Introduction

GW is a valuable and essential natural resource, flowing in underground geological aquifers or onto the surface. Thus, its chemical composition is linked to the existing environment ^1,2. Understanding HG is important to establish the source of GW chemical composition and its interaction with rocks ³. It is also known that water quality is a significant factor influencing both human and animal health ⁴. Likewise, natural processes and human activity affect water quality ^5,6. Therefore, the knowledge of GW characteristics is vital for its management in the studied area ⁷. Indeed, different ionic ratios can be employed to describe water chemical composition, like some researchers have done, to point out GW chemistry evolution ^8-10.

Various investigations have discovered that several geochemical procedures, such as ion exchange, evaporation, rock weathering and mineral dissolution influence GW chemistry proprieties ¹¹. Then, whether water is for drinking or agriculture purposes depends on MW chemical composition; numerous investigators have examined surface water and GW hydrochemistry to assess if they are potable ^12,13.

However, GW in deep aquifers generally retains its stable isotopic signature (δ¹⁸O, δ²H) without being mixed with waters of different isotopic composition. Thus, stable isotopic configuration is used to distinguish fresh GW of isotopically separated bases from meteoric waters ¹⁴, such as vaporized water, during perpendicular transfer to unconfined or surface aquifers ¹⁵. However, it is known that fresh water δ²H/δ¹⁸O fraction differs, due to latitudinal and chronological climatic systems, which allow for precipitation waters dispersion throughout geographic areas ¹⁶. In addition, this fraction strongly depends on T, altitude, latitude, precipitation (which affects tropical and equatorial areas), and it is a function of climatic hazards ^17-19.

In this study, to determine geochemical processes contributing to water mineralization, major constituents and stable isotopes of GW from SS Moroccan L-D region were evaluated using chemical technics and AAS.

Results and discussion

Table 1 presents results obtained from the analysis of physicochemical parameters for L-D area studied water. It is noteworthy that pH values of GW from the studied region ranged from 6.88 to 7.75, with an average of 7.34. This suggests a neutral nature of the investigated GW. EC and TDS values are significantly high, 3341.53 µs/cm and 2505.66 mg/L, respectively.

Ct of Ca²⁺ and Mg²⁺ varied from 82.76 to 942.48 mg/L and from 14.58 mg/L to 269.73 mg/L, with an average of 291.83 and 68.03 mg/L, respectively. In addition, Ct of Na⁺ and K⁺ varied from 123.92 to 1460.32 mg/L and from 3.91 to 72.34 mg/L, with an average of 290.47 and 15.033 mg/L, respectively. Table 1 shows that main anions in the studied zone alternated from 200.65 to 2327.29 mg/L, 95.11 to 2378.69 mg/L, 109.84 to 976.30 mg/L and 0 to 56.43 mg/L, for Cl^-, SO₄ ^2-, HCO₃ ^- and NO₃ ^- ions, respectively. In fact, NO₃ ^- levels in most wells were low, under the boundary value defined by Moroccan standard (50 mg/L), apart from wells W26 and W27, of which maxima values were 56.43 mg/L. NO₃ ^- presence in this GW is due to the activities performed there, such as fertilizers application on the farms, plants break down, discharge of human wastewater and domestic effluents ²⁸. On the other hand, it is seen that the main cations mean molar has the following order: Na⁺ > Ca²⁺ > Mg²⁺ > K⁺. Anions trend as follows: Cl^- > SO₄ ^2- > HCO₃ ^- > NO₃ ^-.

HG facies of the investigated water

To determine HG facies of the GW samples, Piper diagram was used, and Fig. 2 represents the obtained results.

GW samples have a dominant percentage of 86.66% Na-Cl, probably due to evaporates deposits, and a main value of 13.33% Ca-SO₄. In addition, the slight trend near facies of Ca-SO₄ is perhaps due to gypsum lens dissolution situated in marly formations of Miocene, or by schist in the south of the studied zone. This indicates and confirms that GW quality is affected by several parameters, such as chemistry, reservoir rocks geology and anthropogenic factors ^29,30.

HG procedures determination

It is known that water-rock interfaces play a significant role in GW quality modification. They are also beneficial for identifying GW sources. To explain various HG intricate actions for developing water chemistry, the relationship between presented ionic elements was studied using dispersion plots ³¹. When the relationship is very adjacent to the right 1:1, the dependency grade is rectilinear between variables ³². This investigation may tell the source of diverse waters, and the procedures that created their chemical composition ^10,33. In addition, to determine occurred HG procedures, numerous graphical standards are often employed. Fig. 3 represents Ct of Cl vs. Ct of Na diagram for the studied waters.

This diagram is often employed to determine the procedure responsible for water salinity ³⁴. It is found that most samples are situated alongside NaCl halite solubility line (i.e., where the Na/Cl molar quotient = 1), which is related to the marine spray dissolved by rainwater (sea salt), or to evaporites modification ³⁵.

