Quantitative and Qualitative Assessment of Groundwater from Semi-Arid Zones in the Context of Climate Change, Example of Essaouira Region, Morocco

This study aims to assess the effect of climate change on water resources in semi-arid environments, taking the Essaouira region as an example. Analysis of climate data shows a downward trend in precipitation of 12 to 16% and an increase in air temperature of 2.3 °C over the past three decades. The piezometric study shows a continuous drop in the piezometric level which exceeds 12 m for the Cenomanian-Turonian aquifer, 17 m for the Plio-Quaternary aquifer, around 8 m for the Barremian-Aptian aquifer and 5 m for the Hauterivian. Hydrogeochemical analysis shows that (i) the groundwater mineralization is controlled by the dissolution of evaporitic and carbonates minerals, by the reverse ions exchange phenomenon, and by the marine intrusion, especially at Plio-Quaternary aquifer; (ii) the groundwater quality in the study area deteriorates gradually over time and space. The isotopic technique shows that (i) the groundwater recharge in the study area is ensured by precipitation of oceanic origin without signicant evaporation and that (ii) climate change has no remarkable effect on the isotopic content of the groundwater of the study area. However, the results of this article reect that the Essaouira basin is very vulnerable to climate change.


Introduction
Climate change is a global problem, involving the long term complex interactions between environmental factors and economic, social, technological and political conditions which cause signi cant effects at regional level (Lebel and Vischel 2005;Alpert et al. 2008;Misra 2014), including the Maghreb region El Kharraz 2012;Ouhamdouch et al. 2018Ouhamdouch et al. , 2020Ragab and Prudhomme 2002).
In arid and semi-arid regions, rainfall is one of the determining factors of climatic characterization. The study of recent evolution in climate is an essential tool to determine optimal general solutions to the problems resulting from the relationship between water requirement and their availability, and therefore better management of water resource (Bahir et al. , 2019Carreira et al. 2018;Ragab and Prudhomme 2002).
Studies on climate change show that global warming in the Maghreb country is signi cant than the global average. Indeed, on a global scale, the increase estimated at 0.74 ° C in the 20th century, while it was oscillated between 1 and 2 ° C on the Mediterranean scale and the region of North Africa ( GIEC 2007;Green et al. 2011;Ouhamdouch et al. 2018, Ragab andPrudhomme 2002). As for precipitation, it has decreased in the Mediterranean region, in the Sahel, in southern Africa and in certain parts of South Asia at different temporal and spatial scales (Alpert et al. 2008;IPCC 2013).
Morocco, like the Mediterranean countries (Vicente-Serrano 2006), have suffered from several periods of drought (Bahir et al. 2002;Driouech 2010;Babqiqi 2014). Its water resources are limited; they are estimated at 20 billion cubic meters, or an average of 700 m 3 /year/inhabitant, which corresponds to a situation of fairly high water stress. The number of years in rainfall de cit is greater than the number of wet years (Driouech 2010;Stour and Agoumi 2009;Sinan et al. 2009), especially the cycles of 1980-dolomitic sandstones or bioclastic limestones (Duffaud 1960;Duffaud et al. 1966) (Fig.3). The transmissivity is of the order of 1.5*10 -3 m²/s (Mennani 2001).

