Zeuss-Koutine aquifer, located in south-eastern Tunisia, has been used intensively as a primary source to meet the growing needs of the various sectors. Detailed knowledge of the geochemical evolution of groundwater and water quality can enhance understanding of the hydrochemical system, promoting sustainable development and effective management of groundwater resources.
To this end, a hydrochemical and statistical investigation were conducted. Groundwater samples have been collected from 13 wells from 1995 to 2005; to understand the sources of dissolved ions and assess the chemical quality of the groundwater.
Results demonstrate that among the cations, only the mean concentrations of Na+ exceed guideline values set by the World Health Organization (WHO). Regarding the analyzed anions, the concentration of SO42- and Cl- are above the WHO allowable concentrations. TDS in most groundwater samples is greater than 1000 mg l-1, the limit set by the WHO, indicating slightly saline or moderately saline water. In general, a significant increase in the degree of water mineralization was observed in the direction of the southwest to northeast following the regional ï¬‚ow direction.
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Calculated saturation indices show that all water samples were oversaturated with respect to Aragonite, calcite and dolomite, and undersaturated with respect to anhydrite, gypsum and halite. Based on hydro-chemical facies, two types of water that predominates in the study area. The first is Ca-Na-SO4-Cl in type and located in the southwest which corresponds to recharge part of the aquifer. The second type is Na-Ca-Cl-SO4 and located in the eastern, which correspond to the discharge part.
Key words: Zeuss-Koutine aquifer; drinking water quality; hydrochemical facies; saturation indices; trend evolution; multivariate statistical analysis.
Nowadays, in the current world, a sustainable socioeconomic development of every community depends much on the sustainability of the available water resources. Water of adequate quantity and quality is required to meet growing household, industrial and agricultural needs (Das et al. 2009).
In arid zones, water is a rare and precious resource. The current situation of the water resources and their uses in Tunisia present stakes which are common to many areas of the Mediterranean basin: limited and already largely exploited resources to answer the growth of the needs, the increasing use of so-called non conventional resources, a competition between sectoral uses of water, an increasing merchandising of the resources, constraining climatic conditions which come to reinforce the tensions around water (Romagny et al. 2004).
The southern Tunisian region is semi-arid and arid with mean annual rainfall of 100-230 mm and evaporation rates of 1300-1600 mm per annum and the climatic water balance is almost negative around the year (Ouessar et al. 2004). The average annual rainfall decreases from north to south from approximately 1300 to less than 200 mm. In northern Tunisia, the population use dam water as source of freshwater for agricultural and domestic purposes, whereas communities in the south, where surface waters are very scarce or absent, rely mainly on groundwater of variable quality for their domestic supplies.
Since groundwater represents the main source of water in the dry areas of Tunisia, as it is the case in many arid and semi-arid regions around the world, the aquifers have been massively pumped to meet the growing needs of the various sectors (drinking, industry, agriculture, aquaculture, tourism, etc.) during the last decades. The Zeus-Koutine aquifer, which extends over 920 kmÂ², is the first source for fresh water for drinking purposes. It has been pumped heavily since the early 1970s. In recent years, the overuse of groundwater has led to lowering of groundwater levels and worsening its quality (OSS 2005). The quality of ground water is equally important as that of quantity. Due to growing population, agriculture expansion, urbanization, groundwater resource management has become increasingly important for sustainable development of these regions. It is therefore necessary to subject the water to routine quality analyses in order to assess its suitability for human consumption. An appropriate assessment of the suitability of groundwater for domestic water supplies requires that the concentrations of some health parameters (inorganic constituents), as well as esthetic parameters are compared with the World Health Organization (WHO) guideline values set for potable water (WHO 2004).
Little is known about the natural phenomena that govern the chemical composition of groundwater or anthropogenic factors that presently affect them. Generally, the quality of groundwater is controlled by many factors that include composition of recharge water, geological structure and mineralogy of the watersheds and aquifers, the residence time and reactions that take place within the aquifer (Fetter 1994; Appelo and Postma 2005). The interaction of all factors leads to various water types. Increased knowledge of geochemical evolution of groundwater quality could lead to improved understanding of hydrochemical systems in such areas, leading to sustainable development of water resources and effective management of groundwater resource.
