9 to 14.7 mg L−1, but did not differ significantly between groups Again, the highest value belonged to spring water, being the maximum value for groundwater of 8.7 mg L−1 The levels of oxygen in the aquifer suggest an aerated system Regarding water quality for urban use, 64% and 40% of the samples did not comply with the Mexican drinking water standard (NOM-127SSA1-1994) for Cl− and Na+ concentrations, respectively, while in the 2− case of NO− (23%) and SO4 (4%) the majority of the samples fell within the legal range The water quality for irrigation was generally low Around 87% of the samples had EC values higher than 700 μS cm−1, presenting thus a slight restriction for irrigation purposes (Ayers and Westcot, 1992) In fact, 28% of samples (all from Group 1) presented severe restrictions for irrigation purposes (EC N 3000 μS cm−1) Sodium adsorption ratio (SAR) values ranged from 1.17 to 12.43, and were significantly higher in Group (average of 4.91) This indicates a low to medium sodium hazard (e.g., Fetter, 2001) in soils irrigated with these waters Thus, despite the high salinity, groundwater abstracted from the La Paz aquifer system does not present a high sodium hazard The geochemical explanation for this finding is presented in the following sections 4.2 Hydrogeochemistry and controlling processes Regarding major cations and anions in solution, an evolution in the + − hydrochemical facies from low salinity Ca2 +-HCO− type to Na -Cl with increasing salinity was observed (Fig 2) In general, samples with low salinity were located further from the coast, whereas samples with higher salinity were located near the coast or in those areas highly affected by the hydraulic gradient inversion (Fig 1; Tamez-Meléndez et al., 2016) Chloride can be considered a conservative tracer of water salinization, since it is not significantly affected by sorption processes of biological transformations (e.g., Flury and Prapitz, 1993) In addition, chloride concentrations in seawater (ca 560 mmol L− 1, Stumm and Morgan, 1996) are significantly higher than those expected in fresh unpolluted groundwater (b3 mmol L−1) In this sense, the chloride concentration in the La Paz aquifer system may be related to either chloride influx with recharge water (marine aerosols rich in chloride due to sea proximity), the dissolution of chlorine-bearing mineral in the aquifer matrix, or some degree of mixing with seawater Although no chlorine-bearing minerals were reported in the reservoir, the prevailing arid climate conditions in the study area make it possible to find some chloride salts in the unsaturated zone The molar ratio between Na+ and Cl− was close to in low salinity samples, and generally lower in salinity-enriched samples (Fig 3A) For most of the samples with moderate to high salinity, the Cl− vs Na+ crosses clearly plot below a theoretical seawater mixing line, suggesting that some Na+-removing processes take place in the aquifer matrix Cation exchange between Na+ and Ca2 + (Na+ leaves the solution, through sorption in the mineral phase) has been reported in other aquifers affected by seawater intrusion (Appelo and Postma, 2005) The quantity of Ca2+ in samples with increased salinity also indicates a supplementary source of this alkaline earth element (Fig 3B) Potassium presented a similar pattern to that of sodium (Fig 3C) whereas magnesium evolution with higher salinity was closer to the theoretical mixing line behavior (Fig 3D), i.e., only very salty samples presented more magnesium than would be expected from seawater contribution To confirm the importance of cation exchange processes, the difference in concentration Δ n between the observed values and those Fig Piper diagram showing the hydrochemical facies of groundwater in the La Paz aquifer system Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx Fig Major cations in groundwater of the La Paz aquifer system and their relationship with chloride concentration (Cl−) Sodium (A), calcium (B), potassium (C) and magnesium (D) A theoretical mixing line between fresh groundwater and seawater is presented with 2% increases in seawater proportion with each diamond expected if mixing with seawater was the only process was obtained through the following mixing equation (Pennisi et al., 2006): "
