ABSTRACT
Groundwater plays a critical role in supporting a wide range of human activities. However, it faces substantial challenges due to the growing demand for water and reduced precipitation resulting from climate change. Various studies have revealed the high vulnerability of Morocco's groundwater to contamination from multiple anthropogenic sources. This review focuses on assessing nitrate and salinization pollution in Moroccan groundwater, with a specific emphasis on the Gharb region. A comprehensive analysis of research conducted from 2010 to 2023, using reputable databases, underscores the pressing need to address groundwater pollution in Morocco, especially in the Gharb region. The results highlight the significant challenges faced by Morocco's groundwater resources. Agricultural practices and poorly designed irrigation systems are identified as primary contributors to nitrate contamination. Additionally, salinization in the region is influenced by factors such as seawater intrusion, hydrogeological characteristics, and irrigation practices. An integrated approach, combining laboratory analysis, remote sensing, geospatial tools, modeling, and geographic information systems technology, has proven to be effective in addressing the complex issues of assessing groundwater pollution due to nitrate and salinization. This survey presents a comprehensive framework for future research and decision-making processes aimed at achieving sustainable water resource management, preserving the groundwater heritage and safeguarding public health.
HIGHLIGHTS
The study highlights Morocco's groundwater vulnerability to nitrate and salinization pollution.
Agricultural practices and irrigation systems are pinpointed as major contributors to nitrate contamination.
The survey offers a comprehensive framework for future research and policy development, aiming to achieve sustainable water management and agricultural development in the region.
INTRODUCTION
Water is a vital natural resource that covers about 70% of the Earth's surface, yet only 3% of the total water available is freshwater, which is essential for sustaining life. The distribution of water resources is not uniform globally, resulting in disparities in water demand and availability (Mbaki et al. 2017). With the ever-growing global population and the increasing demand for food production, water requires special attention and careful management (Lahmar et al. 2019). Throughout Earth's history, the quantity and quality of water have been continuously deteriorating (Bouita et al. 2021). Understanding the significance of water quality is crucial as it extends beyond its impact on freshwater availability and human health; it holds intrinsic value (Machiwal & Jha 2015). Additionally, the complex interplay of population growth, climate change, and inefficient irrigation practices has further exacerbated the issue of water scarcity (Sefiani et al. 2019). Experts consider water scarcity as one of the most critical challenges facing civilization in the twenty-first century (Simonovic 2002). This problem is particularly significant in semi-arid regions where groundwater extraction for drinking water and irrigation plays a pivotal role in socio-economic development, as seen in Morocco and other Mediterranean countries (Carneiro et al. 2010). In Morocco, the challenges faced by water resources are significant, primarily due to the combined effects of climate change and rapid population growth. The country's natural water resources are among the lowest globally, with Morocco ranking 139th out of 179 countries in terms of pressure on available freshwater resources (El Oumlouki et al. 2018). The estimated potential of Morocco's natural water resources is 22 billion m3/year, with 4.2 billion m3/year for groundwater and 18 billion m3/year for surface water (El Oumlouki et al. 2018). However, the excessive utilization of water resources has led to water and soil degradation, resulting in soil salinization issues. Approximately 160,000 ha or 16% of irrigated land is already affected to varying degrees by salinization (El Oumlouki et al. 2018). Moreover, the overuse of water resources has also resulted in problems such as sodification, deterioration of soil structure, and nitrate pollution. The scarcity and uneven distribution of conventional water resources have raised concerns about the country's water crisis, which aligns with the findings of the United Nations World Water Resources Development Report. With a mobilizable water rate of 650 m3/year, Morocco ranks 155th out of 180 countries (Bouita et al. 2021).
Groundwater plays an important role in fulfilling water demands in Morocco, but it faces significant challenges. The combination of increasing water demands and reduced precipitation caused by climate change has imposed considerable stress on groundwater resources in the country. Over the past three decades, there has been a persistent imbalance between groundwater extraction and recharge, leading to a substantial decline in groundwater levels, with depths reaching 20–65 m. This rapid decline, averaging 0.5–2 m/year, is primarily attributed to limited groundwater recharge and the expansion of agricultural activities (Hssaisoune et al. 2020). Groundwater supports approximately 40% of the water used for irrigation in Morocco, sustaining about 75% of the country's vegetable crops and orchards earmarked for export (Seif-Ennasr et al. 2022). Agriculture heavily depends on groundwater, accounting for 80–95% of water usage, with at least 40% sourced from groundwater (Hssaisoune et al. 2020). In the southern basins, this reliance exceeds 70% (Warner et al. 2013; Hssaisoune et al. 2017). Groundwater assessments reveal an annual extraction of 4,226 million cubic meters (MCM) of water, while the replenishment potential is approximately 3,404 MCM, resulting in an annual groundwater deficit of around 862 MCM for the entire country. However, it is important to note that the situation varies across different basins and from north to south. Furthermore, the issue is exacerbated by the alarming rate of illegal well drilling, with nearly 30% of groundwater withdrawals being unauthorized. Groundwater pollution in Morocco is a significant concern, with approximately 31% of the country's groundwater being impacted by human pollution and natural degradation (Ez-zaouy et al. 2022). The quality of groundwater is a pressing issue, as 22% is categorized as having moderate quality, while 47% is considered good to very good quality, indicating a threat to 53% of the resources due to various natural and anthropogenic processes. The coastal aquifers in Morocco have particularly experienced adverse effects on groundwater quality due to the accelerated decrease in groundwater levels (Ahmed et al. 2021). Factors such as seawater intrusion, water–rock interaction, evaporation, leakage, irrigation water return, anthropogenic activities, nitrate pollution from fertilizers, sewage, manure, as well as natural salinity changes from evaporation and dissolution, contribute to the existing challenges (Ez-zaouy et al. 2022). Previous studies have primarily focused on inorganic compounds, heavy metals, and organic compounds, revealing severe pollution issues in Morocco's groundwater (Marouane et al. 2014a; El Bouzaidi et al. 2020; El Bouzaidi et al. 2023).