Materials And Methods
In this investigation, the results of nine campaigns 1990, 1995, 1997, 2009, 2015, 2016, 2017, 2018 and 2019 were used to assess the quality of groundwater in the Essaouira region in the context of climate change. Electrical conductivities, temperatures, pH and nitrates were measured in situ with a portable conductivity meter (HI9829 multiparametric instrument), and the depth of the water level was measured using a 200 m piezometric probe.
The analyses of the chemical elements were carried out at the Laboratory of Hydrogeology at the Faculty of Sciences Semlalia of Marrakech (Morocco) for the campaigns 1990 to 2009. As for that of 2015 to 2019, the analyses were carried out at the Laboratory of Geosciences and Environment-ENS at the Ecole Normale Superieure of Marrakech (Morocco). The SO 4 2anion contents were determined by the nephelometric method (Rodier et al. 2009). Concentrations of Ca 2+ and Mg 2+ cations were measured by the complexometry method (EDTA) and those of Clby the Mohr method (Rodier et al. 2009). The Na + and K + contents were determined by ame photometry (Rodier et al. 2009). As for HCO 3 contents, they were determined by titration using a sulfuric acid solution. All the samples display an ion balance of less than 10%, which allowed us to validate the obtained results. The obtained results are grouped in Appendix 1.
A total of 46 points collected, in April 2016 (22 points) and in May 2018 (24 points) were analyzed for stable isotopes (oxygen-18 and deuterium) and radioactive (Tritium). The 22 samples taken in 2016 represent the Cenomanian-Turonian aquifer (the upstream part of the study area) and 24 water points taken in 2018 represent the Plio-Quaternary aquifer (18 samples), 4 the Turonian aquifer (4 samples) and surface water (2 samples).
The analysis of stable isotopic elements (δ²H and δ 18 O) were carried out at the Nuclear Technology Institute in Lisbon (Portugal) and the Laboratory of Radio-Analysis and Environment (LRAE) at the National School of Engineers of Sfax (Tunisia) by applying the mass spectrometry method (Friedman et al. 1953;Epstein and Mayeda 1953). The tritium ( 3 H) content was determined by the technique of electrolytic enrichment followed by the liquid scintillation counting method (Lucas and Unterweger 2000) at Nuclear Technology Institute in Lisbon (Portugal).
A Geographic Information System (GIS) was used to map the spatial distribution maps of the electrical conductivity and the physicochemical elements.
The PHREEQC program (Parkhurst and Appelo 1999) was used to calculate the saturation indices (SI) using the following formula: SI = log (KIAPKSP) With: K IAP is the product of the ionic activity of ions. K SP is the mineral solubility product. The saturation index corresponds to the deviation from the equilibrium of the water from the mineral phase. If SI = 0, the water is in equilibrium; SI is negative, the water is undersaturated with respect to the mineral; SI is positive, the water is supersaturated with respect to this mineral.

Climatic parameters (Precipitations and temperatures)
The climate parameter data used in this study were obtained from the Tensift Hydraulic Basin Agency (ABHT).
Analysis of precipitation data for an observation period of 38 years  for the study area reveals signi cant variability on an annual scale (Fig.4).
Indeed, this rainfall is subject to uctuations from one year to another, with wet and other dry periods of two to ve consecutive years. The height of the precipitated sheet of water varies between a minimum of 135 mm, measured in 2008 and a maximum of 707 mm measured in 1996 with an average of 304 mm.
The application of the Pettitt test (Pettitt 1979) (Table 1), with a 90% con dence level shows the presence of a break in the pluviometric series in 1999. This test made it possible to split the rainfall series into two sub-series. The average of annual rainfall before and after this break is A 1 = 313.8 ( rst sub-series) and A 2 = 263.4 (second sub-serie) mm, respectively. This makes it possible to estimate a rainfall de cit of 16%. The results of the Mann-Kendall trend test (Table 1) displays a negative multivariable standard normal (U MK ) (U MK = -1.09). This re ects a downward trend in precipitation and con rms the results of the Pettitt test.
The evolution study of annual atmospheric temperatures was carried out over for 28 years . Maximum temperatures range between 29.3 and 37.2 °C with an average of 34.2 °C. As for the minimum temperatures, they range between 2.4 and 9.3 °C with an average of 7.4 °C. While the average temperatures, vary between 17.7 and 22.4 °C with an average of 20 °C (Fig.5a).
The application of the Pettitt test with a signi cance level equal to 5% shows the existence of a signi cant break in the series of maximum, average and minimum annual temperatures, respectively in 1999, 2000 and1994 (Fig.5b). For the maximum annual temperatures, the average before and after this break is 32.75 and 35.53 °C, with an increase of 2.8 °C. As for the mean annual temperatures, the average before and after the break equal to 18.85 and 21.13 °C, respectively, with a warming of 2.3 °C. For minimum annual temperatures, the average before and after the break is 5.66 and 8.14 °C, respectively, with an increase of 2.5 °C. The rupture date of the maximum and the mean temperature series are approximately the same and this could be explained by the fact that these two parameters exhibit the same evolution during the study period. As for the series of minimum temperatures, it presents an early break (1994). This could be explained by the very cold temperatures experienced by the study area in 1988, 1989 and 1990 (start of the series).
This upward trend is corroborated by the Mann-Kendall test with a positive multivariable standard normal U MK for annual maximum temperatures (U MK = +5.24), for annual mean temperatures (U MK = + 5.65) and annual minimum temperatures (U MK = + 4.65).
The Gaussen diagram corresponds to the intersection of the monthly average precipitation curve and the monthly average temperature curve for the same station. When the precipitation curve is above that of temperatures, we talk about a wet period. While the dry period takes place when the precipitation curve is below that of temperatures.
This diagram distinguishes a dry period from a wet period (Bagnouls and Gaussen 1953;Daget 1977;Hannachi and Fenni 2013). For this study and during the period 1987-2000, Figure 6 shows a dry period from April to September and a wet period from October to March. However, groundwater recharge could, therefore, take place mainly during this wet period. By comparing the length of the dry period during the two periods 1987-2000 and 2001-2014, we can see that it experienced an extension of about one month. This will undoubtedly in uence the groundwater recharge.

