Assessment of Groundwater Quality in the Semi-Arid Environment: Implications of Climate Change

Abstract

The hydrogeochemical properties and evolution of groundwater in the Essaouira syncline basin in northwestern Morocco were investigated in this study, with a total of 447 samples during different campaigns (April 2017, May 2018, March 2019, and July 2020). These samples were analyzed for major ions and stable and radioactive water isotopes (δ²H, δ¹⁸O, and ³H). With decreasing rainfall from climate change in Morocco, it is crucial to assess the sustainability of groundwater reserves. This shortage leads to the degradation of water and soil quality. To ensure sustainable water management and preserve the environment in the study area, it is necessary to assess groundwater quality for drinking and irrigation, take precautions, and establish management plans. This study assessed groundwater quality using two water quality index methods (WQI and IWQI). Several natural processes control groundwater mineralization, including the dissolution of evaporite and carbonate minerals, cation exchange phenomena, evaporation, and seawater intrusion. According to the results obtained using the WQI method, all groundwater samples in the study area are generally of poor quality and must be treated before being used for domestic purposes. Based on the results obtained by the IWQI method, the samples are suitable for use as irrigation water, especially for plants resistant to high salinity concentrations. Stable isotope measurements (δ²H and δ¹⁸O) indicate that Atlantic precipitation continuously recharges the recharge areas of the Essaouira Basin. Thus, the low values of tritium (³H) in groundwater mean that the freshwater in the Essaouira Basin is ancient.

1. Introduction

Groundwater plays a crucial role in supporting global flora and fauna survival, with Morocco standing out as one of the largest consumers, primarily for agriculture and drinking water [1,2,3,4]. Agriculture constitutes a cornerstone of the Moroccan economy, heavily relying on hand pumps and tube wells for irrigation and drinking water in rural areas [5,6,7,8]. Evaluating groundwater quality is paramount, given its global use for various domestic, agricultural, and industrial purposes [9]. The escalating degradation of freshwater sources, which is exacerbated by pollution and climate change, i.e., the limited availability of water of acceptable quality, is generally caused by natural and anthropogenic factors and presents a significant challenge that negatively impacts drinking water supply [10,11].

Evaluating water quality and identifying suitable or unsuitable locations for human consumption and agricultural activities play a crucial role in ensuring sustainable management and socio-economic development. This assessment helps mitigate issues related to the quality and quantity of groundwater, addresses degradation and over-extraction, and reduces the risks of soil degradation.

In Morocco, with precipitation becoming scarce in this context of climate change [9], it is essential to examine the quantity and quality of groundwater reserves and their long-term sustainability. The lack of rainfall in these areas naturally leads to the deterioration of both water and soil quality and quantity.

Various methodologies, including multivariate statistical techniques and hydrodynamic, hydrochemical, and isotopic analyses, have been developed to assess groundwater quality variation [12,13,14,15,16]. The water quality index (WQI) is widely used to evaluate the quality of surface and groundwater, providing a concise and accurate assessment of its suitability for human consumption [17,18,19]. It is a powerful method for assessing drinking water quality and regulating water sources. For irrigation purposes, the irrigation water quality index (IWQI) has been developed to classify water suitability based on various risks (salinity, ion toxicity, trace elements, etc.) [20].

Over the past few decades, Morocco has transitioned significantly from “water stress” to “water scarcity”, with the average freshwater supply per capita declining to 650 m³ annually, compared to 1000 m³ in the early 2000s and 2500 m³ in 1960 [21]. This situation is exacerbated by deteriorating water quality resulting from industrial and agricultural pollution. Due to its geographical location in a semi-arid zone, Morocco is particularly vulnerable to the adverse effects of climate change, as evidenced by more frequent and intense drought periods and irregular precipitation [22,23,24,25].

The purpose of this research is to provide good-quality water to urban and rural populations that rely entirely on this resource. The main objective is to understand recent variations in groundwater quality and identify the key factors influencing these changes. Specific objectives include evaluating groundwater quality for the drinking water supply and irrigation using the WQI and IWQI indices. Additionally, this study aims to characterize the geochemical mechanisms governing the evolution of water quality and determine the recharge sources of the Essaouira Basin aquifers through isotopic (δ¹⁸O, δ²H) and radioactive (³H) signatures. Finally, this study seeks to provide farmers in the region with irrigation water that meets salinity standards in semi-arid environments while also contributing to food self-sufficiency.

2. Study Area

The study area (Figure 1) has a semi-arid climate, which is characterized by two seasons: one rainy season from November to March and the other dry season from April to October. The average annual rainfall does not exceed 300 mm, temperatures are around 20 °C, and the average evapotranspiration is about 910 mm/year. Potential evapotranspiration is calculated over a period of 28 years for the stations of Essaouira (1987–2015) and Igrounzar (1987–2014) based on the Thornthwaite formula (Thornthwaite 1948). The research region is crossed by four rivers—Igouzouline Wadi, Ouazzi Wadi, Ksob Wadi, and Igrounzar. Wadi, which makes up the western terminus of the High Atlas Mountains, is a 3500 km² region in southwest Morocco. It is home to 10% of the nation’s aquifers, including the Plio-Quaternary, Turonian, Cenomano-Turonian, and Hauterivian, whose resources and structures are frequently poorly understood. The Ouazi Basin, the Meskala Basin, the Ksob Basin, and the Igouzoullene Basin are the four sub-basins that make up the Essaouira Basin.

The Hadid Anticline, Igouzouline Wadi, the Bouabout Region, and the Atlantic Ocean form this basin’s northern, southern, eastern, and western boundaries, respectively. It is separated into two sections, the “coastal zone” (downstream section) and the “Meskala-Ouazzi” (upstream section) (Figure 1).

