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Groundwater salinization process in the coastal aquifer Sidi Abed-Ouled Ghanem (Province of El Jadida, Morocco)
Accepted Manuscript Groundwater salinization process in the coastal aquifer Sidi Abed-Ouled Ghanem (Province of El Jadida, Morocco)
Sara Mountadar, Abdelkader Younsi, Abdelkader Hayani, Mustapha Siniti, Soufiane Tahiri PII:
S1464-343X(18)30180-8
DOI:
10.1016/j.jafrearsci.2018.06.025
Reference:
AES 3252
To appear in:
Journal of African Earth Sciences
Received Date:
22 January 2018
Accepted Date:
04 June 2018
Please cite this article as: Sara Mountadar, Abdelkader Younsi, Abdelkader Hayani, Mustapha Siniti, Soufiane Tahiri, Groundwater salinization process in the coastal aquifer Sidi Abed-Ouled Ghanem (Province of El Jadida, Morocco), Journal of African Earth Sciences (2018), doi: 10.1016/j. jafrearsci.2018.06.025
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ACCEPTED MANUSCRIPT
Groundwater salinization process in the coastal aquifer Sidi Abed-Ouled Ghanem (Province of El Jadida, Morocco)
Short title: Coastal groundwater salinization
Sara Mountadar 1,*, Abdelkader Younsi 2, Abdelkader Hayani 1, Mustapha Siniti 3, Soufiane Tahiri 1
1 Laboratory
of Water and Environment, Department of Chemistry, Faculty of Sciences of El
Jadida, University Chouaïb Doukkali, P.O. Box 20, El Jadida (24000), Morocco. 2
Laboratory of Water and Hydrogeology, Department of Geology, Faculty of Sciences of El Jadida, University Chouaïb Doukkali, P.O. Box 20, El Jadida (24000), Morocco. 3
Team of Thermodynamic, Catalysis and Surfaces, Department of Chemistry, Faculty of Sciences of El Jadida, University Chouaïb Doukkali, P.O. Box 20, El Jadida (24000), Morocco.
*Corresponding Author: [email protected]
ACCEPTED MANUSCRIPT Abstract In this work, the salinization mechanisms were highlighted in the coastal area between Sidi Abed and Ouled Ghanem (El Jadida Province, Morocco). This study was based on analyzing and discussing the physicochemical data of water samples from 73 wells distributed in the whole studied region. Firstly, the results of the principal component analysis (PCA) and hierarchical cluster analysis (HCA) showed that the study area is composed of three main groups. This classification is firstly based on the proximity of the wells to the seawater. In fact, it was found that the first and second groups of wells which are located in the coastal fringe are characterized by high concentration of sodium and chloride and EC values whereas lower values are found in the third group that is located in upstream. Moreover, the results also showed the influence of the hydrogeological characteristics of the area where the wells are located on the composition of the water samples. The evaluation of the hydrochemical facies using Piper trilinear diagram have also confirmed these results and showed the probable contribution of seawater intrusion in the modification of wells composition by ion exchange process and by increasing the concentration of sodium and chloride. Then, the groundwater quality were assessed using Sodium Adsorption Ratio (SAR) and sodium percentage (Na %) parameters. In order to define the salinization processes occurring in the aquifer, the binary diagrams between chloride and the other elements were plotted. It was found that the groundwater is contaminated by seawater intrusion which is accompanied by ions exchange depending on the hydrogeological characteristics of the area. Keywords: Coastal aquifer; Groundwater; Hydrogeology; Salinization; Salt intrusion; Statistic analysis, Sahel Doukkala, Morocco.
ACCEPTED MANUSCRIPT 1. Introduction
Salinity always exists in groundwater. The type and quantity of salts reflect the environment, movement and source of groundwater (Bowen 1986). However, coastal aquifers are increasingly threatened by saline intrusion due to groundwater exploitation and global sea level rise (Tomaszkiewicz et al 2014). Several researchers have studied the origin of salinity in coastal location by analyzing the hydrochemical results and by understanding the geochemical process behind this pollution (Mdiker et al 2009; Hamzaoui Azaza et al 2012; Zghibi et al 2013, Hayani 2014). Sahel coastal aquifer (El Jadida Province), like many other semi-arid regions, relies heavily on its pluviometric potential. Hence, water resources in the study area are limited. The coastal aquifer is affected by intensive pumping for irrigation and drinking water. Consequently, the fresh water level is lowered and the ocean intrudes further into the aquifer. In order to study the probable salinity intrusion in the study area, sampling of groundwater and physicochemical analysis were carried out. Due to existence of different factor that could impact the salinization processes, different methods were used: the principal component analysis (PCA), hierarchical cluster analysis (HCA), Piper trilinear diagram, the spatial distributions of ions and the binary diagrams using the seawater fraction.
2. Materials and Methods
2.1. Study area The study area is a part of Sahel basin which belongs to Meseta Western Morocco and lies between latitudes 33°50’N and 32°10’N and longitudes 8°60’E and 8°30’E. It is bounded by the Atlantic Ocean in the West and North, by Sidi Abed locality in the Northeast and Jamaat
ACCEPTED MANUSCRIPT Ouled Ghanem in the Southwest. The sector is about 120 km2, 4 km far from the Ocean and extends over a length of 30 km. The zone climate is semiarid with hot and dry summer (MayNovember) and humid winter (November-April). The Sahel area is a part of the geological unit known as the Moroccan coastal meseta. It contains sub-tubular sedimentary series from Mesozoic and Cenozoic era that are based on Paleozoic land pleated during the Hercynian orogeny (Gigout 1951; Ouadia 1998). Regarding the hydrogeology, there are two units in the study area. The unit 1 consists of a table circulating in Pliocene-Quaternary calcarenite and in limestone of the lower Cretaceous. The unit 2 is composed of water table circulating in the upper Cretaceous consisting of fissured to karstified limestone (Chtaini 1987; Aboumaria 1993). This table is fed from surface infiltration. The human activities in the coastal strip are characterized by the dominance of vegetable growing. Its irrigation is done by pumping water from water table. Cattle breading is also found in this zone. These whole activities require more and more water and then push farmers and ranchers to dig more wells where the pumping is intensive.
2.2. Sampling method and water analysis The choice of the measuring network was based on a mesh of the study area with one to two wells for each km2 with a relatively higher density near the ocean (Fig. 1). In total, 73 wells were chosen. These water points were the subject of many periodic piezometric surveys over the years 2011, 2012 and 2013. These campaigns were carried out during high tide (March April) and low tide (August September) periods, biannually. To this end, a number of devices and documents were used: a Global Positioning System GPS (Garmin nuvi 2008), a piezometric probe (100 m length) and topographic maps 1/50 000e.
ACCEPTED MANUSCRIPT Regarding the physicochemical analysis, it was based on samples carried out in 73 wells. Many periodic campaigns of sampling were fulfilled from September 2011 to December 2013, biannually. The whole sampling were in situ collected in 250 ml polyethylene bottles and conserved in low-temperature cooler and then in the laboratory refrigerator. The analyses carried out are electrical conductivity, pH and majors elements (Ca2+, Mg2+, Na+, K+, Cl-, SO42- and HCO3-). The physical parameters were periodically measured in situ using conductivity meter HACH (443600 model) and pH meter Best.-Nr. 100 787. Measure of Mg2+ and Ca2+ concentrations was performed using complexometry (AFNOR NTF 90-003) while Na+ and K+ ions were determined using a flame spectrophotometer. Concerning bicarbonates, and chloride, they were respectively titrated by volumetric method using sulfuric acid (H2SO4 0.02N) (AFNOR T 90-036) and silver nitrate (AgNO3 0.02N) (AFNOR T 90-014) as a titrand, while sulfates concentrations were measured using gravimetric method (AFNOR T 90-009).