 nX seaị nX ẳ nX sị nX freshị ỵ nClsị nClfreshị nClseaị # 2ị where nX(s), nX(ugw), nX(sea) refer to the concentration of component X in the sample (s), fresh groundwater (fresh), and seawater (sea) As seen, the contribution of seawater is derived from the conservative ion Cl− (Appelo and Postma, 2005) Fig represents the relationship between sodium and calcium differences with a theoretical conservative mixture with seawater Most observed values fell within the minus one slope line, that is, cationic exchange represented in the following equation: intrusion were the only process in action (Fig 5B) Given the agricultural activities carried out in the lower parts of La Paz valley, nitrogen leaching from agricultural soils would explain the high NO− concentrations observed In fact, a subtle relationship was detected between NO− and SO2− , once removed the amount of sulfate provided in the theoretical mixing with seawater (Fig 6) A strong correlation between NO− and SO2− is typically detected in agricultural areas with no other significant salinization processes in action (Nakano et al., 2008; Menció et al., 2016) because of the application of fertilizers In this case, the variability in fertilization management along with salinization due to seawater intrusion justifies the lack of a stronger relationship 1 CaX2 ỵ Naỵ NaX ỵ Ca2ỵ 2 Regarding major anions, sulfate concentrations displayed a wide range of variation, from 0.08 to 5.10 mmol L−1 There were significant differences between Group and the other two groups (p b 0.01; Fig 5A) Deviations from the theoretical seawater mixing line can be justified either by an extra supply of sulfate or by sulfur-removing processes Removing sulfate processes (sulfur reduction) have been reported in other coastal aquifers (e.g., Fidelibus, 2003) However, in the La Paz aquifer system DO concentrations observed suggest that these processes were of little importance In contrast, additional sources of sulfate are possible Gypsum dissolution was rejected since there is no evidence of gypsum in the aquifer matrix and the sulfate vs calcium plot (not shown) indicated an evolution that could not be easily explained by the dissolution of this mineral However, for most of the collected samples, NO− presented values clearly over those expected if seawater Fig Relationship between calcium and sodium differences (as calculated by Eq (2)) from a conservative mixture with seawater Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx Fig Anions in the La Paz aquifer system groundwater and their relationship with concentrations of chloride (Cl−): sulfate (A), nitrate (B), bicarbonate (C) and bromide (D) A theoretical mixing line between fresh groundwater and seawater is presented with 2% increases in seawater proportion with each diamond Bicarbonate concentrations ranged from 1.7 to 12.9 mmol L−1, being significantly higher in Group of samples (p b 0.01; Fig 5C) Most of the high-salinty samples plotted over the seawater mixing line, suggesting that some geochemical processes provided HCO− These processes were related to the lower pH of samples from Group 1, and are discussed in the following section Bromide concentrations in the La Paz aquifer system ranged from 0.002 to 0.09 mmol L−1, and correlated significantly with chloride, plotting over the theoretical seawater mixing line (Fig 5D) Br− concentration in seawater averages 0.8 mmol L−1, approximately 200 times the top values reported for natural fresh waters (below 0.004 mmol L−1; Flury and Prapitz, 1993), although higher values have been reported in coastal areas In addition, Br− is present in many pesticides and fertilizers, and consequently its leachate can reach groundwater (Flury and Prapitz, 1993) Despite this feasible origin, the strong correlation observed between Br− and Cl− concentrations (Fig 5D) and the fact that they plotted in the theoretical mixing line between fresh groundwater and seawater suggest that the increase of Br− in the aquifer system is mainly due to seawater intrusion Thus, the hydrochemical exploration of the data suggests that seawater has partially displaced fresh water in the influence area of abstraction wells located near the coast The variability in well depth, abstraction rates and hydrogeological context, along with the existence of other hydrochemical processes, makes it difficult to obtain an overall quantification of the process Coupled with the seawater intrusion, a significant agricultural