Regarding geology (Figure 2), the study area’s upstream region is distinguished by the outcrop of strata from the Middle and Upper Cretaceous periods, particularly the Albian-Vraconian, Cenomanian, and Turonian Formations [26,27]. These formations are made of marl and sandstone benches surrounded by limestone and dolomitic benches. Sandstone and dolomitic limestones alternate with sandstone banks and sandy clays in the Albian-Vraconian formations. Alternating marls represent the Cenomanian with anhydrite, lumachellic, and dolomitic limestones (approximately 200 m in thickness). Regarding the Turonian Formation, it is made up of silica-rich limestones. The Cenomanian-Turonian aquifer, which is still the most significant in the area, is a significant water reservoir in these synclines [28,29,30,31,32,33,34].

According to Jalal [35], this aquifer has transmissivities varying between 2.2 × 10⁻⁴ and 2.7 × 10⁻¹ m²/s.

Two significant aquifers can be found in the downstream portion: (i) the Plio-Quaternary and (ii) the Turonian in the northern part, between Ksob Wadi and Tidzi Wadi and (iii) the Hauterivian, which serves as the study area’s southern boundary, is situated between the Amssittene Anticline and Igouzoullene Wadi (Figure 1). The calcareous sandstone matrix that makes up the Plio-Quaternary aquifer is distinctive. It has a significant water table, which is the wall created by Senonian marls in the synclinal structure.

According to Mennani [36], the transmissivities of this water table range from 6.1 × 10⁻² m²/s to 4.5 10⁻⁵ m²/s. The Turonian, which is represented by limestones, has an aquifer that is captive beneath the Senonian marls in the synclinal structure and is likely in close contact with the Plio-Quaternary at the structure’s edges. Its transmissivity ranges from 0.8 × 10⁻⁴ m²/s to 2.7 × 10⁻² m²/s [36].

The Hauterivian Aquifer, which has a thickness of around 200 m, is made up of marly clays, more or less fractured dolomitic limestones, and siliceous limestones [26,37]. According to Mennani [36], the transmissivities of this aquifer range from 1.6 × 10⁻⁵ to 6.7 × 10⁻⁵ m²/s.

3. Material and Methods

3.1. Sample Collection and Physiochemical Analysis

We conducted an annual campaign during the low-water period to assess the state of the resource, as well as to monitor the decline in the groundwater level. Four hundred forty-seven groundwater samples were gathered throughout four campaigns (2017, 2018, 2019, and 2020), as shown in

Table 1. Physicochemical results of analyzed samples.

	pH	T	EC	Ca2+	Mg2+	Na+	K+	HCO3−	Cl−	SO42−	NO3−	IB
		°C	µS/cm	mg/L								%
Cenomanian-Turonian Aquifer 1
April 2017 campaign with 69 samples
Min	6.9	17.9	550	56.1	36.5	23	0	183.1	113.4	9.6	6.2	−9
Max	8.5	29.3	6776	535.1	306.3	761	82.1	683.4	2470.9	1441.4	192.2	9
Average	7.45	21.5	2282.7	172.7	109.3	191.2	4.2	398.8	544.6	272.8	45.5	−4
May 2018 campaign with 62 samples
Min	7.2	16.2	601	66.1	30.4	11.5	0	189.2	85.1	4.8	6.2	−9
Max	9.6	24.6	6845	583.2	329.4	561	35.2	689.5	1705.1	2219.7	167.4	9
Average	8	20.9	2481.8	177.2	115.6	152.9	5.4	382.2	495.5	283.5	37	−3
March 2019 campaign with 57 samples
Min	7	14.9	615	82.2	31.6	11.5	0	244.1	113.4	14.4	0	−10
Max	8.4	24	5738	769.5	297.8	540.3	168.1	897	1818.6	1941	173.6	5
Average	7.5	20.5	2428.4	213.6	118.5	166.7	13.9	499.2	573.8	339	37.4	−6
July 2020 campaign with 67 samples
Min	7	19.6	541	68.7	14.7	22	0.6	197.5	40.4	26	0.1	−9
Max	8.17	26.7	5285	699.1	285	614.3	214.6	680	1502	2106	940.5	8
Average	7.454	22.226	2129.2	193.6	92.8	156.1	10.4	356.6	424	274.6	56.9	0
Plio-Quaternary Aquifer
April 2017 campaign with 35 samples
Min	7.1	18.9	724.0	44.1	31.6	85.1	3.9	115.9	170.2	9.6	6.2	−9.0
Max	9.0	27.4	41,560.0	412.8	1422.1	12,504.3	277.6	567.5	23,205.6	2166.8	124.0	7.0
Average	7.7	22.6	3279.5	137.8	116.9	597.8	21.8	320.8	1292.5	157.5	32.2	−4.1
May 2018 campaign with 32 samples
Min	6.0	17.6	48.0	2.0	2.4	0.0	0.0	12.2	3.5	0.0	0.0	−9.0
Max	8.7	27.1	9744.0	364.7	238.2	1464.5	66.5	549.2	3158.6	408.4	396.9	3.0
Average	7.6	22.5	2652.7	134.1	69.2	271.3	11.2	260.8	627.1	122.4	37.4	−1.8
March 2019 campaign with 34 samples
Min	7.1	17.3	880.0	64.1	7.3	85.1	0.0	152.6	226.9	28.8	0.0	−10.0
Max	9.2	26.5	12,250.0	849.7	260.1	1949.6	74.3	659.0	4799.9	831.2	403.1	5.0
Average	7.7	21.8	2766.9	161.6	66.9	344.2	11.9	382.9	799.1	143.5	42.0	−5.8
July 2020 campaign with 34 samples
Min	6.5	20.5	775.0	51.6	12.9	31.9	1.8	144.0	74.4	23.5	0.2	−7.0
Max	8.4	32.0	23,850.0	454.7	715.9	6079.3	214.3	470.9	10,741.2	2098.0	149.7	3.0
Average	7.5	24.1	2837.5	151.8	78.5	411.7	14.9	302.7	801.8	213.8	40.4	−1.2
Hauterivian Aquifer
April 2017 campaign with 13 samples
Min	6.9	19.4	1134.0	106.2	59.6	50.6	3.9	244.1	127.6	57.7	6.2	−9.0
Max	8.4	24.8	2203.0	158.3	178.7	216.1	11.7	518.7	638.1	514.1	18.6	−1.0
Average	7.3	22.9	1647.6	126.7	90.3	114.8	7.8	445.4	345.8	189.2	10.5	−5.4
May 2018 campaign with 14 samples
Min	7.1	18.1	668.0	51.3	20.4	45.0	2.7	207.5	113.6	38.4	2.3	−8.0
Max	8.3	24.3	3688.0	246.9	186.6	196.7	9.0	549.1	894.6	340.8	15.3	3.0
Average	7.6	21.9	1803.6	126.8	83.1	92.0	5.3	425.2	302.6	152.0	8.5	−3.8
March 2019 campaign with 12 samples
Min	7.1	18.0	687.0	82.2	38.9	27.6	3.9	244.1	120.5	43.2	0.0	−8.0
Max	8.2	24.8	2405.0	190.4	109.4	115.0	15.6	659.0	411.2	374.8	18.6	−1.0
Average	7.5	22.5	1558.5	155.3	80.8	79.7	7.2	498.8	290.1	176.2	8.8	−5.3
March 2019 campaign with 12 samples
Min	6.6	21.5	481.0	60.7	25.6	39.2	1.5	178.1	51.7	66.0	0.2	−9.0
Max	8.5	32.0	27,000.0	341.1	817.5	7117.6	231.0	510.0	11,574.5	2320.0	940.5	4.0
Average	7.7	24.8	3136.0	145.8	97.4	507.7	25.3	372.7	894.4	274.3	66.1	−2.5