Nitrate pollution has become a significant issue, particularly in irrigated areas with intensive agricultural practices. In Morocco, the use of nitrogen fertilizer is extensive, with an estimated consumption rate of around 51 kg/ha (Marouane et al. 2015). Although nitrate is a natural component of vegetables and a regular part of the human diet, its levels can significantly increase in groundwater due to excessive use of nitrogen fertilizer, discharge of wastewater, and leakage from sewage networks (Gao et al. 2020). Modern agricultural practices heavily reliant on nitrogen-based fertilizers are the primary contributors to elevated nitrate concentrations in groundwater (Panneerselvam et al. 2021). The presence of nitrate in groundwater has a profound impact on its quality (Mohamed & Rachid 2022). This global concern over nitrate contamination in groundwater has gained attention in numerous studies that aim to assess its impact on groundwater resources and highlight observed changes (Rawat et al. 2022). In Morocco, specifically, nitrate pollution in groundwater is a growing problem, particularly in areas with intensive agricultural practices and irrigation (Warner et al. 2013). This form of pollution not only diminishes the availability of high-quality water resources but also poses health risks to the rural population (Zhao & Pei 2012).
Groundwater pollution due to salinization is a growing concern with significant implications for both groundwater and soil quality. Globally, the loss of agricultural land at a staggering rate of approximately 10 ha/min is largely attributed to salinization, accounting for 30% of this loss, which translates to an annual reduction of 1–2% of irrigated land. This issue is particularly pronounced in semi-arid and arid regions, where the intensive utilization of water resources leads to substantial evaporation, resulting in the salinization of soils and groundwater (Oumara & El Youssfi 2022). Disturbingly, the affected area continues to expand at a rate of 10% per year, driven by factors such as high evaporation rates, low rainfall, inadequate irrigation practices, and other unsustainable human activities. It is projected that by 2050, salinization will impact more than 50% of the world's arable land (Seif-Ennasr et al. 2022). In Morocco, the issue of groundwater salinization has garnered significant attention in recent years, with extensive research conducted to understand the processes, extent, and progression of seawater intrusion, vulnerability, and pollution risks in coastal aquifers (Karroum et al. 2017; Ez-zaouy et al. 2022). Despite scientific interest and policy relevance, salinity poses a substantial obstacle to meeting the water demands in the country. The salinization process is influenced not only by climatic factors but also by human activities, specifically intensive agriculture driven by economic motivations and the efficiency of drainage systems. These irrigation practices in Morocco pose an elevated risk of salinization, with around 160,000 ha (16%) of irrigated land already affected by varying degrees of salinity (Oumlouki et al. 2016). The rapid escalation of salinization and sodification in agricultural areas can be largely attributed to the utilization of marginal and poor-quality water, primarily derived from groundwater sources. This situation is not unique to Morocco, as approximately 30% of the world's irrigated land is impacted by salinity, and the Mediterranean region alone accounts for an extensive area of approximately 16 million ha (Shin et al. 2022). Addressing the issue of groundwater pollution caused by salinization is of utmost importance to preserve water resources and mitigate the adverse effects on agricultural productivity in Morocco.
Groundwater vulnerability assessment plays a crucial role in safeguarding groundwater resources, and it is essential to incorporate regional groundwater vulnerability evaluation early in the risk assessment process (Uricchio et al. 2004). Consequently, the assessment of groundwater quality becomes imperative at both spatial and temporal scales to effectively manage this vital resource, particularly in water-scarce regions. Matgat first introduced the concept of groundwater vulnerability in 1968, and since then, numerous studies have addressed this issue. Starting from the 1980s, groundwater vulnerability investigations have focused on two key aspects: specific vulnerability and intrinsic vulnerability (Meng et al. 2020). Specific vulnerability pertains to the sensitivity of groundwater vulnerability to pollutant properties, human activities, and physical parameters, while intrinsic vulnerability relates to the extent to which pollutants diffuse through the soil surface and reach the groundwater (Babiker et al. 2005). Various methods are employed to assess groundwater vulnerability, including the DRASTIC method, GOD index (Arauzo 2017), SINTACS method (Meng et al. 2020), and GALDIT (Boufekane et al. 2022). The generation of a groundwater vulnerability map necessitates the integration of climatic, geological, and hydrogeological factors. Furthermore, data collection through water sampling campaigns, digitalization, data entry, and treatment are facilitated by the use of geographic information systems (GIS) (ArcView and MapInfo) (Sanad et al. 2024a). Moreover, models such as GLEAMS, HYDRUS, or indices such as the water and nitrogen losses (LOS) provide valuable insights into water movement from agricultural land and soil profiles (Kazakis & Voudouris 2015). Assessing groundwater quality, understanding its hydrochemical evolution, and determining its suitability for irrigation are crucial steps in the sustainable management of water resources (Brindha & Kavitha 2015; Zouahri et al. 2015; El Oumlouki et al. 2018; Srivastava 2019; Sehlaoui et al. 2020; Sunkari et al. 2020; Zhou et al. 2020). These studies have made significant contributions to the research on groundwater hydrogeochemistry, providing valuable tools to combat pollution and effectively manage these natural resources while ensuring the sustainability of agricultural and economic development. Furthermore, evaluating aquifer vulnerability, susceptibility, and potential pollution risks is crucial for supporting government agencies in their planning efforts to protect these resources. The availability of accurate and up-to-date groundwater quality maps is emphasized as an essential requirement for the effective management of water resources (Matzeu et al. 2017).
This paper has several key objectives. It aims to investigate the sources and factors contributing to nitrate contamination and assess groundwater salinization, with a specific focus on the Gharb region. Additionally, the study intends to evaluate the impacts and consequences of nitrate and salinization pollution. It will also provide an overview of the approaches and tools used for assessing groundwater pollution. The findings from this research will contribute to the existing knowledge base and facilitate the implementation of sustainable groundwater management practices in the region.
METHODS
Study area
Geographical and geological characteristics
The Gharb region, located in the northwestern part of Morocco, exhibits unique geographical and geological features that influence the dynamics and quality of groundwater (Figure 1). Bordered by the Drader-Souier plain to the north and the Maamora plateau to the south, the region is encompassed by the vast Atlantic Ocean to the west. To the east, conglomeratic outcrops define the boundaries of the basin, adding to its geological complexity. With elevations varying from 4 to 25 m, the Gharb region consists of distinct zones, including a coastal area, continental borders, and the central alluvial plain of the Sebou River (El Bouzaidi et al. 2023). This river serves as a crucial water source within the region. Moreover, the study area is precisely situated at 34°18′33″ N latitude and 6°18′41″ W longitude (Lahmar et al. 2020).
