Assessment of the quality and irrigation purpose suitability of alluvial water in Erfoud, Morocco

The fieldwork was carried out in June 2017, just before opening of water gates in the Al Hassan Addakhil Errachidia Dam, during the end of the dry season. According to irregular flow in the Ziz River, there is no seasonal variation (in regular intervals) of the characteristics of groundwater. Episodic changes due to the periodic flooding of the area is present. Nevertheless, it is hard to capture because of lack of a public information of opening of the dam and water flow in the river. During field studies 20 wells were investigated: 18 dug wells, reaching groundwaters (between 1 and 10 m deep) and 2 drilled wells, reaching waters located 30 m (E12) and 20 m (E17) deep. The investigated wells were selected due to the different depths of the water table and the diversified electrical conductivity. The field studies involved the basic measurement of physico-chemical parameters of groundwater, such as temperature (T) and specific electrical conductivity (EC). Both T and EC were measured with a conductivity meter (CC-401, Elmetron), with automatic compensation to the reference temperature of 25°C and the accuracy of ±1% and ±1°C, respectively. Samples of water from all investigated wells were taken for further laboratory analyses, collected in tight-capped polyethylene bottles and transported in cold storage.

Before each analysis, the samples were filtered through a glass microfiber filter Whatman GF/D. The analysis of chemical composition (the main anions and cations) was carried out using a ion chromatograph (DIONEX ICS 3000) with analytical columns: IonPac CS16 3×250 mm (for cations) and IonPac AS18 2×250 mm (for anions). The levels of detection for the studied parameters were: for Ca, 0.4 mg dm⁻³, and for other ions, 0.1 mg dm⁻³. The calibration curve was plotted based on Dionex six cation II standard and Dionex seven anion standard II. Heavy metals (Al, As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) were analyzed using an ICP MS TOF spectrometer (Optimass 9500 GBC). The calibration curve was plotted based on certified reference material (CLMS 2AN). All

the solutions were prepared using 1% nitritic acid Suprapur. The following solutions were prepared for calibration: 0 (blank), 1, 10, 50, 100 and 1000 μg dm⁻³. The conductivity and pH were measured with a multi-parameter water quality meter (Hach HQ 40d), whose electrodes were calibrated before each measurement series with Hamilton standards with pH 4, 7 and 9 and conductivity 15 μS m⁻¹.

Certified reference material KEIJM-02 from Environment Canada and ERM-CA713 from Institute for Reference Materials and Measurements, Belgium, were used for the purpose of quality control of the obtained results. The obtained values of concentration of the analyzed elements are presented in Table 1.

The water quality of each sample was plotted on a classical Piper diagram, referring to the concentrations (meq dm⁻³) of hydrochemical components (Piper 1944). This diagram provides a convenient method to classify and compare water types based on the ionic composition of different water samples (Hem 1985). To assess the usability of the waters for irrigation the following ratios were used: sodium total percentage (%Na) (Wilcox 1955), permeability index (PI) (Ragunath 1987), magnesium hazard (MH) (Szabolcs, Darab 1964), Kelly’s ratio (KR) (Kelly 1963), and sodium adsorption ratio (SAR) (Richards 1954). The type of ion exchange was deduced from the base exchange index (BEI), given by the following formula: BEI = ([Cl]−[Na+K])/[Cl] (meq dm⁻³). BEI < 0 indicates the fixation of Ca and Mg and solubilization of Na and K. BEI > 0 indicates the fixation of Na and K and solubilization of Ca and Mg.

The adequacy of groundwater for agricultural and domestic uses is directly related to the different physiochemical parameters and their concentrations. Table 2 presents detailed characteristics of the wells from which water was sampled for chemical composition analyses. The water table lies at the height of 742 to 867 m a.s.l. The height of the water column differs from well to well and ranges from below 1 m to approx. 10 m. The mean temperature of water is 22°C (from minimum 18.3°C to maximum 24.4°C). The study wells are recharge with alluvial water, which is often found in the area of Morocco. Wells of this type of supply are also found in other regions,

for example, oued Draa river (Warner et al. 2013), oued Tassaout River (Rochdane et al. 2015), oued Souss and Massa rivers (Bouchaou et al. 2008). Alluvial waters in Erfoud are characterized by a similar chemical composition as alluvial waters of other regions.