2.3. Data analysis methods Groundwater hydrochemical groups were defined by Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) using XLSTAT software. Hierarchical cluster analysis was performed for grouping groundwater samples in a way that each group or cluster is homogeneous with respect to certain characteristics and distinct from other clusters regarding the same characteristics (Davis 2002). In order to homogenize the data base, the later was centered and reduced. Then, the HCA dendrogram was obtained in the following way: the first step is to group the closest individuals into a class. This notion of proximity therefore presupposes the definition of a distance between two individuals that will be calculated. The Euclidean distance was used in this case. Then, the resulting classes were grouped with other classes until a single class gathers all the individuals. These stages of successive clustering of classes assume the definition of a second distance (called the
ACCEPTED MANUSCRIPT aggregation criterion) measuring the proximity not between two individuals but between two classes of individuals. There are several methods of aggregation. In this study, Ward's method was used. In addition, an automatic truncation has been generated which makes it possible to determine the number of homogeneous groups. After the HCA, the results were interpreted using PCA. Based on the principal component scores, PCA can examine multivariate relationship and explain the variance in the data while reducing the number of variable to several groups of individuals (Custodio and Bruggeman 1987). This technique is quite similar with the correlation or regression analysis methods, and can transform the data set, with many variables, into a set of comprehensive principle components. PCA allows a considerable reduction in the number of variables and the detection of structure in the relationships of different variables. Concerning the used PCA process, the variables were firstly normalized. Afterward, this method makes it possible to calculate new variables - linear combination of the initial variables - called principal components (factorial axes), which by synthesizing the information, highlight the interdependencies of the data. The choice of the number of factorial axes to study depends on the eigenvalues that accompany the PCA. The criteria adopted was the Kaiser criterion which says that axes with eigenvalues greater than 1 are considered significant and are thus retained. The variables used for this analysis were EC, Ca2+, Mg2+, Na+, K+, Cl-, SO42- and HCO3- in 73 observations (wells). Furthermore, the seawater fraction was used to study the ocean intrusion in the wells. This fraction is often estimated in the groundwater using chloride concentration since this ion has been considered as a conservative tracer, not affected by ion exchange (Custodio and Bruggeman 1987). It is calculated as follows (Appelo and Postma 2005):
f sea
CCl , sample CCl , fresh CCl , sea CCl , fresh
(Eq.1)
ACCEPTED MANUSCRIPT where, CCl,sample is the Cl- concentration of the sample, CCl,sea is the Cl- concentration of the Atlantic Ocean and CCl,fresh represents the Cl- concentration of the fresh water. The fresh water sample was chosen considering the lowest measured value of the electrical conductivity (Slama et al 2010). In fact, Cl- ion is not usually removed from the system due to its high solubility (Appelo and Postma 1993). The only inputs are either from the aquifer matrix salts or from a salinization source like seawater intrusion, etc. (Kouzana et al 2009). Once calculated, the seawater fraction is used to calculate the theoretical concentration of each ion i resulting from the conservative mixing of seawater and the fresh water:
Ci,mix f sea x Ci,sea ( 1 f sea ) x Ci,fresh
(Eq.2)
where, Ci,sea and Ci,fresh are the concentration of the ion i of respectively seawater and fresh water. For each ion i, the difference between the concentration of the conservative mixing Ci,mix and the measured one Ci,sample simply represents the ionic deltas (Δ) (Fidelibus 2003) resulting from any chemical reaction occurring with mixing:
Ci Ci , sample Ci , mix
(Eq.3)
In addition, the calculation of these ionic deltas is important for determining and quantifying the hydrogeochemical processes and potential chemical reactions that take place in the aquifer (Slama et al 2010; Pulido-Leboeuf 2004; Grassi and Cortecci 2004). So, when ΔCi is positive, groundwater is getting enriched of ion i, whereas a negative value of ΔCi indicates a depletion of the ion compared to the theoretical mixing (Slama et al 2010; Andersen et al 2005).
3. Results and discussion Groundwater hydrochemical results are presented in Table 1. The charge balance error (CBE) (Eq. 4) was verified in order to justify the validity of the data. It was found that the data is within acceptable limits (less than 5% in all cases).
ACCEPTED MANUSCRIPT CBE = ((sum of cations - sum of anions)/ (sum of cations + sum of anions))*100 (%) (Eq.4) Moreover, the well sampling analysis shows a high electrical conductivity that varies between 1.72 mS/cm and 5.01 mS/cm with 61% of the samples having an EC greater than 3 mS/cm. Groundwater pH is ranging from 7.07 to 8.14 with an average of 7.57 showing its alkaline nature. Sodium and chloride range from 7.5 to 29 meq/L and 6.53 to 35.10 meq/L, respectively. Magnesium and Calcium range from 2.8 to 18.10 meq/L and 5.99 to 16.30 meq/L, respectively. The bicarbonate and sulfate concentrations vary from 1.56 to 4.92 meq/L and 4.28 to 21.78meq/L, respectively. Potassium is characterized by low concentrations compared to others cations; it is ranging from 0.05 to 0.25 with an average of 0.119 meq/L (Table 2). The relationship between various elements was studied using Pearson correlation matrix (Table 3). The correlation between EC and the elements revealed that the main contributors to the groundwater salinity are Ca2+, Mg2+, Na+, K+ and Cl- ions. The hierarchical cluster analysis showed that there are three main groups of wells in the study area (Fig 2). The majority of the first group, containing 28 wells, is located in Southwest in the first hydrogeological unit. The second group with 16 wells is located at the North of the area in the second hydrogeological unit. These two groups of wells sited in the coastal fringe, have an average conductivity of about 3.8 mS/cm and are characterized by the high concentrations of sodium and chloride as Table 4 shows. However, group 1 is more enriched by calcium, magnesium and sulfate than group 2. Concerning the third group, it contains 29 wells and is located in upstream with a lower average conductivity of about 2.3 mS/cm. This group has lower concentration of chloride and sodium than the two first groups of wells. However, the concentration of these two ions is still the dominant among the other elements. Concerning the PCA, the results show that the two first principal components, retained for this study, accounted for 84.11% of the total variance of the original variables. On one hand,
ACCEPTED MANUSCRIPT the analysis of the correlation circle shows that the studied variables are positively correlated to the first axis F1. In fact, the EC and the ions Ca2+, Mg2+, Na+, K+, Cl- and SO42-define this factor. Therefore, this axis represents the salinity degree of the studied wells and consequently differentiates well groups according to their salinity. On the other hand, it can be concluded from the second axis that F2 represents seawater intrusion. Actually, according to this axis, some wells are enriched with sodium, chloride while in opposition hardness (calcium and magnesium), sulfate and bicarbonates characterize others (Fig 3). This means that each group has different salinization origin: a marine origin for chloride and sodium and an intrinsic origin (dissolution of the reservoir rock). Regarding the projection of the wells on the principal plane of PCA, it shows a wide dispersion reflecting the variability in their physicochemical parameters (Fig 4). As the HCA showed before, three groups are identified. On one hand, the majority of the first and second groups of wells both are positively correlated to the first axis. Since F1 represents the salinity degree, these two first groups of wells are characterized by high salinity. However, the group 1 is richer in Ca2+, Mg2+, SO42- and HCO3- than the group 2 according to the second axis. This could be due to differences in hydrogeological characteristics of the area where each group is located. Moreover, the majority of the wells in these groups are near to the coastal fringe which indicate that there high salinity is due to eventual seawater intrusion towards the studied aquifer. On the other hand, the third group is formed of wells with low EC. In this case, the wells are located upstream in agriculture area more far from the sea and consequently are less contaminated. The obtained results showed that the groundwater quality is differently impacted. Firstly, it was found that the proximity of the wells to the sea is the main parameter influencing the composition of the studied wells. As the statistical results showed before, the wells situated in the coastal fringe are the most contaminated ones since they are characterized by high