pollution is inferred In addition, the overexploitation of the La Paz aquifer system for irrigation purposes over the past decades may have increased groundwater salinity as a consequence of recirculation of irrigation water, i.e., irrigation returns flows enriched in salts from evapo-concentrates and salts stored in the soil reaching groundwater Evapo-concentration was detected in the La Paz aquifer system by means of environmental isotopes in a previous work (Tamez-Meléndez et al., 2016) 4.3 Speciation-solubility calculations Fig Relationship between excess sulfate and nitrate (as calculated by Eq (2)) over the theoretical seawater mixing line Speciation-solubility calculation provided the saturation indices (SI) of several mineral phases (calcite, dolomite, gypsum and halite) and the CO2 (g) partial pressure in the water samples from the La Paz aquifer system Calcite (Fig 7A) and dolomite (not shown) presented a rather Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx similar behavior For low-salinity samples (below 20 mmol L−1 of Cl−), subsaturation and equilibrium (SI below or around zero) with respect to calcite and dolomite dominated Thus, low-salinity water was in equilibrium with carbonate minerals present in the aquifer matrix, such as the calcareous cement of conglomerates and sandstones With increasing salinity, the degree of oversaturation increased, and all samples were above 20 mmol L−1 of chloride in the upper fringe of equilibrium or oversaturated with respect to calcite This finding may be explained by the increase in Ca2+ due to cation exchange, as outlined in the previous section In consequence, carbonate mineral precipitation is likely in the La Paz aquifer system, especially in zones near the seashore La Paz groundwater was undersaturated with respect to gypsum (Fig 7B) and halite (not shown) SI values of both mineral phases increase with increasing salinity Although always clearly undersaturated, gypsum SI reaches values relatively close to the equilibrium with gypsum (SI around zero), due to the increase in both SO2− (seawater mixing and marine aerosols) and Ca2+ (released from cationic exchange pool) Gypsum dissolution was discarded in the previous section In contrast, the groundwater was clearly undersaturated with respect to halite, even in the most saline samples (SI halitemax = −4.3) Finally, partial pressure of CO2 (g) in groundwater ranged from 10−2.72 to 10−1.23, although it was 10−3.23 for the spring sampled, and much closer to atmospheric value of 10−3.4 (Fig 7c) CO2 (g) content increased with increasing salinity, supporting the calcite precipitation hypothesis, since CO2 (g) is one of the products of the calcite precipitation chemical reaction (Kehew, 2001; Appelo and Postma, 2005) This increase in dissolved CO2 is the main reason for the lower pH and higher bicarbonate concentrations in samples from Group 1, as discussed in Sections 4.1 and 4.2, respectively 4.4 Strontium concentration and isotopic signature Strontium is known to have a hydrogeochemical behavior similar to that of calcium (McNutt, 2000) It occurs in nature as positively charged solutes, and is therefore affected by cationic exchange processes (Bullen and Kendal, 1998; Fidelibus, 2003; Faure and Mensing, 2005) In addition, the 87Sr and 86Sr isotope ratio is not affected by fractionation processes, because of their minor difference in atomic weight (Ingraham et al., 1998) In consequence, 87Sr/86Sr can be used as a tracer of the source of dissolved strontium Strontium concentrations in the La Paz aquifer system ranged from 3.2 to 92.8 μmol L−1, with a median value of 16.5 μmol L−1 In general, the higher the salinity in a sample, the higher the Sr2+ concentration, as depicted in the significant linear relationship (p b 0.01) between chloride and strontium concentrations (Fig 8A) The measured 87Sr/86Sr ratios of the 26 available samples of groundwater were within a relatively narrow range between 0.7054 and 0.7062 (median 0.7059, Fig 8B), with no significant differences between groups The obtained 87Sr/86Sr values in this study were congruent with those reported by Rosales-Ramírez (2012), who measured strontium concentrations and isotopic ratios in 19 samples collected in the La Paz aquifer system in February 2011 Sr2+ concentrations between 3.9 and 146.1 μmol L− 1, with a median value of 27.7 μmol L− 