Piezometry
The evolution of the groundwater piezometric surface is closely related to the variation of precipitation Ouhamdouch et al. 2016), the degree of exploitation and the contributions of surface water.
The -For the upstream part of the basin, the groundwater has a general ow direction from SE to NW for the southern part and from NE-SW for the northern part (Fig.7a). This ow is conditioned by the substratum of the reservoir studied. Over a 24-year observation period (1995-2019), the groundwater maintains the same direction of ow with a decline in the piezometric level. This drawdown is manifested, for example, by the offset of the piezometric curves 450 and 600 m more and more upstream, and this on the two piezometric maps (Fig.7a). Monitoring the evolution of the piezometric level of the wells whose water level was measured during October 2007 -For the downstream part, the general direction of groundwater ow of the Plio-Quaternary aquifer and that of the Barremian-Aptian is generally from south-east to north-west ( Fig.7b and c). The groundwater ow within the Plio-Quaternary aquifer is imposed by the inclination of its substratum, while for the Barremian-Aptian, the ow is imposed by the north ank of the Amssittene anticline and the uplift of the substratum of the lower Cretaceous formations. Concerning the Hauterivian aquifer, the general direction of ow is from the northeast to the southwest and this follow the southern ank of the Amssittene anticline. The same remark observed for the downstream part, the groundwater keeps the same direction of the ow with a decline in the piezometric level. Over a 29-year observation period (1990-2019) (Fig.7b), the groundwater of the Plio-Quaternary aquifer keeps the same direction of ow with a decent piezometric level. This situation is materialized, for example, by the shift of the isopiezes 40 and 180 m more and more upstream, and this on the two piezometric maps. The evolution of the piezometric level of the wells capturing this aquifer and having experienced measurements of their water body during 1990, 1995, 2000, 2004, 2009, 2015, 2017, 2018 and 2019 (Table 2) shows a reduction in the plan of water at these wells. It reached 17 m at well 261/51 and 6.6 m at well 140/51, between 1990 and 2019. The drought of 1995, the driest year in Morocco during the 20th century, led to a general decline in the water level.
Over a 43-year observation period (1976-2019) (Fig.7c), the groundwater of the Barremian-Aptian and Hauterivian aquifer keeps the same direction of ow with a decent piezometric level. With a total of nine wells, in which six wells capture the Barremian-Aptian aquifer and three captures the Hauterivian aquifer, having experienced measurements of their water body during 1976,1997,2015,2017,2018 and 2019, the study of the groundwater level of the aforementioned aquifers shows a decrease in piezometric levels ( Table 3) Following the absence of industrial activity in the study area and the agricultural activity of "food type" practiced by the population, the decrease in the piezometric level could only be explained by the decrease in precipitation under the effect of climate change.

Hydrogeochemistry
A hydrogeochemical approach is a valuable tool for characterizing groundwater chemistry. The latter is largely in uenced by the characteristics of the host rock, the hydrodynamics of the aquifers and also by the climatic and exploitation conditions.