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Therefore, our objective in this study was to identify the processes controlling the geochemical evolution of groundwater quality and to characterize water/rock interaction and its effects on the chemistry of the water. Finally, to aid the management and future development of groundwater resources in the region, the hydrochemical approach and statistical methods were also applied to divide the territory in into areas with distinct groundwater quality. This may help in the long run to support new and more efficient remedial measures to combat the deterioration of water quality.
2. Study area
The study area is situated in the Mediterranean Sea region of the South-eastern part of Tunisia and lies between longitudes 10Â° 16' and 10Â° 61'E and latitudes 33Â° 39' and 33Â° 75'N.
The aquifer of Zeuss-Koutine is situated in the north of the city of Medenine (Fig 1). It stretches out from the south-west, in the Matmata mountains near Béni Khdache and Toujane, going through the Jeffara plain, into the Gulf of Gabès, actually ending in the saline depression (Sebkhat) of Om Zessar. The geological formations consist of alternating continental and marine origin. The oldest submerging layers are represented by a marine, superior Permian, and the most recent ones are of the recent Quaternary (Morhange and Pirazzoli 2005). Between the Permian and Quaternary formations is a confined strata of different age, which is generally declining in northward direction (Gaubbi 1988). Zeuss-Koutine region is bordered by three main structures that define the South-Eastern Tunisia: the Dahar monocline, West and North-West, the Medenine Tebaga monocline, South, and the plain of the Djeffara, East and North.
PLACE FIGURE 1 HERE
The climate is of the Mediterranean type and is characterized by hot and dry periods in summer and by cold and rainfall periods in winter, which is typical for the arid and semi-arid zones. The coldest months are those of December, January and February with occasional freezing (up to -3 Â°C). June, July and August are the warmest period of the year during which the temperature could reach as high as 48 Â°C (in the shade). The rainfall is characterized by low averages, high irregularity (both in time and space) and torrentially. Surface water hydrology in the study area is represented by the following Rivers: Zeuss, Koutine-Om Ezzassar and Zigzaou. There is no perennial river in the study area.
The Zeuss-Koutine aquifer constitutes an important water resource used for various purposes; potential resources are estimated to 350 l s-1. The exploitation of this reservoir started in 1962 (Gaubbi 1988). The continuous monitoring of the exploitation of this aquifer has shown that the withdrawal rate increased from 102 l s-1 in 1974 to 440 l s-1 in 2005 which resulted in a decline of the mean piezometric level of 0.33 m in 1982 and 1.02 m in 2005 (DGRE 2005).
Results of aquifer tests indicate that transmissivity of the aquifer varies between 0.055 and 0.2 m2 s-1 (OSS 2005).
The aquifer geometry is controlled by Medenine fault (Fig 2) and differs from one locality to another. The Zeuss-Koutine aquifer is constituted by the following litho-stratigraphic units (Yahyaoui 2004):
- Upper Jurassic limestone and dolomite (Callovian Oxfordian);
- Albo-Aptian calcitic dolomites;
- Turonian dolomites and dolomitic limestone;
- Lower Senonian limestone which is divided into two distinct entities:Â
1- The marly-gypsitic Senonian intermediate limestone
2- The Summit limestone of the top.
PLACE FIGURE 2 HERE
Leakage between different layers of the Zeuss-Koutine aquifer is made through intermediary faults, being the Mednine fault the most important. These aquifers are intercepted by several wells at various depths. However, in Medenine region, Jurassic constitutes the main layer because of its considerable thickness reaching up to 120 m. this layer is dolomic and karstified, while the Upper Jurassic layer (Oxfordian-Kimmeridgian), which is marlacious and dolomitic, forms the top of this aquifer.
