5. Analyses and evaluation of pumping test data

A pumping test is a tool to determine the hydraulic characteristics of water-bearing formations such as transmissivity, storage coefficient, and any relevant hydrogeological properties. Such a test is called an aquifer test.

Analytical methods were applied to determine the aquifer type and hydrogeological properties of the Kuwait Group aquifer of the study area. These methods are Theis type curve [18], Cooper and Jacob straight line method for confined aquifer [19], and Walton method for semiconfined aquifer [20]. In effect, the pumping test data analyses indicated that the Kuwait Group aquifer is confined to semi-confined aquifer as shown in Figure 2 for the well AT-15 and Figure 3 for the well AT-18. The aquifer transmissivity ranges between 62.03 and 320.51 m2 /day, where the estimated storage coefficient equals 7.5 <sup>10</sup><sup>4</sup> . The flow net analysis shows that the groundwater flows from the southwest to the northeast. Recharge to the aquifer is primarily from subsurface flow from adjacent bed rocks and by leakage from the underlying Dammam Formation aquifer. The presence of an aquitard layer (i.e., sandy clay) that bounds the aquifer from the top acts as a semipermeable layer and can leak water into the aquifer in the direction of the hydraulic gradient.

6. Mechanisms of controlling groundwater chemistry

Figure 3. Time-drawdown curve of well no. AT-18, Walton method.

increase the concentration of total dissolved in groundwater.

the first ratio is for the cations [(Na<sup>+</sup> + K<sup>+</sup>

anions Cl/ (Cl + HCO

It is important to study the relationship between the water chemistry and the aquifer lithology. Gibbs as mentioned in [12] suggested a diagram that represents the ratio of dominant anions and cations plotted against the value of TDS. These ratios can be divided into two formulas,

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functional sources of dissolved constituents such as precipitation dominance, rock dominance, and evaporation dominance. The chemical analyses of the study area are plotted in Gibb's diagram as shown in Figure 4, and they showed that the predominant samples fall into the category of rock-water interaction field and few samples are located in evaporation-dominance field and precipitation-dominance field, which revealed that the chemical weathering of rockforming minerals is influencing the groundwater quality by dissolution of rock through which there is circulation, while the data in the evaporation-dominance field indicate that the increasing ions of Na+ and Cl are in relation with the increasing of the TDS, as evaporation will

)/(Na<sup>+</sup> + K<sup>+</sup> + Ca2+)] and the second ratio is for the

3) as a function of TDS. This diagram is widely used to evaluate the

Figure 2. Time-drawdown curve of well no. AT-15, using Cooper and Jacob straight line method.

Hydrogeology and Groundwater Geochemistry of the Clastic Aquifer and Its Assessment for Irrigation, Southwest… http://dx.doi.org/10.5772/intechopen.71577 113

Figure 3. Time-drawdown curve of well no. AT-18, Walton method.

5. Analyses and evaluation of pumping test data

test is called an aquifer test.

112 Aquifers - Matrix and Fluids

the direction of the hydraulic gradient.

320.51 m2

A pumping test is a tool to determine the hydraulic characteristics of water-bearing formations such as transmissivity, storage coefficient, and any relevant hydrogeological properties. Such a

Analytical methods were applied to determine the aquifer type and hydrogeological properties of the Kuwait Group aquifer of the study area. These methods are Theis type curve [18], Cooper and Jacob straight line method for confined aquifer [19], and Walton method for semiconfined aquifer [20]. In effect, the pumping test data analyses indicated that the Kuwait Group aquifer is confined to semi-confined aquifer as shown in Figure 2 for the well AT-15 and Figure 3 for the well AT-18. The aquifer transmissivity ranges between 62.03 and

shows that the groundwater flows from the southwest to the northeast. Recharge to the aquifer is primarily from subsurface flow from adjacent bed rocks and by leakage from the underlying Dammam Formation aquifer. The presence of an aquitard layer (i.e., sandy clay) that bounds the aquifer from the top acts as a semipermeable layer and can leak water into the aquifer in

. The flow net analysis

/day, where the estimated storage coefficient equals 7.5 <sup>10</sup><sup>4</sup>

Figure 2. Time-drawdown curve of well no. AT-15, using Cooper and Jacob straight line method.