Chemical facies
To specify the groundwater chemical facies in the study area, the major element composition has been plotted on the Piper diagram (Piper 1944).
-For the Cenomanian-Turonian aquifer, representing the upstream part of the basin studied, the projection of the analysed samples on the Piper diagram (Fig.9a) shows that the waters have a mixed facies between Cl-Na, Cl-Ca-Mg, SO 4 -Ca-Mg, and HCO 3 -Ca-Mg. In 1995, the majority of the samples presented a  .9b) shows that the groundwater facies of the Cenomanian-Turonian aquifer have not experienced any remarkable change.
-For groundwater of the downstream part, the analysis of the Piper diagrams for the Plio-Quaternary and Turonian aquifers ( Fig.10a and b) shows that they are classi ed under a mixed facies between Cl-Na and Cl-Ca-Mg. The regrouping of the points of the Plio-Quaternary aquifer near the Turonian aquifer suggests an interconnection between these two aquifers.
The comparison between the results of 1990 and 2019 is presented in Figure 10c. This shows that there is a slight evolution in the chemical facies of the Plio-Quaternary groundwater. Indeed, on the cations triangle concerning the 1990 campaign, the majority of the points have a percentage higher than 50% in Na + with a tendency towards the Na pole. However, in 2019, the majority of the points do not exceed 50% in Na + with a tendency towards the center of the sorting "no dominant cations". For the anion triangle, a clear dominance of Clis noted, whether in 1990 or in 2019. The position of certain samples relative to the sample representing seawater on the Piper diagram suggests that the Plio-Quaternary aquifer is probably affected by the marine intrusion.
The groundwater of the Barremian-Aptian and those of the Hauterivian generally present three types of chemical facies: Cl-Na, Cl-Ca-Mg, and HCO 3 -Ca-Mg with the dominance of the second facies ( Fig.11a and   b). The dominance of Cl over HCO 3 could be explained by the in uence of Triassic saliferous formations.
The comparison between the water points sampled in 1997 and 2019 (Fig.11c) shows a remarkable evolution in the groundwater chemistry of the Barremian-Aptian and Hauterivian aquifers from the mixed facies Cl-Na and Cl-Ca-Mg to the facets Cl-Ca-Mg.

Groundwater mineralization
To determine the origin and the main processes responsible for the groundwater mineralization of the study area, the correlations between the main major elements have been studied.
Chloride is a conservative ion that is always found in natural waters at very variable contents (Fetter 1993) and sodium is generally associated with chlorides. Chlorides concentrations in groundwater of the upstream part vary widely from 113 to 1818 mg/l with an average of 574 mg/l. As for those of sodium, they vary between 12 and 541 mg/l with an average of 167 mg/l. According to the Piper diagram (Fig.9), it can be seen that the Clions are the most dominant in the waters. For the downstream part, the Clcontents vary between 120 and 4800 mg/l with an average of 620 mg/l and the Na + concentrations vary between 28 and 1950 mg/l with an average of 261 mg/l. The highest Na + and Clcontents are observed at the Plio-Quaternary aquifer.
The Na + vs Clcorrelation diagram (Fig.12a) shows a signi cant positive correlation between these two ions. This re ects that these two elements probably have the same origin. Some points are scattered around the halite dissolution line (line 1:1), re ecting the contribution of this mineral in the groundwater mineralization of the study area. This hypothesis is con rmed by negative values of the saturation indices with respect to this mineral (Fig.13). The rest of the samples are located below the line 1:1 and parallel to it, re ecting a Na + de cit. This suggests the contribution of a phenomenon other than the halite dissolution in the groundwater mineralization.
The Na + de cit compared to Clcould be linked to the basic exchange reactions, as shown in the Figure   12f, with the aquifer matrix where the Na + ions are released from the complex and are replaced by Ca 2+ ions according to equation (1) (Capaccioni et al. 2005): Na++12Ca-X2→Na-X+12Ca2+ (1) With X being the natural exchanger Also, an excess of Na + could be explained by the second type of cations exchange where the Ca 2+ and/or Mg 2+ ions will be released in water and the Na ions will be xed by the matrix according to equation (2): 12Ca2++Na-X→12Ca-X2+Na+ (2) The Ca 2+ contents of the groundwater from the upstream part vary between 82 to 770 mg/l with an average of 214 mg/l. As for those of SO 4 2-, they vary between 13 and 1942 mg/l with an average of 339 mg/l. As for the downstream part, the Ca 2+ concentrations oscillate between 64 and 850 mg/l with an average of 158 mg/l and those of SO 4 2vary between 30 and 830 with an average of 147 mg/l. Figure 12b shows the existence of a signi cant correlation between the Ca 2+ and SO 4 2ions. Indeed, the points whose Ca 2+ /SO 4 2molar ratio is close to or equal to 1, re ect the same origin of these two ions which could be the dissolution of gypsum and/or anhydrite. This is con rmed by negative values of the indices of saturation with respect to gypsum and/or anhydrite (Fig.13). However, the excess of Ca 2+ compared to SO 4 2observed for the majority of the points could be linked to the phenomenon of reverse bases exchange. Also, the saturation indices calculated for these points with respect to carbonate minerals are close to or greater than zero, corroborating that the enrichment of Ca 2+ is mainly due to the bases exchange (Fig.12f).
The Ca 2+ vs Mg 2+ diagram (Fig.12c) shows a positive correlation between these two ions, this re ects that these two elements come from the same origin. The majority of the points are scattered around the dolomite dissolution line (line 1:1), thus suggesting the contribution of the dissolution of this mineral to the groundwater mineralization. Other points are located above the line 1:1, con rming the contribution of the bases exchange process in the groundwater mineralization of the aquifers studied.
The Ca 2+ vs HCO 3 correlation (Fig.12d) shows that these two elements do not have a signi cant correlation and that the majority of the analyzed samples show a Ca 2+ / HCO 3 molar ratio greater than 1.