This aquifer is recharged by water ï¬‚owing from Matmata Mountains, where important outcrops are made up of Jurassic limostone, as well as by local infiltration from the several rivers. Groundwater flow in the study area is towards the Sebkhat Om Jessar and the Mediterranean Sea (Fig 3). The highest contour lines are at the West of study area and lowest contour are close to the Om Jessar Sebkha.
PLACE FIGURE 3 HERE
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Water of the Zeuss-Koutine aquifer is used unevenly by different economic sectors. However, drinking water supply remains the primary use. Anthropogenic activities in the study area rely mainly on agricultural and industrial activities such as construction (brick production, ceramics, etc.) and agro-food industry.
3. 1. Sample collection and analytical techniques
Between 1995 and 2005, groundwater quality monitoring in Zeuss-Koutine aquifer was carried out by collecting water samples from 13 wells capturing the different Zeuss-Koutine layers and representing a homogeneous spatial distribution on the whole aquifer. Figure 1 shows the locations of the selected wells. These wells are numbered from 1 to 13 and specific names are provided in Table 3. Water samples were obtained from one sampling campaign which corresponds to the summer season. This period was chosen because it coincides with the maximum water abstraction from the Zeuss-Koutine aquifer and minimum precipitation and recharge in the area.
General procedures for groundwater sampling, preservation, and chemical analyses were carried out according to the standard procedures described by Eaton et al. (Eaton et al. 1995). All of the water samples were pumped from wells continuously used. Water was only taken from boreholes that were pumping for a significant amount of time to get a representative sample. For deeper wells, a larger volume of stagnant groundwater existed and consequently needed to be drained away from the well area. This was done in order to avoid the sampling of stagnant annulus water that would be in the region of pump and pump systems. During removal of the casing volumes, water stabilization parameters (Temperature and pH) are monitored during purging to determine when the stagnant water was replaced by formation water. When sampling, all water samples were filtered through a 0.45-Âµm Millipore cellulose type membrane. Each sample was collected in two new 500 ml polyethylene bottles. All sampling bottles were washed with de-ionised water and again with filtered sample water before filling it to capacity and then labelled accordingly. For each sample and to prevent precipitation, one bottle is acidified (until pH of samples reached 1) with 35 % nitric acid for cation analysis (Na+, K+, Ca2+ and Mg2+), whereas the other is used for the determination of dissolved anions (Cl-, SO42- and HCO3-). Prior to analysis in the laboratory, the samples were stored at a temperature below 4 Â°C.
Temperature (TÂ°) and hydrogen ion activity (pH) of groundwater were measured in the field, in order to acquire representative values of ambient aquifer conditions. For the pH measurements the electrode was calibrated with a reference buffer solution of pH 4 at each location.
Chemical analyses were performed in certified laboratories (ISO 17025) of national society of drinking water of Ministry of agriculture and hydraulic resources, using standard methods. Chloride was determined by standard AgNO3 titration (Rodier 1996). Bicarbonates were determined by titration with H2SO4 (Rodier, 1996). Sulphate (SO42-) concentration was measured by the gravimeter method using BaCl2, Sodium (Na+) and potassium (K+) was measured by flame photometry, calcium (Ca2+) and magnesium (Mg2+) ions were determined titrimetrically using standard EDTA, ethylene diaminete tracetic acid (Rodier, 1996). Total dissolved solids (TDS) were measured by evaporating a pre-filtered sample to dryness.
The accuracy of the chemical analyses was carefully inspected by repeated analyses (i.e. two analyses) of samples and standards and then calculating their percent charge balance errors (% CBE). The latter was calculated according to the following equation (Freeze and Cherry 1979):
Percent (%) Charge Balance Error= [(âˆ‘z mc - âˆ‘ma)/( âˆ‘z mc + âˆ‘ma)]x100
Where z is the absolute value of the ionic valence, mc is the molality of cationic species, and ma is the molality of the anionic species.
No samples in the database have a CBE greater than Â± 5% (which is an acceptable error for the purpose of this study).
