Water quality assessment and reuse potential of groundwater combined saline solution for irrigation

Groundwater is the main source of freshwater on Earth, accounting for 98% of all freshwater in the liquid form. This water is essential for domestic use and agriculture, particularly in arid and semi-arid regions of the world. Groundwater management is crucial to address various qualitative and quantitative issues and to understand groundwater's behaviour in spatial and temporal terms (Kumar & Remadevi 2006). In addition, the main factors contributing to the continued degradation of this natural resource are climate change, rapid population growth, mismanagement, and a misperception of the nature of the resources due to poor coordination and integration of approaches (Ravikumar et al. 2011). In the north of the Sahara (Touggourt and El-Oued), the shallow aquifer was the main source of drinking water supply. However, due to the exponential increase in the amount of water needed because of urbanization, population increase, and the need for economic development. The mineral quality of deep groundwater at the level of the continental terminal aquifer is decreased during the last few years (Kadri & Chaouche 2018).

Many authors have evaluated the groundwater quality of the terminal complex and continental intercalary for various uses, most notably for drinking: Bouselsal & Saibi (2022) found that the water quality does not meet drinking water standards. The shallow aquifer is highly mineralized.

Also, Zobeidi & Moussaoui (2013) revealed that the water quality does not meet drinking water standards and reported that the shallow aquifer is highly mineralized. While Zaiz et al. (2017) showed through the hydrochemical study that the groundwater of the terminal complex is too mineral and extremely hard, Bouselsal et al. (2014) and Belksier et al. (2015) showed that the water in the aquifer is of very poor quality for drinking, highly mineralized, and very hard, exceeding the World Health Organization (WHO) guidelines.

Barkat et al. (2021) found that the water quality index (WQI) values of the terminal groundwater complex indicated unsuitability for drinking. According to their results, the groundwater is too mineral for drinking and exceeds the WHO guidelines. Therefore, the demineralization process of groundwater is an optimal solution to provide drinking water. Hence, the Algerian state has created groundwater demineralization stations.

For demineralization stations in desert areas, brine is discharged into the environment. The discharge of brine directly into the environment, which returns to the unconfined aquifer, and the lack of a natural outlet have led to the rise of groundwater to the surface or near the surface; the consequences of this phenomenon are harmful to humans and the environment, Also, the strong mineralization of water was caused by the intense evaporation of open water bodies and the dissolution of salts. Since the study area is an agricultural area and the saline solution has adverse impacts on the environment, this article aims to study the reuse of the saline solution for irrigation.

A concentrated salt solution (brine) with salinity up to twice that of the original seawater or groundwater is discharged as effluent through the demineralization (desalination) process (Tamim 2009). The concentrated brine negatively affects the receiving environment due to its high salinity (Hoopner & Widdelberg 1996; Pérez Talavera & Quesada Ruiz 2001). The characteristics of the desalination plant and its brine discharge will determine the extent of this influence (Mushtaque et al. 2000; Latteman & Höpner 2003). Recent research indicates that brine is discharged into the sea by desalination plants in coastal cities (Fernández-Torquemada et al. 2009).

Total dissolved solids, major cations. and major anions are generally used to define irrigation water quality (Etteieb et al. 2017). The rise in salinity, the decline in permeability, and the specific ion toxicity are the three global issues connected to poor water quality that are most frequently encountered (sodium, chloride and boride) (Singh et al. 2018).

Irrigation water quality information is crucial for understanding variations in product quality and necessary adjustments in water management (Ramakrishnaiah et al. 2009). As such, it falls under irrigation systems' planning, design, and operation (Mirabbasi et al. 2008). When assessing the irrigation water quality, it is important to consider the product's effectiveness and the potential for the emergence of hazardous soil conditions.

Many researchers (Cieszynska-Semenowicz et al. 2012; Li et al. 2012; Brindha & Elango 2013; Fakhre 2014) have used several techniques such as hydrochemical indices (sodium adsorption ratio (SAR), sodium percentage (Na%), Kelly index (KI), permeability index (PI), residual sodium carbonate (RSC), and magnesium hazard ratio percentage (MH%)) for the assessment of irrigation water quality and to determine their influence on soils and plants. These combined chemical analyses of all ions are expected to produce better results than using a single parameter (Hem 1985). In addition, the irrigation water quality index (IWQI) uses several indicators and simplifies the water quality to a single value, which is considered a preferable approach (Saeedi et al. 2010).

