Hydrogeochemical Tool to Identify Salinization or Freshening of

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Environ. Sci. Technol. 2010, 44, 4096–4102

Hydrogeochemical Tool to Identify Salinization or Freshening of Coastal Aquifers Determined from Combined Field Work, Experiments, and Modeling A M O S R U S S A K †,‡ A N D O R I T S I V A N * ,† Department of Geological and Environmental Sciences, Ben Gurion University, Beer Sheva 84105, Israel, and Geological Survey of Israel, Jerusalem 95501, Israel

Received February 2, 2010. Revised manuscript received April 26, 2010. Accepted April 26, 2010.

This study proposes a hydrogeochemical tool to distinguish between salinization and freshening events of a coastal aquifer and quantifies their effect on groundwater characteristics. This is based on the chemical composition of the fresh-saline water interface (FSI) determined from combined field work, column experiments with the same sediments, and modeling. The experimental results were modeled using the PHREEQC code and were compared to field data from the coastal aquifer of Israel. The decrease in the isotopic composition of the dissolved inorganic carbon (δ13CDIC) of the saline water indicates that, during seawater intrusion and coastal salinization, oxidation of organic carbon occurs. However, the main process operating during salinization or freshening events in coastal aquifers is cation exchange. The relative changes in Ca2+, Sr2+, and K+ concentrations during salinization and freshening events are used as a reliable tool for characterizing the status of a coastal aquifer. The field data suggest that coastal aquifers may switch from freshening to salinization on a seasonal time scale.

Introduction The water quality of coastal aquifers is of great global concern because they supply fresh water to a large portion of the population. Salinization due to seawater intrusion is one of the major threats to coastal aquifers and an early warning for such an event at a specific point of time and space is invaluable to water authorities around the world. Coastal aquifers contain distinct fresh-saline water interfaces (FSIs) that separate between dense seawater at the bottom and fresh water at the top. At a given distance from the shore, the FSI may fluctuate vertically because of changes in the aquifer’s water balance (1, 2). The water balance is controlled by a suite of natural and man-made mechanisms such as the following: recharge by precipitation, which pushes the FSI downward (3); big sea storms, which may push the FSI upward (4); tides with fluctuating FSI (e.g., ref 5); seawater intrusion due to a global rise in sea level (transgression), which elevates the FSI (6); and seawater intrusion due to overpumping of fresh groundwater, which also elevates the FSI (e.g., ref 7). The numerous controlling mechanisms, as * Corresponding author phone: 972-8-6477504; fax: 972-8-6472997; e-mail: [email protected]. † Ben Gurion University. ‡ Geological Survey of Israel. 4096

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listed above, introduce relatively large variability into the data collected on the vertical position of the FSI. Thus, identifying the salinization/freshening status of a coastal aquifer may be possible by long-term monitoring of the vertical position of the FSI. Previous attempts to identify salinization and freshening events on short-term monitoring were based on the Mg2+ to Ca2+ ratio and Na+ to Cl- ratio (8, 9). In addition, few indices based on the excess or deficit of cations compared to Cl- using only one sample were suggested to identify ongoing freshening or salinization (e.g., ref 10). The chemical composition of groundwater in coastal aquifers is controlled by a conservative mixing between seawater and fresh water and a variety of water-rock interaction processes such as cation exchange, sulfate reduction, and carbonate mineral dissolution/precipitation (e.g., refs 3, 4, 11-17). Various experiments were conducted to simulate the geochemical processes within the coastal aquifers (14, 18-20). When seawater interacted with montmorillonite, Ca2+ and Mg2+ were enriched and Na+ and K+ were depleted because of cation exchange (14). Injection of fresh water into wells drilled in a brackish-water aquifer produced a clear chromatographic pattern of cation exchange (20). Laboratory simulations of seawater intrusion showed that Ca2+ increased to a concentration higher than that of seawater (18, 19). The same experiment design, only for freshening simulation, showed the opposite tendencysCa2+ and Mg2+ concentrations decreased below their concentrations in fresh water, before increasing back to their freshwater concentrations (18). The authors of that study emphasized that the flow rate of the eluted solutes plays an important role in the cation exchange process (18). At very high flow rates the solutes may not reach equilibrium with the column solids, and at extremely low rates the diffusion may blur the chromatographic separation. The objective of this work was to characterize salinization and freshening events in a coastal aquifer through combined field-work across the FSI, laboratory simulations using the same sediments, and modeling of the experimental data. Major ion concentrations were used to identify the status of the aquifer (salinization or freshening) and the time frame for these processes. The carbonate system and its coupled processes were also investigated in this study, where the isotopic composition (δ13C) of dissolved inorganic carbon (DIC) was determined for the first time in experiments of this type.