The physico-chemical parameters (TÂ°, pH, and salinity) and the concentrations of major ions were analyzed and average concentrations during the 1995-2005 period are used for their interpretation. Spatial variation of groundwater salinity data was analyzed using a geographic information system (GIS).
3. 2. Geochemical modeling
The geochemical modeling program PHREEQC (Parkhurst and Appelo 1999) was used to calculate aqueous speciation and the thermodynamic equilibrium conditions (mineral saturation indices) of waters with respect to the main mineral phases present in the aquifer which are indicators of the saturation state of a mineral with respect to a given water composition. The mineral reaction mode (dissolution or precipitation) is constrained by the saturation indices for each mineral (Deutsch 1997). The saturation indices (SI) describe quantitatively the deviation of water from equilibrium with respect to dissolved minerals and are expressed as S.I. = Log (IAP/Kt), where IAP is the ion activity product and Kt is the equilibrium solubility constant. If the water is exactly saturated with the dissolved mineral, SI equals to zero. Positive values of SI indicate supersaturation and the mineral would tend to precipitate, and negative one indicates undersaturation and the mineral would tend to dissolve (Cidu et al. 2009; Subyani 2005).
3. 3. Multivariate statistical analysis
Multivariate statistical analysis is widely used to identify the sources of solutes in a groundwater system (Zhang et al. 2009).
The application of various multivariate approaches such as cluster analysis (CA) and principal component analysis (PCA), offers a better understanding of water quality and allows comparison of different samples of waters (Yidana et al. 2008). The groundwater quality data (TDS, pH, temperature, Na+, Cl-, Ca2+, Mg2+, SO42-, K+ and HCO3-) were analyzed with a multivariate statistical method in order to test the reliability of the different processes that affect the mineralization of the groundwater aquifer system.
The combined use of PCA and CA enabled the classification of water samples into distinct groups based on their hydrochemical characteristics. These techniques will find groups and sets of variables with similar properties.
The parameters used in these multivariate statistical methods (PCA and CA) referred to different units of measurement. Therefore, their values should be standardized with the transformation function (Medina-Gomez and Herrera-Silveira 2003), Z = (x-Âµ)/Ïƒ (where the Z is standardized value, x indicates the original value of the measured parameter, the Âµ is mean of the variable and the Ïƒ is standard deviation). Standardization tends to minimize the effect of the difference of variance in variables, eliminates the influence of different units of measurement and renders the data dimensionless (El Yaouti et al. 2009).
3. 3. 1. Principal components analysis (PCA)
In order to investigate the role of each variable and, at the same time, to simplify the original data structure, the first step of our analysis was the extraction of principal components from the original data set. Principal components analysis (PCA) allows defining eigen vectors of a variance-covariance or a correlation matrix from a data set corresponding to a raw matrix of N rows of observations by P columns of variables (Davis 1986). In our study, PCA was applied to chemical data from the Zeuss-Koutine aquifer to extract the principal factors corresponding to the different processes that control water chemistry and sources of variation in the data. In fact, PCA reduces a large number of variables (measured physical parameters, and major elements in water samples) to a small number of variables (Cloutier et al. 2008).
3. 3. 2. Cluster analysis
Cluster analysis (CA) is a data classification technique and one of the most powerful tools for analyzing water hydro-geochemical data (De Andrade 2008). Cluster analysis techniques allow relationship investigation between the observations or the variables of a dataset, in order to recognize the existence of groups. The approach used in our study is based on hierarchical clustering. Besides, CA was used to split the standardized physico-chemical data into groups (clusters) based on similarities (or dissimilarities) so that each cluster represents a specific process in the aquifer system (Templ et al. 2008). There are two types of cluster analysis: R and Q-modes. In the present study Q-mode cluster analysis was performed on the water chemistry data to group the samples in terms of water quality.