Statistical characteristics of the tested waters are presented in Table 3. The water had neutral or weakly alkaline reaction. Specific EC of the studied waters ranged from 3900 to 15800 μS cm⁻¹), which shows that the waters are saline. The main cations and anions display very similar spatial variation. Regarding spatial variation of selected water parameters in the Erfoud oasis, the highest sodium cation concentrations were observed in wells no. E11 and E12 in the northern part of Erfoud oasis and in wells no. E4, E8 and E15 in the southern part. In the case of sulphate anions, the highest values were also observed in wells no. E11 and E12 and in wells no. E4, E8, E13 and E15. The lowest concentrations of the main ions were found in well no. E17. This well is a complex one, both dug (around 10 m) and drilled (another 10 m), and is often used to water crops.

High salinity of waters, which are used for domestic and agricultural purposes, is a big problem in the arid and semi-arid zone of Morocco. The occurrence in groundwater of high salinity values is described in many regions in Morocco (Laftouhi et al. 2003, El Bakouri et al. 2008, Fakir et al. 2001) and also in other places (Batista et al. 2002, Cerejeira et al. 2003, Morvan et al. 2006, Baran et al. 2007, Hildenbrandt et al. 2008, Andrade, Stigter 2009). This salinity is associated with natural and anthropogenic contaminant. The natural factor is primarily the presence of evaporates (halite, gypsum) in the geological structure, which contain easily dissolved salts. In addition to the lithological nature of the reservoir, the intensive irrigation may contribute to an enhanced salinity.

The concentrations of trace elements and nitrogen forms (nitrates and the ammonium ion) display a mosaic character. There is a possibility that the content of nitrogen, as well as some heavy metals, is associated with anthropogenic pollution due to the lack of proper water treatment system. In the case of nitrates, a concentration higher than the World Health Organization standard for drinking water (WHO 2017) was only exceeded in well no. E11 and was 59.3 mg dm⁻³. However, higher concentration, 155 mg dm⁻³, was observed in well no. E20, which is located out of Erfoud and is not included in the above spatial distribution (Fig. 1, Tab. 3). As shown by Lgourna et al. (2014), based on the two

Fig. 2

field campaigns carried out during May 2009 and April–May 2010, concentrations of nitrates in the area of the Ziz basin ranged from 0 to 316 mg dm⁻³; in total 25% of wells exceed the recommended limit of 50 mg dm⁻³. This may imply anthropogenic contamination derived from local agriculture.

The major ions in this study area were plotted in the Piper diagram (Piper 1944) to identify the different types of the groundwater and understand their relation. Based on the tested samples we found that two hydrochemical types of water dominate in the study area: Na-Cl-HCO₃- SO₄ and Na-Cl-Mg-SO₄; there are also waters with multiple ions: Na-Mg-Cl-SO₄-HCO₃ (Fig. 2). The dominant type of groundwater, common in dry climates, not only in Morocco but also i.a., in Tunisia, is Na-Ca-Cl-SO₄, (Charef et al. 2009, Hamzaoui-Azaza et al. 2011, Lachaal et al. 2013, Alaya et al. 2016).

To assess the type of ion exchange, the base exchange index (BEI) was used. The BEI index (Tab. 4) shows the following types of ion exchange in groundwater: Na and K are replaced by Ca and Mg in 65% the samples (BEI>0), while Ca and Mg are replaced with Na and K in 35% samples (BEI<0).

An increase in sodium and chlorides with the increase of water salinity was found. The correlation coefficient, however, was 0.46, lower than in areas located south of Erfoud, where it reaches 0.90 (Warner et al. 2013). This may be connected with other processes, which determine the hydrochemical type of the water. The diagrams of Na/Cl versus Cl (Fig. 3 A) and Ca+Mg versus Cl (Fig. 3 B) clearly show that salinity increases with the decrease in Na/Cl and increase in Ca+Mg, which may be due to reverse ion exchange in the weathered layer (Appelo, Postma 1994). The figure 3B shows two outliers. The reason for their dissimilarity is more likely that the well no. 20 is located a few km north than the other ones, and the temperature of water in the well no. 9 is the lowest (18.3°C, whereas others temperatures reach more than 20°C).