This article aims to assess the water quality and reuse potential of the combined groundwater saline solution for irrigation in demineralization stations located in the northern Algerian Sahara. The study also seeks to identify areas for improvement in managing brine discharge according to the defined scientific criteria. The study involves several components, including:

• (1)

  Dilution of the saline solution using groundwater

• (2)

  Calculation of the amount of the groundwater needed to achieve the desired total dissolved salts (TDS) value for irrigation

• (3)

  Assessment of water quality

• (4)

  Evaluation of the potential for reuse of the combined saline groundwater solution for irrigation

• (5)

  Water quality assessment using various quality indices such as SAR, MH%, Na%, RSC, PI%, KI, and IWQI to determine the suitability of the water for irrigation purposes.

In this study, the applied methodology adheres to the steps shown in the graphical abstract and is detailed in a sequential order. After defining the study area, the El-Oued demineralization plant and the Touggourt demineralization plant, samples of groundwater and salt solution were taken and analysed in a laboratory, and data were then collected.

El-Oued state is located in the southeast of Algeria. El-Oued State is frequently described as a low-lying Sahara due to its low height in a central, sizable synclinal basin. Due to its low location between 32°30′00″ and 34°12′00″ N latitude and 6°15′00″ to 7°20′00″ E longitude, it is known as the Low Sahara.

El-Oued was established as a municipality in 1957 and an official province in 1984. The El-Oued State now has 18 municipalities spread across an area of 11,738 km², comprising 18 municipalities. Based on National Statistics Office statistics from 2015, the population of El-Oued exceeds half a million, mainly concentrated in a group of large cities in the region (41.41 inhabitants/km²) (Khezzani & Bouchemal 2016). East of El-Oued lies the Republic of Tunisia, northeast is the Province of Tebessa, Biskra and Khenchla to the north, and Biskra to the northwest. In addition, it is bordered by Djelfa and Touggourt to the west and southwest, respectively.

Touggourt is part of the largest Sahara in southeast Algeria, specifically the northeastern Sahara, which the Saharan Atlas encloses to the north, to the province of Djelfa by the northwest, the Wilayas of Tamanrasset and Illizi by the south, the Wilaya of Ghardaia by the west, and El-Oued and Ouargla by the east. It starts in the south at the Blidet Amor village of El Goug. It ends 150 km to the north at the village of Oum Thiour (100 km from Biskra), passing through the Merouane Chott, which is thought to be the lowest point in the northern part of Africa at −31 m below the sea level (Hacini 2006). It is located 500 km southeast of Algiers, the country's capital, at latitude 32°49′ to 34°3′ north and longitude 05°10′ to 06°14′ east.

According to the National Agency of Hydrographic Network (ANRH), the El-Oued is characterized by continental sand dunes that formed during the most recent Quaternary and is situated on the northern side of the Great Oriental Erg. The Quaternary formations (sand dunes) are primarily composed of fine-grained, compact, homogenous, and uniform sand that covers the whole area, particularly in the south, where the dunes can reach heights of up to 100 m. There are corridors between the sand dunes that form deepened plateaus (known as Sahans); they are sometimes very extensive, occasionally stony, and coated with Quaternary gypsum deposits. The extensive lower Saharan basin's basal region is occupied by the saline depressions (Sebkha), which are found in the study area's northernmost region. The El-Oued area is located in the Sahara's northeastern Mesozoic basin (also called the Triassic basin), northeast of the Sahara platform. Sediment deposits dating from the Lower Cretaceous to the Quaternary are found in this area. Water-rich marine formations from the Paleozoic epoch, which are the foundation of the sedimentary basin, are covered by a series of strata that can reach over 2,000 m. Despite the geology and geomorphology of the region, the prevailing climatic conditions hinder the development of surface water resources. Therefore, the only accessible water source in the El-Oued region that may be used for various purposes is groundwater.

