Assessment of Groundwater Sources and Their Impact on Sustainable Development in the Tindouf Region, Algeria

Examination of the piezometric map of the study area drawn up in April 2022 (Figure 4) shows more or less curved equipotential with variable spacing indicating a radial-type unconfined aquifer. The overall groundwater flow is from the NW to the SE in the direction of the overall loss of reservoir land from the NW to the Sebkha of Tindouf and the Sebka of Tinjouk. The streamlines converge towards the Graret El Hareth perimeter.

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Figure 4. Piezometric map of the study area

The streamlines show that the drained water comes from the northern flanks. In the center: A South-West North-East flow at the level of the Oued El Faai zone to go towards the agricultural perimeter Grarte El Harth, with a piezometric depression which corresponds to a piezometric catchment zone fed by water coming from the northern part of Hamada, it is a water catchment area at the piezometric level is less than 275 m, the streamlines are converging towards the boreholes of Grarte El Harth. According to the hypsometric curves, the lowest point of the aquifer is located in the area of the Sebkha of Tindouf where the aquifer evaporates and forms a zone of water accumulation, constituting the Sebkha.

The modulus of the hydro-isohypse curve spacing is not constant, which reflects local variations in permeability with an average hydraulic gradient of 2.10^-3. Drought that leads to evaporation can have significant repercussions on the flow of surface and groundwater. The continental tertiary in the study area is characterized by:

• saturated thickness > 60 m,
• effective thickness > 40 m,
• a piezometric level varies from 2.60 m to 31.60 m.

The piezometric depression observed in this area is due to:

• The increase in demand over time due to population growth.
• The strong quantitative and qualitative pressure on resources (growth in needs) and the irrational exploitation of this resource at the level of two agricultural areas, Grarret el Harth and wadi m'haya with a decrease in rainfall of 35.44mm/year in this arid zone. The excessive demand for the establishment of new agricultural areas and the increase in random drilling operations as well as the overexploitation of the aquifer, knowing where these waters are fossil and non-renewable waters, which has had a negative impact on the reservoir.

Water chemistry analysis is an essential complement to the hydrogeological study. Changes in groundwater geochemistry may be a function of changes in hydrogeological properties or mineral content of host rocks [23, 24]. It provides a great deal of information on the aquifer environment [25], the nature of the surrounding rock, the supply and circulation zones, as well as the potability of the water. This hydrochemical study is based on the interpretation of chemical analyzes of samples from water points (boreholes and wells). These analyzes were carried out in the water chemistry laboratory, and focused on the following major chemical elements:

• pH,
• Electrical conductivity,
• Mineralization and dry residue,
• Cations: Ca⁺⁺, Mg⁺⁺, Na⁺, and K⁺,
• Anions: HCO₃^--, SO₄^--, Cl^--, and NO₃.

Compared to Algerian and, the world health organiztion (W H O) (Table 2 and Table 3) standards of potability the water [26] remains within the standards with regard to cations and anions approximately 96.29% of the water points are drinking water.

The data used in this part come from a field campaign in the study are carried out on March 2022. The physico-chemical analyzes of eight samples measured were carried out in the laboratory; the procedure was followed according to the following protocol:

• The sampling for the analysis of the major chemical elements (anions and cations) was carried out in 150ml polyethylene bottles and, stored at 4℃. The anions (Cl^-, SO₄^-2 and, NO₃^-) were analyzed by liquid phase ion chromatography and the bicarbonates (HCO₃^-) by alkalinity titration. The cations (Ca⁺², Mg⁺², Na⁺ and K⁺) are analyzed by atomic absorption spectrometer (AAS).
• During sampling, the physical parameters (electrical conductivity and pH) were measured in situ.

Table 2. Algerian potability standards

Table 3. World Health Organization standards for drinking water (WHO, 2011)

In the Table 4, the abbreviations represent:

(1) Max: the largest value in the analysis results of the chemical parameter study.

(2) Min: the smallest value in the analysis results of the chemical parameter study.

(3) Mean= $=\mu$: arithmetic mean Eq. (1), for the current study (x_i: denotes a value of the statistical series of samples).

$\mu=\frac{x_1+x_2++\cdots+x_n}{N}=\frac{\sum_{i=1}^n x_i}{N}$                         (1)

(4) To calculate the standard deviation, we proceed as follows:

• We calculate the square of the deviation from the mean of each of the values of the series:

$\left|x_i-\mu\right|^2$                         (2)

• We calculate the sum of the values obtained:

$\sum_{i=1}^n\left|x_i-\mu\right|^2$                         (3)

• We divide by the size of the series:

$\frac{\sum_{i=1}^n\left|x_i-\mu\right|^2}{N-1}$                         (4)

• Calculate the square root of the result:

$\sigma=\sqrt{\frac{\sum_{i=1}^n\left|x_i-\mu\right|^2}{N-1}}$                         (5)

(5)

Variance: $\sigma^2$                         (6)

(6) Standard Error: it is the ratio between the standard deviation and the square root of height of sample (n), it is given by the following Eq. (7):

$E_s=\frac{\sigma}{\sqrt{n}}$                         (7)

The results of the analysis of physical-chemical parameters of groundwater in the study area (Table 2) show values of electrical conductivity oscillating between 1.97 ms/cm and 2.68 ms/cm with a very low standard deviation of about 0.26. Its variation is related to the nature of dissolved salts and their concentration. The Ca⁺² contents are included between 58 mg/l as the minimum value, they will reach the maximum value of 147,5 mg/l. The maximum content of Mg⁺² is 158 mg/l, and the minimum concentration is 30.2 mg/l. The maximum concentration of Na+ in our study is between 80.78 and 237 mg/l with a significant standard deviation of about 57.87 mg/l, and the maximum content of K⁺ is 11 mg/l, the minimum concentration is 3.69 mg/l. The maximum Cl^- measurement is 3 83.4 mg/l, and the minimum concentration is 230 mg/l. SO₄^- values range from 230.57 mg/l to 495 mg/l. Concentrations of the NO₃^- are low, ranging from 0.18 mg/l to 85 mg/l with a standard deviation of about 34.87 mg/l.

Table 4. The physical and chemical parameters of the water in the study area