Groundwater Flow Dynamics and Distribution of Hydrochemical Facies Using GIS in Hodna Plain, M’Sila, Southeastern Algeria

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Figure 2. Spatial distribution of EC concentrations in the study area

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Figure 3. Spatial distribution of TDS concentrations

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Figure 4. Spatial distribution of Ca²⁺ concentrations in the study area

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Figure 5. Spatial distribution of Mg²⁺ concentrations in the study area

EC and TDS range from 1025 μS∙cm^-1 to 3300 μS∙cm^-1 and 810 mg∙l^-1 to 2815 mg∙l^-1, with mean values of 2328 μS∙cm^-1 and 1863 mg∙l^-1, respectively (Table 1, Figures 2 and 3). Ca²⁺ ranges also from 98 mg∙l^-1 to 452 mg∙l^-1, with a mean value of 209.1 ± 106.9 mg∙l^-1 (Table 1, Figure 4). Mg²⁺ varies from 68 mg∙l^-1 to 228 mg∙l^-1, with the mean value of 116 ± 35.4 mg∙l^-1 (Table 1, Figure 5). Na⁺ varies from a minimum of 46 mg∙l^-1 to a maximum of 554 mg∙l^-1, with a mean value of 239 ± 170.6 mg∙l^-1 (Table 1, Figure 6). K⁺ ranges from 1 mg∙l^-1 to 8 mg∙l^-1, with a mean value of 2.8 ± 1.8 mg∙l^-1 (Table 1, Figure 7). Alkalinity varies from 204 mg∙l^-1 to 736 mg∙l^-1, with a mean value of 370.2 ± 172.9 mg∙l^-1 (Table 1, Figure 8). SO₄^2- ranges from 248 mg∙l^-1 to 1207 mg∙l^-1, with a mean value of 678.4 ± 318.2 mg∙l^-1 (Table 1, Figure 9). Cl^– varies from 102 mg∙l^-1 to 845 mg∙l^-1, with a mean value of 397.9 ± 232.6 mg∙l^-1. NO₃^– varies from 1 to 33 mg∙l^-1, with the mean value of 10.7 ± 10.2 mg∙l^-1. Higher mean values of Na⁺, Cl^–, and SO₄^2- indicate a likely contamination by high salinity waters of the lake and a dissolution of evaporate minerals.

The abundance of the major anions is SO₄^2–>Cl^–>HCO₃^–, 68% of samples exceeded the maximum acceptable concen­tration of SO₄^2– for drinking water (400 mg∙l^-1), and 36% of samples overcame the maximum acceptable concentration of Cl^– for drinking water (500 mg∙l^-1). The abundance of the major cations is Na⁺>Ca²⁺>Mg²⁺>K⁺ and 52% of samples exceeded the maximum acceptable concentration of Na⁺ for drinking water (200 mg∙l^-1), whilst 44% of samples showed values of Ca²⁺ higher than the maximum acceptable con­centration for drinking water (200 mg∙l^-1) [31].

3.2 Groundwater flow process and chemical facies distribution

Salinity, represented by the EC (Figure 1), increases from the center of the plain to the salt water lake. This is the result of the inversion of the flow from the salt water lake towards the plain, due to the drawdown induced by pumping. For water samples along the northern piedmont, low values (EC<1700 μS∙cm^1-) were reported and constitute 28% of cases (Group 1, G1). As shown by the piezometric map they correspond to the recharge region.

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Figure 6. Spatial distribution of Na⁺ concentrations in the study area

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Figure 7. Spatial distribution of K⁺ concentrations in the study area

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Figure 8. Spatial distribution of HCO₃- concentrations in the study area

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Figure 9. Spatial distribution of SO₄^2- concentrations in the study area

The second group of samples (G2) shows salinity ranging from 1700 to 2900 μS∙cm^1- and makes up 40% of cases. It characterizes those samples that are obtained from the center plain. The third group (G3) has a higher salinity values than this cap and represents 32% of instances, located close to the salt water lake.