According to the authority of agricultural development in Saharan regions (CDARS), the aquifer system of the Northwest Sahara includes the study area; it is the world's second-largest reservoir. Three nations, Algeria, Tunisia, and Libya, share this aquifer system. Its overall area is around 1 million km², of which Algeria accounts for 70%, Tunisia for 6%, and Libya for 24%. The three primary aquifers in the study area's aquifer system have different depths and physical–chemical characteristics. The region's highest aquifer is the phreatic one. Fine sands interspersed locally with sandy clays, and gypsum lenses make up the free water table in this aquifer. The depth from the surface to groundwater is between 1 and 40 m in this aquifer, which has a thickness of roughly 100 m. A clay substrate that is impermeable lies underneath this aquifer. The phreatic aquifer is a groundwater source for thousands of conventionally drilled wells. In 2015, there were more than 35,000 conventional wells. The phreatic aquifer has an average permeability of 10⁻⁴ m/s. According to estimates, the horizontal transmissivity and storage coefficients are 10⁻² m²/s and 0.2, respectively.

The next deeper aquifer is known as the complicated terminal. This complex aquifer is made up of several aquifers in various geological formations. The complex terminal aquifer is a component of the Cretaceous to Miocene continental formations. It typically has a thickness of around 400 m and a depth between 400 and 600 m. Fossil water in this aquifer dates back between 20,000 and 30,000 years. About 182 deep wells were sunk in this aquifer in 2015, 154 for municipal use and drinking water supply and 28 for irrigation.

Touggourt, the study region, is a portion of the lower Sahara, which is bordered to the north by the South Atlas range and the first foothills of the Aures mountains, to the south by the Tinhert Southern Cliff, to the east by the Dahar Cretaceous outcrops, and to the west by the Mzab Ridge. The large sedimentary basin of the lower Sahara is therefore situated between the northern edge of the Hoggar and the southern edge of the Saharan Atlas. Geologically, two structural units make up the lower Sahara; the igneous and metamorphic rocks that make up the Precambrian socle prevailed, thousands of meters of sedimentary layers from the Cambrian to the Quaternary, in which the Paleozoic lands outcrop in the south, between the Hoggar massif and the Tadmait and Tinhert plateau, Tertiary and Quaternary continental deposits occupy the centre of the basin, and the Mesozoic and early Cenozoic terrains make up a large portion of the edge outcrops Tertiary (Figure 3). According to Figure 1, the study region spans from the Barremian to the Quaternary, which is characterized by the absence of major tectonic deformations.

From a hydrogeological perspective, the Northern Sahara basin is made up of several diverse heterogeneous formations that are separated by impermeable formations. The Intercalary Continental and the Terminal Complex are two aquifers that are recognized as non-renewable resources.

The study area comprises the following three layers (from bottom to top): the continental intercalary aquifer, the complex terminal aquifer, and phreatic aquifer.

The level of aquifers now, as reported by Pizzi & Sartori (1984), is a direct result of a drying up since the Holocene. In the Touggourt region, in the complex terminal, three aquifers are well -differentiated: Figure 2 shows the limestone of the Eocene inferior, followed by the first and second strata of Mio-Pliocene sands. It is clear from the figure that the underground flow continues as it moves from one zone to another (UNEP 2003).

From two demineralization stations, El-Oued Station and Touggourt Station, the samples were collected from the groundwater, salt solution, and water mixture. The samples were collected using 1.5 L plastic bottles to determine if this water was suitable for irrigation. All bottles were cleaned using tap water first, followed by distilled water. The sample water itself was used to clean the bottles during field preparation before taking samples. The vials were thoroughly washed with the sample water to ensure that the sample was true to the water source. After collecting the water samples, each vial was properly labelled and packed in a special box to be transported to the ‘Laboratory for the exploitation and development of natural resources in arid zones, University of Ouargla’ for analysis. The samples were analysed for eight parameters such as electrical conductivity (EC), calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), sulphate ⁠, and bicarbonate using standard methods. The physical parameters (pH, temperature (°C), and salinity) were measured on the field. To analyse the water quality variables, other calculation formulas were used. Based on the recommendation of Eaton (1950), Ayres & Westcot (1985), and Azizi et al. (2019), the various water quality parameters were computed and categorized to determine the suitability of irrigation water quality.

