Hydrogeochemical processes, water–rock interactions, and the suitability of groundwater in a semi-arid region, Northeastern Algeria Open Access

Groundwater is essential for the development of the socio-economy and eco-environment in many parts of the world, especially in arid and semi-arid regions (Zongjun et al. 2019; Haohao et al. 2024). Groundwater geochemistry is the main factor in its use for domestic, irrigation, and industrial purposes. However, interaction of groundwater with aquifer mineral species greatly controls the groundwater chemistry (Subramani et al. 2010). The hydrogeochemical processes responsible for the modification of the chemical composition of groundwater vary in space and time and are responsible for spatiotemporal variations of their chemistry (Krishna et al. 2009; Dehnavi et al. 2011). In the aquifer system, these processes help to clarify the contributions of the water–rock interaction as well as anthropogenic influences on the groundwater. Whereas, groundwater chemistry depends on a number of factors, such as the geological nature of aquifer rocks, recharge water quality, decay of organic matter, partial pressure of soil carbon dioxide (CO₂), the dissolution of mineral species, and the degree of chemical weathering of rocks (Lakshmanan et al. 2003; Krishna et al. 2009; Refat & Humayan 2021). These factors and their interactions result in complex groundwater quality (Simge & Aysen 2013; Lakhvinder et al. 2019; Ratri et al. 2022).

In the Laouinet-Morsott region (Medjerda-Mellegue watershed) as in other semi-arid regions in Algeria, groundwater constitutes the main source exploited for various needs of the populations due to the shortage of surface water caused by weak and irregular rains. The demand for groundwater in this region has increased significantly in recent years due to rapid urbanization, significant expansion of irrigation, as well as increasing domestic needs of the population. The overexploitation of groundwater is revealed by the reduction of piezometric surfaces and the degradation of water quality. The Laouinet-Morsott aquifer is exploited extensively and continuously, and the chemical load of its waters is spatially variable, generally high, in the absence of means of treatment and/or softening of these waters, which constitutes a permanent concern for all populations of these areas. This situation requires searching for solutions to this worrying observation which involves a better knowledge of water chemistry and its determining factors.

Around the world, extensive research has been conducted on evaluating hydrogeochemical processes, water–rock interactions, and the suitability of groundwater for different purposes. Unfortunately, there is a lack of research on groundwater chemistry covering the Laouinet-Morsott area, with the exception of a study carried out in 2009 on part of the plain (Fehdi et al. 2009). This study examines the Laouinet-Morsott plain under different temporal and environmental conditions than the previous study, in order to provide a new database for scientific management and sustainable development, particularly for the protection of groundwater resources. This study aims to:

• 1. Identify groundwater chemistry characteristics, origins, and water–rock interactions controlling hydrogeochemical processes using physicochemical parameters, multiple graphical approaches, chloro-alkaline indices (CAI-1 and CAI-2), and the saturation index (SI).

• 2. Assess and classify the groundwater quality for drinking and irrigation using the water quality index (WQI), SAR, %Na, and QGIS. Overall, these studies provide useful insights and data for rational development and protection of groundwater resources in the region, and also seek to understand the current status of these resources to identify potential challenges and opportunities for their management.

In this study, 65 water samples from wells and boreholes were collected in March 2021, covering scattered settlements and agricultural plots spread over most of the plain. The depth of the water points ranged from 30 to 200 m. Geographical coordinates were obtained using a GPS Status (Figure 1). Groundwater was sampled after pumping for more than 15 min. Water samples were collected in polyethylene plastic bottles. Before sampling, all bottles were cleaned three times with the water to be sampled, then refrigerated and promptly sent to the laboratory for analysis. Physical parameters such as pH, electrical conductivity (EC), and total dissolved solids (TDS) were measured in the field using a portable multi-parameter instrument (HORIBA U-5000, Kyoto, Japan). Total hardness (TH) was determined by a titrimetric method using EDTA (ethylene diamine tetra-acetic acid) at 0.01 mol/L. Major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃⁻, Cl⁻, SO₄²⁻, and NO₃⁻) were analyzed by ion chromatography using a HACH DR-3900 at the Quality Control and Compliance Laboratory in Annaba, Algeria (Berrahmoune Laboratory). The ion balance percentage error for most water samples was within the acceptable limit of 5%, indicating the reliability of the laboratory analyses (WHO 2011).

In this study, the Piper diagram, Gibbs diagrams, ion exchange, CAIs, plots and ratios of major cations and anions, and the SI of major minerals were commonly used to describe the water chemistry, understand the hydrogeochemical processes resulting from rock–water interaction, and the factors controlling these processes.