ACCEPTED MANUSCRIPT concentration of sodium and chloride and high EC in comparison with those located upstream. Since the studied region is an agricultural area, the excessive pumping could be the major cause of seawater intrusion into the aquifer which leads to an increasing in the salinity of the groundwater. Moreover, the obtained group of wells showed that the differentiation in their ions composition could also be due to the hydrogeological characteristics. The concentration of major cations and anions was plotted in the Piper trilinear diagram, using the geochemical software Diagrammes, in order to evaluate the variation in hydrochemical facies. Figure 5 shows that the majority of the first and third groups of wells are plotted in the Ca-Mg-Cl type. The later propose an ion exchange process by which the cations from the aquifer are exchange by sodium from the groundwater. Regarding the second group, it is plotted in the Na-Cl type. This type of water and the cited exchange process are usually observed in seawater intrusion. The difference in the type of water between these groups could be due to the hydrogeological characteristics where they are located. In fact, the first and third group are situated in the same hydrogeological unit (unit 1) while the second group is located in the unit 2. It can also be observed from the plot that Cl- exceeds the other anions for the majority of wells samples while alkalis (Na+ and K+) exceed the alkaline earths (Ca2+, Mg2+) for group 2 although the majority of samples from group 1 and 3 fall into no dominant type. In order to confirm these results, the spatial distribution of different ions was studied by plotting the chemicals maps. Firstly, high values of EC, Na+ and Cl- are observed in the coastal fringe (first kilometers of the shore line) whereas low values are located in upstream (Fig 6 (b), (c), (d)). Since, the coastal fringe is the most contaminated area where the piezometric level is lower than the marine "zero" which promotes the sea intrusion. In these vulnerable areas to marine invasion, it seems that there is a reversal in the underground flows. Instead of table pouring into the Atlantic Ocean (its natural outlet), it is the marine waters that
ACCEPTED MANUSCRIPT flow to the aquifer, resulting a marine intrusion and salinization of groundwater (Fig. 6 (a)). Indeed, the piezometric map shows the existence of isopiestic 0 m inland. This confirms that in the areas between the shoreline and the isopiestic 0 m, it is the marine waters that flow to the groundwater. In order to assess the suitability of groundwater for irrigation purposes, Sodium Adsorption Ratio (SAR) and percent sodium (Na %) were calculated. Sodium is an important ion used for the classification of irrigation water due to its reaction with soil. When the concentration of sodium ion is high in irrigation water, Na+ tends to be absorbed by clay particles, displacing magnesium and calcium ions. This exchange process of sodium in water for Ca2+ and Mg2+ in soil reduces the permeability. The Na+% are calculated as follows (Todd 1980): Na K Na % 2 2 Ca Mg Na K
x 100
(Eq.4)
where the ions concentrations are expressed in meq/L. The sodium percentage in the study area varies from 32.46% to 65.51%. Wilcox (1955) graph based on sodium percentage (Na%) was used to assess groundwater quality. As Figure 7 (a) shows, wells from group 1 and 2 are considered as unsuitable water with high conductivity. Concerning wells from group3, they are characterized by lower conductivity with doubtful to unsuitable water. SAR is also a factor which determines the suitability of water for irrigation. More this factor is higher; less suitable the groundwater is for irrigation. The SAR is calculated as follows (Richards 1954): SAR
Na Ca 2 Mg 2 2
(Eq.5)
where the ions concentrations are expressed in meq/L. The value of SAR in the groundwater samples of the study area ranges from 3.05 to 9.89. Moreover, as Figure 7 (b) shows, group1 and 2 are belong to C4S2 and C4S3 classes. Consequently, these groundwater groups are
ACCEPTED MANUSCRIPT unsuitable for irrigation purposes. Concerning group 3, it’s in C4S2, C3S1 and C3S2 classes. Therefore, this group is less contaminated as the results showed before. The obtained results showed that the groundwater quality is differently impacted. The mineralization of the coastal aquifer can be caused by several possible salinization processes such as the mixing of the freshwater with seawater, water-rock interaction or anthropic sources (agriculture contamination). Correlations between major ions concentrations with chloride content help in understanding the possible water-rock interaction and mixing processes (Marie and Vengosh 2001). The concentration of major chemical species versus chloride are plotted and shown in Fig. 8. The relation between Na+ and Cl- (Fig. 8(a)) shows that most of the groundwater samples from the study area plot below and along the saltwater–freshwater mixing line. This shows that there is ions exchange occurring in the aquifer that leads to Na+ eliminating. Concerning the other ions, they are differently distributed with respect to the group of wells and location. In unit 1, it can be seen that for group 1, all ions are beyond de mixing line while for group 3 they are plotted near and around it. This result confirms the effect of the wells location near to the sea. In unit 2, that are located near to the industrial area with less agricultural activities and with different hydrogeological characteristics, it can be seen that the ions of group 3 plot near (Mg2+) or under (SO42- and Ca2+) the mixing line (Fig. 8 (b), (c), and (d)). So that the hydrochemical processes taking place in the wells water could be determined, the ionic deltas were plotted (Fig 9). It could be concluded that the wells from group 1 and 3, with the same hydrogeological characteristics, are enriched with Ca2+, Mg2+, and SO42-ions (Fig. 9 (b), (c), (d)). Moreover, wells from group 1 are more enriched with these ions since this group is the most contaminated one. However, concerning group 2, the depletion of Ca2+ and SO42ions could be explain by the hydrogeological characteristics of the area which is different from the one where the wells from group 1 and 3 are located.
ACCEPTED MANUSCRIPT The enrichment of these wells with these ions suggests other possible salinization process with seawater intrusion. The depletion of Na+ (Fig. 9 (a)), which have negative ionic delta, suggests a direct cation exchange usually observed in similar situations when the saltwater is replacing fresh water (Appelo and Postma, 2005). In order to confirm this result, Ca2++Mg2+ in function of HCO3-+SO4 2- was plotted. The ionic exchange in groundwater due to seawater intrusion could be concluded when there is an enrichment of Ca2++Mg2+ in comparison to HCO3-+SO42- (Najib et al., 2016 ;Wanda et al., 2011; Nasher et al., 2013; Fadili et al., 2015). Fig. 10 (a) shows that wells samples are plotted above the equilibrium line. Moreover, a negative slope between [(Ca2++Mg2+) - (HCO3-+SO42-)] and [Na++K+-Cl-] can also confirm the existence of ionic exchange (Wanda et al., 2011; Nasher et al., 2013; Najib et al. 2016). Fig. 10 (b) shows that the majority of samples are negatively correlated; subsequently Ca2+ and Mg2+ enrichment and Na+ depletion is caused by cation exchange.