Three dairy wells' groundwater is treated at the El-Oued demineralization station. These wells' groundwater salinity is around 2 g/l. A reverse osmosis technique with a 75% conversion rate is used to treat groundwater. After treatment, we obtain 25% saline solution and 75% drinking water. The station processes 30,000 m³/d groundwater to produce 22,500 m³/d potable water and discards 7,500 m³/d salt solution. The reverse osmosis system consists of three parallel, identical lines with a 75% conversion rate. The concentrated salt water that is left after demineralization is called brine. Brine is a demineralization plant's fluid effluent with a high concentration of dissolved salts and minerals. The brine was diluted with groundwater according to Law 1, obtaining a mixture of 29,359.2 m³/day.

Two dairy wells' groundwater is treated at the Touggourt demineralization station. These wells' groundwater salinity is around 1.6 g/l. After treatment, there is 25% saline solution and 75% drinking water. The station processes 4,000 m³/d groundwater to produce 3,000 m³/d potable water and discards 1,000 m³/d salt solution. The station runs 8 h a day and receives water from the wells for 8 h daily. The reverse osmosis system consists of three parallel, identical lines with a 75% conversion rate. The concentrated salt water that is left after demineralization is called brine. Furthermore, it is a demineralization plant's fluid effluent with a high concentration of dissolved salts and minerals. The brine was diluted with groundwater according to Law 1 to obtain a mixture of 8,488.3 m³/day.

Several indices and parameters are utilized in the hydrochemical study to determine whether the water quality is suitable for irrigation. All the parameters and calculated indices are displayed in Figures 6 and 9, and the formulae are utilized for the calculation. RSC is often used to assess the water quality for irrigation usage. The PI is typically used to understand how soil and aquifer interact. In irrigated soils, an excessive Mg²⁺ concentration can often exchange the Na⁺ content. The MH, which is a mixture of Ca²⁺ and Mg²⁺ ions, is frequently employed to evaluate this phenomenon. The soil structure is often harmed by greater Mg²⁺ concentrations, which causes the water to take more Na⁺ and salinity and affects the yields of crops. Water is categorized as having an excess of sodium or a shortage by measuring sample amounts of Na⁺ against Ca²⁺ and Mg²⁺ and calculating KI. The water samples are considered appropriate for irrigation if Kelly's index is less than 1. The total hardness, which depends on the amounts of Ca²⁺ and Mg²⁺, is a measure of the mineral content in a water sample. Water samples may be categorized into four groups: soft, moderately hard, hard, and very hard. An important parameter for detecting Na⁺ diffusion is sodium percentage (Na%), also known as soluble sodium percentage (SSP). The relationship between risk and water hardness and sodium values is inverse. The SAR can be used to assess the sodium danger in irrigation water; it can be determined how much soil particles with a negative charge are adhered to one another or flocculated by looking at the SAR, which is the balance between calcium (Ca²⁺), magnesium (Mg²⁺), and sodium (Na²⁺) ions. Because flocculation makes it more difficult for plant roots and water movements, it is advantageous.

According to SAR values, there are four classes of soil sodicity hazard potential: low if SAR is less than 10, medium if it is between 10 and 18, high if it is between 18 and 26, and very high if it is greater than 26. The WQI was often used to assess the groundwater's suitability for irrigation and drinking. The IWQI model was used on the data. This model was created by Jerome & Pius (2010) in accordance with the standards provided by Ayers & Westcot (1999) and the irrigation water quality parameters suggested by the University of California Committee of Consultants (UCCC). A non-dimensional number was used to represent the water quality parameters; the higher the value, the better the water quality. A diagram was drawn based on the results of chemical analyses, and then the types and indices of water suitability for irrigation were identified.

It became apparent to us after receiving the results of the groundwater and salt solution analyses of the two stations that the salt percentage in the raw water of the two stations is acceptable. However, the salt percentage in the salt solution is quite high.

According to Kelley (1940) and Paliwal (1967), values less than 1 indicate that water is acceptable for irrigation, and if it is 1–2, it indicates that it is marginally suitable. According to this study, according to the KI values obtained, the water at the Touggourt station and water rejection at the El-Oued station (KI < 1) is good quality water for irrigation, whereas groundwater at the El-Oued station is just marginally suitable for irrigation (1 < KI < 2).