The region is characterized by a dynamic geological setting with tectonic and gravity-induced subsidence that has persisted over time (Aguedai et al. 2022). The sedimentary infill in the Gharb plain is thickest in the central part and gradually thins towards the southern edge (Kili et al. 2008). Recent geoelectrical investigations have provided valuable insights into the aquifer's characteristics and the influence of sedimentation from the Maamora basin and other paleo-rivers (E.l Bouhaddioui et al. 2016). The geological composition features schists and quartzites in the central plain and a significant impermeable layer of ‘blue marls’ acting as a barrier within the Gharb aquifer system (Jeddi et al. 2017). The coastal zone of the Gharb Plain features extensive sandy soils, covering approximately 39,000 ha or 15% of the total area. These sandy soils play a significant role in influencing groundwater recharge and the overall hydrological behavior of the region (Aziane et al. 2020b).
Hydrogeological system
The climate of the region exhibits variations, with a sub-humid climate prevailing in the north and west, while the south and east experience a semi-arid climate. The average annual rainfall in the area is approximately 500 mm, with a gradient that decreases from east to west. As one moves from the coastal areas towards the inner basin, the temperature tends to increase, ranging from 23 to 35°C (Nezha et al. 2016).
The hydrographic network in the Gharb region is dominated by the Sebou River, which is one of the principal rivers in the kingdom, along with its tributaries including Ouerrha, Beht, Rdom, and Tiflet (Al Mazini et al. 2018). The Gharb region in Morocco is renowned for its significant hydrogeological importance, particularly as one of the prominent basins in the country (Mohammed et al. 2022).
The hydrogeological system of the Gharb region exhibits distinct characteristics with two types of aquifers. The first aquifer is a deep water table, covering an extensive area of 3,500 km2 and located at depths exceeding 100 m. The second aquifer, known as the groundwater table, found between 8 and 15 m deep in over 75% of the plain. Recharge to the deep water table primarily occurs through vertical drainage, while rainwater and irrigation water infiltration contribute significantly, accounting for 90–95% of its inflows (Nouzha et al. 2016).
Data collection
This review aimed to provide a clear insight into groundwater pollution assessment by focusing on nitrate and salinization in the Gharb region of Morocco. Groundwater contamination encompasses various types of contaminants, including nitrate, salinization, fertilizers, pesticides, and heavy metals, originating from different sources such as agriculture, industry, and seawater intrusion. To ensure a comprehensive understanding of groundwater contamination, relevant publications were identified using specific keywords. The selected keywords included ‘groundwater,’ ‘nitrate pollution,’ ‘salinization,’ ‘Morocco,’ and ‘Gharb region,’ as well as groundwater vulnerability modeling and pollution assessment methods. Emphasis was given to articles indexed by Web of Science and Scopus. A literature search was conducted for studies published between 2010 and 2023, utilizing electronic databases such as Google Scholar, World Cat, Science Direct, Scopus, Web of Science, Springer, Researchgate, academic libraries, and Taylor & Francis. The research approach consisted of two main phases. In the first phase, the relevancy of articles to the study's purpose was assessed, followed by an evaluation of their accuracy and consistency. Inconsistencies, misclassifications, and typos were identified and corrected during the review process. The selected titles were organized in an MS Excel sheet, eliminating duplicate entries. It is worth noting that certain research papers were excluded due to their low correspondence with the review's objective or their similarity to other papers. Out of the initial search results, a total of 128 papers were deemed relevant and subsequently discussed in this research.
RESULTS AND DISCUSSION
Bibliometric data analysis
The size of each item on the map was determined by its weight, which corresponds to its occurrence and importance in the literature. Items with higher weights are displayed more prominently, while the color of an item indicates the cluster it belongs to. The lines between items represent the strongest links between them. Additionally, the item density visualization incorporated colors to indicate the density of items at specific points (van Eck & Waltman 2023). By employing these methodologies, a comprehensive understanding of the research landscape and key concepts related to groundwater pollution was achieved.
In addition to the keyword analysis, Figure 3, the density visualization map, further confirms the prominence of certain elements. Each point on the map is color-coded to indicate the density of elements at that point, ranging from blue and green to red and yellow. Points closer to red and yellow indicate higher weights and a greater number of neighboring elements, while points closer to green and blue suggest fewer elements in the neighborhood. This visualization underscores the significance of ‘groundwater vulnerability,’ ‘groundwater,’ ‘vulnerability,’ ‘groundwater resources,’ ‘nitrate pollution,’ ‘salinization,’ and the ‘Gharb region’ as hot topics within the study.
Overlay visualization generated in VOSviewer based on the keywords analysis.
Nitrate pollution in the Gharb region
Sources of nitrate pollution
Groundwater contamination by nitrate () is a significant global environmental problem, particularly associated with agricultural activities (Zhao & Pei 2012). This pollution is often attributed to agricultural activities, particularly in Mediterranean regions, where intensive irrigation systems contribute to nitrate leaching from the soil into groundwater (Marouane et al. 2014b). Studies have shown a rise in nitrate concentrations in intensively cultivated areas, especially in arid and semi-arid regions with prevalent irrigated agriculture (Malki et al. 2016; Barakat 2020). It is important to differentiate between anthropogenic and natural sources of nitrate contamination, with human activities and geological formations being major contributors (Matzeu et al. 2017). While nitrates themselves are not considered carcinogenic, the World Health Organization sets a maximum allowable concentration of 50 mg/L for nitrate in drinking water (Aziane et al. 2020b). Nitrate, the dominant form of nitrogen in streams and groundwater, can have detrimental effects on aquatic ecosystems when present in excessive levels (Barakat 2020; Gao et al. 2020). Nitrites and nitrates are naturally occurring ions that can easily infiltrate underground water through the soil and reach streams through runoff (Seif-Ennasr et al. 2022). Nitrate pollution is a global concern, influenced by factors such as precipitation, land-use practices, redox conditions, hydrogeological conditions, and fertilizer application. Long-term effects of nitrogen losses from agricultural land contribute to elevated nitrate concentrations in groundwater (Baghapour et al. 2014; Mohamed & Rachid 2022).