4. Conclusion In the studied region, the groundwater quality is starting to be affected due to salinization. Several methods were used in order to define the origin of the aquifer pollution: hydrogeology, hydrochemistry, statistical treatments, ionic spatial distribution and binary diagrams. All of these tools and their combination have shown a better adaptation to our problem, allowing us to check our initial hypotheses on the potential sources of the high salinity of groundwater in the region between Sidi Abed and Oulad Ghanem (Province of El Jadida, Morocco). To this end, 73 wells distributed throughout this region, were analyzed and monitored. Statistical analysis has shown that the wells are differently affected depending on their location and the characteristics of the area. As for the study of salinization processes in the area, it has been clearly demonstrated that the salinization of the aquifer is mainly caused by marine intrusion, and secondarily by the dissolution of the aquifer rock. Consequently, the
ACCEPTED MANUSCRIPT concentrations of the ions in the groundwater vary due to cation exchange. In order to avoid the damage that this pollution can do to the irrigated land and to human and animal health, cares should be taken in order to properly exploit this natural water resource.
Acknowledgements This work was carried out with the support of scholarship awarded by the National Center for Scientific and Technical Research (CNRST) – Morocco.
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Table 1. Average values of physicochemical characteristics of groundwater for 73 wells studied. Wells
Ca2+ (meq/L)
Mg2+ (meq/L)
Na+ (meq/L)
K+ (meq/L)
HCO3(meq/L)
Cl(meq/L)
SO42(meq/L)
EC (mS/cm)
pH
Wells
Ca2+ (meq/L)
Mg2+ (meq/L)
Na+ (meq/L)
K+ (meq/L)
HCO3(meq/L)
Cl(meq/L)
SO42(meq/L)
EC (mS/cm)
pH
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
6.80 8.00 7.60 7.60 7.98 8.10 8.00 8.18 8.54 8.44 6.19 6.09 7.54 7.39 7.24 6.70 7.24 7.49 8.74 10.50 9.38 6.49 16.30 14.30 13.80 10.53 8.40 7.30 5.99 7.40 6.40 7.20 7.70 13.10 15.40 16.20
7.60 6.40 5.20 2.80 3.54 7.90 8.80 8.64 9.97 10.05 7.58 7.74 7.25 7.00 6.59 6.90 6.68 9.22 9.22 11.10 10.70 7.25 18.10 15.30 14.20 14.66 9.20 7.10 4.53 7.80 8.00 7.20 7.50 13.30 14.20 14.20
18.50 17.50 11.50 9.01 11.01 15.50 21.50 21.50 24.00 29.00 20.50 26.02 16.02 19.01 18.50 13.50 19.01 24.02 25.01 16.01 15.01 13.63 21.50 22.50 14.01 12.01 13.01 11.02 10.01 14.97 11.50 10.50 12.01 22.02 27.01 24.01
0.15 0.10 0.10 0.10 0.10 0.15 0.20 0.15 0.20 0.20 0.20 0.25 0.15 0.15 0.15 0.10 0.15 0.15 0.15 0.15 0.10 0.15 0.15 0.15 0.10 0.10 0.10 0.10 0.05 0.10 0.05 0.05 0.10 0.15 0.15 0.15
3.61 3.12 3.45 2.80 3.13 1.64 3.77 3.64 3.51 3.30 3.20 1.56 3.04 3.25 3.16 3.12 3.45 3.47 3.12 4.10 3.77 4.35 4.84 4.92 4.59 3.45 3.35 3.20 3.36 3.20 3.12 2.95 3.45 3.53 3.78 3.69
21.63 19.63 12.02 11.22 13.97 19.23 25.24 25.16 28.24 35.10 24.12 30.78 18.37 21.75 21.30 14.82 21.47 28.52 29.76 17.63 16.42 16.11 27.64 26.04 16.03 12.74 15.22 11.62 11.03 17.23 12.02 10.82 14.03 26.44 32.05 28.44
5.87 7.79 7.23 4.28 4.61 8.68 7.46 7.52 9.15 7.81 5.80 6.29 7.85 7.01 6.97 7.61 6.21 7.11 8.55 14.57 12.88 5.04 21.78 18.69 20.42 19.96 11.10 9.71 5.05 8.52 9.98 10.46 8.88 17.19 19.54 21.10
3.30 2.66 2.03 1.90 1.90 2.68 3.90 3.70 4.10 4.20 4.01 5.01 3.30 3.90 3.75 2.44 3.35 4.30 4.03 3.31 3.23 3.31 4.04 3.84 3.19 3.11 2.49 2.11 2.13 2.79 1.95 2.02 2.32 3.94 4.70 4.37
7.65 7.67 7.60 7.72 7.64 7.07 7.70 7.54 7.65 7.43 7.55 7.22 7.88 8.01 7.83 7.25 7.44 7.51 7.59 7.66 7.49 7.36 7.33 7.30 7.34 7.28 7.28 7.55 7.53 7.59 7.64 7.53 7.42 7.38 8.14 7.42
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73
15.97 14.17 6.35 7.20 7.10 9.80 7.14 7.04 7.69 9.03 6.49 7.74 13.90 13.33 13.00 13.23 12.98 11.93 11.23 11.98 14.43 12.98 9.48 12.48 10.98 8.98 8.20 12.48 10.48 11.00 10.00 6.75 14.47 10.00 10.48 9.48 10.99
11.94 12.76 6.45 5.60 7.30 10.20 7.33 6.34 8.23 8.31 8.98 3.38 14.90 13.17 11.00 13.59 12.51 14.41 15.64 14.82 10.87 10.70 8.32 10.70 4.94 6.42 8.60 11.11 7.41 13.00 8.40 5.35 10.71 10.80 9.88 11.53 9.06
23.50 24.01 12.50 12.01 16.97 25.01 16.50 16.01 16.50 17.01 15.50 7.50 23.01 22.50 18.50 24.01 24.02 26.01 27.50 26.01 22.50 18.50 15.01 17.01 12.50 13.01 12.50 16.01 11.02 20.01 16.01 7.50 24.01 20.01 19.50 21.01 14.50
0.15 0.15 0.10 0.05 0.10 0.15 0.15 0.10 0.10 0.10 0.10 0.05 0.15 0.15 0.10 0.15 0.10 0.15 0.15 0.15 0.10 0.10 0.05 0.10 0.05 0.05 0.05 0.10 0.05 0.10 0.05 0.05 0.15 0.10 0.10 0.15 0.10
3.82 4.02 3.12 3.25 3.45 3.86 3.12 2.87 2.55 3.45 3.53 2.22 3.94 3.70 3.12 4.59 3.86 3.94 2.87 2.55 2.79 2.71 2.55 4.10 3.61 2.55 2.87 2.95 2.63 3.28 4.10 2.46 3.04 3.61 2.55 2.30 2.39
27.96 28.25 13.22 12.42 20.03 29.64 19.79 18.77 18.65 20.07 16.96 6.53 26.84 26.27 22.43 28.25 28.53 31.07 32.48 30.79 26.56 21.20 16.93 18.94 13.87 14.15 13.22 17.80 11.61 23.23 17.63 7.10 28.53 22.83 22.89 24.01 17.25
18.43 17.36 7.29 7.97 6.92 9.64 6.84 6.83 10.16 9.67 8.80 9.38 19.92 17.74 15.89 16.60 15.52 16.04 17.62 18.27 17.92 16.84 12.97 15.46 10.51 10.34 11.52 17.08 14.27 15.09 10.89 9.59 17.36 12.28 12.58 14.09 13.66
3.95 3.88 2.16 2.01 2.86 4.21 2.98 2.49 2.66 2.79 2.37 1.72 4.01 3.89 3.34 4.12 4.04 4.21 4.35 4.28 3.89 3.44 2.65 3.52 2.12 2.31 2.17 3.51 2.15 3.66 2.65 1.72 4.11 3.52 3.74 3.91 1.35
7.42 7.41 7.54 7.85 7.41 7.37 7.55 7.67 7.74 7.62 7.49 7.79 7.31 7.34 7.37 7.37 7.48 7.49 7.53 7.49 7.75 7.51 7.81 7.63 7.81 7.82 7.71 7.70 7.77 7.65 7.67 7.92 7.71 7.62 7.84 7.88 2.88
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Table 2. Descriptive statistics. Variable pH EC (mS/cm) Ca2+ (meq/L) Mg2+ (meq/L) Na+ (meq/L) K+ (meq/L) HCO3- (meq/L) Cl- (meq/L) SO42- (meq/L)
Observations Minimum Maximum 73 7.07 8.14 73 1.72 5.01 73 5.99 16.30 73 2.80 18.10 73 7.50 29.00 73 0.05 0.25 73 1.56 4.92 73 6.53 35.10 73 4.28 21.78
Average Standard deviation 7.575 0.202 3.227 0.846 9.743 2.900 9.427 3.274 17.898 5.360 0.119 0.044 3.306 0.648 20.692 6.817 11.753 4.835
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Table 3. Correlation matrix (Pearson n). Variables EC ClHCO3Na+ K+ Mg2+ Ca2+ SO42-
EC
Cl- HCO31 0.934 0.267 1 0.243 1
Na+ 0.937 0.995 0.241 1
K+ 0.797 0.762 0.164 0.749 1
Mg2+ 0.705 0.664 0.459 0.668 0.378 1
Ca2+ 0.527 0.510 0.408 0.505 0.141 0.796 1
SO420.448 0.387 0.316 0.398 0.041 0.872 0.924 1
Table 4. Group’s barycenter. Groups 1 2 3
Cl(meq/L) 24,73 24,81 14,53
HCO3Na+ (meq/L) (meq/L) 3,59 21,14 3,34 21,12 3,02 13,01
K+ Mg2+ (meq/L) (meq/L) 0,13 12,85 0,17 8,19 0,08 6,80
Ca2+ (meq/L) 12,82 7,58 7,96
SO42CE (meq/L) (mS/cm) 17,08 3,83 7,19 3,83 9,13 2,31
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Fig. 1. Map of spatial distribution of studied wells located between Sidi-Abed and OuledGhanem.
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Group 1 Group 3
Group 2
Fig. 2. Dendrogram of the cluster analysis between wells.
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Fig. 3. Correlation circle (axes F1 and F2).
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Fig. 4. Plot of axis F1 versus F2 (projection of the wells).
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Fig. 5. Piper diagram of wells samples.
ACCEPTED MANUSCRIPT 280
280
Sidi Abed
Sidi Abed
(a)
(c)
275
275
270
270
Sidi Moussa
265
Sens d'écoulement de la nappe.
12
260
Ouled Ghanem
Sidi Moussa
265
isopièze 12 m.
Equidistance = 2 m.
Echelle : 0
850
260
Ouled Ghanem
Equidistance = 100 mg/l.
Echelle : 0
2 km
255
isochlorures 850 mg/l.
2 km
255 175
180
185
190
280
195
175
185
190
280
Sidi Abed
195
Sidi Abed
(d)
(b) 275
275
270
270
Sidi Moussa
265
180
Sidi Moussa
265
3.2
Ouled
isoconductivité électrique 3.2 mS/cm.
Equidistance = 0.4 mS/cm.
260 Ghanem
Echelle : 0
450
260
Ouled Ghanem
isosodium 450 mg/l.
Equidistance = 50 mg/l.
2 km
Echelle : 0
2 km
255
255 175
180
185
190
195
175
180
185
190
195
Fig 6. Spatial distribution: (a) Flow direction of the water table; (b) Electrical conductivity; (c) Chloride concentration; (d) Sodium concentration.
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(a)
(b)
Fig. 7. Groundwater assessing for irrigation purpose: (a) Plot of Na%; (b) Plot of SAR.
ACCEPTED MANUSCRIPT (b)
(a)
(c)
(d)
Fig. 8. (a) Na+/Cl- relationship; (b) Ca2+/ Cl- relationship; (c) Mg2+/Cl- relationship; and (d) SO42-/Cl- relationship.
ACCEPTED MANUSCRIPT (a)
(b)
(c)
(d)
Fig. 9. (a) ΔNa+, (b) ΔCa2+, (c) ΔMg2+ and (d) ΔSO22- versus chloride content for wells of the three groups.