According to the water quality diagram shown in Figure 6 and Wilcox (1955), the groundwater El-Oued and Touggourt strata are classified as having doubtful uses for irrigation. On the other hand, salt solutions in El-Oued and Touggourt stations are considered unsuitable for irrigation.

According to the computed PI values, the groundwater at El-Oued and Touggourt and water rejection at the El-Oued station come into class II, ‘suitable’ for irrigation, and salt solution in the Touggourt station is grouped into class III, ‘unsafe’ for irrigation.

Values less than 20 indicate excellent irrigation water, while values between 20 and 40 are deemed good, 40–60 are permissible, and 60–80 are regarded as doubtful. In this study, the groundwater and water rejection at the Touggourt station was lower than 20, suggesting that the water was excellent for irrigation. Moreover, the groundwater and water rejection at the El-Oued station was greater than 40 and lower than 60, suggesting that the water was permissible for irrigation.

Groundwater quality may be classified into four categories: excellent if the SAR is less than 10, good if it is between 10 and 18, dubious if it is between 18 and 26, and unsuitable for irrigation if it is greater than 26. In this study, groundwater and water rejection at El-Oued and Touggourt stations were greater than 0 and lower than 10, respectively, suggesting that the water was low sodic hazard (S1).

According to the USSL diagram, Touggourt station groundwater falls into the C4–S1 (extremely high salinity with low sodium) group. Moreover, groundwater from the El-Oued station falls into C4–S2 (very high salinity with medium sodium) group. Very high salinity dangers are present in the groundwater at El-Oued and Touggourt stations, along with low to medium alkali hazards. According to the USSL diagram, the groundwater stations at El-Oued and Touggourt are suitable for irrigation in all soil types. The water rejected (salt solution) at El-Oued and Touggourt stations is unclassified and unfit for irrigation.

The sodicity hazard of irrigation water may be effectively measured using RSC. The soil's calcium and magnesium ions tend to precipitate as a result of the anions and in irrigation water, which increases the proportion of sodium ions. RSC was therefore seen as a sign of the sodicity hazard of water. It divides the water into three classes: water containing less than 1.25 meq/l is suitable for irrigation. Water with RSC levels of 1.25–2.5 meq/l was deemed marginal, and irrigation is not recommended for water with RSC levels higher than 2.5 meq/l. In this study, groundwater and water rejection at El-Oued and Touggourt stations was lower than 1.25, suggesting that the water was good for irrigation.

If the percentage of MH is less than 50%, it indicates that the water is safe for irrigation purposes; however, if the percentage is greater than 50%, it indicates that the water quality is unsuitable for irrigation. According to this index, the groundwater and water rejection samples from the El-Oued station fall into suitable water. The groundwater and water rejection samples from the Touggourt station fall into harmful and unsuitable water.

IWQI: The model of IWQI stipulates that irrigation water quality needs may vary from field to another based on the type of crop being grown there and the local climatic and soil conditions. The WQI has been extensively utilized in recent years to assess the appropriateness of water for irrigation and drinking. The IWQI model was used on the collected data. This model was developed by Meireles et al. (2010). The parameters which were first selected were those that were deemed to be more pertinent to irrigation usage. The second phase was defining the terms quality measurement values (q_i) and aggregation weights (w_i). According to the standards set by Ayers & Westcot (1999) and the irrigation water quality parameters supplied by the UCCC, values of q_i were estimated based on each parameter value. The results are shown in Table 1. A non-dimensional number was used to represent the water quality parameters; the greater the value, the higher the water quality.