Agricultural activities are recognized as the main contributors to nitrate pollution in water. Factors such as thin soil cover, over-fertilization, excessive use of fertilizers, runoff and infiltration of excess fertilizers, drainage systems, extensive bare soil due to frequent plowing, and the use of organic fertilizers from animal production and wastewater reuse contribute to nitrate pollution (Barakat et al. 2020). Past studies have demonstrated that agriculture accounts for 60% of global groundwater contamination, and the application of nitrogen fertilizers is strongly associated with elevated nitrate concentrations (Rawat et al. 2022). Groundwater nitrate pollution in recharge and runoff areas primarily originates from livestock manure and landfill leachate (Chen et al. 2016; El Alfy & Faraj 2017; Barakat 2020; Panneerselvam et al. 2021).
Hydrogeological settings, seasonal trends, and human activities play crucial roles in the mobility and accumulation of nitrates. High levels of soluble nitrates can be transported into groundwater during rainfall or irrigation events, eventually reaching rivers and causing further environmental damage (Wang et al. 2015; Gao et al. 2020). Nitrates generally move slowly in soil and groundwater, with a lag time between pollution activities and detection in groundwater (El Khodrani et al. 2020). The migration rate of nitrates varies depending on the subsoil's nature, averaging 1–2 m/year (Bouita et al. 2021).
Domestic wastewater is another source, as it is often discharged untreated into rivers and spilled on land, with only a small portion undergoing treatment (Kanga et al. 2020; Bouita et al. 2021). Industrial pollution also plays a role, resulting from the extraction and processing of raw materials. Wastewater generated during manufacturing operations contributes to nitrate pollution (Jeddi et al. 2017). Furthermore, leachate from animal waste, glacial sediments, geogenic action, and sewage intrusion are major sources of nitrate contamination in groundwater (Malki et al. 2016; Panneerselvam et al. 2021).
In the Gharb plain, one of the major irrigated areas in Morocco, the intensification of agriculture has led to the deterioration of soil and the receiving environments, including surface water and groundwater (Bendra et al. 2012). Intensive agricultural practices in the Gharb region have resulted in high nitrate concentrations in some wells (Al-Qawati et al. 2018). Physicochemical studies conducted in this region over the past decades have revealed a substantial increase in nitrate pollution over time (Marouane et al. 2014a; Nouzha et al. 2016; Jeddi et al. 2017; Aziane et al. 2020b; El Khodrani et al. 2020; Bouita et al. 2021). The Mnasra zone in the Gharb region is characterized by intensive agricultural activities, such as the cultivation of vegetables and industrial crops (Omrania et al. 2019; Aziane et al. 2020b). In Mnasra and the city of Kénitra, elevated nitrate concentrations, averaging approximately 27.89 mg/L, were reported by Nouzha et al. (2016). These higher values are attributed to the intermittent and scattered discharge of livestock products, coupled with the use of chemical fertilizers and pesticides in agricultural practices, leading to infiltration into the groundwater. According to Kanga et al. (2020), nitrate pollution in the Sebou River basin, which includes the Gharb region, primarily arises from agricultural activities, urban growth, and industrial processes. The assessment of nitrogen pollution in groundwater within the Maamora Gharb aquifer, as described by Bouita et al. (2021), reveals alarmingly high concentrations of nitrate. The study found an average nitrate concentration of 90.25 mg/L. The research attributes these elevated concentrations to intense agricultural activities that generate a substantial load of organic nitrogen from animal effluent and fertilizers.
However, mismanagement of irrigation can lead to water misuse and have detrimental environmental consequences, including nitrate pollution and eutrophication (El Khodrani et al. 2020). Researchers (Marouane et al. 2014b) have found a higher frequency of excess nitrate in wells located at a distance of 3.2 km from an irrigated area compared to those without irrigation. Excessive irrigation or heavy rainfall can result in the leaching of accumulated nitrate into the subsoil, posing a substantial risk of groundwater nitrate contamination when combined with high fertilizer and water inputs (Benkaddour et al. 2020). According to El Bouzaidi et al. (2023), the study indicates that active substances can be detected in 65% of wells, predominantly concentrated in the central part of the Gharb irrigated zone. The presence of these substances is attributed to the excessive use of insecticides within the irrigated perimeter. Alami et al. (2010) confirm that the overuse of nitrogen fertilizers, combined with inadequately designed irrigation systems, are the main contributing factors to nitrate contamination.
Factors contributing to nitrate contamination
Nitrate pollution in groundwater is influenced by various factors. The relationship between groundwater nitrate contamination and nitrogen sources at the land surface is complex, as described by Malki et al. (2016). A significant amount of nitrogen from fertilizer applications can be leached into groundwater through irrigation or rainfall (Van Meter et al. 2016). The application of fertilizers leads to an increase in organic matter in surface soil, affecting the adsorption abilities of pesticides on the soil and microbial degradation (Oumara & El Youssfi 2022). In cases where fertilizer is applied excessively compared to the needs of the vegetation, soluble substances may infiltrate into groundwater, leading to pollution (El Khodrani et al. 2020). According to Wang et al. (2015), a reduction in fertilizer application does not lead to a rapid decline in nitrate concentrations in groundwater. Additionally, nitrate contamination levels in groundwater are influenced by climate conditions, the type of fertilizers used, practices related to manure application, and soil properties (El Khodrani et al. 2020).
The depth of aquifers has been identified as a significant factor impacting nitrate content, with surface water often containing higher concentrations compared to deep water due to autotrophic metabolisms under specific redox conditions (Aziane et al. 2020b). Shallow aquifers are particularly vulnerable to nitrate leaching as their shallow depth facilitates contact between leached nitrates and groundwater, while deep aquifers indirectly connected to the soil surface are better protected against such leaching (El Khodrani et al. 2020). Factors such as soil texture, permeability, rainfall intensity, groundwater recharge rate, fluctuations in the piezometric level, depth of the water table, evapotranspiration process, and irrigation practices can also contribute to NO3 contamination and the infiltration of nitrates into groundwater (Barakat et al. 2020). Controlling groundwater recharge in areas with high rainfall is crucial to minimize nitrate leaching and reduce the risk of groundwater contamination (Wang et al. 2015). In their study, Marouane et al. (2015) found alarming levels of nitrate contamination in the groundwater of the Mnasra zone. The survey revealed that 89.7% of the monitored wells exceeded the nitrate concentration limit set by the World Health Organization. The main causes of the elevated nitrate levels were identified as the sandy soil composition and shallow water depth in the area.