If Q_n = 0 implies the complete absence of contaminants, while 0 < Q_n < 100 implies that the contaminants are within the prescribed standard. When Q_n > 100 implies that the contaminants are above the standards (Poonam et al. 2013; Randriamahefa et al. 2020; Talhaoui et al. 2020). The calculated WQI was divided into five categories, as shown in Table 1.

Statistical analysis of the major ions (Mg²⁺, Na⁺, K⁺, Ca²⁺, ⁠, ⁠, and Cl⁻) is the basis of understanding the hydrogeochemical characteristics of groundwater (Zhou et al. 2016; Zongjun et al. 2019). The statistics of groundwater chemical parameters are presented in Table 2. The pH values ranged from 4.38 to 8.40, with an average of 7.52. Most of the sampled groundwater was a basic type (89.23%) and 10.76% was an acidic type. The variation in pH is essentially due to the change in geological facies and the variation in the CO₂ content dissolved in the water during its infiltration into the soil and subsoil. EC ranged from 530 to 31,700 μS/cm with an average value of 4,101.01 μS/cm. In most cases, the EC value increases when it rains; rainwater dissolves various salts present in rocks and other chemical elements, which adds charged ions to groundwater, not forgetting the contribution of high evaporation from the topsoil to increasing the concentration of chemical parameters (Alsharifa et al. 2017; Ratri et al. 2022).

Table 2

Descriptive statistics, WHO standards, and assigned unit weights of the major parameters of groundwater

Parameters .	Maximum .	Minimum .	Average .	WHO standards (2011) .	Relative weight (Wi) .
pH	8.4	4.38	7.52	6.5–8,5	0,41721716
EC (μS/cm)	31,700	530	4,101.04	1,000	0,00354635
TDS (mg/L)	19,300	376	2,451.98	500	0,00709269
TH (mg/L)	4,800	340	1,111.23	300	0,01182115
Ca2+ (mg/L)	1,090.4	61	225.80	75	0,04728461
Mg2+ (mg/L)	595.6	45.96	133.25	50	0,07092692
Na+ (mg/L)	4,780	46.2	267.63	200	0,01773173
K+ (mg/L)	190.42	7	31.39	12	0,29552882
(mg/L)	3,610.83	218.79	562.73	120	0,02955288
Cl− (mg/L)	9,470.92	103	619.66	250	0,01418538
(mg/L)	897.94	85	321.76	250	0,01418538
(mg/L)	69.7	7	26.11	50	0,07092692
CAI-1	0.56	−0.54	0.21	–	–
CAI-2	2.13	−0.33	0.24	–	–
SI (Halite)	−3.14	−6.93	−5.96	–	–
SI (Gypsum)	−0.37	−1.84	−1.14	–	–
SI (Anhydrite)	−0.59	−2.06	−1.36	–	–
SI (Aragonite)	2.9	3.18	0.83	–	–
SI (Calcite)	3.05	−3.03	0.97	–	–
SI (Dolomite)	6.17	−5.84	2.11	–	–
Total					1

Parameters .	Maximum .	Minimum .	Average .	WHO standards (2011) .	Relative weight (Wi) .
pH	8.4	4.38	7.52	6.5–8,5	0,41721716
EC (μS/cm)	31,700	530	4,101.04	1,000	0,00354635
TDS (mg/L)	19,300	376	2,451.98	500	0,00709269
TH (mg/L)	4,800	340	1,111.23	300	0,01182115
Ca2+ (mg/L)	1,090.4	61	225.80	75	0,04728461
Mg2+ (mg/L)	595.6	45.96	133.25	50	0,07092692
Na+ (mg/L)	4,780	46.2	267.63	200	0,01773173
K+ (mg/L)	190.42	7	31.39	12	0,29552882
(mg/L)	3,610.83	218.79	562.73	120	0,02955288
Cl− (mg/L)	9,470.92	103	619.66	250	0,01418538
(mg/L)	897.94	85	321.76	250	0,01418538
(mg/L)	69.7	7	26.11	50	0,07092692
CAI-1	0.56	−0.54	0.21	–	–
CAI-2	2.13	−0.33	0.24	–	–
SI (Halite)	−3.14	−6.93	−5.96	–	–
SI (Gypsum)	−0.37	−1.84	−1.14	–	–
SI (Anhydrite)	−0.59	−2.06	−1.36	–	–
SI (Aragonite)	2.9	3.18	0.83	–	–
SI (Calcite)	3.05	−3.03	0.97	–	–
SI (Dolomite)	6.17	−5.84	2.11	–	–
Total					1