Table 1

Parameter limiting values for quality measurements (q_i) (Meireles et al. 2010)

qi .	EC (μs/cm) .	SAR (meq/l)1/2 .	Na+ .	Cl− .	.
			Meq/l .
85–100	200 ≤ EC < 750	SAR < 3	2 ≤ Na < 3	Cl < 4	1 ≤ HCO3 < 1.5
60–85	750 ≤ EC < 1,500	3 ≤ SAR < 6	3 ≤ Na < 6	4 ≤ Cl < 7	1.5 ≤ HCO3 < 4.5
35–60	1,500 ≤ EC < 3,000	6 ≤ SAR < 12	6 ≤ Na < 9	7 ≤ Cl < 10	4.5 ≤ HCO3 < 8.5
0–35	EC < 200 or EC ≥ 3,000	SAR ≥ 12	Na < 2 or Na ≥ 9	Cl ≥ 10	HCO3 < 1 or HCO3 ≥ 8.5

qi .	EC (μs/cm) .	SAR (meq/l)1/2 .	Na+ .	Cl− .	.
			Meq/l .
85–100	200 ≤ EC < 750	SAR < 3	2 ≤ Na < 3	Cl < 4	1 ≤ HCO3 < 1.5
60–85	750 ≤ EC < 1,500	3 ≤ SAR < 6	3 ≤ Na < 6	4 ≤ Cl < 7	1.5 ≤ HCO3 < 4.5
35–60	1,500 ≤ EC < 3,000	6 ≤ SAR < 12	6 ≤ Na < 9	7 ≤ Cl < 10	4.5 ≤ HCO3 < 8.5
0–35	EC < 200 or EC ≥ 3,000	SAR ≥ 12	Na < 2 or Na ≥ 9	Cl ≥ 10	HCO3 < 1 or HCO3 ≥ 8.5

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IWQI is a dimensionless parameter with a value between 0 and 100; q_i is the quality of the ith parameter, a number from 0 to 100, a function of its measurement or concentration; and w_i is the normalized weight of the ith parameter, the function of its relative importance to groundwater quality. The suggested WQI was used to divide classes based on existing water quality indices. Classes were defined considering the risk of salinity problems and soil water infiltration reduction. In addition, plant toxicity is seen in the classifications provided by Bernardo (1995) and Holanda & Amorim (1997). The classification of restrictions to water usage classes is presented in Table 3.

Table 3

Water quality index characteristics (Meireles et al. 2010)

IWQI .	Water use restrictions .	Recommendation .
		Soil .	Plant .
85–100	No restriction (NR)	May be used for the majority of soils with a low probability of causing salinity and sodicity problems, being recommended leaching within irrigation practices, except for in soils with extremely low permeability.	No toxicity risk for most plants
70–85	Low restriction (LR)	Recommended for use in irrigated soils with light texture or moderate permeability, being recommended salt leaching. Soil sodicity in heavy texture soils may occur, being recommended to avoid its use in soils with high clay	Avoid salt-sensitive plants
55–70	Moderate restriction (MR)	May be used in soils with moderate to high permeability values, being suggested moderate leaching of salts.	Plants with moderate tolerance to salts may be grown
40–55	High restriction (HR)	May be used in soils with high permeability without compact layers. High-frequency irrigation schedule should be adopted for water with EC above 2,000 μS/cm and SAR above 7.0.	Should be used for irrigation of plants with moderate to high tolerance to salts with special salinity control practices, except water with low Na, Cl, and HCO3 values.
0–40	Severe restriction (SR)	Should be avoided use for irrigation under normal conditions. In special cases, it may be used occasionally. Water with low salt levels and high SAR requires gypsum application. In high saline content water, soils must have high permeability, and excess water should be applied to avoid salt accumulation.	Only plants with high salt tolerance, except for waters with extremely low values of Na, Cl, and ⁠.

IWQI .	Water use restrictions .	Recommendation .
		Soil .	Plant .
85–100	No restriction (NR)	May be used for the majority of soils with a low probability of causing salinity and sodicity problems, being recommended leaching within irrigation practices, except for in soils with extremely low permeability.	No toxicity risk for most plants
70–85	Low restriction (LR)	Recommended for use in irrigated soils with light texture or moderate permeability, being recommended salt leaching. Soil sodicity in heavy texture soils may occur, being recommended to avoid its use in soils with high clay	Avoid salt-sensitive plants
55–70	Moderate restriction (MR)	May be used in soils with moderate to high permeability values, being suggested moderate leaching of salts.	Plants with moderate tolerance to salts may be grown
40–55	High restriction (HR)	May be used in soils with high permeability without compact layers. High-frequency irrigation schedule should be adopted for water with EC above 2,000 μS/cm and SAR above 7.0.	Should be used for irrigation of plants with moderate to high tolerance to salts with special salinity control practices, except water with low Na, Cl, and HCO3 values.
0–40	Severe restriction (SR)	Should be avoided use for irrigation under normal conditions. In special cases, it may be used occasionally. Water with low salt levels and high SAR requires gypsum application. In high saline content water, soils must have high permeability, and excess water should be applied to avoid salt accumulation.	Only plants with high salt tolerance, except for waters with extremely low values of Na, Cl, and ⁠.