Studies have shown that seasonal changes, particularly during spring and summer, have a positive correlation with nitrate levels (Marouane et al. 2015). Spring precipitation plays a crucial role in transporting pollutants from the surface to deeper soil layers and subsequently to the water table. During summer, favorable conditions, such as evapotranspiration and high temperatures, facilitate the conversion of N-ammonia into N-nitrate. This leads to higher nitrate concentrations during these seasons. The impact of spring precipitation and high air temperatures on the movement and transformation of nitrates emphasizes the potential for nitrate leaching (Aziane et al. 2020b). Additionally, heavy rainfall events contribute to the penetration of soluble nitrates into the soil and subsequent leaching into groundwater (Wang et al. 2015). However, while initially, groundwater flow increases and nitrate concentrations rise, over time, the nitrate concentration starts to decline as the infiltration flow diminishes.
Salinization in the Gharb region
Salinization, a significant environmental issue globally, poses a threat to coastal aquifers. This problem is exacerbated by both climate change and human activities, leading to a reduction in soil quality, crop productivity, and cultivated land (Seif-Ennasr et al. 2022). This phenomenon also affects water bodies connected to the sea, including lagoons, rivers, torrents, and wetlands (Kazakis et al. 2019). Coastal aquifers, defined as groundwater in direct contact with seawater, are particularly vulnerable to degradation through salinization and contamination due to their hydrogeological characteristics (Aguedai et al. 2022). Coastal regions, especially in arid and semi-arid areas, face significant challenges related to salinization of groundwater resources (Chafouq et al. 2018).
Impacts of seawater intrusion
Seawater intrusion poses a significant challenge worldwide, particularly in coastal areas, where the rising demand for freshwater exacerbates this phenomenon (Boufekane et al. 2022). The combination of population growth, urban development, and excessive groundwater pumping exceeding natural recharge has led to increased vulnerability (Momejian et al. 2019; Prusty & Farooq 2020). The propagation of saline water from the sea into coastal surface bodies is driven by differences in water density, along with factors such as sea level rise, tides, storms, and waves (Kazakis et al. 2019). Climate change and sea level rise also play significant roles in controlling seawater intrusion (Hssaisoune et al. 2020). Changes in precipitation patterns and the inter-annual variability of atmospheric precipitation impact groundwater recharge. The intrusion of seawater into coastal aquifers is influenced by various geological factors, including lithology, geomorphology, and structural features. The flow of water inland depends on the geological formations and their varying porosity and permeability, affecting the water-holding capacity of the aquifers (Ez-zaouy et al. 2022). Seawater intrusion alters groundwater salinity through water–rock interactions and the mixing of saline and freshwater, further contributing to groundwater salinization (Sun et al. 2018). Several studies highlight the issue of groundwater salinization by seawater intrusion and its impact on water quality in different regions (Najib et al. 2017; Mountadar et al. 2018; Hssaisoune et al. 2020; Mariami et al. 2022; Dakak et al. 2023). The primary objective of the study done by Aguedai et al. (2022) was to emphasize the significance of marine intrusion and identify areas prone to such intrusion in the Mnasra basin coastline. The findings of the study revealed that salinization in the area is influenced by various sources of water mineralization, including marine influence, water–aquifer interaction, and water irrigation. These results highlight the vulnerability of the aquifer to marine intrusion and suggest that subsurface structures may play a substantial role in facilitating the intrusion.
Effects of anthropogenic activities
Salinization, a growing environmental concern, results significantly from anthropogenic activities (Lahmar et al. 2020). The rapid population growth and the expansion of agricultural activities have intensified soil exploitation, leading to discernible alterations and degradation of the soil's inherent characteristics, particularly salinization (Oumara & El Youssfi 2022). Furthermore, pollution from human sources, such as septic tank effluent and the use of synthetic fertilizers, significantly contributes to groundwater salinity and degradation of water quality (Agoubi 2021). The percolation of irrigation water, carrying nutrients and fertilizers, further compounds the salinization issue, exacerbated by the sandy soil nature, large irrigation doses, and heavy rainfall events (Sefiani et al. 2019; Shen et al. 2020; Agoubi 2021; Mohamed & Rachid 2022; Oumara & El Youssfi 2022). The utilization of poor-quality, high-salinity water for irrigation purposes, combined with the lack of maintenance of drainage canals and inappropriate crop practices, contributes to the problem (El Hamdi et al. 2022). The introduction of salt from the irrigation water reacts with soil elements and particles, resulting in changes to soil physicochemical properties (Lahmar et al. 2020). This alteration affects the patterns of soil water and salt movement, ultimately influencing water availability and salt distribution in the soil (Sun et al. 2018).
In the Sfafaa perimeter, El Khodrani et al. (2017) found that the water table depth ranged from 9 to 12 m, and 70.6% of wells were deemed unsuitable for irrigation due to high salinity levels. Similarly, Lahmar et al. (2020) reported that in the Sidi Yahya perimeter, the water table depths ranged from 6 to 96 m, with approximately 51.5% of wells considered unsuitable for irrigation due to high or extreme salinity levels. Additionally, El Khodrani et al. (2020) demonstrated that the waters in the Sfafaa zone exhibited higher salinity compared to the waters in M'nasra, while Marouane et al. (2014b) highlighted that salinity distribution primarily occurred in the groundwater, which was further confirmed by the salinity evolution of drainage water during rainfall events. Another study, conducted by Mariami et al. (2022), analyzed the irrigation water quality in the Skhirat region of Morocco. The results revealed significant salinization, with prevailing salinity and alkalinity classes of irrigation water classified as average to poor quality, poor quality, and very poor quality.