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(a)
(b)
Fig. 10. (a) Ca2++Mg2+ versus HCO3 - +SO42- ; (b) (Ca2++Mg2+)-( HCO3 - +SO4 2-) versus [Na++K+ – Cl-]
ACCEPTED MANUSCRIPT Highlights
- The salinity of groundwater in the region Sidi AbedOulad Ghanem was evaluated - The origin of groundwater salinization in this coastal aquifer was studied - Several tools and methods were applied to draw relevant conclusions - The salinization is mainly due to marine intrusion and aquifer rock dissolution - Some cares should be taken in order to properly exploit this natural water resource
Sara Mountadar, Abdelkader Younsi, Abdelkader Hayani, Mustapha Siniti, Soufiane Tahiri PII:
S1464-343X(18)30180-8
DOI:
10.1016/j.jafrearsci.2018.06.025
Reference:
AES 3252
To appear in:
Journal of African Earth Sciences
Received Date:
22 January 2018
Accepted Date:
04 June 2018
Please cite this article as: Sara Mountadar, Abdelkader Younsi, Abdelkader Hayani, Mustapha Siniti, Soufiane Tahiri, Groundwater salinization process in the coastal aquifer Sidi Abed-Ouled Ghanem (Province of El Jadida, Morocco), Journal of African Earth Sciences (2018), doi: 10.1016/j. jafrearsci.2018.06.025
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ACCEPTED MANUSCRIPT
Groundwater salinization process in the coastal aquifer Sidi Abed-Ouled Ghanem (Province of El Jadida, Morocco)
Short title: Coastal groundwater salinization
Sara Mountadar 1,*, Abdelkader Younsi 2, Abdelkader Hayani 1, Mustapha Siniti 3, Soufiane Tahiri 1
1 Laboratory
of Water and Environment, Department of Chemistry, Faculty of Sciences of El
Jadida, University Chouaïb Doukkali, P.O. Box 20, El Jadida (24000), Morocco. 2
Laboratory of Water and Hydrogeology, Department of Geology, Faculty of Sciences of El Jadida, University Chouaïb Doukkali, P.O. Box 20, El Jadida (24000), Morocco. 3
Team of Thermodynamic, Catalysis and Surfaces, Department of Chemistry, Faculty of Sciences of El Jadida, University Chouaïb Doukkali, P.O. Box 20, El Jadida (24000), Morocco.
*Corresponding Author: [email protected]
ACCEPTED MANUSCRIPT Abstract In this work, the salinization mechanisms were highlighted in the coastal area between Sidi Abed and Ouled Ghanem (El Jadida Province, Morocco). This study was based on analyzing and discussing the physicochemical data of water samples from 73 wells distributed in the whole studied region. Firstly, the results of the principal component analysis (PCA) and hierarchical cluster analysis (HCA) showed that the study area is composed of three main groups. This classification is firstly based on the proximity of the wells to the seawater. In fact, it was found that the first and second groups of wells which are located in the coastal fringe are characterized by high concentration of sodium and chloride and EC values whereas lower values are found in the third group that is located in upstream. Moreover, the results also showed the influence of the hydrogeological characteristics of the area where the wells are located on the composition of the water samples. The evaluation of the hydrochemical facies using Piper trilinear diagram have also confirmed these results and showed the probable contribution of seawater intrusion in the modification of wells composition by ion exchange process and by increasing the concentration of sodium and chloride. Then, the groundwater quality were assessed using Sodium Adsorption Ratio (SAR) and sodium percentage (Na %) parameters. In order to define the salinization processes occurring in the aquifer, the binary diagrams between chloride and the other elements were plotted. It was found that the groundwater is contaminated by seawater intrusion which is accompanied by ions exchange depending on the hydrogeological characteristics of the area. Keywords: Coastal aquifer; Groundwater; Hydrogeology; Salinization; Salt intrusion; Statistic analysis, Sahel Doukkala, Morocco.
ACCEPTED MANUSCRIPT 1. Introduction
Salinity always exists in groundwater. The type and quantity of salts reflect the environment, movement and source of groundwater (Bowen 1986). However, coastal aquifers are increasingly threatened by saline intrusion due to groundwater exploitation and global sea level rise (Tomaszkiewicz et al 2014). Several researchers have studied the origin of salinity in coastal location by analyzing the hydrochemical results and by understanding the geochemical process behind this pollution (Mdiker et al 2009; Hamzaoui Azaza et al 2012; Zghibi et al 2013, Hayani 2014). Sahel coastal aquifer (El Jadida Province), like many other semi-arid regions, relies heavily on its pluviometric potential. Hence, water resources in the study area are limited. The coastal aquifer is affected by intensive pumping for irrigation and drinking water. Consequently, the fresh water level is lowered and the ocean intrudes further into the aquifer. In order to study the probable salinity intrusion in the study area, sampling of groundwater and physicochemical analysis were carried out. Due to existence of different factor that could impact the salinization processes, different methods were used: the principal component analysis (PCA), hierarchical cluster analysis (HCA), Piper trilinear diagram, the spatial distributions of ions and the binary diagrams using the seawater fraction.
2. Materials and Methods
2.1. Study area The study area is a part of Sahel basin which belongs to Meseta Western Morocco and lies between latitudes 33°50’N and 32°10’N and longitudes 8°60’E and 8°30’E. It is bounded by the Atlantic Ocean in the West and North, by Sidi Abed locality in the Northeast and Jamaat
ACCEPTED MANUSCRIPT Ouled Ghanem in the Southwest. The sector is about 120 km2, 4 km far from the Ocean and extends over a length of 30 km. The zone climate is semiarid with hot and dry summer (MayNovember) and humid winter (November-April). The Sahel area is a part of the geological unit known as the Moroccan coastal meseta. It contains sub-tubular sedimentary series from Mesozoic and Cenozoic era that are based on Paleozoic land pleated during the Hercynian orogeny (Gigout 1951; Ouadia 1998). Regarding the hydrogeology, there are two units in the study area. The unit 1 consists of a table circulating in Pliocene-Quaternary calcarenite and in limestone of the lower Cretaceous. The unit 2 is composed of water table circulating in the upper Cretaceous consisting of fissured to karstified limestone (Chtaini 1987; Aboumaria 1993). This table is fed from surface infiltration. The human activities in the coastal strip are characterized by the dominance of vegetable growing. Its irrigation is done by pumping water from water table. Cattle breading is also found in this zone. These whole activities require more and more water and then push farmers and ranchers to dig more wells where the pumping is intensive.
2.2. Sampling method and water analysis The choice of the measuring network was based on a mesh of the study area with one to two wells for each km2 with a relatively higher density near the ocean (Fig. 1). In total, 73 wells were chosen. These water points were the subject of many periodic piezometric surveys over the years 2011, 2012 and 2013. These campaigns were carried out during high tide (March April) and low tide (August September) periods, biannually. To this end, a number of devices and documents were used: a Global Positioning System GPS (Garmin nuvi 2008), a piezometric probe (100 m length) and topographic maps 1/50 000e.
ACCEPTED MANUSCRIPT Regarding the physicochemical analysis, it was based on samples carried out in 73 wells. Many periodic campaigns of sampling were fulfilled from September 2011 to December 2013, biannually. The whole sampling were in situ collected in 250 ml polyethylene bottles and conserved in low-temperature cooler and then in the laboratory refrigerator. The analyses carried out are electrical conductivity, pH and majors elements (Ca2+, Mg2+, Na+, K+, Cl-, SO42- and HCO3-). The physical parameters were periodically measured in situ using conductivity meter HACH (443600 model) and pH meter Best.-Nr. 100 787. Measure of Mg2+ and Ca2+ concentrations was performed using complexometry (AFNOR NTF 90-003) while Na+ and K+ ions were determined using a flame spectrophotometer. Concerning bicarbonates, and chloride, they were respectively titrated by volumetric method using sulfuric acid (H2SO4 0.02N) (AFNOR T 90-036) and silver nitrate (AgNO3 0.02N) (AFNOR T 90-014) as a titrand, while sulfates concentrations were measured using gravimetric method (AFNOR T 90-009).