¹ In the study area, we encountered two situations: either the Turonian aquifer on its own or as part of the Cenomanian-Turonian system. Given the scarcity of water points associated with the Turonian aquifer, we have included it within the Cenomanian-Turonian system.

. A 200 m piezometric probe was used to measure the water levels. Using a HANA multiparameter system, the following parameters were measured in situ: temperature (T), hydrogen potential (pH), and electrical conductivity (EC) (HI9828). All wells were pumped for 5–10 min before groundwater monitoring to remove the influence of standing water. After sampling, groundwater samples were labeled, saved, and sent to the lab for chemical analysis. Afterward, groundwater samples were saved at each station in 250 mL polyethene bottles that were kept at 4 °C [38]. The principal chemical components analysis for the 2020 campaign was then examined at the Mohammed VI Polytechnic University in Morocco’s International Water Research Institute (IWRI) utilizing a SKLAR San⁺⁺ continuous flow analyzer (CFA).

The analysis of major chemical elements was analyzed in ENS (Marrakech, Morocco). The TH (as CaCO₃) and Ca²⁺ were analyzed volumetrically using standard EDTA. The Mg²⁺ was calculated, taking the difference value between TH and Ca²⁺. A flame photometer was used for the estimation of Na+ and K⁺ ions. The HCO₃⁻ and CO₃²⁻ were estimated using the volumetric method, with HCl as the standard solution. The Cl⁻ was analyzed by titrating it with standard AgNO₃. The SO₄²⁻ was determined using the turbidimetric procedure, and the NO₃⁻ was determined using the colorimetric method. The EC is expressed in micro-siemens per centimeter (μS/cm) at 25 °C. The chemical ions are expressed in milligrams per liter (mg/L), as well as in milliequivalent per liter (meq/L).

The reliability of the results found is evaluated using the ionic balance method. However, the accepted results are those that do not exceed the acceptable limit of 10% [39].

Stable water isotopes collected from 77 samples (wells, spring, surface water, and dam) of the campaign (2020) were analyzed in the laboratory at the Center “Centro de Ciencias e Tecnologias Nucleares (C²TN)”, Campus Tecnológico e Nuclear” in Lisbon (Portugal).

3.2. Drinking Water Quality Index (WQI)

The WQI, which is regarded as a helpful index to estimate the overall quality of groundwater for usage, is used to evaluate groundwater for drinking purposes [9,40,41,42]. According to Equation (1), the WQI is determined using the arithmetic weight technique (

Table 2. Weight and relative weight of each parameter used for the WQI calculation.

Physico-Chemical Parameters	WHO Standard [44] 1	Weight (wi)	$\mathbf{Relative} \mathbf{Weight} ({\mathbf{W}}_{\mathbf{i}}) {\mathit{W}}_{\mathit{i}}=\frac{{\mathit{w}}_{\mathit{i}}}{{\displaystyle \sum _{\mathit{i}}^{\mathit{n}}\mathit{w}\mathit{i}}}$
pH	8.5	4	0.114
EC (µS/cm)	1500	4	0.114
TDS (mg/L)	1000	5	0.142
Cl− (mg/L)	250	3	0.086
SO42− (mg/L)	250	4	0.114
NO3− (mg/L)	45	5	0.142
HCO3− (mg/L)	120	3	0.086
Na+ (mg/L)	200	2	0.057
Ca2+ (mg/L)	75	2	0.057
Mg2+ (mg/L)	50	1	0.029
K+ (mg/L)	12	2	0.057
		35	0.998

¹ The WHO standard is relevant to the method of this study as it serves as a comparison reference for the sampling results.

and

Table 3. Water quality classification based on WQI.