Nitrates contamination
The main source of nitrate in water is the leaching of nitrogenous products in the soil following the decomposition of organic matter or synthetic and/or natural fertilizers. The nitrate content of unpolluted natural waters is highly variable, varying from 1 to 15 mg/l depending on the season and the origin (Chenaker et al. 2017).
The NO 3 contents in groundwater of the Cenomanian-Turonian aquifer (upstream part) measured in March 2019 vary from 0 to 175 mg/l with a punctual spatial distribution (Fig.14a). Generally, levels are high in the Meskala region and exceed the threshold (50 mg/l) set by the World Health Organization (WHO 2011). Also, high values have been noted in some other wells such as 613/52 upstream O37 and 75/52 west of the Kourimat and O56 downstream.
For the Plio-Quaternary and Turonian aquifers, the NO 3 contents vary, respectively between 0 and 400 mg/l and between 0 and 65 mg/l (Fig.14b). As for the Barremian-Aptian aquifer, it has NO 3 contents varying between 5 and 60 mg/l. While the Hauterivian, has concentrations varying between 3 and 16 mg/l (Fig.14c).
The very weak correlation between Cland NO 3 - (Fig.12e)  The contamination (<50 mg/l) of the other wells at the level of the aquifers studied could be explained by traditional methods of drawing. These result in a signi cant amount of water owing around the catchment wells, constituting quasi-permanent pools that are enriched in NO 3 by livestock waste during watering. Note also that the number of contaminated wells in the Plio-Quaternary and Turonian aquifer (northern part of the downstream part) is greater than that of the Barremian-Aptian and Hautarivian aquifers (southern part of the downstream part). This is mainly due to the concentration of inhabitants in the northern part, where the water points are located in the middle of the agglomerations, while in the southern part and because of highly uneven geology, most wells are far from the places habitat.

Evolution of groundwater salinity
The groundwater salinization is a very marked phenomenon in areas of water scarcity, especially the Saharan, arid and semi-arid zones. The scarcity or even the absence of surface water and the increasing demand for water as well as the decrease in precipitation have created enormous pressures on groundwater which have thus resulted in the degradation of their quality.
The spatial-temporal distribution of salinity was studied to assess the impact of climate change on the groundwater quality by using the results of the campaigns of 1995, 2007, 2016, 2017, 2018, and 2019.  (Fig.15).
From the analysis of the maps in Figure 15, it can be seen that the salinity values become more important by advancing in time and going from east to west and this during the six campaigns. Taking, for example, the region of Sebt Kourimat, recharge area of the Cenomanian-Turonian aquifer, the salinity values uctuate around 0.46 g/l in 1995 to reach 2.9 g/l in 2019. However, the general spatial-temporal evolution of salinity shows an increasing trend.
For the downstream part, the groundwater from the Plio-Quaternary aquifer has salinity values varying between 0.6 and 3.4 g/l with an average of 1.7 g/l in 1990, between 0.9 and 3 g/l with an average of 1.6 g/l in 1995, from 0.4 to 4.1 g/l with an average of 1.3 g/l in 2004, between 0.9 to 2.2 g/l with an average of 1.4 g/l in 2009, from 0.3 to 4.7 with an average of 1.5 g/l in 2015, between 0.4 and 4.8 g/l with an average of 1.53 g/l in 2017, between 0.5 and 6.5 g/l with an average of 1.6 g/l in 2018 and between 0.46 and 8.4 g/l with an average of 1.7 g/l in 2019 (Fig.16). From the maps of Figure 16, the highest values are observed in the southern and western part and this further to the remoteness to the recharge zones, to the residence time, to the in uence of the Triassic terrains, and to the in uence from the sea (marine intrusion (well 11/51)). While the low values of salinity are recorded in the north (along the Ksob wadi) and in the east of the Plio-Quaternary aquifer which represent the recharge zones. These low values are due to the fact that these places represent the recharge zones of this aquifer. The temporal evolution of groundwater salinity of the Plio-Quaternary aquifer shows an upward trend going from year to year and consequently deterioration in the groundwater quality. As for the Turonian aquifer, the minimum values of salinity are around 0.8 g/l and the maximum values are around 1.3 g/l with an average of 1.1 g/l, and this for 2004,2009,2015,2017,2018 and 2019 campaigns (Fig.16). The temporal evolution of the groundwater salinity of this aquifer does not show a signi cant trend, this could be explained by its signi cant depth and its captive nature.
Concerning the Barremian-Aptian aquifer, the salinity values vary between 0.2 and 3.2 g/l with an average of 1.1 g/l for the points representing the 1997 campaign, from 0.3 to 2.1 g/l with an average of 1.1 g/l for the samples collected in 2015 and 2017, between 0.4 to 2.8 g/l with an average of 1.1 g/l for the points of 2018 campaign waters and between 0.7 and 2.4 g/l with an average of 1.2 g/l for 2019 campaign (Fig.17).
The spatial-temporal distribution of the groundwater salinity of the Barremian-Aptian aquifer (Fig.17) shows As the study area is under a semi-arid climate, with a tendency towards an arid climate in recent years accompanied by a decrease in precipitation and an increase in the temperature, which frequently causes intense periods of drought resulting in evaporation that affects surface and groundwater, especially the shallow waters, the degradation of the groundwater quality is mainly due to this situation and the decrease in the piezometric level caused by climate change.