1, and 87 Sr/86Sr ratios between 0.7055 and 0.7066, with a median value of 0.7060, were reported in that study These values not differ significantly from those obtained in our study, and therefore suggest similar controlling processes The strontium isotope ratios for different rocks in the study area (Schaaf et al., 2000) were mostly below the range of the ratios in water samples (0.7036–0.7039, Table 2) Only intrusive granite reached values closer to those obtained in water samples (0.7050) RosalesRamírez (2012) reported an 87Sr/86Sr value of 0.7054 for the Comondú Formation calcareous sinter, the formation constituting the lower member of the intergranular aquifer of La Paz system In contrast, modern seawater contains 92.4 μmol L− of Sr2 + and 87Sr/86Sr ratios of 0.70918 ± 0.00001 (2 SD) (Stumm and Morgan, 1996; Faure and Mensing, 2005), significantly higher than those found in the system (Fig 8B) As a result, groundwater from the La Paz aquifer system with high Sr2+ concentrations did not show significant influence of seawater in its 87Sr/86Sr values In comparison, a theoretical mixture of fresh groundwater (mean of Group 3) and seawater is presented in Fig 8B following the formulation given in Faure and Mensing (2005) Given the exposed information and the patterns observed in concentration and isotopic data, the strontium in solution in the aquifer appears to originate mainly from continental sources, i.e., the weathering of igneous rocks and the dissolution of strontium present in the intergranular aquifer However, there was no detectable trend in the strontium isotopic ratio when compared with the strontium concentration, and all samples presented similar isotopic composition (Fig 8B) This suggests that the sources of Sr2+ remain relatively stable In contrast, the observed increase in Sr2 + concentration was significantly higher than that would be expected if seawater were the only input (Fig 8A), indicating additional sources of strontium According to Bullen and Kendal (1998), an equilibrium between the dissolved strontium and the strontium present in the exchange pool may be achieved given enough time Under this scenario, the strontium available in the exchange pool has the same isotopic composition as that in the dissolved phase due to negligible fractionation processes in strontium geochemistry A hypothetical cation exchange pool in equilibrium with fresh groundwater from the La Paz aquifer system (Group 3), when in contact with seawater, would retain Na+ and K+ and release Ca2+, Mg2+ and Sr2+ into the solution (Table 3) This pattern is clearly identified in this study (Section 4.2; Figs and 8A) and would explain the increase in Sr2+ concentration and its non-variable isotopic signatures with increasing concentrations (Fig 8B) Therefore, intrusion of seawater produces a shift in the chemical composition of the exchange pool, including the release of significant Fig Saturation indices (SI) of calcite (A) and gypsum (B), and partial pressure of CO2 (C) against salinity (as depicted by chloride concentration) The shaded areas indicate the ranges of uncertainty for the calculated saturation indices; ±0.22 for gypsum (Langmuir and Melchior, 1985), and ±0.35 for calcite (Plummer et al., 1990) Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 10 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx Fig A) Relationship between chloride (Cl−) and strontium (Sr2+) concentrations in the La Paz aquifer system B) Relationship between strontium concentration and 87Sr/86Sr The rectangular shaded area represents measured whole rock 87Sr/86Sr in the study area (Schaaf et al., 2000; Rosales-Ramírez, 2012) A theoretical mixing line between fresh groundwater and seawater is presented with 2% increases in seawater proportion with each diamond quantities of Sr2+ According to the data, the increase in Sr2+ concentration is mainly due to these exchange reactions, and is clearly higher than the increase provided by seawater (Fig 8A) This finding was supported by the good relationship found between differences in calcium and strontium from the theoretical mixture with seawater (Fig 9) Similar patterns in exchangeable strontium and its relatively constant isotopic composition with increasing concentrations have been previously reported for a coastal aquifer in Tuscany, Italy (Pennisi et al., 2006) An alternative explanation