Methods Study Area. The study area was the coastal aquifer of Israel, which is located along the Eastern Mediterranean coastline. The aquifer’s geological section consists of alternating layers of calcareous sandstone (Kurkar), red loam (Hamra), and marine clays of Pleistocene age (21), which overlie impervious marine clays of Pliocene age (the Saqiye Group). The field work was conducted in the Netanya area (∼30 km north to Tel-Aviv) in a monitoring well (Poleg 1) that is located 70 m from the shoreline (3.1 m above sea level). The Poleg well was designed especially for studying the FSI by drilling into the phreatic aquifer without adding any mud or fluid to avoid contamination (3). The 2′′ polyvinyl chloride (PVC) pipe was perforated throughout the saturated zone, crossing the FSI, and extending down to a depth of 46 m (3). The FSI depth is represented by the point where the salinity is half of seawater salinity (between 20 to 25 m), while the thickness of the FSI zone is 3 to 7 m. Field Work. Electrical conductivity (EC) profiles were conducted approximately every month from December 2006 10.1021/es1003439

 2010 American Chemical Society

Published on Web 05/12/2010

to September 2007 to study the seasonal fluctuation of the FSI. The seasonal chemical variations were examined in the Poleg well by sampling groundwater using a multilevel sampler (MLS; (22)). The MLS is a 3 m rod with 22 holes for sampling cells. The cell volume is 15 cm3 and has a membrane (0.2 µm) at each base. The distance between every cell is about 15 cm, which enables high sampling resolution. The cells were filled with distilled water before insertion into the rod. Then, the MLS was fitted into the FSI in the well for 1 month to reach equilibrium between the water in the cells and the groundwater. Seasonal samplings were conducted also using a 1 m long and 3.8 cm diameter stainless steel bailer. The data gathered by the bailer was used to determine the chemical composition of the end member components of fresh water and saline water in the well. Laboratory Experiments. Salinization and freshening events of the aquifer were simulated by laboratory experiments using columns packed with aquifer sediments. The sediments were sampled adjacent to the Poleg well with a manual drill at a depth of 3-4 m, which is in the oxic zone, so no special treatment was needed. Fresh groundwater was collected at a depth of 6 m, and seawater was collected from the seashore near the research well. The experimental column was 20 cm long with a 4 cm internal diameter packed with the sediment sampled with the manual drill. The experimental solution (Poleg fresh groundwater or Mediterranean seawater) was pumped through the column with a peristaltic pump and proper tubing. The water exiting from the column was sampled regularly at predetermined periods. The column was equilibrated with fresh water before each salinization experiment and before each freshening experiment the column was equilibrated with seawater. The waters sampled were analyzed for major ions (including Sr2+), alkalinity, DIC and δ13CDIC. Three salinization experiments were conducted at three different flow rates, 50, 200, and 1000 m · y-1. The last two experiments (200 and 1000 m · y-1) were immediately followed by a freshening cycle at the same flow rate as the previous salinization experiment. It should be noted that the high flow rate experiments were 10 times faster than suggested in a previous study (18), to examine if, indeed, at a high flow rate the cation exchange process does not occur. Modeling. The aqueous geochemical modeling code PHREEQC-2 (23) was used to model the results of the salinization/freshening scenarios simulated with an experimental column, as was conducted in previous works (18, 24). The 20 cm column length was divided into 40 cells (0.5 cm thick). The PHREEQC database (phreeqc.dat) was used. The chemical compositions of end members were the average compositions of the fresh water and seawater used in the column experiments. Cation exchange parameters, such as the selectivity coefficients, were taken from the program database. The cation exchange capacity (CEC) was calculated by the empirical equation using the clay and organic carbon content of the aquifer sediments (11). Dispersivity was calculated by the Fickian model of dispersion (25). The time step was determined by the experiment’s flow velocity and the time required for full salinization or freshening. Analytical Methods. Cations and sulfate concentrations were analyzed by inductively coupled plasma-atomic emission (ICP-AES, P-E optima 3300) with a precision of 2%. Clwas measured by titration using a 0.01 N AgNO3 solution as a titrant, and small samples were analyzed by ion chromatography (Dionex 4000i). The errors calculated by averaging duplicate samples in both methods were less than 3%. Water subsamples for alkalinity, DIC, and δ13CDIC measurements were filtered through 0.45 µm filter and poisoned with HgCl2 to prevent any biological activity. Total alkalinity was measured by titration of a 0.01 N HCl solution as a titrant. The error calculated by averaging duplicate samples was