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Groundwater and water rejection samples obtained from El-Oued and Touggourt stations were analysed using the SAR, SSP, Na%, and PI indices shown in Figure 8. Water quality indicators for irrigation have shown that groundwater at El-Oued and Touggourt stations is acceptable for irrigation.

Regarding water rejection (salt solution), water in El-Oued and Touggourt stations is unsuitable for irrigation.

After evaluating the quality of the brine and groundwater at the El-Oued and Touggourt stations, it was discovered that the brine's salinity was quite high and unsuitable for irrigation. The aforementioned methods were used to reduce salinity by combining groundwater and brine. Also, TDS values in accordance with irrigation standards were determined. After that, the amount of groundwater to be added was calculated using Law 1. The appropriate water mixture was analysed in the laboratory. Finally, the irrigation water quality indices were calculated and discussed based on the analysis results.

Hydrogeochemical facies, according to the plotted Piper diagram shown in Figure 9, water after mixing in El-Oued is Cl-_Na + _K + , whereas water after mixing in Touggourt is Cl⁻__Ca²⁺_Mg²⁺.

KI: According to the KI values obtained, the water after mixing at El-Oued and Touggourt stations (KI < 1) is good quality water for irrigation.

PI: According to the computed PI values, after mixing in El-Oued and Touggourt stations, the obtained water comes into class II, ‘suitable’ for irrigation.

Na% or SSP: The results show that water after mixing at the Touggourt station was lower than 20, suggesting that the water was excellent for irrigation. Conversely, water after mixing at the El-Oued station was greater than 40 and lower than 60, suggesting that the water was permissible for irrigation.

According to SAR calculation results, water after mixing at El-Oued and Touggourt stations was greater than 0 and lower than 10, suggesting that the water was low sodic hazard (S1).

RSC: After mixing at El-Oued and Touggourt stations, the water founded to have RSC values lower than 1.25, suggesting that the water is good for irrigation.

MH: According to this index, water after mixing from the El-Oued station falls into suitable water. In contrast, after mixing from the Touggourt station, water falls into harmful and unsuitable water.

IWQI: The IWQI for irrigation usage is shown in Figure 11. In El-Oued and Touggourt stations, water after mixing was classified as having ‘severe restriction (SR)’ for irrigation and may be used in soils with high permeability.

The SAR, SSP, Na%, and PI indices were used to evaluate the water after mixing taken from El-Oued and Touggourt stations to determine that they are appropriate for irrigation. Water quality indicators for irrigation have shown that water after mixing from the El-Oued and Touggourt stations is suitable for irrigation. KI < 1 indicates that the water is suitable for irrigation. Moreover, as the RSC in the water from the Touggourt and El-Oued stations is <1.25, the water is suitable for irrigation.

According to the MH values determined from the El-Oued station, MH < 50% means that the water is suitable for irrigation, and the values of MH > 50% were observed in the Touggourt station. This issue may arise when irrigation water contains relatively more magnesium ions than calcium ions.

Figures 5 and 10 show the EC values for samples taken from El-Oued and Touggourt's demineralization stations. The concentration of salts in the water increases with the EC. It is obvious that the EC has decreased the water quality standards for irrigation where the concentration of EC in the brine leaving the demineralization station is 7,690 and 7,620 μs/cm, respectively, and the concentration of groundwater is 2,790 and 2,990 μs/cm, respectively. After water mixing, the EC became 4,041.73 and 3,535.45 μs/cm, respectively.