## 6. Mechanisms of controlling groundwater chemistry

It is important to study the relationship between the water chemistry and the aquifer lithology. Gibbs as mentioned in [12] suggested a diagram that represents the ratio of dominant anions and cations plotted against the value of TDS. These ratios can be divided into two formulas, the first ratio is for the cations [(Na<sup>+</sup> + K<sup>+</sup> )/(Na<sup>+</sup> + K<sup>+</sup> + Ca2+)] and the second ratio is for the anions Cl/ (Cl + HCO 3) as a function of TDS. This diagram is widely used to evaluate the functional sources of dissolved constituents such as precipitation dominance, rock dominance, and evaporation dominance. The chemical analyses of the study area are plotted in Gibb's diagram as shown in Figure 4, and they showed that the predominant samples fall into the category of rock-water interaction field and few samples are located in evaporation-dominance field and precipitation-dominance field, which revealed that the chemical weathering of rockforming minerals is influencing the groundwater quality by dissolution of rock through which there is circulation, while the data in the evaporation-dominance field indicate that the increasing ions of Na+ and Cl are in relation with the increasing of the TDS, as evaporation will increase the concentration of total dissolved in groundwater.

Figure 4. Gibbs plots represent groundwater chemistry and geochemical process in the study area.

#### 6.1. Hydrochemical facies

Hydrochemical facies interpretation using Piper trilinear diagram is a useful tool for determining the flow pattern and origin of chemical histories of groundwater. The Piper trilinear diagram is presented in Figure 5. One principal hydrochemical water type has been delineated. The majority of the groundwater samples of the study area fall in Ca 2+– Na<sup>+</sup> � Cl� water type, where alkaline earth (Ca2+ + Mg2+) exceeds the alkaline (Na+ + K+ ) and strong acid (Cl� and SO4 <sup>2</sup>�) exceeds the weak acid (HCO3 � and CO3 <sup>2</sup>�), and non-carbonate hardness exceeds 50%.

#### 6.2. Saturation index

Geochemical models are tools used to calculate chemical reaction in groundwater system such as dissolution and precipitation of solids, ion exchange, and sorption by clay minerals. In this study, the speciation model has been applied to the groundwater samples of Al-Atraf field to determine the saturation index (SI) of minerals. The SI for a given mineral measures the degree of saturation of that mineral with respect to the surrounding system. The degree of saturation index is defined as follows [21]:

$$\text{SI} = \log \frac{\text{K}\_{\text{iap}}}{\text{K}\_{\text{sp}}} \text{ \tag{1}$$

groundwater is being saturated with respect to the mineral and incapable of dissolving more of the minerals. The oversaturation can also be produced by incongruent dissolution, common

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Figure 5. Piper trilinear diagram representing the chemical analysis of the study area.

Table 1 shows the saturation indices of anhydrite, calcite, gypsum, dolomite, halite, and silica along with Pco2. Nearly, all groundwater samples of the study area are undersaturated with respect to anhydrite, gypsum, halite, and silica and oversaturated with respect to calcite and

The partial pressure of the carbon dioxide value (Pco2) of the study area ranges between 1.32 <sup>10</sup><sup>3</sup> and 8.23 <sup>10</sup><sup>3</sup> atm., with an average value of 3.78 <sup>10</sup><sup>3</sup> atm. This indicates that the groundwater of the Kuwait Group aquifer becomes charged with CO2 during infiltration through the soil zones. According to Appelo et al. [22], when Pco2 values range between 102.5

ion effect.

dolomite.

where "iap" is the ion activity product of the dissociated chemical species in solution and "Ksp" is the solubility product of the mineral. When SI is <0, it indicates that the groundwater is undersaturated with respect to that particular mineral. When SI > 0, it means that the Hydrogeology and Groundwater Geochemistry of the Clastic Aquifer and Its Assessment for Irrigation, Southwest… http://dx.doi.org/10.5772/intechopen.71577 115

Figure 5. Piper trilinear diagram representing the chemical analysis of the study area.