WQI Range	Water Type
Class 1: <50	Excellent water
Class 2: 50–100	Good water
Class 3: 100–200	Poor water
Class 4: 200–300	Very poor water
Class 5: >300	Water unsuitable for drinking

). When assessing the groundwater quality, weights between 1 and 5 were assigned, depending on how significant a role they played [43].

$$WQI={\displaystyle \sum _{i=1}^n{q}_i}{W}_i$$

(1)

where q_i is the sub-quality index of each parameter, W_i is the weight unit of each parameter, and n is the number of parameters according to Equation (2).

$$q_i={\displaystyle \frac{V_i-V_0}{S_i-V_0}}\times 100$$

(2)

S_i is the value of each parameter’s standard allowable limit, V_i is each parameter’s excellent value, and V₀ equals zero for all parameters except pH, which has a value of 7 [43]. Equation (3) calculates W_i for each parameter following the suggested guidelines [43].

$$W_i=K/S_i$$

(3)

where K is the proportionality constant.

3.3. Irrigation Water Quality Index

The main goal of establishing whether water is suitable for irrigation is to comprehend the water’s salinity levels, which impact soil structure and agricultural productivity. The electrical conductivity (EC), sodium adsorption ratio (SAR), sodium ion concentration (Na⁺), chloride ion concentration (Cl⁻), and bicarbonate ion concentration (HCO₃⁻) were the five water quality parameters used to create the irrigation water quality index (IWQI) [21]. Before the data analysis began, the concentration units were changed from [mg/L] to [meq/L].

The values of the accumulation weights (W_i) recommended by Meireles [21] were defined in the first phase based on their respective significance for irrigation water quality.

Table 4. Weights for the IWQI parameters according to Meireles [21].

Parameters	Weight (wi)
EC	0.211
Na+	0.204
HCO3−	0.202
Cl×	0.194
SAR	0.189
Total	1

displays its normalized values and total as being equal to one. The q_i value was calculated in the second phase using several values suggested by Ayers and Westcot [45], as shown in

Table 5. Limiting values of (qi) calculations [46].

HCO3−	Cl−	Na+	SAR	EC	qi
meq/L			meq/L	μS/cm
1–1.5	1–4	2–3	2–3	200–750	85–100
1.5–4.5	4–7	3–6	3–6	750–1500	60–85
4.5–8.5	7–10	6–9	6–12	1500–3000	35–60
<1 or ≥8.5	<1 or ≥10	<2 or ≥9	<2 or ≥12	<200 or ≥3000	0–35

. A higher value denotes better water quality and vice versa, representing a non-dimensional number. Equation (4)) was used to determine the q_i value.

$$q_i=q_{i\max }-\left[(x_{ij}-x_{\inf })\times q_{iamp}\right]/q_{amp}$$

(4)

where

q_max is the upper-class value of q_i.

x_ij represents the data points of the parameters shown in Table 1 (observed value of each parameter).

x_inf denotes the lower limit value of the class to which the observed parameter belongs.

q_iamp represents the amplitude of the classes for the q_i classes.

x_iamp is the amplitude of the class to which the parameter belongs.

Finally, the IWQI was determined using the following relationship (Equation (5)):

$$IWQI={\displaystyle \sum _i^n{q}_i\times W_i}$$

(5)

where n represents the number of parameters considered, in this case, 5 parameters. The values in

Table 6. Irrigation water quality index characteristics [21].

Recommendation		Water Use Restrictions	IWQI
Plant	Soil
No toxicity risk for most plants	May be used for the majority of soils with a low probability of causing salinity and sodicity problems. Leaching is recommended within irrigation practices, except for in soils with extremely low permeability.	No restriction (NR)	85–100
Avoid salt-sensitive plants	Recommended for use in irrigated soils with light texture or moderate permeability. Salt leaching is recommended. Soil sodicity in heavy-texture soils may occur. It is recommended to avoid its use in soils with high clay contents.	Low restriction (LR)	70–85
Plants with moderate tolerance to salts may be grown	May be used in soils with moderate to high permeability values. Moderate leaching of salts is suggested.	Moderate restriction (MR)	55–70
Should be used for the irrigation of plants with moderate to high tolerance to salts, with special salinity and control practices, except water with low Na, Cl, and HCO3 values	May be used in soils with high permeability without compact layers. High-frequency irrigation schedule should be adopted for water with an EC above 2000 µS cm−1 and a SAR above 7.0.	High restriction (HR)	40–55
Only plants with high salt tolerance, except for water with extremely low values of Na+, Cl−, and HCO3−	Should be avoided for irrigation under normal conditions. May be used occasionally in special cases. Water with low salt levels and high SAR requires gypsum application. In high-salinity water, soils must have high permeability, and excessive water should be applied to avoid salt accumulation.	Severe restriction (SR)	0–40

were multiplied by the corresponding weight of each parameter shown in Table 3 according to Meireles [21].

The IWQI has been considered one of the most effective tools for assessing irrigation water quality for policymakers. It provided a clear classification of irrigation water quality based on its impact on the irrigated soil and its toxicity to plants. The IWQI was classified into five categories, namely, no restriction (IWQI = 85–100), low restriction (IWQI = 70–85), moderate restriction (IWQI = 55–70), high restriction (IWQI = 40–55), and severe restriction (IWQI = 0–40) [21,46] (Table 6).

3.4. Spatial Interpolation

Throughout this study, groundwater parameters and their spatial distributions were known, as were the sampling locations, thanks to the use of ArcGIS software (version 10.8). Inverse distance weighting (IDW) interpolation was used in this study. This technique calculates surrounding points within a user-defined boundary to estimate a value for each individual cell [32,47,48,49,50]. However, to reliably generalize the information at the basin scale, and given the absence of water points in a context of prolonged drought, unfortunately, it would be necessary to conduct a study involving mathematical modeling of groundwater flow. Only a hydrodispersive model can ensure the accurate spatiotemporal monitoring of water quality.