to the Sr2+ observed values is related to the leaching from irrigated soils Some fertilizers can have significant Sr concentrations, ranging from b to approximately 4500 ppm (Otero et al., 2005) Part of this Sr can reach the saturated zone when applied irrigation water exceeds the soil retention capacity The 87Sr/86Sr of fertilizers varies widely, from 0.702 to 0.835 (Vitòria et al., 2004) However, given the unknown specific fertilization practices in the study area, it is difficult to further assess the contribution of agricultural soils to groundwater Sr2+ 4.5 Boron concentration and isotopic signature The Mexican regulations for water quality not establish a limit value for boron The World Health Organization recommends a limit of 2.4 mg L− (222 μmol L− 1) as a guideline, but recognizes that in most sites, natural levels are below 0.5 mg L−1 (WHO, 2011) In contrast, the range between boron deficiency and boron toxicity is very narrow, so boron applications can be extremely toxic to some agricultural Table Strontium isotopic compositions (87Sr/86Sr) and concentrations of strontium (Sr) and rubidium (Rb) in several rocks sampled in the study area (Schaaf et al., 2000; RosalesRamírez, 2012) Sr [ppm] 87 Sr/86Sr [−]a Reference Comondú formation CS Calcareous sinter 862 0.705420 ± 08 Rosales-Ramírez, 2012 Deformed granitoids LCB-1 Granite 750.3 0.705029 ± 30 Schaaf et al., 2000 Undeformed granitoids east of La Paz LCB-3 Tonalite 450.7 0.703949 ± 31 Schaaf et al., 2000 Sample Rock Mafic to ultramafic intrusives from the El Novillo complex LCB-5 Qz-diorite 364.6 0.703777 ± 31 LCB-6 Qz-diorite 523.0 0.703603 ± 30 LCB-7 Tonalite 400.1 0.703943 ± 31 LCB-8A Gabbro-norite 271.1 0.703555 ± 38 LCB-8B Hornblendite 232.4 0.703620 ± 41 a Deviation (±) refers to last two significant digits Schaaf et Schaaf et Schaaf et Schaaf et Schaaf et al., 2000 al., 2000 al., 2000 al., 2000 al., 2000 crops at concentrations only slightly above optimum levels for others (Gupta et al., 1985) Contaminated irrigation water is one of the main causes of boron toxicity in plants, as the concentration build-up of boron in soils, especially in arid regions with high evapo-transpiration rates (Gupta et al., 1985) As a sample, different crops tolerance to boron toxicity along with other effects of boron (human health, livestock or ecosystems) are available in Moss and Nagpal (2003) Thus, the processes leading to an increase in boron concentration in groundwater are of paramount importance in La Paz aquifer system, given the agricultural use of the abstracted groundwater Boron in natural waters is normally present either as boric acid, the B(OH)3 planar trigonal complex or as borate, the B(OH)− tetrahedral complex The proportion of each species depends on pH, concentration, temperature, salinity, the presence of other cations, and the ionic strength (Vengosh and Spivack, 2000; Hemming and Hönisch, 2007) In most situations, B(OH)3 is enriched in the heavy isotope 11B with re11 spect to B(OH)− , with differences in their δ B value between 19 and 23‰ (Vengosh and Spivack, 2000; Faure and Mensing, 2005) In contrast, the tetrahedral B(OH)− is preferentially selected in sorption processes, especially in clay mineral surfaces In consequence, adsorbed boron is usually enriched in the light isotope, while residual boron in the aqueous phase is enriched in the heavy one In the collected samples, boron concentrations ranged from 6.5 to 299.1 μmol L−1, with a median value of 25.5 μmol L−1 (Fig 10A) Only one sample in this study was over the WHO recommendation for boron levels in drinking water The highest concentrations were found near the coastline, and the lowest in remote recharge areas In general, the more salinity in a sample, the higher the boron concentration This finding is supported by the significant correlation between Cl− and B concentrations (p b 0.01), although the correlation coefficient was lower than that obtained for other correlations (e.g., Cl−-Ca2+ or Cl−Sr2 +) For the samples with isotopic analysis, the δ11B varied from +27.8‰ to + 54.8‰ (median + 38.3‰) (Fig 10B) Boron concentrations and isotopic signatures in Group of the samples were significantly higher than in the other groups (p b 0.01) According to Vengosh and Spivack (2000), there are several potential sources of boron in an aquifer system, and each presents a range of isotopic