6.1. Hydrochemical facies

114 Aquifers - Matrix and Fluids

(Cl� and SO4

exceeds 50%.

6.2. Saturation index

index is defined as follows [21]:

Hydrochemical facies interpretation using Piper trilinear diagram is a useful tool for determining the flow pattern and origin of chemical histories of groundwater. The Piper trilinear diagram is presented in Figure 5. One principal hydrochemical water type has been delineated. The majority of the groundwater samples of the study area fall in Ca 2+– Na<sup>+</sup> � Cl�

Geochemical models are tools used to calculate chemical reaction in groundwater system such as dissolution and precipitation of solids, ion exchange, and sorption by clay minerals. In this study, the speciation model has been applied to the groundwater samples of Al-Atraf field to determine the saturation index (SI) of minerals. The SI for a given mineral measures the degree of saturation of that mineral with respect to the surrounding system. The degree of saturation

SI ¼ log

where "iap" is the ion activity product of the dissociated chemical species in solution and "Ksp" is the solubility product of the mineral. When SI is <0, it indicates that the groundwater is undersaturated with respect to that particular mineral. When SI > 0, it means that the

Kiap Ksp

� and CO3

) and strong acid

<sup>2</sup>�), and non-carbonate hardness

<sup>0</sup> (1)

water type, where alkaline earth (Ca2+ + Mg2+) exceeds the alkaline (Na+ + K+

Figure 4. Gibbs plots represent groundwater chemistry and geochemical process in the study area.

<sup>2</sup>�) exceeds the weak acid (HCO3

groundwater is being saturated with respect to the mineral and incapable of dissolving more of the minerals. The oversaturation can also be produced by incongruent dissolution, common ion effect.

Table 1 shows the saturation indices of anhydrite, calcite, gypsum, dolomite, halite, and silica along with Pco2. Nearly, all groundwater samples of the study area are undersaturated with respect to anhydrite, gypsum, halite, and silica and oversaturated with respect to calcite and dolomite.

The partial pressure of the carbon dioxide value (Pco2) of the study area ranges between 1.32 <sup>10</sup><sup>3</sup> and 8.23 <sup>10</sup><sup>3</sup> atm., with an average value of 3.78 <sup>10</sup><sup>3</sup> atm. This indicates that the groundwater of the Kuwait Group aquifer becomes charged with CO2 during infiltration through the soil zones. According to Appelo et al. [22], when Pco2 values range between 102.5


and 106.4 atm., it represents a closed system. Since the Kuwait Group aquifer is acting as a confined to semi-confined aquifer, it is more likely that the groundwater represents a deep, closed environment system. The mass-balance modeling as mentioned in Ref. [11] has been applied to specify the amounts of reacting minerals and to determine the possible source of the

Figure 6. (a) Relation between total cations and (Na+K) in the study area. (b) Relation between total cations and (Ca+Mg)

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in the study area.

Table 1. Results of thermodynamic speciation calculation of the study area.

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Figure 6. (a) Relation between total cations and (Na+K) in the study area. (b) Relation between total cations and (Ca+Mg) in the study area.

and 106.4 atm., it represents a closed system. Since the Kuwait Group aquifer is acting as a confined to semi-confined aquifer, it is more likely that the groundwater represents a deep, closed environment system. The mass-balance modeling as mentioned in Ref. [11] has been applied to specify the amounts of reacting minerals and to determine the possible source of the

Table 1. Results of thermodynamic speciation calculation of the study area.

116 Aquifers - Matrix and Fluids

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 likely to occur in aquifer. The ratio Ca2+ / Ca2+ + SO4 <sup>2</sup> of most groundwater samples ranged 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 dissolution. According to reference [11], waters with HCO3 /SiO2 < 5 indicated mainly silicate weathering. However, the ratio HCO3 /SiO2 of the groundwater samples found to be ranged 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.