In addition, the correlation factor between chemical constituents (Na⁺ and Cl) in the investigated GW was very high (R = 0.98). This finding indicates that their correlations are quite robust, since they have an identical source. Indeed, it is noticed that the source has a dry location, with high-speed evapotranspiration, which causes the formation of salt deposits that are splashed into GW ³⁶.

This diagram is often employed to determine the procedure responsible for water salinity ³⁴. It is found that most samples are situated alongside NaCl halite solubility line (i.e., where the Na/Cl molar quotient = 1), which is related to the marine spray dissolved by rainwater (sea salt), or to evaporites modification ³⁵.

In addition, the correlation factor between chemical constituents (Na⁺ and Cl) in the investigated GW was very high (R = 0.98). This finding indicates that their correlations are quite robust, since they have an identical source. Indeed, it is noticed that the source has a dry location, with high-speed evapotranspiration, which causes the formation of salt deposits that are splashed into the GW ³⁶.

Source of Ca ²⁺ , Mg ²⁺ , HCO ₃ ^- and SO ₄ ^2- ions

Fig. 4 represents the plot of SO₄ ^2- + HCO₃ ^- vs. Ca²⁺ + Mg²⁺ ions.

It is seen that GW samples are dispersed on line 1:1 (least mineralized) and beyond. In addition, samples adjacent to 1:1 display that calcite, dolomite and gypsum dissolution is the main mechanism in the system ³⁷.

However, in CO₃²⁻ media, simultaneous Ca²⁺ ions enhancement and Mg²⁺ ions diminution are principally due to interaction phenomena between rocks and water, such as dolomitization, dissolution and precipitation ³⁸.

On the other hand, Fig. 5 is the graphic representation of Ca²⁺ vs. Mg²⁺ ions.

It is seen that some of the studied GW points have a Mg²⁺/Ca²⁺ ratio different from 1:1 line. This finding means that Ct of Ca²⁺ is higher than that of Mg²⁺ ions. Ca²⁺ source mainly comes from gypsum dissolution. It is known that dolomite dissolution results in a 1:1 relationship between alkalinity and Ca²⁺ + Mg²⁺ ions. Indeed, Fig. 6 represents the variation in Ca²⁺ + Mg²⁺ vs. HCO^3- ions and Ca²⁺ vs. HCO^3- ions.

It was noticed that the relationship between alkalinity and Ca²⁺ + Mg²⁺ ions is weak. This finding shows that dolomite dissolution was not the principal source of Ca²⁺ and Mg²⁺ ions in the studied GW area. Moreover, the relationship between alkalinity and Ca²⁺ ions is quite weak, indicating HCO^3- ion reduction.

Nevertheless, Fig. 7 represents the plot of Ca²⁺ vs. SO₄ ^2- ions. It is seen that the bulk of investigated GW is dispersed nearby the gypsum dissolution line. This confirms the source of Ca and SO₄ ions dissolution (gypsum). Additionally, the obtained association between Ca and SO₄ ions proposes the discharge of gypsum and anhydrite, when water runs underground.

For more details, Fig. 8 shows the representation plot of Na⁺ ions vs. HCO₃ ^- ions. A poor correlation between alkalinity and Na⁺ ions was obtained. This result suggests that Na₂CO₃ dissolution is not significant in the investigated area.

In fact, the studied SS zone is in the arid region, which offers an advantageous circumstance for evaporation and condensation. Due to evaporation, Cl^- ions in GW from arid zones is high, which reflects its mineralization degree. Additionally, Fig. 9 shows the plot of TDS vs. Cl^- ions. It is seen that Ct of Cl^- rises with TDS growth, indicating that the investigated GW was affected by the evaporation phenomenon.

Mineral dissolution/precipitation: Gibbs diagram

It is known that the interaction between GW and aquifer minerals has an essential role in water quality, which is also suitable for determining its source 39. Gibbs 40 proposed a diagram where cations (Fig. 10a)) (Na + K)/(Na + K + Ca) and anions Cl/(Cl + HCO₃) proportion (Fig. 10b)) is expressed in the relationship with TDS.

Fig. 10a and b represent obtained Gibbs plot for investigated GW from Moroccan L-D zone. It is seen that most of the studied samples were found at the evaporation area, where its effect foremost occurs in the investigated GW region. Therefore, it is concluded that the evaporative dissolution procedure governs GW chemistry in this area. In addition, it is noticed that most tester walls are in the evaporation area, which may be due to climatic circumstances (arid weather).

SI at chemical stability

According to literature, fresh-water quality and exchanges of water with soil and rocks through percolation and packing in aquifers play a very important role in the determination of GW chemistry ⁴⁰. Indeed, SI are generally employed to define the trend of water towards precipitation or salt dissolution. Fig. 11 represents SI evolution of the investigated GW.

According to geochemical modeling results on GW development, there is an important relation between SO₄ ^2- ions and SI of carefully chosen minerals. These findings reveal that CO₃²⁻ precipitation is intensely influenced by evaporative mineral dissolution.