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Na⁺ is the dominant cation present in the collected water samples, and the concentration of Na⁺ varied from 46.2 to 4,780 mg/L, with an average of 267.63 mg/L. The dissolution of halite is the main source of sodium concentration in samples (Fehdi 2008; Halimi & Djabri 2024). In addition, the dissolution of silicates and cation exchange can increase the concentration of Na⁺ in groundwater (Fehdi 2008; Li et al. 2016). Ca²⁺ and Mg²⁺ were the secondary cations in groundwater, with concentrations ranging from 61 to 1,090 mg/L and 45.96 to 595.6 mg/L, respectively. The concentration of K⁺ was found to be relatively low compared with other cations and ranged between 7 and 190.42 mg/L. The source of potassium in the groundwater of the study area is the weathering of potassic clays and leaching of evaporites as well as the dissolution of chemical fertilizers used in agricultural activities. The average concentrations of Ca²⁺, Mg²⁺, and K⁺ are 225.80, 133.25, and 31.39 mg/L, respectively. The average concentration of cations in the groundwater samples for this study was Na²⁺ > Ca²⁺ > Mg²⁺ > K⁺.

The geological formations, water–rock interaction, and relative mobility of ions are prime factors influencing the geochemistry of groundwater (Lakshmanan et al. 2003; Shajedul & Mostafa 2022). Where the ion concentrations in groundwater depend on the hydrogeochemical processes involved in the aquifer system. Water–rock interaction reflects the differences in mineral composition of the aquifer, and existence of fissures, faults, and cracks which affect groundwater movement in the subsurface medium (Krishna et al. 2009; Awadh et al. 2016). Generally, different chemical processes occur during rock–water interaction, which includes dissolution/precipitation, rock weathering, ion exchange processes, oxidation, and reduction (Dehnavi et al. 2011).

All values in meq/L.

In general, if the ratio between Na⁺ and Cl⁻ equal to 1 implies halite dissolution (Equation (15)) (Dehnavi et al. 2011), while the increased concentration of Na⁺ than Cl⁻ means that Na⁺ comes from silicate weathering and/or the ion exchange process (Mayback 1987). As shown in Figure 8(b), most of the water samples are distributed along the line x = y = 1, which indicates that halite dissolution is the main source of Na⁺ and Cl⁻ in groundwater. However, in the study area, the average annual rainfall reached 362 mm. In this situation, the free halite might not be available for dissolution in the soil zone, but the irrigation in the study area could increase the concentration of Na⁺ in the groundwater.

Datta & Tyagi (1996), Rajmohan & Elango (2004), and Sandow (2009) suggest that using the diffusion diagram [Ca²⁺ + Mg²⁺ vs. + ] can help to determine the sources of these ions by evaluating the trend line formed by these ions (Figure 8(d)). If water samples are located above the equiline (1:1) indicate a dominance of carbonate weathering. A dominance of silicate weathering if the water samples are distributed below the 1:1 line, while those falling along the equiline (Ca²⁺ + Mg²⁺ = + ⁠) imply that these ions are due to both carbonate weathering (Equations (18)–(20)) and silicate weathering (Equation (21)) (Dehnavi et al. 2011; Collins 2021).

Figure 8(e) shows the bivariate plot [ to Ca²⁺ and Ca²⁺ + Mg²⁺], the equivalent ratio of [/Ca²⁺] would be 1:1 to 2:1 depending on the partial contribution of carbonic acid and strong acid (nitric acid and sulfuric acid), while this ratio equals 1:1 to 1:2 implies weathering of dolomite (Equation (20)) (Jianwei et al. 2020; Ali et al. 2023). If the equivalent ratio of [/(Ca²⁺ + Mg²⁺)] equals 1:1 to 2:1, the Ca²⁺, Mg²⁺ and ions come only from the carbonate weathering, while water samples located on the 1:1 line indicate that the dissolution of dolomite takes place in the chemical composition of the groundwater. As shown in Figure 8(e), the dissolution of calcite and dolomite are the main processes affecting the chemistry of these samples. Additionally, a few water samples falling below the 1:1 line show decreased concentrations of Ca²⁺ and Mg²⁺ relative to ⁠, suggesting the presence of other hydrogeochemical processes such as silicate weathering and/or cation exchange (El Alfy et al. 2017; Zongjun et al. 2019; Refat & Humayan 2021).