The key causes are the introduction of residual solids into the aquifer, and the water flow through low soluble mineral sediments.

Samples of groundwater were plotted in a diagram (Cl^–vs Ca²⁺+Mg²⁺) [32] to display the different forms of water in the study region (Figure 10). This diagram shows that the chemical character in general falls within the following types of water:

- Fresh groundwater with low salinity concentrations, located in the northern part of the study area;

- Saline water with high salinity concentrations, located in the southern part.

Situated in the nearby Hodna Lake, these salt water samples reflect the effect of salty water intrusion and related processes (halite and gypsum dissolution) and anthropogenic contamination at the well head.

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Figure 10. Cl versus Ca and Mg in mmol/l for groundwater samples

The different water samples have been classified according to their chemical composition using the Piper diagram (Figure 11). The analysis showed that the water grade calcium-bicarbonate (Ca²⁺-HCO₃^–) characterized 24% of the samples, primarily located in the north of the region of study. The presence of fractured and karst carbonate formations on the northern boundary indicates a rainwater infiltration where they develop a calcite facies, which explains the existence of low salinity water (EC<1700 μS∙cm^1-) on the northern part. SO₄^2–-Cl^–-Na⁺ water type characterizes 32% of all the samples collected, and this condition is mainly detected along the salt water lake, in the southern part of the study area. This explains the presence of high salinity (EC>2900 μS∙cm^1-) water in that portion of the study area. These facies are linked to sandy formations, such as marl and clays around the salt lake. The remaining water samples (40%) are intermediate types of water between those mentioned above, and are sulfate-dominant types (SO₄^2–-HCO₃^–-Ca²⁺, SO₄^2–-Ca²⁺-Mg²⁺, SO₄^2–-Ca²⁺, SO₄^2–-Na⁺-Ca²⁺, and SO₄^2–-Cl^–). In this category the electrical conductivity ranges from 1700 to 2900 μS∙cm^1-, pecurial of the mixed water. Therefore, the water type Ca²⁺ -HCO₃^– reflects the replenishment and regeneration from the recent meteoric water, while the water type SO₄^2–-Cl^–-Na⁺ suggests leaching of the salt deposit aquifer matrix (Figure 12).

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Figure 11. Diagram Piper applied to water samples of Hodna aquifer

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Figure 12. Schematic section along the groundwater flow path

3.3 Statistical analysis

3.3.1 Factor analysis

The FA was conducted on 25 individuals and 9 variables (EC, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^–, SO₄^2–, HCO₃^–, and NO₃^–). Table 2 displays the individual values of the variables derived and the proportion of the overall survey variation described by the variables. The study yielded 10 variables, but only three variables have been preserved, accounting for 81% of the overall variance. The parameter’s weights for the three factors of the dataset are given in Table 2. EC, Na⁺, Cl^– and SO₄^2– marked factor 1, which explained 52% of the variance, it has a strong to moderate positive loading in EC, Na⁺, Cl^– and SO₄^2–, which are 0.77, 0.78, 0.63, and 0.70 respectively. High positive charges indicated a strong linear correlation between the parameters and the factor. Factor 1 may therefore be regarded as a cause for salinization. Simultaneous drought and overexploitation caused the groundwater level to deteriorate. Such variables suggest this is the aquifer's effect on various forms of soil. Groundwater aquifer, containing evaporite host rocks, can achieve large Cl^– and SO₄^2– concentrations.

Factor 2 explains 22% of the total variance, it has a moderate negative loading in Mg²⁺, Ca²⁺ and HCO₃^–, which are –0.68, –0.56 and –0.55 respectively. These variables indi­cate the existence of an influence of the aquifer by the types of carbonate rock.

Table 2. Variance explained and component matrixes

3.3.2 Cluster analysis

Clusters can be defined using two separate approaches, namely R or Q-modes [34]. R-mode is typically applied to water quality variables for their interactions highlighting, while Q-mode shows the relationships between the tested samples. In this study eight calculated hydrochemical variables (Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^–, SO₄^2–, HCO₃^–, and NO₃^–) were utilized.