signatures Those possible in this study case were: (1) boron provided by the dissolution of soluble phases in igneous rocks (δ11B between − and + 3‰); (2) boron supplied by marine aerosols in the recharge water (up to 30‰); (3) seawater boron introduced with seawater intrusion (427 μmol L−1 with a stable δ11B of 39.5‰); (4) boron present in brines (+ 39‰ and higher); and (5) boron from domestic wastewater or synthetic borate fertilizers (ca 90 μmol L−1 and approximately −15 to +10‰) In addition, sorption processes can significantly modulate both concentrations and isotopic signature (e.g., Pennisi et al., 2006) Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx 11 Table Molar concentration (M), relative molar proportion of cations in water (F) and exchanger pool in equilibrium (X, as computed using PHREEQC) of selected cations in fresh groundwater (GW) of La Paz aquifer system and seawater (SW); expected behavior in relation to cation exchange pool in the aquifer Cation MGW mmol L−1 FGW % XGW % MSW mmol L−1 FSW % XSW % XGW/XSW Exchanger behavior Na+ K+ Ca2+ Mg2+ Sr2+ Ba Total 2.398 0.066 1.022 0.700 0.005 0.010 4.201 57.08 1.57 24.33 16.66 0.12 0.24 100 4.86 0.71 65.64 28.38 0.41 n.a 100 480.18 10.46 10.54 54.41 0.09 0.43 556.11 86.35 1.88 1.90 9.78 0.02 0.08 100 70.54 7.68 5.29 16.43 0.06 n.a 100 0.07 0.09 12.41 1.73 6.83 ca 3a Highly retained Retained Highly released Released Released Releaseda n.a.: Not available (no equilibrium constant in PHREEQC database) a Boron in most natural waters is in boric acid (H3BO3) or borate (H4BO− ) form The behavior is estimated from actual water concentrations instead of proportions in the exchange pool In samples with low B concentration (Group 3), marine aerosols contributing to recharge can explain the isotopic signature Igneous rocks were discarded as a potential source because they have δ11B values close to zero Some samples contained concentrations and isotopic signatures explained by a minor fraction of wastewater (Group 2, Fig 10B) These samples were collected in urban areas, and therefore a contribution of wastewater in their chemistry was most likely In contrast, in high-concentration samples (Group 1), an influence of seawater can be inferred Nevertheless, the observed boron concentration values were significantly higher than those expected from mixing with seawater (Fig 10A) A desorption process from the aquifer matrix may explain the observed results Desorption of boron from sediments following seawater intrusion has been reported in other coastal aquifers (Bianchini et al., 2005; Pennisi et al., 2006), and was explained by the increase in water ionic strength along with a shift in the chemical equilibrium between aqueous phase and sediment According to Goldberg et al (2000), soils with a high cationic exchange capacity exhibit a higher B adsorption capacity In the La Paz aquifer system, a high cationic exchange capacity is expected from the lithology and supported by the observed cationic exchange patterns for Na+, Ca2+, K+, Mg2+ and Sr2+ However, such a process is expected to produce a decrease in δ11B in the water phase In the La Paz aquifer system, this pattern was not observed, since the δ11B of group samples was higher than both fresh groundwater and seawater (Fig 10a) An alternative hypothesis that may explain the observed isotopic signature in this group is related to the precipitation of calcite covered in previous sections Group samples presented a saturation index with respect to calcite which was significantly higher than that seen in the other groups (Fig 11), along with higher levels of dissolved CO2 (Fig 7C), this indicates that some degree of calcite precipitation occurred in these water samples Carbonate precipitation tends to incorporate the tetrahedral complex B(OH)− to the crystalline structure of calcite The precipitated carbonates have δ11B values ca 17‰ lower than those present in solution (Hemming and Hönisch, 2007) In consequence, the preferential removal of this isotope will produce an increase in the δ11B in the remaining soluble fraction However, this hypothesis does not explain the great increase in boron concentration observed in the La Paz aquifer system As reported for Sr, fertilizers may also be a significant source of B Otero et al (2005) reported concentrations on the order of 100 ppm of B in fertilizers commonly