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Assessment of groundwater quality using Physico-chemical analyses of Sahel-Doukkala region

Water quality preservation represents one of the biggest challenges the world is facing nowadays. In Morocco, water quality decline is among the significant problems facing the country's water sector. Geographic Information System (GIS) is an effective and useful tool for interpreting, evaluating and displaying spatial data for water resources management. In order to assess the Physico-chemical characteristics of groundwater of the Sahel-Doukkala aquifers, water samples were collected from 50 points well distributed in the study area, analysed according to standard methods, and the results were interpreted using the geographic information system (GIS) technique. There was an important spatial variability in the studied parameters and element concentrations (T°, pH, EC, Cl, Ca, Mg, Sr, B and Na), revealing that the aquifers lithology, sea intrusion and the agriculture methods are the main factors influencing the water nature in the study area.


Introduction
Water quality degradation is among the most challenging issues the world is currently facing. The demographic expansion and the increasing demand for water resources for different purposes, including agriculture, industrial activities, and tourism, lead to the water resources depletion, especially with the climate change effects (short runoff season, less reliable water resource base, increase of water temperature, increase of water pH) [1]. Under social, economic and environmental pressures on water resources, the water demand has grown in the last decades. According to Wada et al., 2016, the worldwide water demand has increased by 600% compared to the past 100 years. Whereas, in the recently published report of the United Nations World Water Development (WWRD), it was reported that about 6 billion peoples around the world would suffer from clean water scarcity by 2050 while the global water demand is estimated to increase by 20 to 30% per year in the next three decades [2]. In Morocco, the groundwater resources suffer from the nature of Moroccan climate (Dry and semi-dry climate) [3], climate change effect [4][5][6], anthropogenic pressures [7,8], and the mismanagement of freshwater sources [9]. According to the World Bank report (2017), it is projected that urban water demand would increase by 60% to 100% in the biggest cities by 2050. Many studies were carried out in order to assess the groundwater quality in Moroccan cities and regions * Corresponding author:Ayoub.benmed@gmail.com such as Hssaisoune [3,[10][11][12]. Sahel Doukkala is located in the central part of the Moroccan Atlantic Coast. This region knows steadily increasing water use, especially with the population expansion [13,14] and the extensive pumping from aquifers, especially in the coastal fringe where almost all the agriculture activities are concentrated [15]. Consequently, the water resources in this region suffer from both overexploitation and quality decline due to human activities. In this context, the present work assesses the hydrochemical characteristics of groundwater of the Sahel Doukkala and reviews the change in its quality during the last decades by investigating the latest research work carried out in the studied area.

Study area
The Sahel Doukkala is a coastal region located in the provinces of Safi and El Jadida and subdivided into two large areas on the Sahel and Doukkala situated between latitudes 32 ° 15' and 33 ° 15' North and longitudes 8° and 9°15' covering an area of 7700 Km 2 with about 150 km of coastline. It is limited to the North and east by the wadiOumErRabia, to the west and in the North-West by the Atlantic Ocean, in the South by the base of the hills of Mouissate and in the South-East by the primary massif of Rehamna. The climate in the Doukkala Sahel is semi-arid while the minimum temperature (T°) is around 18°C in winter and maximum (T°) is 23°C in summer. According to the El-Jadida station, the average yearly rainfall is 317 mm, while the maximum rainfall was registered in December (59.8) [16].

Sampling
A field mission was carried out in January 2013 in the Sahel of Doukkala, where 50 groundwater samples were collected through a selected sampling network to provide representative data on the term of spatial variability of groundwater quality in the study area. To avoid the contamination of the sample, the water sampling was carried out by using Polyethylene bottles and gloves. The sampling collection was carried out according to Rodier's protocol [17]. Five water samples were collected at each sampling point and stored in five Polyethylene bottles for a different type of analysis.

Laboratory analysis
The water samples were analysed by the Ion chromatography (IC) technique (type DIONEX). Cations such as calcium (Ca 2+ ), magnesium (Mg 2+ ), sodium (Na + ), potassium (K + ) and anions such as bicarbonates (HCO 3-), chlorides (Cl -), bromide (Br -) and sulphates (SO4 2-) were measured in each water sample. Bore and Strontium concentrations were determined by using the Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in the laboratory of Unit climate and water at CNESTEN Regarding the reliability of the results, we proceeded to apply the ionic balance (by taking into account the existing relationship between the total cations and the total anions) with an acceptable error margin of 10% [18,19].

GIS
Modelling was performed in several steps. ArcGIS 10.7.1 was used as an effective tool for mapping and visualising the spatial distribution of groundwater quality parameters, understanding the natural environment and managing water resources.

Temperature
The water quality as a function of temperature is presented in figure 3. The obtained values of temperature are ranged from 11, 8 to 23,8 while the water quality varied between good and excellent. The samples retrieved from the NE part of the study area exhibit a good water quality, while the samples showed excellent water quality in the SW part. The samples retrieved from the coastal fringe showed good quality in the South and excellent quality (Wells: P37, P47, Borehole: P46) in the North (Borehole: P20, Wells: P19, P22, P38). Fig.3. Spatial distribution of underground water temperature in the studied area.

pH:
The pH values are ranged from 6, 88, recorded in (P45) and 8, 44 (P14). The measured values of pH for almost all samples are corresponding to natural water where according to High Commission for Water and Forestry and the Fight Against Desertification [20], in natural waters, the pH is usually ranging between 6 and 8,5.
The obtained values are in good accordance with the values measured in groundwater in the Sahel of Doukkala region during the wet season for samples retrieved between 2016 and 2018 [21]. Almost all the sampling point showed neutral water, while few samples are slightly alkaline located in the NW (P2, P4, P14, P16, P17, P19, P23, P38 and P44).