(0.03 meq · L-1. δ13CDIC was measured by conventional mass spectrometry (GS-IRMS) with (0.1‰ precision and reported on the VPDB scale. DIC was measured together with the δ13CDIC using NaHCO3 as standard. The precision of DIC analysis was (0.1 mmol · L-1. All water samples were kept in 4 °C until analysis. A sediment sample taken from the study area was analyzed as well. Particle-size distribution (PSD) analysis was conducted using the laboratory sieve shaker method (26) and laser diffraction system (MS-2000, Malvern), which measures particle sizes in the range of 0.02-2000 µm. The porosity was determined directly from measurements of the particle density and bulk density. The clay percentage was determined by the pipet method, filtering the clay from a sediment sample based on Stokes’ law. X-ray diffraction (XRD) was used to characterize the mineralogical and clay composition of the sediment. Calcium carbonate percentage was determined by the pressure calcimeter method (27). Organic carbon content was measured by the wet oxidation and titration methods (28).

Results and Discussion Sediments of the Coastal Aquifer at the Poleg Site. The sediments of the coastal aquifer in the Poleg area are composed mainly of fine (0.2-0.4 mm) grained calcareous sandstone, consisting of 83% quartz, 15.7% calcium carbonate, 0.7% clay, and 0.6% organic carbon. The typical mineralogical composition of the sediment is dominantly quartz, secondary calcite, and traces of aragonite, feldspars, and phyllosilicates. The clay composition is mainly Illitesmectite, secondary kaolinite, and traces of chlorite. The porosity is about 0.37. These sediments were used to pack the columns for the simulation experiments described below. Geochemical Characterization of Salinization and Freshening. Laboratory Experiments. In the freshening experiments, the major ions seemed conservative, since they generally showed a linear mixing between the two end members: fresh groundwater and seawater (Figure 1a-f). However, in magnification of the fresh part (Cl- between 20 to 30 meq · L-1) it can be seen that Mg2+ and K+ are actually enriched compared to their fresh groundwater values, while Ca2+ and Sr2+ are depleted (inset graphs in Figure 1b-e). In the salinization experiments Na+, Mg2+, and SO42 seem relatively conservative (Figure 1a,b,f). On the other hand, the non-conservative behavior of Ca2+, Sr2+, and K+ is clearly exhibited (Figure 1c-e). Ca2+ and Sr2+ form a high peak (ca. twice the seawater concentration) above the conservative mixing line. K+, however, shows a clear depletion with respect to the conservative mixing line. The alkalinity and DIC results show no clear pattern; however, the δ13C results show depletion during salinization (SI-T 1.3). The most reasonable explanation for the enrichment or depletion of Na+, K+, Ca2+, Mg2+, and Sr2+ during salinization or freshening is cation exchange. To examine the role of the cation exchange process compared to other processes affecting the cations, mass balance calculations were conducted (Figure 2). These mass balances revealed a comparable change in Ca2+ and change in other cations (K+ and Na+ during salinization experiments and Mg2+ and K+ during freshening experiments), whereas Sr2+ was omitted from the calculation because of its very low concentrations. They were calculated from the difference between the measured concentrations of the cations and the expected concentrations from conservative mixing between fresh water and seawater. In the salinization experiments, K+ and Na+ balanced the 2+ Ca enrichment. The Na+ depletion was relatively small and seemed almost conservative, even though quantitatively the depletion was significant (Figure 1a). The Ca2+ enrichment was balanced by K+ and Na+ depletion, having a trend-line VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Major ions versus Cl- in laboratory experiments and model: (a) Na+; (b) Mg2+; (c) Ca2+; (d) Sr2+; (e) K+; (f) SO42-. The model results are plotted as a red solid line for salinization simulation and as a blue dashed line for freshening simulation. For Mg2+, Ca2+, Sr2+, and K+ the fresh part is magnified as inset graphs, which show that during the last stage of the freshening experiment the Ca2+ and Sr2+ are depleted below the mixing line (black dashed line) between fresh water (black circle) and seawater, while Mg2+ and K+ are enriched. slope of -1 (Figure 2a). A slope of -1 indicates that with enrichment of Ca2+, K+ and Na+ are equivalently depleted. Thus, during salinization the cation exchange is the major geochemical process controlling the variation in these cations. A similar mass balance was conducted for the data in the end part of the freshening experiment, when the Cl- values reached the fresh groundwater values (20 to 30 meq · L-1). In this part of the experiment, as noted before, the values of Ca2+, Mg2+, and K+ clearly deviated from their conservative expected values (Figure 1b,c,e). The mass balance results showed that the depletion in Ca2+ was equivalent to the enrichment in Mg2+ and K+ (Figure 2b). This indicates that during freshening Ca2+, Mg2+, and K+ are controlled by cation exchange with the aquifer sediments. It does not mean that cation exchange process occurs only in the last part of the freshening experiment, but only that the cation exchange process is noticeable there. Comparison of the total exchange of the cations during salinization and freshening cycles (the salinization experiment and the following freshening experiment) indicate that the cycles are reversible for the major cations. For example, the total equivalents of Ca2+ on the column sediments during freshening experiments (average of 0.74 meq) were nearly equal to the total equivalents desorbed from the column during the following salinization experiments (0.64 meq). Although the desorption magnitude was higher than the adsorption magnitude, the time of the adsorption was much 4098