4. Results and Discussion

4.1. Groundwater Geochemistry

Table 1 shows the outcomes of the physicochemical examination of groundwater samples taken from the research region.

The Plio-Quaternary Aquifer’s groundwater sample temperature ranges from 20.5 to 32 °C. In comparison, the Hauterivian Aquifer’s temperature ranges from 22.2 to 27.7 °C, with a mean value of 24.6 °C, and the Cenemano-Turonian Aquifer’s temperature ranges from 19.6 to 26.7 °C, with a mean value of 22.22 °C.

The majority of water samples have pH levels that are close to neutral. The average values for the Plio-Quaternary, Hauterivian, and Cenemano-Turonien aquifers are 7.5, 7.7, and 7.4 (Table 1).

The electrical conductivity (EC) of the groundwater of the Plio-Quaternary Aquifer ranges from 775 to 23,850 μS/cm, with an average value of 2837 μS/cm. The groundwater of the Hauterivian Aquifer ranges from 481 to 27,000 μS/cm, with an average value of 3136 μS/cm. The Cenemano-Turonien ranges from 541 to 5285 S/cm, with an average value of 2129.2 μS/cm. These findings demonstrate that the Hauterivian Aquifer’s groundwater is the least mineralized, and the Plio-Quaternary aquifer’s groundwater is the most mineralized (Table 1).

Chloride ions dominate the chemical makeup of groundwater samples from the Plio-Quaternary Aquifer, followed by bicarbonate for the anions, and calcium and sodium for the cations. Except for two samples, the Hauterivian Aquifer is characterized by the dominance of chlorides for anions and the dominance of calcium ions for anions.

4.2. Hydrochemical Facies

The Piper diagram was projected with the milliequivalent percentages of the chemical elements to highlight the hydrochemical facies of groundwater. This diagram makes it possible to track the development of the chemical makeup of the water [2].

The Piper diagram’s representation of the analyzed samples (Figure 3) reveals that (i) the groundwater in the Essaouira Basin’s upstream region is composed of three types, including Ca-Mg-Cl, Na-Cl, and Ca-HCO₃, with the first type predominating, and (ii) the groundwater in the basin’s downstream region is highly mineralized, with Na-Cl predominating. The study area’s diverse groundwater chemical facies reflect the area’s varied geological formations and the likelihood of a connection between surface water and groundwater.

4.3. Origin of Groundwater Mineralization

Along groundwater flow routes, water–rock interactions cause gradual changes in groundwater chemistry [51]. Understanding the geochemical processes that lead to groundwater mineralization can be aided by using ion ratios [52,53,54,55].

In dry and semi-arid areas, the Na⁺ vs. Cl⁻ relationship has frequently been employed to pinpoint the mechanisms of mineralization acquisition [56]. The Essaouira Basin’s significant association between Na⁺ and Cl⁻ (Figure 4a) shows that halite (NaCl) is likely dissolved by groundwater. The high Na⁺ and Cl⁻ concentrations and, in particular, a Na/Cl ratio close to one are indicative of this. Negative saturation indices support the disintegration of halite (Figure 5). Reverse cation exchange, which results in the fixing of Na⁺ and the release of Ca²⁺ and Mg²⁺ from the aquifer matrix, is what causes the excess of Cl⁻ over Na⁺ (Figure 6a) [55]. Groundwater in the coastal zone is gradually impacted by surface water recharge, human activity, evaporation, and seawater intrusion in the discharge zone (downstream portion of the research area), leading to increased Na⁺ and Cl⁻ concentrations.

Negative values are displayed using the gypsum and anhydrite saturation indices (Figure 5). This illustrates how the dissolution of gypsum and anhydrite influences groundwater mineralization. The Ca²⁺ against SO₄²⁻ correlation map reveals that while most samples are positioned above the 1:1 line, which indicates an excess of Ca²⁺ over SO₄²⁻, several locations from the four campaigns (2017, 2018, 2019, and 2020) are aligned with the gypsum and/or anhydrite dissolving line. Reverse cation exchange may explain the excess Ca²⁺ (Figure 4a).

A significant positive association between Ca²⁺ and Mg²⁺ can be seen (Figure 4c). This illustrates how these two elements most likely share a common ancestor. The calculated dolomite saturation indices are close to zero, indicating that the groundwater is saturated with dolomite.

The contribution of calcite dissociation and reverse cation exchange to groundwater mineralization in the Cenomanian-Turonian Aquifer system is shown in the Ca²⁺ against the HCO₃⁻ correlation diagram (Figure 4d). Values near zero are seen in the calcite saturation index (Figure 5), which represents an equilibrium between calcite dissolution and precipitation. The interaction between Ca²⁺ and HCO₃⁻ (Figure 4d) is negligible for all four campaigns (2017, 2018, 2019, and 2020). This demonstrates how little calcite dissolution contributes to groundwater mineralization in the studied area.

The plot in Figure 6b, “(Ca²⁺ + Mg²⁺) vs. (HCO₃⁻ + SO₂⁻)”, demonstrates that cation exchange is not the only factor affecting groundwater composition. Three categories can be identified in the diagram (Figure 6b). The samples are aligned along the 1:1 axis and display gypsum, calcite, and dolomite dissolution [56]. Samples from the second group exhibit a slight predominance of Ca²⁺ + Mg²⁺ over HCO₃⁻ + SO₄²⁻, while samples from the third group exhibit a notable increase in Ca²⁺ + Mg²⁺ concentration. Reverse ion exchange may cause an increased concentration of Ca²⁺ + Mg²⁺ compared to HCO₃⁻ + SO₂⁻ [57].