Impact of nitrate and salinization pollution
Groundwater contamination poses significant risks to ecosystems, biodiversity, and natural habitats (Oueld Lhaj et al., 2024; Sanad et al., 2024b). Nitrate pollution can lead to eutrophication in aquatic ecosystems, resulting in excessive algal growth, depleted dissolved oxygen levels, and oxygen-deprived zones (El Alfy & Faraj 2017; Barakat 2020; Barakat et al. 2020; Gao et al. 2020; Hssaisoune et al. 2020). It can also disrupt nutrient balances in soils, impacting plant communities, composition and diversity (Sehlaoui et al. 2020). High nitrate levels in groundwater not only affect ecosystems but also impact agriculture. Excessive nitrate accumulation in soils can harm plant growth and reduce productivity (Malki et al. 2016; Lahmar et al. 2019). Additionally, improper fertilizer and pesticide use can lead to pollutants that affect the physicochemical and biological quality of water bodies and aquifers (Omrania et al. 2019; Barakat 2020; Omari et al. 2023). This combination of factors endangers both natural environments and agricultural systems.
Elevated groundwater salinity levels can significantly affect various organisms, especially those adapted to freshwater habitats. This increased salinity has the potential to impede the survival, growth, and reproduction of these organisms, ultimately resulting in a loss of biodiversity. This, in turn, poses a substantial threat to the sustainability of land-use systems and overall ecosystem health (Mariami et al. 2022). Furthermore, salinization, a form of soil chemical degradation, negatively impacts environmental quality and agricultural production by altering soil structure, permeability, and bulk density (Bedoui et al. 2022). The effects of salinity on soil and plant health vary among plant species and at different developmental stages (Shin et al. 2022). Compounding this issue is the exacerbation of salinity problems by climate change, including increased temperatures, fluctuations in precipitation, and higher evapotranspiration rates, all of which reduce salt leaching (Bedoui et al. 2022; Shin et al. 2022). Drinking water with high salinity levels can lead to increased sodium intake, disrupting electrolyte balance and potentially contributing to hypertension and cardiovascular diseases. The excessive salt intake from contaminated groundwater can also strain the kidneys, resulting in kidney stones and renal dysfunction. Furthermore, the consumption of salinized groundwater may lead to dehydration due to the osmotic imbalance caused by the high salt content (Bedoui et al. 2022; Shin et al. 2022).
Groundwater contamination by nitrate presents significant health risks and implications for human populations, both in the short term and over an extended period (El Alfy & Faraj 2017; Gao et al. 2020; Bedoui et al. 2022; Shin et al. 2022). Consumption of water contaminated with elevated nitrate levels can result in immediate health effects, particularly in infants. Infants are especially vulnerable to a condition known as methemoglobinemia or ‘blue baby syndrome’ due to their lower levels of enzyme activity (Barakat et al. 2020; Gao et al. 2020; Li et al. 2021). Nitrosamines can react with hemoglobin, potentially causing asphyxia in infants, or combine with nitrogen derivatives, increasing the risk of digestive system cancer (Benkaddour et al. 2020). It's important to emphasize that specific vulnerable groups, including pregnant women, infants, and individuals with compromised immune systems, are particularly at risk from the health effects of nitrate pollution (Benkaddour et al. 2020; El Khodrani et al. 2020; Panneerselvam et al. 2021; El Bouzaidi et al. 2023). Moreover, excessive nitrate presence in the soil can lead to water source contamination and potential risks for animals (Alami et al. 2010). A study conducted in China by Gao et al. (2020) focused on assessing the potential health risks associated with nitrate contamination in groundwater. Groundwater samples were collected during both wet and dry seasons, and the nitrate content was analyzed. The results revealed that more than half of the groundwater samples exceeded the drinking water standard of China, which is set at 10 mg/L (measured as nitrogen). Furthermore, the study found that children and infants faced greater health risks compared to adults at the same nitrate concentration in groundwater. Additionally, the results indicated that adult females had a higher health risk than adult males during both wet and dry seasons. This observation suggests that the order of health risk was as follows: infants > children > adult females > adult males.
Evaluating nitrate and salinization pollution
An integrated approach that combines laboratory analysis, remote sensing, geospatial tools, modeling tools, continuous monitoring systems, and GIS technology has proven invaluable in addressing the complex challenges of assessing groundwater pollution due to nitrate and salinization.
Laboratory analysis methods
Laboratory analysis methods play a pivotal role in assessing groundwater quality and are essential for sustainable water resource management, particularly in regions with high economic activities (Kanga et al. 2020). The identification of areas where groundwater quality is degraded is crucial as it enables appropriate actions to be taken to reduce pollution. To assess the extent and impact of groundwater pollution caused by nitrate and salinization, a combination of assessment techniques and monitoring approaches is employed (Teng et al. 2019; Kanga et al. 2020; Azzirgue et al. 2021). One widely used technique involves the analysis of water samples collected from monitoring wells or groundwater abstraction points. Through these analyses, the concentrations of nitrates and salinity levels are determined, providing critical information about groundwater quality. The examination typically includes the determination of various parameters such as electrical conductivity (EC), pH, and concentrations of ions including ,
,
,
,
,
,
-,
,
, and
(Marouane et al. 2014a; Zouahri et al. 2015; El Khodrani et al. 2016; Malki et al. 2016; Oumlouki et al. 2016; El Oumlouki et al. 2018; Sasakova et al. 2018; Aziane et al. 2020a; Lahmar et al. 2020; El Hamdi et al. 2022; Dakak et al. 2023).
EC, a measure of water's electric conduction, serves as an indicator of water mineralization and salinity. High EC values in groundwater can arise from elevated concentrations of mineral ions resulting from factors such as dissolved mineral electrolytes, water–rock interactions, and the infiltration of saltwater or industrial discharges into aquifers (Oumlouki et al. 2016; El Oumlouki et al. 2018). The possibility of seawater intrusion in coastal aquifers further contributes to higher EC levels (Al-Qawati et al. 2018). The increase in EC can lead to decreased water uptake by plants, resulting in reduced productivity and yield reductions, particularly for sensitive crops (Zouahri et al. 2015).