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longer. Hence, the total equivalents were equal. The change in K+ during the salinization was 0.17 meq and during freshening 0.18 meq. It is hard to calculate the change in Na+ because of its relatively small change during the salinization/ freshening processes, but it seems that it shows reversibility too (salinization -0.5 meq and freshening -0.4 meq). The Mg2+ seems also reversible since the amount of Mg2+ desorbed from the sediment in the last part of the freshening experiments (0.31 meq) is close to the amount of Mg2+ adsorbed to the sediment before (0.25 meq). Field Observations. The field geochemical data collected in the Poleg observation well in the MLS profiles are presented as cations versus Cl- plots (Figure 3a-d). Na+ (Figure 3a), Mg2+, and SO42- (not shown) seem relatively conservative when plotted versus Cl-, similarly to their behavior in the salinization and freshening experiments (Figure 1a,b,f). Moreover, K+, Ca2+, and Sr2 (Figure 3b-d) seem nonconservative as seen in the salinization experimental results (Figure 1c-e). The DIC concentrations are above a conservative line, while the δ13C values are below it (Figure 3e,f respectively). The patterns of Ca2+ and Sr2+ were very similar, which means that their exchange characteristic in this system is nearly identical. Similar behavior of Ca2+ and Sr2+ was documented in an earlier sorption study using synthetic solutions (29). In contrast, two earlier experimental studies showed that Mg2+ was adsorbed during the salinization

fresh water displaces seawater, as was found in laboratory experiments (31). It was suggested that small water-clay configurations are formed and behave as rigid particles (31). Identifying Salinization and Freshening Events. The laboratory experiments and model results show that there is a clear difference between the cation behavior in freshening as compared to salinization, when plotting the concentrations of cation in pore water versus Cl- (Figure 1). These results are remarkably similar to the seasonal field data (Figure 3). This similarity and the distinct pattern of salinization and freshening processes indicate the presence of both events in the coastal aquifer of Israel. The two processes can be distinguished using the difference in the Ca2+, Sr2+, and K+ patterns during salinization and freshening. Ca2+ and Sr2+ enrichment and K+ depletion characterize salinization of the aquifer, and freshening looks like a “conservative” behavior in low resolution. It should be noted, however, that enlargement of the freshwater end-member shows actually nonconservative behavior, as mentioned above. We propose a salinization index (I) for cases that cation exchange is the main process in the system, which is based on the opposite cation exchange characteristics of Ca2+ and K+ during salinization and freshening. This method enables distinguishing between salinization and freshening using only one sample, and considering the local saline end member and not average seawater composition. Assuming that the saline end-member is always seawater may lead in some cases to erroneous conclusion. In addition, the index calculation is based on two parameters with opposite characteristics (Ca2+ and K+). Therefore, the overall change in the index is larger than the change of any one of them separately, making the index a sensitive sensor for salinization and/or freshening. The salinization index I is calculated as follows: FIGURE 2. Plot of the equivalent change in Ca2+ versus (a) K+ and Na+ during salinization experiments; (b) Mg2+ and K+ during freshening experiments. The mass balance was conducted by calculating the difference between the measured concentrations of the cation and the expected concentrations from conservative mixing between fresh water and seawater. The slope trend line (close to -1) indicates that the enrichment or the depletion in Ca2+ is equivalent to the depletion or the enrichment in the other cations.

