7. Geochemical evolution of groundwater

The initial composition of groundwater originates from rainfall with low concentrations of dissolved ions. During its return path to the ocean, the water composition is altered by rock weathering and evaporation causing more Ca2+, Mg2+, Na+ , SO4 <sup>2</sup>, HCO3 , Cl, and SiO2 to be added. The concentration of these ions depends on the rock mineralogy that the water encounters and its rapidity along the flow path. The abundance of the major cations in Al-Atraf field is in the order of Na+ > Ca2+ > Mg2+ > K<sup>+</sup> . The sequence of the anions is in the order of Cl > SO4 <sup>2</sup> > HCO3 . The majority of the groundwater samples of the study area (75.36%) exhibited NaCl water chemical type, followed by (23.19%) of Na2SO4 and (1.45%) of CaSO4 water chemical types. The average TDS of 4441 mg/l represents brackish groundwater as presented in Table 2. Calcium and magnesium present in the groundwater are mainly due to the dissolution of gypsum and anhydrite, the most rock-forming minerals of the Kuwait Group aquifer of the study area. Calcium ions are derived also from cation exchange process. The concentration of calcium ions in the study area ranges from 332 to 743 mg/l with an average value of 484.23 mg/l and magnesium ranges from 85 to 203 mg/l, with an average value of 140.87 mg/l. This indicates that the Ca2+ ion concentration in the study area is relatively higher than magnesium ion. Alkalinity is the quantitative capacity of an aqueous solution to neutralize an acid. The ideal range of the total alkalinity is from 80 to 140 mg/l. In natural environment, carbonate alkalinity tends to make up most of the total alkalinity due to the common occurrence and dissolution of carbonate rocks and the presence of carbon dioxide in the atmosphere. The total alkalinity of the study area ranges between 51.2 and 127 mg/l as CaCO3 with an average value of 91.39 mg/l. Figure 7a represents Ca2+ + Mg2+ versus alkalinity + SO4 2 in e.p.m., suggesting that these ions have resulted from weathering of carbonate and sulfate

Table 2. Report of physico-chemical parameters of the studied groundwater samples of the study area.

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major ions in a specific groundwater system. This is in order to deduce groundwater source rock and to determine the nature and the extent of the geochemical reactions that occur in this system, that is, water-rock interaction. By the application of Hounslow concept, all groundwater samples of the study area showed that Cl > Na+ indicating that the reverse ion exchange is

between 0.41 and <0.5 indicating calcium removed by ion exchange or calcite precipitation, and few groundwater samples show a range value of 0.5–0.6, which is due to gypsum disso-

between 1.45 and 4.77 which indicate that silicate weathering is a dominant chemical process in the aquifer. Dissolved silica data show the influences of silicate weathering on water chemistry in the study area. Participation of silicate minerals in the chemical reactions plays a vital role in groundwater chemistry. Silicate weathering can be evaluated by estimating the ratio between Na+ + K<sup>+</sup> and the total cation (e.p.m) as shown in Figure 6a. This reveals that the silicate weathering contributes mainly Na+ and K<sup>+</sup> ions to groundwater [23]. Further, the plot of Ca2+ + Mg2+ versus total cations of the groundwater samples as in Figure 6b has a linear spread, indicating that some of these ions (Ca2+ + Mg2+) are resulted from the weathering of silicate minerals. In addition, all the groundwater samples exhibited an oversaturation with respect to calcite, which suggest the prevailing of calcite precipitation process in the aquifer.

The initial composition of groundwater originates from rainfall with low concentrations of dissolved ions. During its return path to the ocean, the water composition is altered by rock

added. The concentration of these ions depends on the rock mineralogy that the water encounters and its rapidity along the flow path. The abundance of the major cations in Al-Atraf field is

exhibited NaCl water chemical type, followed by (23.19%) of Na2SO4 and (1.45%) of CaSO4 water chemical types. The average TDS of 4441 mg/l represents brackish groundwater as presented in Table 2. Calcium and magnesium present in the groundwater are mainly due to the dissolution of gypsum and anhydrite, the most rock-forming minerals of the Kuwait Group aquifer of the study area. Calcium ions are derived also from cation exchange process. The concentration of calcium ions in the study area ranges from 332 to 743 mg/l with an average value of 484.23 mg/l and magnesium ranges from 85 to 203 mg/l, with an average value of 140.87 mg/l. This indicates that the Ca2+ ion concentration in the study area is relatively higher than magnesium ion. Alkalinity is the quantitative capacity of an aqueous solution to neutralize an acid. The ideal range of the total alkalinity is from 80 to 140 mg/l. In natural environment, carbonate alkalinity tends to make up most of the total alkalinity due to the common occurrence and dissolution of carbonate rocks and the presence of carbon dioxide in the atmosphere. The total alkalinity of the study area ranges between 51.2 and 127 mg/l as CaCO3 with an average value of 91.39 mg/l. Figure 7a represents Ca2+ + Mg2+ versus alkalinity + SO4

in e.p.m., suggesting that these ions have resulted from weathering of carbonate and sulfate

, SO4

. The majority of the groundwater samples of the study area (75.36%)

<sup>2</sup>, HCO3

. The sequence of the anions is in the order of

, Cl, and SiO2 to be

2

<sup>2</sup> of most groundwater samples ranged

/SiO2 < 5 indicated mainly silicate

/SiO2 of the groundwater samples found to be ranged

likely to occur in aquifer. The ratio Ca2+ / Ca2+ + SO4

lution. According to reference [11], waters with HCO3

7. Geochemical evolution of groundwater

in the order of Na+ > Ca2+ > Mg2+ > K<sup>+</sup>

<sup>2</sup> > HCO3

Cl > SO4

weathering and evaporation causing more Ca2+, Mg2+, Na+

weathering. However, the ratio HCO3

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Table 2. Report of physico-chemical parameters of the studied groundwater samples of the study area.