The pH value reflects the concentration of hydrogen ions in water and is influenced by various factors, such as temperature and hydrochemical interactions (Lahmar et al. 2019). It is also linked to the buffering system established by carbonates and bicarbonates, which contributes to the stability of the equilibrium between different forms of carbonic acid (Nouzha et al. 2016). Considering pH is essential in all applications involving water, as it influences the geochemical and biological processes that control the natural water composition (Al-Qawati et al. 2018).
The presence of magnesium () and calcium (
) in groundwater is attributed to the lithology of the aquifer and the hydrolysis of silicate minerals, which are common constituents of groundwater (Omrania et al. 2019). These ions significantly contribute to the hardness of groundwater (Lahmar et al. 2019).
Potassium () is an essential element for humans, but its presence in groundwater is typically below levels of health concern. Its occurrence in natural water is mainly attributed to the chemical weathering and erosion of potassium-rich rocks and minerals (Nouzha et al. 2016; Al-Qawati et al. 2018). However, excessive potassium can lead to salt transformation and percolation into groundwater, potentially causing pollution (El Khodrani et al. 2017; Al-Qawati et al. 2018). Additionally, high potassium concentrations can induce magnesium deficiencies in crops. Sodium (
), on the other hand, is commonly found in groundwater and its concentration varies depending on soil and rock types (Omrania et al. 2019). In irrigation, high sodium concentrations can lead to ion exchange in the soil, replacing calcium and magnesium ions and reducing permeability, resulting in poorly drained soil conditions (Zouahri et al. 2015).
Chlorides () are widely distributed in the environment and can be associated with sodium or potassium. They originate from the weathering and leaching of sedimentary rocks and soils, anthropogenic activities, irrigation water infiltration, and the dissolution of salt deposits (Omrania et al. 2019). While chlorides are conservative elements in groundwater, their high concentrations in coastal aquifers may indicate saltwater intrusion (Moukhliss et al. 2021). Agricultural activities can also contribute to the leaching of agricultural amendments, introducing sodium chloride into groundwater (Mouna et al. 2016).
Sulfate () is naturally present in groundwater and is closely associated with major cations such as
,
, and
(Al-Qawati et al. 2018). In natural water, sulfate content primarily originates from various sources (Lahmar et al. 2019).
Nitrates (), a part of the natural nitrogen cycle, are mainly derived from fertilizers and organic waste (Nouzha et al. 2016; Lahmar et al. 2019; Omrania et al. 2019). Ammonium (
), another component of the nitrogen cycle, is a soluble gas in water and serves as an indicator of water pollution from organic discharge of agricultural, domestic, or industrial origin (Moukhliss et al. 2021). Water contaminated with
and
at the surface can infiltrate through the soil into the aquifer. Additionally, the natural supply of mineral nitrogen in the soil can contribute to increased nitrogen pollution risk in groundwater from nitrates (Nouzha et al. 2016).
Ion-specific measurements and chemical analyses are instrumental in determining ion concentrations within groundwater. These analyses are fundamental in uncovering the origins and potential ramifications of these contaminants on groundwater quality. A range of water quality indicators is employed to assess groundwater pollution, each serving a specific purpose. These indicators encompass metrics such as the sodium absorption ratio, residual sodium carbonate, sodium percent, permeability index, magnesium adsorption ratio, Kelly ratio, base exchange index, magnesium hazard, potential salinity, and residual sodium bicarbonate (Sehlaoui et al. 2020; Panneerselvam et al. 2021).
Policy- and decision-makers face numerous challenges in ensuring the quality of water supply sources and implementing effective measures to improve groundwater quality (Kanga et al. 2020). To address these challenges, the water quality index (WQI) has become a widely used approach (Panneerselvam et al. 2021). The WQI integrates physical, chemical, and biological parameters to provide a comprehensive assessment of water quality that is easily understood by decision-makers (Sanad et al. 2024a). This index serves as an effective tool for evaluating water quality for domestic usage, as well as monitoring surface and groundwater pollution (Hammoumi et al. 2022). Furthermore, the spatialization of the WQI using GIS offers decision-makers a valuable tool to evaluate the effectiveness of water management projects and take appropriate actions to mitigate the impacts of human activities on groundwater resources (Kanga et al. 2020). The nitrogen pollution index (NPI) is a valuable tool used to assess the level of nitrate pollution in groundwater resulting from human activities. By applying the NPI, the extent of nitrate pollution can be determined, and water quality can be classified into different categories based on NPI values (Sarra et al. 2018; Mohamed & Rachid 2022).
One commonly used graphical tool is the Piper diagram, which not only highlights the influence of geology on water quality but also facilitates the estimation and classification of chemical element percentages (Mouna et al. 2016; Shen et al. 2020). The Piper diagram is valuable for classifying different water types based on their ionic composition, enabling the elucidation of key geochemical processes occurring within the groundwater system (Aguedai et al. 2022). Another graphical representation, the Gibbs diagram, aids in identifying the significant factors shaping groundwater quality evolution. It provides a visual depiction of the mechanisms influencing water quality changes, including evaporation, rock-water interaction, and precipitation dominance (Panneerselvam et al. 2021).
Remote sensing and geospatial tools
Remote sensing techniques, including satellite imagery and aerial photography, offer valuable tools for assessing nitrate and salinization pollution in groundwater. In assessing the vulnerability of aquifers to pollutants, scoring and weighting methods have proven to be common and effective approaches (Matzeu et al. 2017). Over the past decades, a wide variety of methods have been developed to evaluate groundwater vulnerability (Busico et al. 2017).
The DRASTIC method, developed by the Environmental Protection Agency (EPA) in 1987, is widely used internationally for evaluating the potential pollution of underground waters (Nezha et al. 2016). It incorporates seven morphological, hydrological, and hydrogeological parameters. It estimates the vertical groundwater vulnerability by assuming that the contaminant is introduced at the ground surface and reaches the groundwater table through precipitation or infiltration. One major advantage of the DRASTIC model is its ability to handle a large number of input data, which helps mitigate the impacts of errors or uncertainties associated with individual parameters on the final output (Baghapour et al. 2014).