2.3. Data analysis methods Groundwater hydrochemical groups were defined by Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA) using XLSTAT software. Hierarchical cluster analysis was performed for grouping groundwater samples in a way that each group or cluster is homogeneous with respect to certain characteristics and distinct from other clusters regarding the same characteristics (Davis 2002). In order to homogenize the data base, the later was centered and reduced. Then, the HCA dendrogram was obtained in the following way: the first step is to group the closest individuals into a class. This notion of proximity therefore presupposes the definition of a distance between two individuals that will be calculated. The Euclidean distance was used in this case. Then, the resulting classes were grouped with other classes until a single class gathers all the individuals. These stages of successive clustering of classes assume the definition of a second distance (called the
ACCEPTED MANUSCRIPT aggregation criterion) measuring the proximity not between two individuals but between two classes of individuals. There are several methods of aggregation. In this study, Ward's method was used. In addition, an automatic truncation has been generated which makes it possible to determine the number of homogeneous groups. After the HCA, the results were interpreted using PCA. Based on the principal component scores, PCA can examine multivariate relationship and explain the variance in the data while reducing the number of variable to several groups of individuals (Custodio and Bruggeman 1987). This technique is quite similar with the correlation or regression analysis methods, and can transform the data set, with many variables, into a set of comprehensive principle components. PCA allows a considerable reduction in the number of variables and the detection of structure in the relationships of different variables. Concerning the used PCA process, the variables were firstly normalized. Afterward, this method makes it possible to calculate new variables - linear combination of the initial variables - called principal components (factorial axes), which by synthesizing the information, highlight the interdependencies of the data. The choice of the number of factorial axes to study depends on the eigenvalues that accompany the PCA. The criteria adopted was the Kaiser criterion which says that axes with eigenvalues greater than 1 are considered significant and are thus retained. The variables used for this analysis were EC, Ca2+, Mg2+, Na+, K+, Cl-, SO42- and HCO3- in 73 observations (wells). Furthermore, the seawater fraction was used to study the ocean intrusion in the wells. This fraction is often estimated in the groundwater using chloride concentration since this ion has been considered as a conservative tracer, not affected by ion exchange (Custodio and Bruggeman 1987). It is calculated as follows (Appelo and Postma 2005):
f sea
CCl , sample CCl , fresh CCl , sea CCl , fresh
(Eq.1)
ACCEPTED MANUSCRIPT where, CCl,sample is the Cl- concentration of the sample, CCl,sea is the Cl- concentration of the Atlantic Ocean and CCl,fresh represents the Cl- concentration of the fresh water. The fresh water sample was chosen considering the lowest measured value of the electrical conductivity (Slama et al 2010). In fact, Cl- ion is not usually removed from the system due to its high solubility (Appelo and Postma 1993). The only inputs are either from the aquifer matrix salts or from a salinization source like seawater intrusion, etc. (Kouzana et al 2009). Once calculated, the seawater fraction is used to calculate the theoretical concentration of each ion i resulting from the conservative mixing of seawater and the fresh water:
Ci,mix f sea x Ci,sea ( 1 f sea ) x Ci,fresh
(Eq.2)
where, Ci,sea and Ci,fresh are the concentration of the ion i of respectively seawater and fresh water. For each ion i, the difference between the concentration of the conservative mixing Ci,mix and the measured one Ci,sample simply represents the ionic deltas (Δ) (Fidelibus 2003) resulting from any chemical reaction occurring with mixing:
Ci Ci , sample Ci , mix
(Eq.3)
In addition, the calculation of these ionic deltas is important for determining and quantifying the hydrogeochemical processes and potential chemical reactions that take place in the aquifer (Slama et al 2010; Pulido-Leboeuf 2004; Grassi and Cortecci 2004). So, when ΔCi is positive, groundwater is getting enriched of ion i, whereas a negative value of ΔCi indicates a depletion of the ion compared to the theoretical mixing (Slama et al 2010; Andersen et al 2005).
3. Results and discussion Groundwater hydrochemical results are presented in Table 1. The charge balance error (CBE) (Eq. 4) was verified in order to justify the validity of the data. It was found that the data is within acceptable limits (less than 5% in all cases).
ACCEPTED MANUSCRIPT CBE = ((sum of cations - sum of anions)/ (sum of cations + sum of anions))*100 (%) (Eq.4) Moreover, the well sampling analysis shows a high electrical conductivity that varies between 1.72 mS/cm and 5.01 mS/cm with 61% of the samples having an EC greater than 3 mS/cm. Groundwater pH is ranging from 7.07 to 8.14 with an average of 7.57 showing its alkaline nature. Sodium and chloride range from 7.5 to 29 meq/L and 6.53 to 35.10 meq/L, respectively. Magnesium and Calcium range from 2.8 to 18.10 meq/L and 5.99 to 16.30 meq/L, respectively. The bicarbonate and sulfate concentrations vary from 1.56 to 4.92 meq/L and 4.28 to 21.78meq/L, respectively. Potassium is characterized by low concentrations compared to others cations; it is ranging from 0.05 to 0.25 with an average of 0.119 meq/L (Table 2). The relationship between various elements was studied using Pearson correlation matrix (Table 3). The correlation between EC and the elements revealed that the main contributors to the groundwater salinity are Ca2+, Mg2+, Na+, K+ and Cl- ions. The hierarchical cluster analysis showed that there are three main groups of wells in the study area (Fig 2). The majority of the first group, containing 28 wells, is located in Southwest in the first hydrogeological unit. The second group with 16 wells is located at the North of the area in the second hydrogeological unit. These two groups of wells sited in the coastal fringe, have an average conductivity of about 3.8 mS/cm and are characterized by the high concentrations of sodium and chloride as Table 4 shows. However, group 1 is more enriched by calcium, magnesium and sulfate than group 2. Concerning the third group, it contains 29 wells and is located in upstream with a lower average conductivity of about 2.3 mS/cm. This group has lower concentration of chloride and sodium than the two first groups of wells. However, the concentration of these two ions is still the dominant among the other elements. Concerning the PCA, the results show that the two first principal components, retained for this study, accounted for 84.11% of the total variance of the original variables. On one hand,
ACCEPTED MANUSCRIPT the analysis of the correlation circle shows that the studied variables are positively correlated to the first axis F1. In fact, the EC and the ions Ca2+, Mg2+, Na+, K+, Cl- and SO42-define this factor. Therefore, this axis represents the salinity degree of the studied wells and consequently differentiates well groups according to their salinity. On the other hand, it can be concluded from the second axis that F2 represents seawater intrusion. Actually, according to this axis, some wells are enriched with sodium, chloride while in opposition hardness (calcium and magnesium), sulfate and bicarbonates characterize others (Fig 3). This means that each group has different salinization origin: a marine origin for chloride and sodium and an intrinsic origin (dissolution of the reservoir rock). Regarding the projection of the wells on the principal plane of PCA, it shows a wide dispersion reflecting the variability in their physicochemical parameters (Fig 4). As the HCA showed before, three groups are identified. On one hand, the majority of the first and second groups of wells both are positively correlated to the first axis. Since F1 represents the salinity degree, these two first groups of wells are characterized by high salinity. However, the group 1 is richer in Ca2+, Mg2+, SO42- and HCO3- than the group 2 according to the second axis. This could be due to differences in hydrogeological characteristics of the area where each group is located. Moreover, the majority of the wells in these groups are near to the coastal fringe which indicate that there high salinity is due to eventual seawater intrusion towards the studied aquifer. On the other hand, the third group is formed of wells with low EC. In this case, the wells are located upstream in agriculture area more far from the sea and consequently are less contaminated. The obtained results showed that the groundwater quality is differently impacted. Firstly, it was found that the proximity of the wells to the sea is the main parameter influencing the composition of the studied wells. As the statistical results showed before, the wells situated in the coastal fringe are the most contaminated ones since they are characterized by high