The temperatures of the water samples taken from El-Oued and Touggourt's demineralization station are shown in Figures 5 and 10. It is clear that the temperatures have dropped to levels that are suitable for irrigation, with the brine temperature in the demineralization station being 31.5 and 28, respectively, and the groundwater temperature being 40.2 and 40, respectively. After water mixing, the temperature decreased to 37.97 and 38.58, respectively.

The TDS values in samples taken from the demineralization station are shown in Figures 5 and 10. The TDS values of groundwater are 1,786 and 1,985 mg/l, respectively, and the TDS values of the brine leaving the demineralization El-Oued and Touggourt stations are 4,922 and 3,810 mg/l, respectively, and the water is not suitable for agricultural use. TDS values at El-Oued and Touggourt stations are 2,587 and 2,200 mg/L, respectively, where the values comply with water quality norms for agricultural use.

By assessing the IWQI of El-Oued and Touggourt stations shown in Figure 13, the water after mixing from the Touggourt station is more suitable for irrigation than from the El-Oued station.

Since El-Oued is a desert with highly permeable soil (sand), water after mixing can be used for irrigation with certain limitations on the kinds of plants that can withstand salt. Water after mixing was used to irrigate the palm trees since the date palm can resist both salt and drought. It can withstand up to 4 dS/m of soil salinity without reducing the yield. Palm planting is prevalent in the southern Algerian Sahara oases, including El-Oued and Touggourt. Moreover, first-degree palm tree cultivation is the focus of the study area. A daily flow of water after mixing of 29,359.2 m³ /day is offered, which is sufficient to irrigate 679.6 hectares of palm trees. It should be mentioned that an area of 226 hectares planted with palm trees consumes 9,763.2 m³/day. The available flow achieves 19,596 m³/day, which is a sufficient flow to irrigate an area of 453.6 hectares of trees. The planting of an equivalent area with palm trees is recommended.

Since Touggourt is a desert with highly permeable soil (sand), the water after mixing can be used for irrigation with certain limitations on the kinds of plants that can withstand salt. Water after mixing was used to irrigate the palm trees since the date palm can resist both salt and drought. It can withstand up to 4 dS/m of soil salinity without reducing the yield. Palm planting is prevalent in the southern Algerian Sahara oases, including El-Oued and Touggourt. Moreover, first-degree palm tree cultivation is the focus of the study area. After mixing, the flow of water is approximately 8,488.3 m³ /day, which is sufficient to supply 196.48 hectares of palm trees, and since an area of 94.24 hectares is planted with palm trees, it consumes 4,071.1 m³/day. Parallelly, a flow of 4,417.2 m³/day is offered, and it is enough to irrigate an area of 102.24 hectares of trees, so planting an equivalent area with palm trees is recommended.

This study focuses on assessing the water quality and reuse potential of a groundwater-combined saline solution for irrigation obtained from demineralization stations located in El-Oued and Touggourt. The salinity of the saline solution was reduced by dilution with groundwater, and the resulting water mixture was analysed in a laboratory. The study evaluated the suitability of the water for irrigation using several water quality indices, including the PI, KI, RSC, MH, Na%, and SAR. The study results and their implications for sustainable water use and management of brine discharge are discussed. The results of the study show that both the groundwater and water mix from the Touggourt and El-Oued stations are suitable for irrigation, as indicated by various water quality indices, including the USSL diagram, KI, and Doneen diagram. However, the groundwater and water mix from the El-Oued station are marginally suitable for irrigation based on the KI. The study highlights the potential for reuse of groundwater-combined saline solutions for irrigation and provides insights into managing brine discharge to promote sustainable water use in the northern Algerian Sahara. The groundwater and water mix at El-Oued and Touggourt stations were found to be ‘severely restricted (SR)’ for irrigation based on the results of the IWQI. Only soil with high permeability, such as sand, should be used with these types of water. To use the water mix for irrigation, there are several restrictions on the types of plants that can tolerate salt. The study found that the salt solution was unsuitable for irrigation, but the water mix is appropriate for irrigation based on the IWQI.

The study found that the groundwater combined saline solution could be reused for irrigation with certain restrictions, but there are also other potential uses for the brine that could be explored to prevent it from being discharged into the sea or natural environment. Further research could be conducted to explore these alternative uses and to develop sustainable management strategies for brine disposal in the future.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.