The results of hierarchical clustering methods depend on the specific measure of similarity and the linking method (De Andrade 2008). In our study, and in order to perform CA, an agglomerative hierarchical clustering was developed using a combination of the Ward's linkage method as a clustering algorithm and Euclidean distances as a measure of similarity. The result of such analyses is a graph, called dendrogram.
3. 3. 3. Software
The computer program ANDAD 6.00, developed by the Geo-Systems Center of Instituto Superior Tecnico, Portugal (CVRM 2000), and STATISTICA (StatSoft) software, were performed to conduct several multivariate statistical analyses including cluster analyses and principal components analyses.
4. RESULTS & DISCUSSION
4. 1. Chemical characteristics
A statistical summary of the physico-chemical and chemical data of all the investigated groundwater during the period 1995-2005 is presented in Table 1.
PLACE TABLE 1 HERE
Groundwater temperatures vary from 12.70 to 29.0 Â°C with mean and standard deviation values of 20.52 and 2.87 Â°C, respectively. This parameter varied with sampling location and year of collection. The pH values are in the range 7.10-8.50 with a mean value of 7.66 and standard deviation of 1.43 indicating that the waters are generally neutral to slightly alkaline. These values were found to be in the permissible range of 6.5 to 8.5 and trend evolution shows minor variations during the period evaluated (1995-2005).
The chemical compositions of the groundwater samples were statistically analysed and the results are given in Table 1. Among the cations, only the mean concentrations of Na+ exceed guideline values set by the World Health Organization (WHO). The sodium ion concentration (Na+) in the groundwater samples range from 144.5 to 867.00 mg l-1 with a mean value of 438.99 mg l-1 and a standard deviation of 182.65 mg l-1 (Table 1). The recommended limit for sodium in natural water is 200 mg l-1 (WHO 2004). Five percent (5%) of the groundwater samples fall above this limit. Regarding the analyzed anions, only the bicarbonate concentrations (HCO3-) in all samples are lower than the WHO allowable bicarbonate concentration of 348 mg l-1.
Figure 4 shows that Na+ and SO42- are the dominant cation and anion, respectively. Among the cations, the concentrations of Ca2+, Mg2+, Na+ and K+ ions ranged from 117 to 374, 62 to 148, 144 to 867 and 8 to 29 mg l-1 with a mean of 241, 108, 439 and 15 mg l-1, respectively. Their concentrations represent on average 30, 13, 55 and 2% of all the cations, respectively. The order of relative abundance of major cations in the groundwater of the Zeuss-Koutine aquifer is Na+ > Ca+2 > Mg2+ > K+. Among the anions, the concentrations of HCO3-, Cl-, SO42- ions lie in between 93 and 243, 209 and 1653, 400 and 1120 mg l-1 with a mean of 173, 744, 818 mg l-1, respectively. The order of their abundance is SO42- > Cl- > HCO3-, contributing on average, respectively, 47, 44, and 9 % of the total anions.
PLACE FIGURE 4 HERE
4. 2. Hydrochemical Facies
The criteria for delineating the hydrochemical processes and defining groundwater types of hydrochemical facies, used in this study, is simple and straightforward. These criteria were derived by constructing piper and Durov diagrams for major dissolved constituents.
Different water types can be defined depending on the composition of the water (i.e., the cation and anion concentrations). The piper diagram in Figure 5a constructed using Aquchem Scientific Software shows the relative concentrations of the different ions from individual water samples. Two main hydrochemical facies have been identified on the basis of major ion concentrations. The first is Ca-Na-SO4-Cl in type (which corresponds to wells 1, 2, 8, 9, 10, 11, 12 and 13) and located in the southwest of the study area which correspond to the downstream part of the aquifer. Whereas the second is Na-Ca-Cl-SO4 in type (which corresponds to wells 3, 4, 5, 6, and 7). This water group dominated by anions Cl- is often localized in the confined aquifers in North and East central part of the Zeuss-Koutine aquifer. This is clearly demonstrated in the Durov diagrams (Fig 5b) where SO42- type waters evolve into Cl- type. The ionic compositions of these groundwater samples show variable but very interesting systematic spatial variation of the chemical facies from south west to north east. The first type is characterized by low and moderate mineralization levels, while the second type groundwater shows relatively higher mineralization. In fact, the total dissolved solids (TDS) contents of the two identified groups vary from 1235 to 2700 and 3200 to 4368 mg l-1 (Table 1) with a mean of 2127 and 3684 mg l-1, respectively. These values are elevated compared to the guideline value of 1000 mg l-1 set by the WHO (WHO 2004) which indicates that the groundwater is generally moderately saline or brackish. Brackish water has the total dissolved solids concentration in the range 1000-10000 mg l-1 (Freeze and Cherry 1979).