I)

[

]

[K] - [K]exp [Ca] - [Ca]exp - 0.5 [Ca]exp [K]exp

(1)

The expected values of Ca2+ and K are calculated and defined as: [M]exp ) [M]slw - ([M]slw - [M]fw)·

[Cl-]slw - [Cl-] [Cl-]slw - [Cl-]fw (2)

experiments (18, 19), while in this study the Mg2+ behavior seems conservative. The difference may stem from either the difference in the composition of the sediments in the columns or from the fresh and saline groundwater composition in this study and those in previous studies. Different sediment composition, as well as different water composition, results in different exchange potential and behavior. For example, the fresh groundwater in this study is more of the NaCl type than that in the previous studies (18, 19) that led to different equivalent fractions (β) of the exchangeable sites (11). Modeling. The input data in PHREEQC-2 included the end member compositions in the experiments and the process of cation exchange. The model adequately reproduced the experimental results (Figure 1a-f). The remarkable correlation between the model and the experiments suggests that cation exchange is indeed the main process occurring during salinization and freshening processes and that the parameters chosen in the model describe the system well. The dispersivities calculated in the experiments and used in the model were 0.005 ((0.0015) m for freshening and 10 times smaller 0.0005 ((0.0001) m for salinization. A similar change in dispersivity, though somewhat smaller, was observed in a previous study (30). The decrease in dispersivity was attributed to the expected decrease in permeability when

where M denotes the species Ca2+ or K+; slw and fw denote saline water and freshwater, respectively, and [Cl-] denotes the chloride concentration of the water sample. Values of the index higher than zero imply a salinization process, while lower values imply a freshening process (Figure 4a). The empirical boundary line of the index between freshening and salinization is 0.5. For the sake of convenience, 0.5 is subtracted from the equation to set the boundary-line value of zero. In gypsum containing aquifers the index should be calculated using only K+ and therefore be subtracted only by 0.15. The results of our study show that the proposed index numbers can be used in water whose salinity range is 10-80% of the saline end member (Figure 4b). Time Scale of Salinization and Freshening Events. Although the three salinization experiments were performed at different flow rates, the results of each ion showed the same pattern. This was also true for the freshening experiments (Figure 1). The ion exchange process occurs during the experiments even at very high velocities (1000 m · y-1), much higher than previously suggested (18). This implies that in our experimental situation, time was probably not a limiting factor in the ion exchange process. The question is, therefore, what is the time scale of the salinization and freshening events in the field? VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Major ions versus Cl- in the Poleg well: (a) Na+; (b) K+; (c) Ca2+; (d) Sr2+; (e) δ13CDIC; (f) DIC. The slw and sw symbols denote saline water in this well and seawater, respectively. This figure combines the results from this study and our previous results in 2000 and 2001 (3). The mixing line between fresh water and the most saline groundwater (slw) is included in the graphs as a black line. The seasonal field results can be used to answer this question. For example, the Ca2+ results from the MLS samplings are not constant throughout the year (Figure 3c). Two MLS sets of data (9/00 and 4/01) seem very similar to a conservative mixing line between fresh water and the saline water in the well. All other MLS sets are different and are situated above that mixing line. The different results of the MLS samplings conducted every 3 months suggest seasonal changes. The correlation between the field and the laboratory results (Figures 1 and 3) enables identifying the MLS results as seawater intrusion or freshening. This implies that active (“loading of the column”) seawater intrusion and freshening processes occur at least on a seasonal time scale in the study site. Carbonate System. Our MLS DIC and δ13CDIC results (Figure 3e,f) indicate an addition of DIC from organic carbon oxidation, whose δ13CDIC value is low in this area (-24‰; (3)). Since the oxygen levels were low below the FSI, it seems reasonable that the oxidation is through anaerobic bacterial sulfate reduction. Organic carbon oxidation by sulfate reduction was suggested to occur in saline groundwater (3, 4) from sulfide enrichment where sulfate seemed depleted (4), the alkalinity and DIC ratio is 1:1 and dissolved oxygen concentration is low (3). The same phenomenon was observed in the column experiment. During the salinization experiment, the δ13CDIC value (-11‰) was lower than expected from a conservative mixing between the freshwater values (-7‰) and seawater values (∼0‰; (32)) (SI-T 1.3). This means that even on the small scale of the column, organic matter oxidation occurs 4100