The breakdown of carbonate minerals is also hypothesized to be caused by elevated calcium and bicarbonate concentrations in groundwater. This implies that weathering and reverse ion exchange reactions are the main mechanisms governing groundwater chemistry due to excess HCO₃⁻ [56,57,58,59].

Dissolving regulates groundwater mineralization in the Essaouira Basin, including the dissolution of evaporite minerals and reverse cation exchange.

It should be noted that we emphasized the last year of 2020, which was particularly dry and marked by reduced precipitation, increased temperatures, and decreased streamflows.

4.4. Assessment of Groundwater Quality

4.4.1. Water Quality for Drinking Purposes

The water quality index (WQI) was used to evaluate the groundwater for drinking (Figure 7). The physicochemical characteristics used to generate this index are the basis for its selection. It is a five-class index (Table 3), with class (1) denoting exceptional water and class (2) denoting decent water, and WQI values ranging from 50 to 100. At the same time, classes 3 (100–200 WQI) and 4 (200–300 WQI) specifically designate water of poor and extremely poor quality, respectively. Water with WQI values greater than 300 (class 5) is not fit for human consumption.

The WQI values range from 55 to 1312 (

Table 7. Results of WQI and its percentage of four campaigns: 2017, 2018, 2019, and 2020.

Range WQI	Water Type	Companion 2017		Companion 2018		Companion 2019		Companion 2020
		Sample No.	%	Sample No.	%	Sample No.	%	Sample No.	%
<50	Excellente water
50–101	Good water	24	21	29	27	8	66.7	8	44.5
100–200	Poor water	73	64	61	58	4	33.3	8	44.5
200–300	Very poor water	12	11	13	12			1	5.5
>300	Unsuitable water for drinking	5	4	2	2			1	5.5

). These values are broken down as follows: one sample is marked as having water unfit for consumption (WQI > 300); one sample has WQI values ranging from 200 to 300 (very poor water), i.e., 5.5% of the total samples capturing the Essaouira Basin; eight water samples are of poor quality (100 < WQI < 200), i.e., 44.5% of the total samples capturing the studied aquifer; and eight water samples are of poor quality (50 < WQI < 101), i.e., 44.5% of the total samples capturing the studied aquifer.

In Figure 7, the WQI’s geographic spread is also shown. As shown by this map, the Essaouira Basin’s upstream region is primarily where the bad and extremely poor water samples are found. This suggests that human activities—including heavy fertilizer use, septic tank leaks, and organic matter effluent—are thought to affect the study area’s groundwater quality significantly. The research area’s groundwater quality also degrades with decreasing elevation (Figure 7). This phenomenon suggests that the geologic environment and human influences will impact groundwater during the flow process. That groundwater flow in the research area is essentially the same as surface water flow, going from a high elevation to a low elevation.

According to this index, the groundwater in all three aquifers is generally of poor quality. This might be explained by the rise in important element contents brought on by the interaction of seawater pollution, evaporite formation dissolution, and the cation exchange and evaporation processes.

4.4.2. Water Quality for Irrigation Purpose

As a result, the Essaouira Basin’s 2020 campaign IWQI values, which vary from 4.07 to 83.94, were categorized as follows: A total of 32.7% of the wells under analysis fell into the category of severe range, which limits the use of groundwater to irrigating only plants with high salt tolerance and forgoes irrigation under normal circumstances. A total of 8.9% of the analyzed wells were in the high restriction category, which restricts the use of irrigation water for plants with moderate to high tolerance in permeable soil without compact layers and taking into account the high frequency of the irrigation system for irrigation water, with EC > 2000 S/cm and SAR > 7, 8.9% of the analyzed wells in the high restriction category. However, 48.5% of the examined wells fell into the category of moderate restriction, which restricts the use of groundwater for irrigation of plants with a moderate tolerance and is advised in soils that are moderate to highly permeable while taking into account intermediate soil leaching processes. Ten wells were classified as having modest restrictions, which suggests avoiding salt-sensitive plants and considering concerns with the irrigated soil’s texture, permeability, and sodicity. In the category of “no restriction”, no wells were found (

Table 8. Classification of groundwater quality for the investigated sites based on IWQI for the four campaigns.

IWQI Values	Type of Restriction	Campaign 2017		Campaign 2018		Campaign 2019		Campaign 2020
		Sample No.	%	Sample No.	%	Sample No.	%	Sample No.	%
85–100	No restriction
70–85	Low restriction	2	15.4	3	23.1	3	3.4	10	9.9
55–70	Moderate restriction	4	30.8	8	61.5	18	20.7	49	48.5
40–55	High restriction	5	38.5			18	20.7	9	8.9
0–40	Severe restriction	2	15.5	2	15.4	48	55.2	33	32.7

).

The GIS distribution map depicts the IWQI’s spatial distribution and offers a clear visual of the research area’s groundwater quality (Figure 8). It can be a valuable tool for decision-makers to pinpoint problem areas with excessive extraction and act as a roadmap for sustainable groundwater management. The previous section went into great detail on the IWQI’s findings and classification. Highly salt-tolerant plants can be grown in the western zone of the research region, which was highlighted in brown on the GIS-IWQI map. Simultaneously, low- to moderate-limitation plants, which are highlighted in blue and green, can primarily be grown in the central and northeastern regions of the Essaouira Basin. Areas near the Atlantic Ocean that were severely restricted were indicated in red. According to the GIS zoning map for the parameter IWQI (Figure 8), groundwater quality for irrigation did not vary over the four years that samples were taken, according to the GIS zoning map for the IWQI parameter (Figure 8).