PLACE FIGURE 5 HERE
Spatial distribution maps of salinity from 1995 to 2005 show the same trend. A typical map is represented in Figure 5. In general, a significant increase in the degree of water mineralization in the direction of flow is observed and the least mineralized water was found closest to the main recharge area. In fact, the map (Fig 6) shows that Ksar Chrarif region (wells 1 and 2) and Koutine region (wells 8 and 9), that are located in the upstream section of the aquifer (recharge zone) have the lowest salinity levels compared to downstream zones.
PLACE FIGURE 6 HERE
4. 3. Correlation matrix
Compositional relations among dissolved species can reveal the origin of solutes and the process that generated the observed water compositions. The degree of a linear association between any two of the water quality parameters (Adhikary et al. 2009), as measured by the simple correlation coefficient (r), is presented in Figure 7. The results show high correlations (> 0.7) between some pairs of parameters.
Figure 7a shows the correlation matrix for the chemical variables of the groundwaters samples. The concentration values of all elements correlate positively with higher similarity level, except HCO3-, which has law correlation with all elements.
PLACE FIGURE 7 HERE
The Na+-Cl- relationship has often been used to identify the mechanisms for acquiring salinity in semi-arid regions (Magaritz et al. 1981; Dixon and Chiswell 1992; Sami 1992). As shown in Figure 7, regarding to Na+-Cl- relationship, a high correlation (r = 0.85) was observed. The high Na+ and Cl- contents measured in majority of samples may suggest the dissolution of chloride salts. Besides, high concentration of sodium may be attributed to base exchange and leaching of sodium salts such as halite during the movement of groundwater through sediments. A Na+/Clâˆ’ molar ratio greater than 1 is typically interpreted as reï¬‚ecting Na+ released from silicate weathering reactions (Meybeck 1987). Silicate dissolution can be a probable source for Na+ in some groundwater samples (1, 9, 10, and 13). The Na+/Clâˆ’ ratio in most groundwater samples (70 %) are less than 1 (average value = 0.96), indicating that Na+ release from silicate weathering is not important in the studied area , on the other hand the process of ion exchange is the predominant process for reduction of the Na+ in these groundwater.
The relationship between Ca2+ and SO42- concentrations is characterized by a relative high correlation coefficient (0.78) and a SO42-/Ca2+ ratio, in moles/l, superior to 1. This indicates that calcium and sulphate are not involved in the same geochemical processes (Hounslow 1995). Actually, sulphates are mostly found in a dissolved form, whereas calcium can undergo a precipitation of calcite and/or a Ca/Na base exchange between clay minerals and water (Desbarats 2009).
The sources of Ca2+ and Mg2+ on groundwater can be determined from the (Ca2+ + Mg2+)/HCO3- ratio. If Mg2+ and Ca2+ only originate from the dissolution of carbonates in the aquifer materials, this ratio would be about 0.5 (Sami 1992). The low Ca2+ + Mg2+ / HCO3- ratio (<0.5) could be the result of either Ca2+ + Mg2+ depletion by cation exchange or HCO3- enrichment. However, high ratios cannot be attributed to HCO3- depletion; under the existing alkaline conditions, HCO3- does not form carbonic acid (H2CO3) (Spears, 1986). In the present study, high ratios suggest that an excess of alkalinity in these waters is balanced by the alkalis Na+ and K+. The excess of Ca2+ + Mg2+ over HCO3- reï¬‚ects an extra source of Ca2+ and Mg2+ and is balanced by Cl- and SO42- .