used in Spain The B isotopic signature of fertilizers depends on the fertilizer origin and varies widely (δ11B between − 10 and + 40‰, Tirez et al., 2010) Nevertheless, most of the δ11B values reported in Tirez et al (2010) were below those observed in the La Paz aquifer system groundwater, with the only exception being B from hog manure Hog's farming is not a common practice in La Paz valley, and observed δ11B values were not likely to be related to fertilization practices Despite the different sources of B in groundwater discussed above, none of the presented hypotheses for boron dynamics completely explains the obtained data A combination of these processes, along with others still unidentified probably plays a significant role in boron hydrogeochemistry In addition, groundwater in the La Paz aquifer system presented 87Sr/86Sr signatures of crustal rocks while its δ11B is derived mostly from seawater Therefore, boron isotopic dynamics were not completely understood in our study, because of the lack of a clear isotopic signature of different sources and the variety of processes affecting it Further studies are required to clarify this point 4.6 Seawater intrusion estimation Regarding human uses, mixing only 2% seawater in a fresh-water aquifer exceeds organoleptic objectives for the upper limit of chloride (water begins to taste salty) If the mix exceeds 4%, the water becomes mostly unusable, and if the mix exceeds 6%, the water can only be used for cooling and flushing purposes In this context, an estimation of average seawater contribution to collected samples was carried out using Cl− and Br−, two halides known for their conservative behavior in groundwater (Flury and Prapitz, 1993) Assuming that there were no contributions other than fresh groundwater (Group 3) or seawater, and that both anions behave in a conservative way, the seawatercontributing fraction to a particular sample can be computed (Appelo and Postma, 2005): f sea ¼ Fig Correlation between differences in calcium and strontium with a conservative mixture with seawater (as calculated by Eq (2)) msample −mfresh msea −mfresh where m is the molar concentration of the anion tested (Cl− or Br−) in the sample, freshwater (f) or seawater (s) However, some increase in groundwater salinity in the aquifer is expected as a consequence of the recirculation of groundwater (pumping for irrigation purposes and infiltration of irrigation return flows) For example, groundwater salinity increased by a factor of three in a heavily pumped aquifer in southern Portugal following the infiltration of irrigation return flows (Stigter Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 12 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx Fig 10 A) Relationship between chloride (Cl−) and boron (B) concentrations B) Relationship between boron concentration and its isotopic composition (δ11B) in the La Paz aquifer system A theoretical mixing line between fresh groundwater and seawater is presented with 2% increases in seawater proportion with each diamond A mixing line for urban wastewater is also presented Grey arrows indicate the expected shift after sorption processes (lower concentration, heavier isotopic composition; Vengosh and Spivack, 2000) et al., 2006) The authors of that study suggested that concentrations may rise up to five times their natural values should the heavy abstraction regimen continue Thus, in our estimation we consider the freshwater salinity as five times that observed in fresh water from the La Paz aquifer system (Group 3), providing a margin of confidence to state whether a well is affected by seawater intrusion Results obtained for both Cl− and Br− anions were in reasonable agreement given the uncertainties of this estimation, with a slope close enough to 1.0 and a regression coefficient of 0.94, as depicted in Fig 12A.The mean of both estimates is used as a measure of seawater contribution According to this estimate, 65% of the studied wells (30 samples) presented seawater contributions below 2%, while 35% surpassed this threshold (Fig 12B) It is notable that 2% of seawater in freshwater implies around 14 mmol L−1 of Cl−, two times higher than the Mexican guideline value for drinking water (250 mg L− or mmol L−1) Seven samples contained at least 6% seawater, being severely affected not only for drinking purposes but also for irrigation