The Ca²⁺, Mg²⁺ and HCO₃^– concentrations may be regarded as the variables representing the phase of dissolution of carbonates that occurs in the boundary areas during the aquifer recharge. On the other hand, the variables Cl^– and SO₄^2– may be defined as being indicative of other alternate salinization of the Hodna aquifer groundwater, as the variables representing the phase of the Hodna Lake salt water intrusion. Thus, Na⁺, K⁺ and NO₃^– coming from the solute return flow from irrigation production may also be viewed as an applied method for salinization. Such factors can be contained in the phases of natural mineralization and anthropogeny. The non-homogeneous existence of the aquifer culminated in numerous hydrogeochemical facies for groundwater such as Ca²⁺-HCO₃^- in the northern fringe and SO₄^2–-Cl^–-Na⁺ in the southern plain.

The G1 has a low salinity (EC<1700 μS∙cm^1-), while G2 shows high salinity (EC>2900 μS∙cm^1-), and G3 both inter­mediate and average salinity (1700<EC<2900μS/cm). These groups of the samples are located in different areas. The G1 is located in the north, near the recharge zone, the G2 in the south, near the salt water lake and the G3, in the center of the plain (Figure 13). The G1 consists of wells 1, 2, 6, 13, 15, 19 and 22, it is defined by both Ca²⁺ and HCO₃^–. It is situated at the geological outcrops to the North. G2 is located primarily in the South of the research region near the salt water lake. This category includes the 5, 7, 10, 14, 16, 17, 18, 21, 24 and 25 wells. This is affected by the salivary deposits of the Triassic as well as by the salt water lake returns flow and is defined by SO₄^2–-Cl^–. G3 is a transitional time between the two severe classes, originating from the wells 3, 4, 8, 9, 11, 12, 20 and 23 and distinguished by Na⁺, K⁺ and NO₃^– from the agricultural activity.

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Figure 13. Dendrogram of cluster analysis of variables and the hydrochemical samples

3.3 Water–rock interaction process

The ions leached out and dissolved in reservoirs through weathering and water absorption in rocks and soils [35]. The geological forms, water-rock contact, and ion relative mobility are prime factors that affect groundwater geochemistry [36]. The use of distributed plots for TDS vs Na / (Na+Ca) and TDS vs. Cl / (Cl+HCO₃) [37] can be used to evaluate mechanisms of rock-water interaction processes. The Gibbs ratios are calculated by the formulae 1 and 2 given below:

Gibbs Ratio I (for Anion) = Cl^–/(Cl^– + HCO₃^–)   (1)

Gibbs Ratio II (for Cation) = Na+/(Na⁺+Ca²⁺)  (2)

where, all ions concentrations are expressed in mmol/l.

The different water samples have been classified according to their chemical composition using Gibbs dia­gram (Figure 14). The predominant samples (64%) fall into mixing and few (20%) samples into fresh water. Gibbs ratio I values in the present study vary from 0.28 to 0.85, with an average value of 0.61, while Gibbs ratio II values vary from 0.13 to 0.95, with an average value of 0.72. The first form of water is constantly modified due to both the effects of the water-rock relationship of the aquifer stone, which is made mostly of evaporite facies, and the impacts of human activities, and finally the return flow of irrigation. The effect is demonstrated by a large rise in concentrations of sodium, sulfate, and chloride in groundwater. The second form of water at northern boundary is affected by carbonate rock.