EC
Electrical conductivity is an excellent tracer of water origin, and it provides information about water mineralisation [20]. The maps a and b presented in Figure 5 show the values of EC in each water sample (Fig.5a) and the spatial distribution of EC all around the study area (Fig.5b). The values are relatively high in the NE (Sidi Bennour, Zemamra) and NW (downstream parts of Oued Oum Erabie and El Jadida city) and low in the SW part of the Sahel of Doukkala. As the EC reflects the degree of water salinity [22], the higher values recorded around El Jadida could be related to the marine intrusion resulting from the excessive pumping activities in good accordance with the findings reported in [23][24][25][26]. Given the distance of the Sahel basin from the sea, the higher values registered around Zemamra and Sidi Bennour are not necessarily associated with marine intrusion. However, they can be related to natural factors such as the flow direction and the concentration of dissolved salts [27] and/or to human activities.

Chloride and Sodium
The spatial distributions of chloride (Cl -) and sodium (NA + ) are presented in Figure 6. They display similar distribution as the EC. The highest values are shown in El Jadida near the coastal fringe and around SidiBennour and Zemamra. The Clvalues are ranged from 24,82 mg/l to 3658 mg/l, while the maximal value was recorded in the North of the Sahel Doukkala aquifer (P15) (Fig.6.a). The Sodium concentrations varied between 12,91 mg/l and 1452 mg/l ( Fig.6.b). The The Cl-Na stoichiometric diagram of Figure 7 indicated that much of the water samples displayed proportions close to 1:1 (into green ellipse), revealing that the molar release of Cland Na + to water comes exclusively from Halite dissolution (NaCl => Na + + Cl -); however, samples with concentrations above 10 meq/L have a tendency of excess of Clover Na + . This tendency must be related to several factors but mainly to: (1) Na + sorption in aquifer fine materials (lime and clays) and/or (2) additional introduction of Clfrom the agriculture activities in the study area. Thus, the origin of salinity cannot be attributed to marine intrusion in the Sidi Bennour and Zemamra areas as the distance to the ocean exceeds 50 Km, but to the wastewater discharges used for irrigation [28,29] and/or related to the process of water-aquifer interaction.  The chloride concentration in water can provide the type of water according to the classification of Stuyfzand [30]. The type of water for each sampling location is presented in Figure 8. The map indicates a wide variation of water from one location to another. The water type is mainly fresh and fresh-brackish, brackish and brackish-salt. Twenty-three samples present brackish water distributed primarily in the Sahel aquifer and the aquifer of the coastal Sahel, while the brackish-salt water is mainly concentrated in the Doukkala aquifer in good accordance with EC distribution (Fig.5.). These results indicate again that the higher salinity values found in the Doukkala aquifer distant from the ocean by 50 km is mainly related to other factors and not necessarily associated with the marine intrusion.

Magnesium and calcium
The spatial distribution of Mg and Ca, are displayed in Figure 9. According to Kaid Rassou [31]; the dissolution of the carbonate formations of the Cretaceous and Plioquaternary, calcite (CaCO3) and dolomite CaMg (CO3)2, and gypsiferous formations (CaSO4, 2H2O) of Jurassic are the main sources of these elements in the groundwater of this region. The distribution of Mg 2+ /Ca 2+ ratios values is displayed in the map of Figure 10. The values are inferior to 1, indicating that there is enrichment by Ca over Mg, which reflect the abundance of limestone facies on the dolomitic facies of the aquifers lithology as reported for the coastal basin of the Oualidia in Kaid Rassou, [31]. Approximatively, similar results were reported in Fadil 2014 [23], where the calculated Mg 2+ /Ca 2+ ratios for wells distant from the ocean by 1 Km were ranging from 0,10 to 1,05. Aris et al., 2012 [33] indicate that all ratios Mg 2+ /Ca 2+ shown values higher than 0,7 designate a mixture of fresh and seawater.   The Strontium is an indicator of evaporates, while the calcium is an indicator of marine inputs. The calculated ratios of Sr 2+ /Ca 2+ were mapped and presented in Figure 12. The Strontium shows strong enrichment over calcium in almost all sampling locations in the coastal Sahel and Sahel aquifers, where the values of the ratios were >1. Some ratios show values <1 observed in the centre of the Sahel aquifer. The obtained results are in good accordance with the analysis done in the Oualidia lagoon sediments, where a strong enrichment of Strontium over calcium was reported in Mejjad 2020 [35]. The same findings were found in the groundwater of the Oualidia Sahel aquifer, where the examination of the rSr 2+ /rCa 2+ ratios have exhibited obvious contamination by evaporates (the Jurassic evaporate deposits); in KaidRassou 2005 [31]).   Figure 13 shows a comparison between water temperature and boron concentrations and electrical conductivity and Boron. The Temperature-Boron plot indicates a profound source of mineralisation from the bottom of the aquifer as the Boron is usually released from deep groundwater and ancient aquifers [36] being highly mineralised [37]. Also, the higher T° reflects deeper water as it was reported that in deep wells, the T° of water generally rises 1 °F for every 60 feet to 100 feet of deepness [37]. Thus, a significantly positive correlation observed between T° and B, suggesting the presence of a deeper source of water highly mineralised. Fig.13. Hydrochemical graphs of water samples taking in the study area for Electrical conductivity vs Boron and temperature vs Boron.

Conclusions
The analyses show that the water is highly mineralised and provide a clear picture of the causes influencing the water type in the Sahel and Doukkala aquifers. The aquifers lithology, sea intrusion and different human activities (agriculture, farms and urban areas) are among the factors influencing the water nature in the study area. It must be added that these analyses highlight the need for implementing projects management and integrated land use planning to conserve and protect this natural resource.