4.5. Origin, Modes, and Elevation of Groundwater Recharge

Stable isotope ratios of the water molecule range from −6.31 to 4.79‰ relative to SMOW (average of −4.38) for δ¹⁸O and from −40.48 to 48.32‰ relative to SMOW (average of −25.11) for δ²H (

Table 9. Results of isotopic analysis of samples collected in the Essaouira Basin for the 2020 campaign.

Aquifer	Sample	pH	EC	Cl−	δ 18O	δ2H	3H
			μS/cm	meq/L
CT	S1	7.03	1109	2.38	−5.87	−34.5	0.13
	S2	7.04	1030	2.11	−6.01	−34.9	0.49
	S3	7.37	1537	4.67	−5.85	−33.8	0.3
	S4	7.23	1597	4.5	−5.19	−31.3	0.43
	S5	7.12	1504	4.61	−5.36	−32.5	0.62
	S6	7.85	636	1.14	−5.12	−32.8	0.49
	S7	7.15	541	1.53	−6.31	−37.9	0.05
	S8	7.6	868	1.44	−6.16	−39.1	0.96
	S9	7.4	806	1.42	−6.07	−39.2	0.83
	S10	7.4	955	1.92	−5.93	−31.9
	CT123	7.3	2993	18.55	−5.24	−28.5
	CT88	7.45	3345	21.44	−5.32	−29.9
	CT60	7.5	1353	5.18	−5.28	−30.9
	CT66	7.7	1754	9.07	−4.37	−26
	809/52	8	666	1.39	−5.87	−31.2
	CT74	8	1390	7.21	−4.54	−28
	1209/52	7.3	985	2.81	−5.4	−34.6
	CT107	7.4	3270	26.8	−4.21	−24.3
	874/52	7.4	4323	36.67	−4.8	−24.9
	E39	7.7	1198	2.8	−5.58	−29.3
	CT39	7.6	1496	5.44	−6.09	−32.5
	75/52	7.9	2338	7.4	−5.46	−30.2
	CT35	7.9	850	2.69	−5.21	−27.2
	CT34	7.6	1148	2.2	−5.92	−32.1
	1112/52	7.5	1440	2.69	−5.99	−35.8
	776/52	7.4	2090	4.86	−6.27	−35.6
	CT40	7.4	1750	6.27	−6.29	−36.6
	CT108	7.1	1157	4.01	−3.16	−15.2
	CT125	7.5	1394	2.38	−5.62	−29.8
	CT49	7.3	2005	12.22	−5.31	−28.2
	O124	7.2	2417	16.11	−5.12	−28.4
	612/52	7.4	3800	33.85	−4.5	−22.7
	CT52	7.5	1970	12.2	−4.92	−24.2
	CT51	7.1	2126	14.41	−4.45	−22
	89/52	7.6	2106	11.73	−4.7	−24.5
	CT54	7.5	1891	10.66	−4.89	−23.6
	CT56	7.2	5285	30.76	−4	−19.6
	O122	7.6	865	1.43	−6.01	−32.2
	Barrage(CT90)	8.17	637	7.22	0.58	2.2
HT	E70	7.1	1407	5.9	−3.3	−12.4
	E71	7.7	4094	30.5	−3.01	−15.9
	HT120	7.2	1541	7	−4.37	−23.3
	HT102	7.5	1551	6.9	−4.34	−24.5
	HT24	8.2	1837	9.1	−1.03	−8.2
	E83	8.12	867	1.5	−1.34	−6.4
	HT115	7.9	1896	8.9	−4.75	−21.6
	HT117	7.7	1161	3	−5.32	−25.9
	HT118	8.1	481	6.1	−4.78	−19.1
	E72	7.6	2224	12.4	−2.81	−16.9
	E76	8.05	1862	10.6	−3.89	−20.8
	HT26	7.2	1579	7	−3.56	−19.3
	P9	7.3	1635	7.1	−4	−20.3
	HT25	7.4	1812	9.2	−5.25	−21.6
	E73	7.4	2340	13.7	−1.59	−11.1
	E74	8.5	27,000	326.5	4.79	20.3
	HT29	8.2	1050	1.5	−3.16	−10.1
PQ	11/51	8.1	23,850	303	0.08	3.13
	3/51	6.5	2033	11.04	−3.88	−20
	15/51	7.5	995	3.53	−3.76	−13
	327/51	7.6	3876	33.64	−3.61	−16.9
	27/51	7.7	775	2.28	−5.26	−29.3
	28/51	7.6	945	3.42	−5.36	−25.6
	E103	7.5	2372	14.63	−4.91	−24.4
	116/51	7.5	2442	15.54	−5.16	−25
	105/51	7.3	4317	33.84	−1.92	−13.8
	262/51	7.38	1714	9.35	−3.24	−18.4
	O5	7.5	1693	8.04	−4.9	−27.9
	148/51	7.5	1425	8.38	−3.87	−17.8
	149/51	7.6	3219	21.45	−1.95	−13.4
	380/51	7.6	2166	12.95	−5.29	−29.9
	386/51	7.6	2397	14.16	−4.29	−20.6
	390/51	7.06	1794	10.4	−4.62	−26
	346/51	7.7	817	2.1	−4.74	−26.3
	6/51	7.2	2472	15.22	−1.79	−8.8
	E3	7.7	1752	11	−5.32	−27.7
	O99	8.4	2804	21.02	−4.81	−26.6
	O121	6	40	*	−5.9	−27.2

* Not measured.

).

Two isotopic groups are present, as seen in Figure 9. The first (G1) samples, which are located above the global meteoric water line (GMWL) between the local meteoric water line of the Essaouira Basin (LMWL: δ²H = 7.96* δ¹⁸O + 11.30; Bahir [27]) and the global meteoric water line (GMWL: δ²H = 8* δ¹⁸O + 10; Craig [60]), highlight the significant contribution of direct rainwater infiltration to aquifer recharge, particularly for points located near the hydrographic network (wadis), which confirms a rapid and recent recharge.