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and affects the isotopic values. The oxidation is probably through sulfate reduction in this anoxic environment. This possibility is supported by the slight sulfate depletion as compared to conservative mixing (Figure 1f). Indeed, mass balance calculation based on the DIC and δ13CDIC data shows that the enrichment in DIC can be explained by organic matter oxidation via sulfate reduction. Implications. The great resemblance between field, laboratory, and model results enabled examining the field data in the light of the experiment and modeling data. The data show that during salinization and freshening cation exchange is the main process affecting the cations Na+, K+, Ca2+, Mg2+, and Sr2+. Another process evident from the results is organic matter oxidation, which occurred during salinization probably through bacterial sulfate reduction. A geochemical index was developed based on the difference in Ca2+ and K+ behavior during salinization (Ca2+ enriched and K+ depleted) and during freshening (Ca2+ and K+ concentrations lie on a mixing line). The index results of the seasonal field sampling indicate that the salinization and freshening processes occur at least on a seasonal time scale. The experimental results also suggest that the salinization and freshening cycle is reversible. The use of the index was tested on data from a few previous studies (9, 18, 33). The studies were chosen because the aquifer’s status was well documented, chemical data was included, and groundwater salinity was in the range of the index’s validity (10-80% of the saline end member). The first study described salinization of the Israeli coastal aquifer (9), the second study described a salinization experiment

chemical analyses, and O. Crouvi from the Geological Survey and G. Biran and E. Shani from Ben Gurion University for their help with the sediment analyses. Special thanks to V. Friedman, D. Ronen, and M. Zilberband from the Hydrological Survey for providing helpful information about the observation wells, the MLS and PHREEQC. We also want to thank the reviewers for their helpful comments, which improved the manuscript significantly. Part of this research was supported by the Israel Science Foundation (#857/09).

Supporting Information Available Additional Supporting Information may be found in the online version of this article. Tables with all the analytical data and the modeling parameters are presented in SI-T1 and SI-T2, respectively. A map and hydrological cross section of the study site are presented in SI-F1. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited

FIGURE 4. Salinization index versus Cl- (in relative concentration to the saline end member): (a) the index of the experiments and field data. It implies salinization when its values are higher than zero, while lower or zero values imply freshening; (b) blow-up of the y axis, emphasizing that the index can be used in water whose salinity range is within 10-80% of the saline end member.

FIGURE 5. Index calculation versus Cl- from data of previous studies. The first study described salinization of the Israeli coastal aquifer (Mercado, 1985, (9)), the second described a salinization laboratory experiment (Appelo et al., 1990, (18)), and the last described a freshening of the aquifer near Venice, Italy (Gattacceca et al., 2009, (33)). The Index results fit quite well the conclusion of these studies.

(18), and the third study described a freshening of the aquifer near Venice, Italy (33). The fitness of the index on data from other studies (Figure 5) confirms its generality as a reliable geochemical tool.

Acknowledgments We would like to thank Boaz Lazar from the Institute of Earth Sciences, Hebrew University, Jerusalem, and Yoseph Yechieli from the Geological Survey of Israel for their invaluable suggestions and help. We thank also H. Hemo for his great help in the field. S. Ashkenazi and Y. Mizrachi from the Geological Survey and A. Katzir from Ben Gurion University helped us as well. We also want to thank O. Yoffe and D. Stiber from the Geological Survey for their help with the

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