ACCEPTED MANUSCRIPT concentration of sodium and chloride and high EC in comparison with those located upstream. Since the studied region is an agricultural area, the excessive pumping could be the major cause of seawater intrusion into the aquifer which leads to an increasing in the salinity of the groundwater. Moreover, the obtained group of wells showed that the differentiation in their ions composition could also be due to the hydrogeological characteristics. The concentration of major cations and anions was plotted in the Piper trilinear diagram, using the geochemical software Diagrammes, in order to evaluate the variation in hydrochemical facies. Figure 5 shows that the majority of the first and third groups of wells are plotted in the Ca-Mg-Cl type. The later propose an ion exchange process by which the cations from the aquifer are exchange by sodium from the groundwater. Regarding the second group, it is plotted in the Na-Cl type. This type of water and the cited exchange process are usually observed in seawater intrusion. The difference in the type of water between these groups could be due to the hydrogeological characteristics where they are located. In fact, the first and third group are situated in the same hydrogeological unit (unit 1) while the second group is located in the unit 2. It can also be observed from the plot that Cl- exceeds the other anions for the majority of wells samples while alkalis (Na+ and K+) exceed the alkaline earths (Ca2+, Mg2+) for group 2 although the majority of samples from group 1 and 3 fall into no dominant type. In order to confirm these results, the spatial distribution of different ions was studied by plotting the chemicals maps. Firstly, high values of EC, Na+ and Cl- are observed in the coastal fringe (first kilometers of the shore line) whereas low values are located in upstream (Fig 6 (b), (c), (d)). Since, the coastal fringe is the most contaminated area where the piezometric level is lower than the marine "zero" which promotes the sea intrusion. In these vulnerable areas to marine invasion, it seems that there is a reversal in the underground flows. Instead of table pouring into the Atlantic Ocean (its natural outlet), it is the marine waters that
ACCEPTED MANUSCRIPT flow to the aquifer, resulting a marine intrusion and salinization of groundwater (Fig. 6 (a)). Indeed, the piezometric map shows the existence of isopiestic 0 m inland. This confirms that in the areas between the shoreline and the isopiestic 0 m, it is the marine waters that flow to the groundwater. In order to assess the suitability of groundwater for irrigation purposes, Sodium Adsorption Ratio (SAR) and percent sodium (Na %) were calculated. Sodium is an important ion used for the classification of irrigation water due to its reaction with soil. When the concentration of sodium ion is high in irrigation water, Na+ tends to be absorbed by clay particles, displacing magnesium and calcium ions. This exchange process of sodium in water for Ca2+ and Mg2+ in soil reduces the permeability. The Na+% are calculated as follows (Todd 1980): Na K Na % 2 2 Ca Mg Na K
x 100
(Eq.4)
where the ions concentrations are expressed in meq/L. The sodium percentage in the study area varies from 32.46% to 65.51%. Wilcox (1955) graph based on sodium percentage (Na%) was used to assess groundwater quality. As Figure 7 (a) shows, wells from group 1 and 2 are considered as unsuitable water with high conductivity. Concerning wells from group3, they are characterized by lower conductivity with doubtful to unsuitable water. SAR is also a factor which determines the suitability of water for irrigation. More this factor is higher; less suitable the groundwater is for irrigation. The SAR is calculated as follows (Richards 1954): SAR
Na Ca 2 Mg 2 2
(Eq.5)
where the ions concentrations are expressed in meq/L. The value of SAR in the groundwater samples of the study area ranges from 3.05 to 9.89. Moreover, as Figure 7 (b) shows, group1 and 2 are belong to C4S2 and C4S3 classes. Consequently, these groundwater groups are
ACCEPTED MANUSCRIPT unsuitable for irrigation purposes. Concerning group 3, it’s in C4S2, C3S1 and C3S2 classes. Therefore, this group is less contaminated as the results showed before. The obtained results showed that the groundwater quality is differently impacted. The mineralization of the coastal aquifer can be caused by several possible salinization processes such as the mixing of the freshwater with seawater, water-rock interaction or anthropic sources (agriculture contamination). Correlations between major ions concentrations with chloride content help in understanding the possible water-rock interaction and mixing processes (Marie and Vengosh 2001). The concentration of major chemical species versus chloride are plotted and shown in Fig. 8. The relation between Na+ and Cl- (Fig. 8(a)) shows that most of the groundwater samples from the study area plot below and along the saltwater–freshwater mixing line. This shows that there is ions exchange occurring in the aquifer that leads to Na+ eliminating. Concerning the other ions, they are differently distributed with respect to the group of wells and location. In unit 1, it can be seen that for group 1, all ions are beyond de mixing line while for group 3 they are plotted near and around it. This result confirms the effect of the wells location near to the sea. In unit 2, that are located near to the industrial area with less agricultural activities and with different hydrogeological characteristics, it can be seen that the ions of group 3 plot near (Mg2+) or under (SO42- and Ca2+) the mixing line (Fig. 8 (b), (c), and (d)). So that the hydrochemical processes taking place in the wells water could be determined, the ionic deltas were plotted (Fig 9). It could be concluded that the wells from group 1 and 3, with the same hydrogeological characteristics, are enriched with Ca2+, Mg2+, and SO42-ions (Fig. 9 (b), (c), (d)). Moreover, wells from group 1 are more enriched with these ions since this group is the most contaminated one. However, concerning group 2, the depletion of Ca2+ and SO42ions could be explain by the hydrogeological characteristics of the area which is different from the one where the wells from group 1 and 3 are located.
ACCEPTED MANUSCRIPT The enrichment of these wells with these ions suggests other possible salinization process with seawater intrusion. The depletion of Na+ (Fig. 9 (a)), which have negative ionic delta, suggests a direct cation exchange usually observed in similar situations when the saltwater is replacing fresh water (Appelo and Postma, 2005). In order to confirm this result, Ca2++Mg2+ in function of HCO3-+SO4 2- was plotted. The ionic exchange in groundwater due to seawater intrusion could be concluded when there is an enrichment of Ca2++Mg2+ in comparison to HCO3-+SO42- (Najib et al., 2016 ;Wanda et al., 2011; Nasher et al., 2013; Fadili et al., 2015). Fig. 10 (a) shows that wells samples are plotted above the equilibrium line. Moreover, a negative slope between [(Ca2++Mg2+) - (HCO3-+SO42-)] and [Na++K+-Cl-] can also confirm the existence of ionic exchange (Wanda et al., 2011; Nasher et al., 2013; Najib et al. 2016). Fig. 10 (b) shows that the majority of samples are negatively correlated; subsequently Ca2+ and Mg2+ enrichment and Na+ depletion is caused by cation exchange.