On the other hand, the second group (G2) is represented by samples with higher enriched isotopic compositions positioned below the GMWL and LMWL. This group relates to samples in which the evaporation impact is predominant, indicating either the sluggish entry of rainwater due to low soil permeability or the return of evaporated irrigation water.

The Cl⁻ versus- δ¹⁸O diagram (Figure 9b) supports the existence of two primary processes that have aided in the mineralization of the Essaouira Basin: (i) the dissolution of evaporating rocks, which are primarily driven by the water–rock interactions mentioned above, and (ii) evaporation. Some samples, including HT29 (dam), HT24 (well), E73, and E74 (wadis), which are mentioned in Figure 9b, exhibit isotopic enrichment, supporting the idea that the evaporation process aids in groundwater mineralization.

We projected the isotopic contents into the altitudinal line graph of precipitation and water sources in Morocco (Figure 10), which has a gradient of 0.25 per 100 m for δ¹⁸O. This value is consistent with that defined for the High Atlas (0.27 per 100 m) [61,62], as well as that of the Essaouira Basin, which is our study area (−0.26 per 100 m) [38,39].

5. Conclusions

As precipitation declines due to climate change in Morocco, it is crucial to assess the quantity, quality, and long-term sustainability of groundwater reserves. The reduced rainfall in these regions inevitably causes deterioration in both water and soil resources.

Understanding the hydrochemical and isotopic variations of groundwater in the Essaouira Basin is crucial for assessing the impact of climate change on water availability and quality in this semi-arid region.

The processes governing groundwater mineralization are evaluated. The groundwater’s suitability for drinking and irrigation purposes is examined using the hydrogeochemical results of groundwater in the Plio-Quaternary, Turonian, Hauterivian, and Cenemano-Turonien Aquifers. The hydrochemical findings demonstrate the following:

The hydrogeochemical analysis revealed that the mixed facies Cl-Ca-Mg, Cl-Ca, Cl-Na, and HCO₃-Ca with most of the mixed facies Cl-Ca-Mg and Cl-Ca are present in the groundwater of the Cenomanian-Turonian Aquifer. The analysis of these facies’ temporal evolution reveals that there have been no notable changes. Cl-Na and Cl-Ca-Mg mixed groundwater can be found in the Plio-Quaternary and Turonian Aquifers. For the Plio-Quaternary Aquifer, the chemical facies have changed from Cl-Na to Cl-Na and Cl-Ca-Mg, and for the Turonian aquifer, it has changed from Cl-Na to Cl-Ca-Mg. The Hauterivian Aquifer typically displays three different chemical facies: Cl-Na, Cl-Ca-Mg, and HCO₃⁻Ca-Mg, with the Cl-Ca-Mg facies predominating. From Cl-Na to Cl-Ca-Mg, a striking evolution of facies was seen for the period under study. Several natural processes regulate groundwater mineralization in the Essaouira Basin, including evaporite and carbonate mineral dissolution, cation exchange, evaporation, and seawater intrusion. The outcomes of water analyses were assessed using the WQI and IWQI methodologies to determine the quality of groundwater used for irrigation and drinking purposes. For water management to be sustainable, understanding the groundwater quality in the area and identifying its use is crucial. According to the WQI method’s findings, all groundwater samples in the research region are unfit for drinking and must be treated before being utilized for domestic purposes. The IWQI method also reveals that the groundwater samples are of poor quality for use as irrigation water. However, the waters in the research region are appropriate for halophytic plants, such as samphire and sinapis, which make mustard because they can withstand high salinity concentrations.

According to the stable isotope signatures, the groundwater samples are of meteoric origin (δ¹⁸O, δ²H). These tracers show that direct infiltration recharges the aquifers in the Essaouira Basin during precipitation without significant evaporation. The hydrochemical approach’s findings are supported by the combination of chemical and isotopic characteristics, particularly Cl⁻ versus δ¹⁸O, which shows that dissolution is one of the primary processes causing groundwater mineralization in the Essaouira Basin. However, the recharge area of the Cenomanian-Turonian Aquifer varies between 375 (Bouzerktoun area) and 1275 m (Bouabout area), the recharge altitude of the Plio-Quaternary Aquifer varies between 225 and 950 m, and for the Hauterivian Aquifer, it varies between 225 and 950 m. These differences could explain the situation following a reduction in annual rainfall recorded in the region.

An essential component of more effective and sustainable management and socio-economic development is assessing groundwater quality and identifying suitable and unsuitable locations for domestic and agricultural use. However, the prudent use of this vital resource must be a top priority to mitigate qualitative and quantitative groundwater issues in arid and semi-arid regions. To enhance groundwater quality in the study area, several solutions are proposed. Firstly, it is crucial to reduce the use of chemical fertilizers by promoting sustainable agricultural practices, such as crop rotation and organic fertilizer use. Additionally, adopting innovative irrigation techniques like drip irrigation helps conserve water in order to address the deficit caused by reduced precipitation due to climate change. The improved management of septic systems is also necessary to prevent leaks and infiltrations into aquifers, requiring regular inspections and proper repairs. The adequate treatment of domestic effluents is essential to reduce organic pollution before discharge into groundwater. Simultaneously, educating and raising awareness among farmers, businesses, and local residents is vital to promote environmentally friendly practices. Strengthening continuous monitoring programs for water quality is also crucial to promptly detect potential issues and implement effective corrective measures. Implementing these integrated solutions can minimize the adverse impact of human activities on groundwater resources and advance sustainable water management in the region.