of most crops (Ayers and Westcot, 1992) These seawater contributions are expected to increase since measures against groundwater overexploitation are lacking in the study area Conclusion Groundwater abstraction rates in the La Paz aquifer system over the past decades have been higher than natural recharge values, producing an inversion in the hydraulic gradient and therefore an advancement of the seawater intrusion front Seawater intrusion has produced an increase in groundwater salinity and a shift in its hydrochemical facies, severely affecting water quality for urban supply and/or irrigation purposes in several wells Although maximum seawater influences were estimated as over 6% in only seven sampled wells, approximately one-third of studied wells showed over 2% of seawater contribution, enough to limit their use for human supplies If corrective measures are not taken, this proportion is expected to increase in most of the wells, due to continued groundwater abstraction The coupled use of major ion chemistry and both Sr and B isotopic signatures allowed the identification of the geochemical process taking place in the La Paz aquifer An important cationic exchange process is occurring in the reservoir, with seawater sodium replacing continental calcium (and strontium) in the aquifer clay surfaces Although with a greater degree of uncertainty, we hypothesize that boron is also being significantly released from clay surfaces to aqueous phase, an issue of relevance given its phytotoxicity This fact has been rarely observed in other study areas Moreover, cation exchange processes may impose long-term water quality problem due to reverse exchange in freshening waters (waters enriched in sodium with its related soil-structure problems), unless current abstraction rates would decrease In contrast, a considerable agricultural pollution is developing in the aquifer, especially in the lower areas of the valley Irrigation return flows have resulted in increasing nitrate concentrations in the aquifer, and probably sulfate and other pollutants not measured in this study (such as pesticides), which can further threaten the reliance on water resources in an area were groundwater is the solely supply source for human and agriculture uses Since seawater intrusion is a severe problem in coastal aquifers, the adequate assessment of its importance is of capital interest for water resources management Relatively simple measures, such as electrical conductivity monitoring in pumping wells can serve as a proxy of seawater intrusion Nevertheless, given the difficulty of planning and law enforcement in groundwater politics, extreme actions must be taken by the water authorities in coastal areas to find a suitable solution to this environmental crisis Acknowledgements Fig 11 Relationship between calcite saturation index and δ11B of groundwater in the La Paz aquifer system The shaded areas indicate the ranges of uncertainty for the calculated saturation index, i.e., ±0.35 for calcite (Plummer et al., 1990) The study was co-funded by Fundación FEMSA and the Water Science and Technology Chair of Tecnológico de Monterrey Fundación FEMSA had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript J Mahlknecht and R Ledesma-Ruiz were supported by funding from European Commission project WATERCLIMA–LAC Coastal Water Management, reference: EuropeAid/135857/DH/ACT/MULTI (RAL&RCA); A Meixner was Please cite this article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic influence using major constituents, Sr and B isot , Sci Total Environ (2017), http://dx.doi.org/10.1016/j.scitotenv.2017.02.137 J Mahlknecht et al / Science of the Total Environment xxx (2017) xxx–xxx 13 Fig 12 A) Agreement in the proportion of seawater estimation with Cl− and Br− tracers B) Histogram of seawater proportion in the sampled production wells in the La Paz aquifer system supported by funding from the Deutsche Forschungsgemeinschaft (DFG Major Research Instrumentation Programme INST 144/308) The authors acknowledge the contribution of C Támez-Meléndez, A 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constituents, Sr and B isot , Sci Total Environ (2017),... article as: Mahlknecht, J., et al., Assessing seawater intrusion in an arid coastal aquifer under high anthropogenic in? ??uence using major constituents, Sr and B isot , Sci Total Environ (2017),