Controls of groundwater's hydrochemical evolution primarily rely on recharging water chemistry, water aquifer matrix activity, or both, as well as groundwater residence period within the aquifer. Two basic mechanisms lead to the deposition of solutes in groundwater: dissolution by evaporation and dissolution of carbonate [38]. Evolving water's chemistry relies not only on the matrix's bulk chemistry, but also on the weathering rate [39]. Therefore, only comparatively low concentrations of carbonates and evaporates may have a substantial effect on water chemistry [26]. We utilized the ion connection diagrams of the major elements produced in molar concentrations for these hydrogeochemical processes clarifying. Variation of the evaporate components influenced the first diagram. The Na⁺ vs Cl^– relation reveals that the majority of water samples contribute to a regression straight line with a 0.71 slope, significantly different from the slope 1 of the halite dissolution (Figure 15a). Therefore, Na+ and Cl^– are largely derived from the dissolution of halite [40], since the geological environment is very rich in evapo­rite minerals. The second diagram Ca²⁺ + Mg²⁺ vs SO₄^2– + HCO₃^– can highlight the origin of Ca²⁺, Mg²⁺ and SO₄^2–. Hence, it is evident that the chemistry of the water in the area is usually determined by the dissolution of calcite, dolomite and gypsum and by the interaction of ions, or the salinity is induced by the impact of the Hodna Lake (Figure 15b). This is also verified in the third Ca²⁺ vs SO₄^2– diagram (Figure 15c). The infiltration of the rain water through the carbonate formations in the northern part of study area allows the dissolution of limestones and dolomites of Cretaceous and Jurassic. During the groundwater flowing, the water allows the dissolution of gypsum and/or anhy­drite in Mio-Plio-Quaternary age. The last diagram Ca²⁺+Mg²⁺+HCO₃^–/Na⁺+K⁺+Cl^–+SO₄^2– vs TDS can demon­strate the global origin of salinity (Figure 15d). In fact, in this diagram two groups can be individualized by their salinity: samples with low salinity (TDS<1700 mg∙l^1-) dominated by carbonates coming from the northern part, and those with high salinity (TDS>1700 mg∙l^1-) dominated by salty minerals from the southern part.

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Figure 14. Gibbs diagrams for water samples of the Hodna aquifer

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Figure 15. Plot of (a) Na⁺vs Cl^–

(b) Ca²⁺+Mg²⁺vs SO₄^2–+HCO₃^– (c) Ca²⁺vs SO₄^2–

(d) TDS vs Ca²⁺+Mg²⁺+HCO₃^–/Na⁺+K⁺+Cl^–+SO₄^2–

3.4 Groundwater isotopes

The use of stable isotopes is key to determining groundwater sources and evolution [41]. Isotopic methodologies are well known nowadays in groundwater network hydrological studies [42]. Stable oxygen and hydrogen isotopes in groundwater give the original isotopic composition of rainwater recharging in the hydrological cycle [41-43]. They can include details on rainfall, evaporated surface water, and sources of seawater within the coordinate framework of ¹⁸O which ²H, and can therefore help to classify the causes of salinity in groundwater [44]. Table 3 indicates the δ¹⁸O and δ²H values of the groundwater samples tested. Stable isotopes of water points sampled in this study present a wide range of variation between –9.4 and –5.2 in δ¹⁸O with a mean of –7.1 ± 1.3‰, and from –62.0 to –42.0 in δ²H with a mean of –52.5 ± 5.6‰. In Figure 16, measured values of δ¹⁸O and δ²H from the investigated groundwater samples are compared with the global meteoric water line (GMWL: δ²H = 8 δ¹⁸O + 10), defined by Craig [45]: the groundwater sam­ples are distributed around a straight line intersecting the GMWL, with a slope equal to δ²H = 2.2 δ¹⁸O –36.1 (Figure 7). The aquifer's isotopic composition in δ²H and δ¹⁸O shows two classes of water that are virtually identical. The first category contains water samples strongly associated with the GMWL, demonstrating no major evaporation isotopic changes and that the aquifer regeneration is very fast. This group comprises primarily of wells (1, 15, 13, 6, 11, 19 and 22) located to the North of the recharge zone and two wells (7 and 14) located to the South of the plain secondarily. The second category is near below the meteoric water level, suggesting that the evaporation of groundwater [46, 47] and the salt waters of the lake have affected it. This group comprises basically of the wells located in the discharge region to the South of the plain (Figure 16).

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Figure 16. δ²H versus δ¹⁸O relationship for groundwater in the Hodna area