One parametric method worth considering is the GOD index, which incorporates three parameters: groundwater confinement (G), overlying strata (O), and depth to groundwater (D) (Arauzo 2017). Unlike the DRASTIC model, the GOD index focuses solely on the risk of pollution based on the attenuation capacity of the unsaturated zone, without considering the movement of pollutants through the saturated zone. While the GOD index evaluates the risk based on the unsaturated zone's characteristics, the DRASTIC index incorporates the risk associated with both the unsaturated and saturated zones. Both methods can be valuable tools for groundwater management, particularly at medium to small scales, providing a general overview and aiding in the identification of new nitrate vulnerable zones (Martínez-Bastida et al. 2010).
One specific method that focuses on seawater intrusion assessment is the GALDIT method. It considers six hydrogeological parameters that influence the potential for seawater intrusion, such as (G) groundwater occurrence, (A) aquifer hydraulic conductivity, (L) depth to groundwater level above the sea, (D) distance from the shore, (I) impact of the existing status of seawater intrusion in the area, and (T) thickness of the aquifer (Recinos et al. 2015).
Another method used to assess the intrinsic vulnerability of groundwater is the SINTACS model, which is partially derived from the widely employed DRASTIC method (Kanga et al. 2020). The intrinsic vulnerability, determined using the SINTACS method, is obtained through overlay mapping of seven layers, which are the sum of the products of scores and weights (Busico et al. 2017).
In addition, the integration of GIS technology improves the spatial analysis and visualization of groundwater pollution data. GIS can be used to overlay various spatial data sets, including geological maps, land-use models, and hydrological features (El Khodrani et al. 2020). The use of remote sensing and geospatial tools enhances our understanding of the complex dynamics of groundwater pollution and aids in decision-making processes for its prevention and mitigation (Zhao & Pei 2012; Hammoumi et al. 2013; Kazakis & Voudouris 2015; Nezha et al. 2016; Arauzo 2017; Matzeu et al. 2017; Kouz et al. 2018; Eljebri et al. 2019; Kazakis et al. 2019; Momejian et al. 2019; Sefiani et al. 2019; Meng et al. 2020; Boufekane et al. 2022).
Modeling tools
Hydrogeological modeling is instrumental for comprehending groundwater pollution dynamics, particularly concerning nitrates and salts. These models utilize geological data, field measurements, and hydrological parameters to depict the movement of contaminants in aquifers, offering insights into spatial distribution and temporal variations of pollutants. Notably, models such as GLEAMS, HYDRUS, and indices such as LOS may underestimate certain hydrogeological aspects, particularly within the vadose zone, focusing primarily on water movement and nitrogen transport from agricultural land and soil (Wallis et al. 2011; Kazakis & Voudouris 2015; Rawat et al. 2022).
The objective of the study by Shekofteh et al. (2013) was to assess nitrate leaching from an agricultural field using the HYDRUS-2D simulation model. The results demonstrated that a well-calibrated and validated model can provide precise estimations of contamination, specifically the movement of nitrate in soil and its potential leaching into groundwater.
In a different study presented by Li et al. (2014), they discussed the calibration of the De Nitrification-De Composition (DNDC) model for a highly agricultural area in Northern China. This study underlines the DNDC model's capacity to effectively simulate and predict nitrate dynamics in agricultural regions, thus contributing to the evaluation and control of nitrate contamination.
Similarly, in the study presented by Das & Nag (2017), the LEACHMN model was utilized to investigate the transport and fate of nitrate within the soil profile and its leaching to subsurface drains in intensive agricultural farmland. By comparing field data with simulation results, the researchers found that the LEACHMN model performed well in simulating nitrate dynamics in both soil and subsurface drainage at the field scale.
The study conducted by Momejian et al. (2019) aimed to assess the uncertainty of two commonly used GIS-based groundwater vulnerability models, DRASTIC and EPIK, in predicting seawater intrusion in coastal urban areas. The study highlighted the challenges faced by conventional vulnerability models, particularly in scenarios involving anthropogenic influences and lateral flow processes like seawater intrusion caused by vertical groundwater extraction.
A modification of the GALDIT method called GALDIT-superficial seawater intrusion (SUSI) was proposed by Kazakis et al. (2019), which incorporates the consideration of SUSI. Moreover, GALDIT-SUSI has the potential to address other salinization mechanisms, enhancing its applicability and usefulness in managing saline groundwater contamination.
Rath et al. (2021) aimed to evaluate the effects of sensor-based irrigation on both crop yield and nitrate contamination in a corn-peanut rotation. Their study employed the soil and water assessment tool (SWAT) model to assess various irrigation and nutrient management strategies.
In a study by Boufekane et al. (2022), a novel approach combining groundwater vulnerability assessment and a numerical model was proposed to predict seawater intrusion in coastal aquifers. The study utilized the GALDIT method in GIS for vulnerability assessment and the MODFLOW model to simulate current and future groundwater levels from 2020 to 2050.
CONCLUSION
This paper has conducted a comprehensive analysis of research outputs published between 2010 and 2023, collated from well-regarded databases such as Web of Science and Scopus. The research specifically concentrates on groundwater pollution caused by nitrate and salinization in the Gharb region, an area that has garnered significant research interest, much like many other arid and semi-arid regions.
The outcomes of this analysis underscore the mounting concern of nitrate contamination, principally stemming from intensive agricultural methods. Contributing factors include the excessive application of nitrogen-based fertilizers, soil coverage, drainage systems, the use of organic fertilizers derived from livestock production and wastewater recycling, and the hydrogeological landscape. Additionally, the review illuminates the escalating apprehension regarding groundwater contamination due to salinization. This is driven by various elements, such as seawater intrusion, the effects of climate change, geological attributes, and human-induced activities such as agricultural practices and irrigation methods. Moreover, numerous studies have delineated how groundwater contamination poses significant threats to ecosystems, biodiversity, and the integrity of natural habitats. Furthermore, the appraisal of assessment methodologies, encompassing laboratory analyses, remote sensing, geospatial tools, modeling techniques, continuous monitoring systems, and GIS technology, offers crucial insights into groundwater quality. These tools also provide substantial support for decision-making processes, both in preventing and mitigating contamination.
Overall, this review serves as a robust foundation for future research initiatives, policy formulation, and the sustainable management of groundwater pollution originating from nitrate and salinization in the Gharb region. By addressing existing research gaps, Morocco can safeguard its invaluable groundwater resources, protect public health, and bolster the long-term sustainable growth of its agricultural sector.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.