The alteration of carbonates by carbonic acid and water saturated with CO₂ is a dense process in the groundwater of the study area. Whereas this water can easily dissolve the carbonate minerals available in its flow path, which leads to an increase in the concentration of calcium, magnesium, and bicarbonate ions in the groundwater. Furthermore, the Na/Ca scatter plot (Figure 8(f)) shows that ion exchange (Ca/Na) also increased the calcium concentration in the groundwater (Dehnavi et al. 2011; Mohsen et al. 2014). The dissolution of magnesium calcite (Equation (20)), gypsum, and/or dolomite provides the magnesium ion, if the weathering of carbonates and silicates (Equation (21)) in the same place, bicarbonate and sodium ions are dominant to other ions present in the groundwater.

Several environmental factors influence the SI, such as evaporation, temperature, pH, and pressure. Evaporation plays a significant role in concentrating dissolved minerals in groundwater, often leading to oversaturation and the precipitation of these minerals. At higher temperatures, the solubility of halite, gypsum, and anhydrite increases, which can lower the SI and prevent precipitation. However, the increase in temperature decreases the solubility of calcite, dolomite, and aragonite, thereby increasing the SI and promoting precipitation.

The pH of water influences the chemical form and solubility of many minerals; alkaline conditions decrease the solubility of calcite, dolomite, and aragonite, increasing the SI and promoting precipitation, whereas acidic conditions increase their solubility and decrease the SI. Generally, halite, gypsum, and anhydrite are less affected by pH changes. Furthermore, pressure impacts the solubility of gases in water, such as CO₂, which can affect the pH and, consequently, the SI. Higher pressure increases the solubility of CO₂, which can form carbonic acid and lower the pH, thus decreasing the SI. Lower pressure decreases CO₂ solubility, potentially increasing the pH and the SI.

Groundwater quality assessment is an important topic around the world because water quality is directly related to human health. In this study, the WQI was used to define the quality of groundwater intended for consumption (Suneetha et al. 2015; Twana et al. 2019; Mohamed et al. 2021). According to the groundwater quality standards of the World Health Organization (WHO 2011), the relative weight (W_i) of each physicochemical parameter (pH, EC, TDS, TH, Ca²⁺, Mg²⁺, Na⁺, K⁺, ⁠, Cl⁻, ⁠, ⁠) were calculated to assess water quality and are represented in Table 2.

Results of the quality of the groundwater for drinking purposes are shown in (Figure 10), using the inverse distance weighted (IDW) interpolation. The WQI values range from 69.44 to 844.7 with a mean of 173.71. According to the WQI classification in Table 1, the WQI results range from II to V, suggesting that the groundwater quality ranges from good to unfit for drinking. The results showed that more than half of the samples were of poor quality, while (26.15%) were good, (13.84%) unfit for drinking, and (7.69%) very poor water.

The poor quality of groundwater in the study area can be attributed to various factors: natural effects like water interacting with different geological formations, anthropogenic influences such as the infiltration of agrochemical products, manures, and wastewater. Without forgetting the overexploitation of aquifers and the drought that has hit the region in recent years, are among the reasons leading to the deterioration of groundwater quality.

The quality of groundwater can also determine its suitability for irrigation, as high levels of ions in the water can affect plants and soil. In order to determine the quality of groundwater for agricultural and irrigation activities, SAR and %Na were calculated, and the results were represented in spatial maps obtained using the IDW method.

Sodium concentration is important in grading water for irrigation purposes because it can reduce the permeability and the structure of the soil (Todd 1980). The results obtained show that irrigation water can be divided into three groups according to the %Na classification standards (Table 2). The %Na values of the groundwater samples range from 22.41 to 79.70. Most of the groundwater samples are good water (73.84%), while 24.61% of the samples belong to permissible water and 3.07% of all samples are classified as doubtful water (Figure 11(b)).

The quality of drinking water and economic development, especially agricultural development on which the region depends are linked to groundwater security. For this reason, groundwater quality should be improved through rational exploitation of resources and control of pollution sources to stimulate economic development.

The hydrogeochemical properties, the water–rock interaction processes, and the groundwater's quality intended for drinking and irrigation uses were evaluated. The main findings are as follows:

• (1) Groundwater in the study area consists of dominant cation and anion sequence Na²⁺ > Ca²⁺ > Mg²⁺ > K⁺ and Cl⁻ > > > ⁠, respectively. The excess mineralization and salinity in almost all of the study areas are due to the influence of natural processes and anthropogenic activities. Most of the water samples belong to very hard-brackish or hard-brackish water, and the dominant water types are mixed Ca–Mg–Cl–SO₄, Na–K–Cl, and Ca–Mg–⁠.