The cation exchange between Ca2+ or Mg2+ and Na+ may also explain the excess Na+ concentration. Ca2+ and Mg2+ can exchange Na+ sorbed on the exchangeable sites of the clay minerals, resulting in the decrease of Ca2+ and Mg2+ and the increase of Na+ in groundwater.
The good correlation between SO42- and Mg2+ (r = 0.73) suggests that a part of the SO42- and Mg2+ may also be derived by the weathering of an Mg2+ sulphate mineral. There is no relation between Ca2+ and HCO3- and correlation coefficient is not significant. This indicates that calcite may not be the source of Ca2+.
Strong correlations exist among the major elements (Fig 7b), Na+, Ca2+, Mg2+ and Cl-, and TDS (r > 0.75). These relationships clearly identify the main elements contributing to the groundwater salinity and their tendency to follow a similar trend. The salinisation of the groundwater would be expected to result from the ionic concentrations increasing due to the effects of interactions between the groundwater and the geological formations.
To identify the processes of mineralization of groundwater, it is also interesting to make the (Ca2+ + Mg2+) vs (HCO3- + SO42-) scatter diagram (Kumar et al. 2009). Data points that all along or close to the 1:1 line are attributed to carbonate and sulfate mineral weathering. Those that fall above the 1:1 line resulted from the effects of the reverse ion exchange processes in the system. Data points that fall below the equiline are deemed to have also resulted from the dissolution of carbonates and sulfates. In Fig. 8, although some of the points (4 samples) fall above the 1:1 line, signifying the effects of cation exchange and silicate mineral weathering. About 0.69 % of all the points fall around and below the equiline, indicating the contribution of carbonate and sulfate mineral dissolution, and cation exchange in the hydrochemistry.
PLACE FIGURE 8 HERE
4. 4. Geochemical modelling
The processes of dissolution and precipitation are actively taking place within the groundwater system. SI describes quantitatively the deviation of water from equilibrium with respect to dissolved minerals. In Table 2 the SI for anhydrite, gypsum, halite, aragonite, calcite and dolomite are shown.
Nearly all water samples were oversaturated with respect to Aragonite, calcite and dolomite, and undersaturated with respect to anhydrite, gypsum and halite, suggesting that these carbonate mineral phases may have influenced the chemical composition of the groundwater of the study area. As saturation state indicates the direction of the process, thus, precipitation of aragonite, calcite and dolomite and dissolution of anhydrite, gypsum and halite are expected.
According to Hidalgo and Cruz-Sanjulian (2001), the oversaturation in calcite is related to incongruent dolomite dissolution which causes precipitation of calcite. Once the system is saturated in calcite, the hydrochemical evolution is affected by the dissolution of gypsum, which will be influencing factor in the process of dolomite dissolution (Jallali 2005). All of the groundwaters are oversaturated with respect to dolomite. Interaction between groundwater, which contains saturated calcite and dolomite and sufficient amounts of Ca2+, with the gypsum layer would lead to the dissolution of gypsum (Feng and Han 2002).
PLACE TABLE 2 HERE
4. 5. Statistical analyses
4. 5. 1. Principal component analysis
To investigate relationships between major elements in groundwaters, PCA was performed on a data set of 13 samples and 10 chemico-physical elements (TÂ°, pH, TDS, Na+, Cl-, Ca2+, Mg2+, SO42-, K+, HCO3-). Principal component analysis (PCA) was applied to the data to rank the main players in the hydrochemistry. Factor loadings were further calculated for each of the sample and used to determine the level of intensity of each if the factors controlling the hydrochemistry at each location. The PCA with varimax rotation resulted in three main components representing three sources of variation in the hydrochemistry of groundwater from the Zeuss Koutine aquifer and explaining 70 % of the variance in the hydrochemistry or information contained in the original data set which is sufficient to give a good idea of the data structure. The eigen values and the percentage of the variance explained by each eigenvector are listed in Table 3.