Figure 7. (a) Relation between Ca+Mg and alkalinity + SO4. (b) Relation between Ca2+ and alkalinity. (c) Relation between Ca2+ and SO2 4.

Table 3. Irrigation water quality parameters for groundwater samples of the study area.

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Table 3. Irrigation water quality parameters for groundwater samples of the study area.

Figure 7. (a) Relation between Ca+Mg and alkalinity + SO4. (b) Relation between Ca2+ and alkalinity. (c) Relation between

Ca2+ and SO2

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4.

minerals (gypsum and anhydrite). However, most of the points are placed in Ca2+ + Mg2+ side, which indicates excess calcium and magnesium derived from other processes such as reverse ion exchange reactions. In Ca2+ versus alkalinity diagram, Figure 7b indicates the contribution of both calcite and dolomite weathering on groundwater chemistry of the study area. Moreover, in Ca2+ versus SO4 <sup>2</sup>� diagram (Figure 7c), most of the sample show excess calcium over sulfate, which reveal that the groundwater samples seem to be derived from gypsum or anhydrite dissolution. Moreover, excess sulfate over calcium in few samples expresses the removal of calcium from the system likely by calcite precipitation. Therefore, silicate weathering and carbonate dissolution are the prevailing geochemical processes in the aquifer of the study area.

#### 7.1. Ion exchange

Ion exchange is one of the important processes responsible for the concentration of ions in groundwater.

$$\text{CAI} - 1 = \frac{\text{Cl}^- - (\text{Na}^+ + \text{K}^+)}{\text{Cl}^-} \tag{2}$$

TH <sup>¼</sup> <sup>2</sup>:5 Ca<sup>2</sup><sup>þ</sup> <sup>þ</sup> <sup>4</sup>:1 Mg2þ: (3)

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<sup>2</sup>� – Ca<sup>2</sup><sup>þ</sup> <sup>þ</sup> Mg2<sup>þ</sup> (4)

� when

where Ca2+ and Mg2+ concentrations are expressed in mg/l as CaCO3. Hardness of water is due to the precipitation of Ca2+ and Mg2+ salts like carbonate, sulfates, and chlorides. Hardness of water causes scaling of pots, boilers, and irrigation pipes. However, the total hardness of the study area is varying from 1326 to 4040 mg/l as CaCO3, with an average value of 1826.22 mg/l as shown in Table 2. The analytical result of TH indicates that the groundwater of the study area is exceeding very hard water type according to [28] as shown in Table 4. Therefore, according to TDS and TH standards the groundwater is not suitable for drinking purposes.

Hydrogeology and Groundwater Geochemistry of the Clastic Aquifer and Its Assessment for Irrigation, Southwest…

The suitability of groundwater for irrigation depends on the effect of mineral composition of water on the soil and plants. The effect of the salt on soils causes change in soil structure,

Residual sodium carbonate has been calculated to determine the hazard effects of carbonate and bicarbonate on the quality of water for irrigation and is expressed by the equation:

Where all ionic concentrations are expressed in meq/l. The classification of irrigation water according to the RSC presents in Table 5 after [29], where water containing more than 2.5 meq/l of RSC are not suitable for irrigation, while those having <1.25 meq/l are good for irrigation. Eaton (1950) indicated that if waters which are used for irrigation contain excess of

evaporation takes place and the pH of the soil increases up to 3 [30]. When total carbonate levels exceed the total amount of calcium and magnesium, the water quality diminished [31]. The calculated RSC values of the groundwater samples of the study area are ranged from �52.63 to

<sup>2</sup>� than its equivalent Ca2+ + Mg2+, there will be a residue of Na+ + HCO3

<sup>3</sup> þ CO3

8.2. Irrigational suitability

8.2.1. Residual sodium carbonate

HCO3

� + CO3

Table 4. Water classes (After [28]).

permeability, and hence it effects on plant growth.

RSC ¼ HCO�

Where all values are expressed in meq/l. When there is an exchange between Ca2+ or Mg2+ in groundwater with Na+ and K+ in the aquifer material, the CAI-1 is negative, and if there is a reverse ion exchange, CAI-1 will be positive [24]. The values of CAI-1 of the study area are positive in most wells, and very few wells show negative, and the CAI-1 ranges from �0.05 to 0.39 with an average value of 0.19 as presented in Table 3. Thus, it reveals that reverse ion exchange is the dominant process in the groundwater, whereas normal ion exchange is also noticed in a very few wells.