Fig. 3

In alluvial waters of the oasis, a high correlation was found between salinity and sulfate concentrations (Fig. 3 C), for which r = 0.84. The high ratio suggests that the source of sulfates may be the dissolution of evaporites, such as gypsum, which can also be confirmed by the correlation between sulfates and strontium, for which r = 0.41.

The location of the sites along the line of SO₄+HCO₃ versus Ca+Mg may also suggest the dissolution of gypsum, as well as calcite and dolomite rock. This could explain the hydrochemical type of the water, in which the dominant cations are the sodium ion and the magnesium ion. The molar ratio of Na/Cl is in general <1, indicating a possible recent recharge due to direct rain water infiltration. The second explanation refers to the salt concentration in the investigated wells. Since it increases above the norm, more sodium is taken onto the soil cation exchange sites, thus lowering the sodium to chloride ratio (Kirchner et al. 2001). This pattern of behavior is consistent with ion exchange buffering of sodium in the catchment soils.

In the study area, agricultural activity is quite intensive. Hence, an attempt is also made to understand the suitability of groundwater for agriculture purposes. Table 5 presents the values of water salinity indicators. The total sodium percentage (%Na) was between 40 and 66, Kelly’s ratio between 0.6 and 1.9, the magnesium hazard between 24 and 64, and permeability index between 44 and 72.

Based on the values of KR and MH coefficients, the water was classified as unsuitable for irrigation purposes (in 90% and 85%, respectively). %Na indicates that the groundwater is permissible. Sodium concentration plays an important role in evaluating the groundwater quality for irrigation, because sodium causes an increase in the hardness of soil as well as a reduction in its permeability (Arumugam, Elangovan 2009, Jalali 2009). Based in the permeability index, the water is classified as moderate quality for agricultural purposes.

Sodium concentration is important when evaluating the suitability of groundwater for irrigation. The values of Sodium Adsorption Ratio (SAR) calculated for the Erfoud area are within the range of 4.2 to 14.1. In order to identify the availability of waters for irrigation, the Wilcox classification diagram was used. Water classification with the SAR vs EC diagram confirms the high and very high alkali and salinity hazard (Fig. 4).

Fig. 4

The samples are located in field C4, indicating high salinity hazard. Regarding alkali hazard, 21% of the water is at risk of medium, 42% of high, and 31% of very high sodium hazard. The location in the C4–S4 area (31% samples) shows that this water is saline and cannot be used on soil with restricted drainage. Even with adequate drainage, special management for salinity control may be required, and plants with good salt tolerance should be selected. In C4–S3 classes (42% samples), salinity is medium and classified as brackish water. The water can be used if a moderate amount of leaching occurs. Plants with moderate salt tolerance can be grown.

The results of hydrochemistry, supported by statistical analysis, suggest mainly the natural contaminants in groundwater of this area. EC, Na, Cl appeared as major chemical quality issues in groundwater of this area. The nutrient load in groundwater was comparatively low as compared to a load of cation and anion concentration. However, locally over-normative concentrations of trace elements and nutrients are observed. These are likely due to locally anthropogenic effects, which explain their spatial variability.

The United Nations (UN) General Assembly declared in 2010 that safe and clean drinking water and sanitation is a human right. The adoption of the Sustainable Development Goals in 2015 by many countries, including Morocco, involves targets and indicators of safe drinking water. Table 6 presents the parameters, which exceeded the health-based guidance level for drinking water determined by the WHO (2017).

The analysed water had an excessive concentration of parameters affecting its heath properties, i.a., NO₃, Cd, Mn, Ni. In the dry areas, elevated concentration of sodium and its negative influence on the ecosystem have been pointed out many times (i.a. Bahaj et al. 2013, Bouchaou et al. 2008, El Mandour et al. 2008, Rochdane et al. 2015, Dłużewski et al. 2017, Latos et al. 2018, Szczucińska et al. 2019). The low quality of alluvial water was observed in areas where wells are located in alluvial formations.