3.1 Chemical characteristics
The results obtained show that the Bordjias plain groundwater exhibits a variable chemical composition (Table 3). Electrical conductivity of all samples varies from 735 to 4940 µS.cm-1. pH values fall within WHO standards (between 6.5 and 8.5)[ 17]. The region water samples can be classified according to their conductivity into three main categories (Table 2); the increase of water samples conductivity from points lying at the southern limit of the Bordjiasplain, can be attributed to the lithological nature of the embankment lands, consisting essentially of clay and marl [18].
Concentrations of Ca
2 + and Mg
2+ in the samples were converted to hardness expressed in mg L
-1 CaCO
3 (TH). Sample hardness was distributed as follows: 05 water points (16.13%) exhibited very hard water (TH > 500 mgL
-1), 22 water points (70.9%), hard water (500 > TH > 300 mg L
-1), and 04 water points (12.9%), a moderately hard water (TH < 300 mgL
-1). Calcium and magnesium can be attributed to the dissolution of calcite and dolomite in carbonate ores [
19]. Sodium levels ranged from 61.4 to 885.5 mg L
-1. For the most part, waters of the Sirat, Mesra and Fornaka regions have Na
+ contents higher than 150 mgL
-1. Potassium contents, on the other hand, vary between 2.7 and 14.1 mg L
-1in all the samples analyzed. The presence of evaporites within the impermeable substratum can account for sodium content modification. In some localized zones, Na
+ ion content exceeds 250 mgL
-1, due to soil enrichment in clay leading to less permeability[
20]. Cl
-, SO
42-, NO
3- and PO
43-ions are inorganic components which can contribute significantly to the deterioration of groundwater quality. Nitrate concentration in the Southwest region water varies between 13.6 and 167.8 mg L
-1. 61.30%, of water samples from 19 localities, showNO
3- levels higher than the 50 mg L
-1recommended standard.
Samples from the Sirat, Mesra and Fornaka regions are, in particular, highly contaminated with nitrates (NO3-) [ 21]. This is probably a consequence of the presence of the water points in farming areas, where ammonium nitrate is often used as a chemical fertilizer. Reuse of groundwater used in irrigation may also be responsible for the increase in groundwater nitrate content due to leaching (leachate) and soil washing processes. PO43- phosphate concentrations in most water samples (0.12 to 0.67 mg L-1) are relatively high, due to phosphate fertilizers presence in crop areas but relatively low concentrations were measured in other water points (0.01 to 0.08 mg L-1), this may be due to the lithological composition. Chloride concentration varied between 106.6 and 1136 mg L-1. 26 of the collected samples (83.90%), had chloride levels higher than 150 mg L-1, knowing that the maximum concentration for good quality water must not exceed 150 mg.L-1.
The high Cl- content in Bordjias Plain groundwater would probably result from the erosion and dissolution of rocks containing evaporites. Water flowing from the Ennaro catchment to the south becomes laden with chloride salts to attain values greater than 150mgL-1, due to high aquifer evapotranspiration (shallow water table). Sulfate concentration (SO42-) ranges from 27 to 599.5 mgL-1, 30 testedwater samples showed sulfate concentrations below the 250 mgL-1standard but for the P2 Siratwater point, a value as high as 600 mg L-1 was found.
Sources of sulfates include rainwater, fertilizers, and especially sulfide minerals dissolution. The presence of the gypsum diapyrat the Ain-Nouissy region could also be responsible for the sulfate presence in the groundwater flowing in the direction of the Sirat. Indeed, the existence of gypsum marls, or even Triassic gypsum, can account forsulfate salts high content [22, 23]. Oxidation of pyrite contained in the gypsum may result in sulfate ions production.
Analysis of Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V elements showed the absence of metallic chemical pollution in the waters analyzed according to World Health Organization standards [19]. Indeed, the relatively low values of trace elements present in these samples indicate that the studied samples of groundwater are situated far from pollution sources and that can be explained by the lithological composition of the region. The concentration of HCO3-was quite high and varied from 201.3 to 373.9 mg L-1 throughout the study area. Natural processes such as the dissolution of carbonate minerals and the dissolution of CO2 in soil are sources of HCO3- to groundwater. The reaction process is as follows;
- CaCO3+CO2+H2O Ca2+ + 2 HCO3− (1)
- CO2+H2 O H+ + HCO3− (2)
In addition, anthropogenic carbon dioxide should be considered as a potential source of bicarbonate in groundwater [24]. Potential sources of CO2 in process (1) come from municipal waste at garbage dumps. For process (3), CO2 is caused by the oxidation of organic matter leaking from old sewage systems. Bicarbonates can also be obtained from the reduction of sulfates by organic matter in the water table according to the reaction scheme (3 and 4) proposed by [25].
CH2O+O2 CO2+H2O (3)
2 CH2O+SO42− H2S+2 HCO3− (4)
3.3 Chemical reactions of alterations
The abundance of limestone at the watersheds of the study area and the rate of erosion and weather resistance suggest that the dissolution of the carbonate minerals will add significant quantities of Ca2 + and Mg2 + to the reservoir. The most common reaction for calcite is a simple dissolution giving a 1: 1 Ca2 +/HCO3- equivalence ratio.