• (2) After correlation analysis and water–rock interaction analysis, the water–rock interactions and evaporation are the predominant processes in the formation of hydrochemical components. In general, the main rocks involved in water–rock interactions are evaporites, carbonates, and silicates. Dissolutions of halite, gypsum, anhydrite, calcite, and dolomite are the predominant processes contributing to defining the groundwater chemistry. Additionally, cation exchange (exchange of Ca²⁺ and Mg²⁺ with Na⁺ and K⁺) and silicate weathering are also the prevalent processes in the water system.

• (3) Through geochemical calculations, the SI of halite, gypsum, and anhydrite in the aquifers is less than zero, indicating that these minerals are unsaturated and can dissolve continuously in the aquifers. In contrast, aragonite, calcite, and dolomite in the aquifers are in a saturated state.

• (4) Based on the WQI, more than 73% of all groundwater samples are categorized as poor, very poor, or unfit for drinking, indicating that the water from this groundwater is not suitable for drinking. In this result, the groundwater must be treated before use. According to SAR and %Na, more than 80% of samples are suitable for the purpose of irrigation.

The studied region, facing an increasing shortage of groundwater, is encountering major challenges in managing its resources. Rapid urbanization, the intensification of agricultural activities, and the overexploitation of groundwater are worsening the situation. Additionally, the infiltration of untreated wastewater and the discharge of pollutants are further degrading the quality of groundwater. To address these issues, urgent and specific measures are required to protect and improve these vital resources:

• (1) Strengthening the control of illegal drilling and sustainable management of aquifers

• • • Action: Implement a strict permit system for all wells, requiring annual reports on groundwater use.

  • • Timeline: Within the first 2 years, all wells should be registered and monitored, with annual follow-up on extracted volumes.

  • • Goal: Reduce illegal wells by 70% within 5 years and maintain groundwater levels at a sustainable threshold over the next 10 years.

• (2) Progressive modernization of wastewater treatment infrastructure

• • • Action: Upgrade wastewater treatment systems in critical industrial and agricultural areas, with a 5-year investment plan to ensure effluents are treated before discharge.

  • • Timeline: Begin infrastructure audits within the first year and complete the modernization program over the next 5 years.

  • • Goal: Achieve 100% wastewater treatment in major urban and agricultural areas within 6 years.

• (3) Establishment of a continuous water quality monitoring network

• • • Action: Install automatic sensors to monitor pollution levels, salinity, and other water quality indicators in key groundwater extraction areas.

  • • Timeline: Implement the monitoring system within 3 years, with annual reports on groundwater quality.

  • • Goal: Obtain continuous data on water quality and adjust policies based on these results to ensure adaptive management over a 10-year period.

• (4) Long-term training and awareness programs for farmers

• • Action: Launch ongoing training programs for farmers on efficient irrigation practices, such as drip irrigation, and reduced use of fertilizers and pesticides.

  • Timeline: Start training within the first 6 months, with annual follow-up to ensure the adoption of sustainable practices over the next 5 years.

  • Goal: Reduce chemical fertilizer use and water consumption for irrigation by 40% in agricultural operations within 7 years.

• (5) Public awareness campaigns on water management and environmental risks

• • • Action: Organize regular (quarterly) awareness campaigns to educate the public on best water management practices, the risks of untreated discharges, and the impact of human activities on aquifers.

  • • Goal: Improve environmental awareness among 80% of residents and farmers over the next 5 years.

• (6) Promotion of climate-adapted crops and long-term sustainable irrigation

• • • Action: Encourage the planting of crops suited to semi-arid climates and the adoption of water-efficient irrigation systems through subsidies and incentives.

  • • Timeline: Expand subsidy programs within the next 2 years, with biennial reviews to assess progress.

  • • Goal: Reduce water consumption in large agricultural operations by 35% within 7 years and increase the proportion of climate-adapted crops by 50% within 10 years.

• (7) Investment in long-term research to adjust policies

• • • Action: Fund scientific research on the impacts of climate change and pollutant discharges on groundwater quality, with studies published every 3 years to adjust policies accordingly.

  • • Timeline: First publication within the next 3 years, followed by regular updates every 3–5 years.

  • • Goal: Update water management policies every 5 years based on research findings.

The rigorous implementation of these recommendations is essential to ensure sustainable water resource management in this region. This approach will promote balanced economic growth, particularly in the sectors of urban development and agriculture, while protecting public health, preserving resources for future generations, and ensuring rational development.

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

The authors declare there is no conflict.