Isotope results

The investigation of stable isotopes from water molecules was widely employed for studying GW. They offer more data on the water source and on how the GW is renewed. They also define small and durable climatic changes, and allow the quantitative valuation of mixing and other procedures, such as those reported by ^42,43. Indeed, GW salinity origin can be found by stable isotopes configurations 44. They are equivalent to that of GMWL: δ²H (‰) = 8*δ¹⁸O (‰) + 10 ⁴⁵. ⁴⁶ revealed that this relation is caused by isotopic water particles fraction through evaporation and condensation phenomena in hydrological cycle. Table 2 shows isotopic analyses of waters from the studied L-D region.

It is seen that O and δ²H values of the thirty studied GW testers changed from -6.96 to -8.93‰, and from -51.5 to -65.56‰, for δ¹⁸O and δ²H isotopes, respectively. These ranges describe an Atlantic source of precipitation. In addition, their findings are almost like those determined by other researchers ²³.

Indeed, evaporation usually occurs through the surface runoff in lakes and reservoirs ⁴⁷. Thus, the effect of isotopic conversion procedure can modify GW isotopic structure, which must also be taken into account ⁴³. Moreover, atmospheric T is a main factor that controls isotopic composition and δD/δ¹⁸O relation of water ⁴⁸.

Fig. 12 represents the diagram of isotopic composition (δ²H, δ¹⁸O) vs. LMWL and GMWL.

Thus, to assess the determined values for both meteoric water lines, it is seen that the complete points of tested GW are strategized to the right, describing a trend with an inclination of 5.35, and reproducing an evaporation incline between 4 and 6 in arid and semi-arid zones ^43,50,51. These results suggest either evaporation of raindrops before hitting the ground ⁵², or overdue infiltration of precipitation, and a mixture of meteoric water with evaporated rain, in some limited samples ²³.

In its turn, determining altitudes and recharge zones in the origin is very important for estimating GW assets. Thus, the stable isotope rank is employed to define fresh-water elevations. In addition, wastewater isotopic sign is frequently used vs. soil T, where the seepage follows ⁵³. For this, the probable relation between δ¹⁸O isotopic quotient and water infiltration altitude was defined. Fig. 13 represents δ¹⁸O values and elevations of various investigated GW. Herein, a line drawn to give local altitude isotopic slope was obtained at -0.52‰ per 100 m.

Thus, the obtained values are dissimilar from those presented in literature. In fact, on Morocco rule, the local altitude isotopic slope was about -0.27‰ per 100 m altitude ⁵⁴. So, water should be recharged at altitudes that do not exceed 300 m above sea level. However, variations in isotopic structures between the aquifer and precipitation (which have been employed to define the slope) cannot be assigned to altitude, and may have been caused by soil T.

Conclusion

Combined HG and stable isotopes (δ¹⁸O, δ²H) practical approach in this investigation led to a good comprehension of the function of GW system in the studied L-D area, where mineralization procedure caused a significant modification in composition. Obtained HG parameters showed that abundant cations and anions followed the trend: Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and Cl^- > SO₄ ^2- > HCO₃ ^- > NO₃ ^-, respectively. In addition, two dominant HG facies were founded for GW of the studied zone: Na-Cl and Ca-SO₄, with 86.66% of the former. It was also revealed that the main procedures in water mineralization are evaporation and minerals dissolution phenomena. Moreover, stable isotope data indicated an important origin of meteoric water recharge linked to a predominance of evaporation in the increase in salt Ct. Finally, it was found that waters are recharged at altitudes not higher than 300 m.

Authors’ contributions

K. Mizeb: collected data; conceived and designed analyses; performed analyses. M. Ghalit: collected data; contributed with data or analyses tools. M. Doubi: conceived and designed analyses; collected data; conceived and designed analyses; wrote the paper. M. El Kanti: conceived and designed analyses; collected data; contributed with data or analyses tools. H. Erramli: conceived and designed analyses; contributed with analyses tools; conceived and designed analyses. R. Touir: conceived and designed analyses; collected data; contributed with data or analyses tools; conceived and designed analyses; wrote the paper.

Abbreviations

AAS: atomic absorption spectroscopy

Ca-SO₄: calcium sulfate

CO₃²⁻: carbonate

Ct: concentration

EC: electrical conductivity

GMWL: global meteoric water line

GW: groundwater

HCO₃ ^-: hydrogencarbonate

HG: hydrogeochemical

IRMS: isoprime precision isotope ratio mass spectrometer

L-D: Laayoune-Dakhla

LMWL: local meteoric water line

MS: mass spectrometry

MW: makeup water (water added to compensate for losses, especially those caused by evaporation)

Na₂CO₃: sodium carbonate

Na-Cl: sodium chloride

NO₃ ^-: nitrate

SI: saturation index

SO₄ ^2- : sulphate

SS: southern Sahara

T: temperature

TDS: total dissolved solids

UV-vis: visible ultra-violet

V-SMOW: Vienna-standard mean ocean water

Symbols definition

‰: per mile

δ²H: deuterium

δ¹⁸O: oxygen-18