Water samples with a Ca2 +/HCO3- ratio less than unity represent 25.8% of the samples analyzed and are just below the 1: 1 ratio line (Fig. 4a). All other water samples, 74.2%, are above this line, which probably indicates the existence of another source of calcium, which may be caused by the erosional interaction between water and rock [27].
The dissolution of gypsum in marl and clay is a potential source of additional SO42− and Ca2 + in the waters. Most samples are above the 1: 1 ratio line (Fig. 4b) with the exception of P2Sirat well. This may denote another source of calcium. Well number 2 is below the 1: 1 Ca2 +/SO42−ratio line, suggesting in this case that calcium may have been incorporated and removed from the system during clay formation [28]. If sodium presence can only be accounted for by the dissolution of evaporite minerals, then Na+ ions would have to balance Cl− ions. Sodium is slightly in excess in most of the analyzed samples and accounts for 67.7% of the total. In 32.3% of the water points, Na+ amount is small compared to Cl−amount. In this case, Na+ ions could be consumed in the formation of clay in this region (Fig. 4c). Thus, the chemical composition of groundwater samples appears to be governed mainly by the dissolution and reprecipitation of minerals in the receiving lithologies of the watershed [29].
3.4 Classification of water samples
The major ions that determine the chemical composition of the studied ground water are plotted on a triangular diagram that Fig. 5 depicts. Nitrates were added to the Piper diagram [30] because of their abundance in groundwater. Water samples from the localities of Sirat and Fornaka except those from drilling (F1. Fornaka) are mainly found in the Na-Cl (NO3 + SO4) zone. The ions (Na + K) and (Cl + NO3) have concentrations ranging from 50–76% meqL-1 and 57-71.5% meqL-1, respectively. A large number of samples from the green region (Ennaro, Souaflia, Saf-Saf, O /Meddah and AinSoltane) are of the Ca-HCO3 (Cl + NO3) type. The concentration of (Ca + NO3) and HCO3ions varies between 45-68.27% meqL-1, 39–55% meqL-1 and 33-43.13% meqL-1, respectively except for water points P1Souaflia, F2.Souaflia and F3.Ennaro, which are of the N-HCO3 (Cl + NO3) type. Water samples from the locality of Mesra, which are mainly of the type Ca-Cl (NO3 + SO4),exhibit more or less high magnesium concentrations (20.55–27.22% meqL-1). Their respective (Ca + Mg) and (Cl + NO3) concentrations vary between 52.82–62.74% meqL-1 and 49.68–54.71% meqL-1. Water samples in this region have varying chemical composition. Indeed, the water flowing to the south-west region (Sirat and Fornaka) would have evolved from the Ca-HCO3 (Cl + NO3) to the Na-Cl (NO3 + SO4) type through the water-rock interactions in the sandy-clay zones. The distribution of points on the diagram shows that various natural reactions are involved in the control of the groundwater chemical composition [30] .
3.5 Evaluation of samples
3.5.1 Suitability of samples for consumption and household Use
According to WHO criteria (2011), it can be said that 77.4% of water samples studied (Table 4) are suitable for human consumption with the exception of water points (P1 Sirat, P2 Sirat, P3Sirat, F2 Sirat, F3Sirat, F3Mesra and P2Fornaka), which are unsuitable for consumption because of high nitrate concentrations although they can be used for household purposes.
3.5.2 Suitability for irrigation
The sodium adsorption ratio (SAR) [31] can be used as an index referring to the suitability of water for irrigation
where ion concentration in irrigation water is in meqL-1. The calculation of the sodium adsorption ratio for a given water provides a useful indicator for the risk (hazard) of sodium presence in water for soils and crops. The samples studied were found to have SAR values ranging from 1.63 to 15.52, which makes them suitable for irrigation (Table 4).
3.5.3 Sample suitability for industrial applications
The saturation index is a very important factor for water quality assessment [32]. This index indicates the extent to which the water flowing through pipes will precipitate or dissolve CaCO3. It is defined as the actual pH of water (pH water) minus the calculated pH (pHc), where water would have equilibrium with calcium carbonate (pHc):
Sat index = pHe–pHc (2)
pHc is calculated by the following equation:
pHc = ( pK’2 – pK’s ) + pCa2+ + pAlk (3)
From the negative logarithms of the dissociation constant of H2CO3 (pK'2), the calcium carbonate solubility product (pK's), the molar concentration of Ca2 + (pCa2 +) and the CO32- plus HCO3- (pAlk). If the saturation index is positive, CaCO3 will precipitate but if it is negative,it will dissolve. The application of this index to the samples (Table 4) indicates that most samples, i.e., 87.1% show a tendency to precipitate CaCO3 (values > 0) with the exception of water points 19, 29, 30 and 31, which show a tendency to dissolve (value < 0).