4. Conclusion In the studied region, the groundwater quality is starting to be affected due to salinization. Several methods were used in order to define the origin of the aquifer pollution: hydrogeology, hydrochemistry, statistical treatments, ionic spatial distribution and binary diagrams. All of these tools and their combination have shown a better adaptation to our problem, allowing us to check our initial hypotheses on the potential sources of the high salinity of groundwater in the region between Sidi Abed and Oulad Ghanem (Province of El Jadida, Morocco). To this end, 73 wells distributed throughout this region, were analyzed and monitored. Statistical analysis has shown that the wells are differently affected depending on their location and the characteristics of the area. As for the study of salinization processes in the area, it has been clearly demonstrated that the salinization of the aquifer is mainly caused by marine intrusion, and secondarily by the dissolution of the aquifer rock. Consequently, the
ACCEPTED MANUSCRIPT concentrations of the ions in the groundwater vary due to cation exchange. In order to avoid the damage that this pollution can do to the irrigated land and to human and animal health, cares should be taken in order to properly exploit this natural water resource.
Acknowledgements This work was carried out with the support of scholarship awarded by the National Center for Scientific and Technical Research (CNRST) – Morocco.
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Table 1. Average values of physicochemical characteristics of groundwater for 73 wells studied. Wells
Ca2+ (meq/L)
Mg2+ (meq/L)
Na+ (meq/L)
K+ (meq/L)
HCO3(meq/L)
Cl(meq/L)
SO42(meq/L)
EC (mS/cm)
pH
Wells
Ca2+ (meq/L)
Mg2+ (meq/L)
Na+ (meq/L)
K+ (meq/L)
HCO3(meq/L)
Cl(meq/L)
SO42(meq/L)
EC (mS/cm)
pH
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
6.80 8.00 7.60 7.60 7.98 8.10 8.00 8.18 8.54 8.44 6.19 6.09 7.54 7.39 7.24 6.70 7.24 7.49 8.74 10.50 9.38 6.49 16.30 14.30 13.80 10.53 8.40 7.30 5.99 7.40 6.40 7.20 7.70 13.10 15.40 16.20
7.60 6.40 5.20 2.80 3.54 7.90 8.80 8.64 9.97 10.05 7.58 7.74 7.25 7.00 6.59 6.90 6.68 9.22 9.22 11.10 10.70 7.25 18.10 15.30 14.20 14.66 9.20 7.10 4.53 7.80 8.00 7.20 7.50 13.30 14.20 14.20
18.50 17.50 11.50 9.01 11.01 15.50 21.50 21.50 24.00 29.00 20.50 26.02 16.02 19.01 18.50 13.50 19.01 24.02 25.01 16.01 15.01 13.63 21.50 22.50 14.01 12.01 13.01 11.02 10.01 14.97 11.50 10.50 12.01 22.02 27.01 24.01
0.15 0.10 0.10 0.10 0.10 0.15 0.20 0.15 0.20 0.20 0.20 0.25 0.15 0.15 0.15 0.10 0.15 0.15 0.15 0.15 0.10 0.15 0.15 0.15 0.10 0.10 0.10 0.10 0.05 0.10 0.05 0.05 0.10 0.15 0.15 0.15
3.61 3.12 3.45 2.80 3.13 1.64 3.77 3.64 3.51 3.30 3.20 1.56 3.04 3.25 3.16 3.12 3.45 3.47 3.12 4.10 3.77 4.35 4.84 4.92 4.59 3.45 3.35 3.20 3.36 3.20 3.12 2.95 3.45 3.53 3.78 3.69
21.63 19.63 12.02 11.22 13.97 19.23 25.24 25.16 28.24 35.10 24.12 30.78 18.37 21.75 21.30 14.82 21.47 28.52 29.76 17.63 16.42 16.11 27.64 26.04 16.03 12.74 15.22 11.62 11.03 17.23 12.02 10.82 14.03 26.44 32.05 28.44
5.87 7.79 7.23 4.28 4.61 8.68 7.46 7.52 9.15 7.81 5.80 6.29 7.85 7.01 6.97 7.61 6.21 7.11 8.55 14.57 12.88 5.04 21.78 18.69 20.42 19.96 11.10 9.71 5.05 8.52 9.98 10.46 8.88 17.19 19.54 21.10
3.30 2.66 2.03 1.90 1.90 2.68 3.90 3.70 4.10 4.20 4.01 5.01 3.30 3.90 3.75 2.44 3.35 4.30 4.03 3.31 3.23 3.31 4.04 3.84 3.19 3.11 2.49 2.11 2.13 2.79 1.95 2.02 2.32 3.94 4.70 4.37
7.65 7.67 7.60 7.72 7.64 7.07 7.70 7.54 7.65 7.43 7.55 7.22 7.88 8.01 7.83 7.25 7.44 7.51 7.59 7.66 7.49 7.36 7.33 7.30 7.34 7.28 7.28 7.55 7.53 7.59 7.64 7.53 7.42 7.38 8.14 7.42
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73
15.97 14.17 6.35 7.20 7.10 9.80 7.14 7.04 7.69 9.03 6.49 7.74 13.90 13.33 13.00 13.23 12.98 11.93 11.23 11.98 14.43 12.98 9.48 12.48 10.98 8.98 8.20 12.48 10.48 11.00 10.00 6.75 14.47 10.00 10.48 9.48 10.99
11.94 12.76 6.45 5.60 7.30 10.20 7.33 6.34 8.23 8.31 8.98 3.38 14.90 13.17 11.00 13.59 12.51 14.41 15.64 14.82 10.87 10.70 8.32 10.70 4.94 6.42 8.60 11.11 7.41 13.00 8.40 5.35 10.71 10.80 9.88 11.53 9.06
23.50 24.01 12.50 12.01 16.97 25.01 16.50 16.01 16.50 17.01 15.50 7.50 23.01 22.50 18.50 24.01 24.02 26.01 27.50 26.01 22.50 18.50 15.01 17.01 12.50 13.01 12.50 16.01 11.02 20.01 16.01 7.50 24.01 20.01 19.50 21.01 14.50
0.15 0.15 0.10 0.05 0.10 0.15 0.15 0.10 0.10 0.10 0.10 0.05 0.15 0.15 0.10 0.15 0.10 0.15 0.15 0.15 0.10 0.10 0.05 0.10 0.05 0.05 0.05 0.10 0.05 0.10 0.05 0.05 0.15 0.10 0.10 0.15 0.10
3.82 4.02 3.12 3.25 3.45 3.86 3.12 2.87 2.55 3.45 3.53 2.22 3.94 3.70 3.12 4.59 3.86 3.94 2.87 2.55 2.79 2.71 2.55 4.10 3.61 2.55 2.87 2.95 2.63 3.28 4.10 2.46 3.04 3.61 2.55 2.30 2.39
27.96 28.25 13.22 12.42 20.03 29.64 19.79 18.77 18.65 20.07 16.96 6.53 26.84 26.27 22.43 28.25 28.53 31.07 32.48 30.79 26.56 21.20 16.93 18.94 13.87 14.15 13.22 17.80 11.61 23.23 17.63 7.10 28.53 22.83 22.89 24.01 17.25
18.43 17.36 7.29 7.97 6.92 9.64 6.84 6.83 10.16 9.67 8.80 9.38 19.92 17.74 15.89 16.60 15.52 16.04 17.62 18.27 17.92 16.84 12.97 15.46 10.51 10.34 11.52 17.08 14.27 15.09 10.89 9.59 17.36 12.28 12.58 14.09 13.66
3.95 3.88 2.16 2.01 2.86 4.21 2.98 2.49 2.66 2.79 2.37 1.72 4.01 3.89 3.34 4.12 4.04 4.21 4.35 4.28 3.89 3.44 2.65 3.52 2.12 2.31 2.17 3.51 2.15 3.66 2.65 1.72 4.11 3.52 3.74 3.91 1.35
7.42 7.41 7.54 7.85 7.41 7.37 7.55 7.67 7.74 7.62 7.49 7.79 7.31 7.34 7.37 7.37 7.48 7.49 7.53 7.49 7.75 7.51 7.81 7.63 7.81 7.82 7.71 7.70 7.77 7.65 7.67 7.92 7.71 7.62 7.84 7.88 2.88
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Table 2. Descriptive statistics. Variable pH EC (mS/cm) Ca2+ (meq/L) Mg2+ (meq/L) Na+ (meq/L) K+ (meq/L) HCO3- (meq/L) Cl- (meq/L) SO42- (meq/L)
Observations Minimum Maximum 73 7.07 8.14 73 1.72 5.01 73 5.99 16.30 73 2.80 18.10 73 7.50 29.00 73 0.05 0.25 73 1.56 4.92 73 6.53 35.10 73 4.28 21.78
Average Standard deviation 7.575 0.202 3.227 0.846 9.743 2.900 9.427 3.274 17.898 5.360 0.119 0.044 3.306 0.648 20.692 6.817 11.753 4.835
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Table 3. Correlation matrix (Pearson n). Variables EC ClHCO3Na+ K+ Mg2+ Ca2+ SO42-
EC
Cl- HCO31 0.934 0.267 1 0.243 1
Na+ 0.937 0.995 0.241 1
K+ 0.797 0.762 0.164 0.749 1
Mg2+ 0.705 0.664 0.459 0.668 0.378 1
Ca2+ 0.527 0.510 0.408 0.505 0.141 0.796 1
SO420.448 0.387 0.316 0.398 0.041 0.872 0.924 1
Table 4. Group’s barycenter. Groups 1 2 3
Cl(meq/L) 24,73 24,81 14,53
HCO3Na+ (meq/L) (meq/L) 3,59 21,14 3,34 21,12 3,02 13,01
K+ Mg2+ (meq/L) (meq/L) 0,13 12,85 0,17 8,19 0,08 6,80
Ca2+ (meq/L) 12,82 7,58 7,96
SO42CE (meq/L) (mS/cm) 17,08 3,83 7,19 3,83 9,13 2,31
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Fig. 1. Map of spatial distribution of studied wells located between Sidi-Abed and OuledGhanem.
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Group 1 Group 3
Group 2
Fig. 2. Dendrogram of the cluster analysis between wells.
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Fig. 3. Correlation circle (axes F1 and F2).
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Fig. 4. Plot of axis F1 versus F2 (projection of the wells).
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Fig. 5. Piper diagram of wells samples.
ACCEPTED MANUSCRIPT 280
280
Sidi Abed
Sidi Abed
(a)
(c)
275
275
270
270
Sidi Moussa
265
Sens d'écoulement de la nappe.
12
260
Ouled Ghanem
Sidi Moussa
265
isopièze 12 m.
Equidistance = 2 m.
Echelle : 0
850
260
Ouled Ghanem
Equidistance = 100 mg/l.
Echelle : 0
2 km
255
isochlorures 850 mg/l.
2 km
255 175
180
185
190
280
195
175
185
190
280
Sidi Abed
195
Sidi Abed
(d)
(b) 275
275
270
270
Sidi Moussa
265
180
Sidi Moussa
265
3.2
Ouled
isoconductivité électrique 3.2 mS/cm.
Equidistance = 0.4 mS/cm.
260 Ghanem
Echelle : 0
450
260
Ouled Ghanem
isosodium 450 mg/l.
Equidistance = 50 mg/l.
2 km
Echelle : 0
2 km
255
255 175
180
185
190
195
175
180
185
190
195
Fig 6. Spatial distribution: (a) Flow direction of the water table; (b) Electrical conductivity; (c) Chloride concentration; (d) Sodium concentration.
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(a)
(b)
Fig. 7. Groundwater assessing for irrigation purpose: (a) Plot of Na%; (b) Plot of SAR.
ACCEPTED MANUSCRIPT (b)
(a)
(c)
(d)
Fig. 8. (a) Na+/Cl- relationship; (b) Ca2+/ Cl- relationship; (c) Mg2+/Cl- relationship; and (d) SO42-/Cl- relationship.
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(b)
(c)
(d)
Fig. 9. (a) ΔNa+, (b) ΔCa2+, (c) ΔMg2+ and (d) ΔSO22- versus chloride content for wells of the three groups.
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(a)
(b)
Fig. 10. (a) Ca2++Mg2+ versus HCO3 - +SO42- ; (b) (Ca2++Mg2+)-( HCO3 - +SO4 2-) versus [Na++K+ – Cl-]
ACCEPTED MANUSCRIPT Highlights
- The salinity of groundwater in the region Sidi AbedOulad Ghanem was evaluated - The origin of groundwater salinization in this coastal aquifer was studied - Several tools and methods were applied to draw relevant conclusions - The salinization is mainly due to marine intrusion and aquifer rock dissolution - Some cares should be taken in order to properly exploit this natural water resource