PLACE TABLE 3 HERE
The scores of variables onto the three principal component axes are plotted in Figure 9, which enables the identification of several processes of water mineralization.
PLACE FIGURE 9 HERE
Factor 1 (F1) explains more than 58.31 % of total variance and contains large loadings on temperature, Na+, Cl-, Ca2+, Mg2+, SO42-, K+. It represents the weathering of halite and evaporates minerals from the underlying geology. Factor-2 represent the weathering carbonate minerals and contributes to 12 % of the total variance and is strongly associated with HCO3-, to a lesser extent Temperature. The pH is described by F3, accounting for a further 10 % of the remaining variance.
4. 5. 2. Cluster analysis
The dendrogram of the Q-mode cluster analysis built on the wells shows three main groups of waters (Fig 10).
The decision on the number of clusters to use in an HCA is somewhat subjective and is based on the expert knowledge of the researcher on the subject matter being investigated. In this study, the differentiation was based on the knowledge of the geology and groundwater ï¬‚ow patterns in the area (Cloutier et al. 2008). These groups are:
Group 1: Made up of the total five water-samples (wells 1, 2, 8, 9, 10, 11, 12 and 13), which have the lowest salinity levels. Based on the overall chemical composition, these waters are characterized by SO42-- Cl-- Ca2+- Na+ facies. This group is located in the upstream zone of the study area where the aquifer system seems to receive most of its meteoric recharge.
Group 2: It consists of eight water samples from Zeuss-Koutine aquifer. This group is characterized by Cl-- Na+ water types and by high salinity. This is located in the downstream of the study area. The most pronounced characteristic of this group is the increase of Na+ and Cl- contents.
PLACE FIGURE 10 HERE
Based on hydrochemical studies, the chemical composition of groundwater differs according to water types. Two types are identified. The hydrochemical types Ca-Na-SO4-Cl and Na-Ca-Cl-SO4 are characteristics of the studied area. The spatial variation of groundwater chemistry in the study site suggests that the hydrogeochemical compositions of groundwater have been mainly controlled by groundwater flow pattern that follows the topography of the study area. In general, a significant increase in the degree of water mineralization was observed in the direction of the water flow (south west to north east). The least mineralized water is found closest to the main recharge area. The water displays a chemical evolution from Ca-Na-SO4-Cl in recharge area towards Na-Ca-Cl-SO4 facies in the discharge area. The concentration of TDS in all groundwater samples is greater than 1000 mg l-1, indicating slightly saline or moderately saline water.
The geochemical modelling study established by calculation of the saturation indices of mineral phases, shows that all water samples were oversaturated with respect to aragonite, calcite and dolomite, and undersaturated with respect to anhydrite, gypsum and halite. Thus, dissolution of halite, gypsum and anhydrite determine Cl-, SO42-, Na+, Ca2+ and Mg2+ contents in the groundwater. Other geochemical processes, such as cation exchange of Na+ for Ca2+ and Mg2+ and leaching of sodium salts during the movement of groundwater through sediment, also contribute to the content of Na+.
By comparing the analytical results with national and international (WHO) drinking water norms, we notice that water quality tends to deteriorates. In fact, water sampled in wells located in north east part of the aquifer which corresponds to the discharge zone of the aquifer (wells 3, 4, 5 and 6) have concentrations exceeding to the drinking water legislation norms and thus should be treated before human consumption. The conclusions from of the current study as well as other available water quality data should be taken into account when developing strategies for safe drinking water supplies.
The results of the statistical analyses (principal component analysis and cluster analysis) corroborate the geochemical methods and results and provide further information about the water quality of Zeuss-Koutine aquifer.
This research has been funded in cooperation with the National Society of Drinking Water in Tunisia (SONEDE) and the Resources Water Direction of Medenine Tunisia (DGRE).