Isotopy
The isotopic approach is of crucial importance in studies of aquifer systems. They make it possible to determine the groundwater origin and their residence times, to identify and quantify the rate of mixing between two types of water and to locate the recharge areas (Fontes 1976).
For the upstream part (Cenomanian-Turonian aquifer), the oxygen-18 contents vary between a minimum of -6 ‰ vs SMOW and a maximum of -3.3 ‰ vs SMOW, with an average of -4.9 ‰ vs SMOW. For deuterium, the maximum value is -20.2 ‰ vs SMOW and the minimum value equal to -34.5 ‰ vs SMOW with an average of -28.4 ‰ vs SMOW (Appendix 2). As for the downstream part, the contents of oxygen-18 vary between a minimum of -5 ‰ vs SMOW and a maximum of -1.8 ‰ vs SMOW, with an average of -3.9 ‰ vs SMOW for the Plio-Quaternary aquifer and between a minimum of -5 ‰ vs SMOW and a maximum of -4.4 ‰ vs SMOW, with an average of -4.7 ‰ vs SMOW for the Turonian aquifer. For deuterium contents, the maximum value is -8.9 ‰ vs SMOW and the minimum value equal to -29.7 ‰ vs SMOW with an average value of -22.6 ‰ vs SMOW for the Plio-Quaternary layer. As for the Turonian, the maximum value is -27.3 ‰ vs SMOW and the minimum value equal to -28.5 ‰ vs SMOW with an average value of -27.9 ‰ vs SMOW (Appendix 2). The comparison of the stable isotope contents of the upstream part and the downstream part of the Essaouira basin shows a slight depletion of the waters of the upstream part compared to those of the downstream part in these isotopes. This is due to the effect of elevation and continentality due to the remoteness of the coast.
In the absence of a local meteorological line characterizing the isotopic composition of the rainwater in the study area, the meteoric line with equation δ²H = 7.95 x δ 18 O + 11.3 was considered by Mennani et al. (2001) as an input function for the aquifer systems of the Essaouira basin. Figure 18, representing the variation of δ²H vs. δ 18 O of the groundwater representing the Cenomanian-Turonian aquifer, shows that some samples are located above the global meteoric water line (GMWL) and around the local meteoric water line (LMWL). This re ects that the aquifer recharge is ensured by the in ltration of precipitation of oceanic origin without signi cant evaporation. While some points are located below GMWL suggesting that these points have evaporated before being in ltrated to the aquifer. The sample of precipitation is an annual average of samples collected in 2004, 2006, 2016 and 2018. As for the seawater sample, we refer to values obtained by Carreira et al. 2014.
The distribution of representative samples of the groundwater representing the Plio-Quaternary and Turonian aquifers (downstream part) on the correlation diagram δ²H vs δ 18 O (Fig.18) shows that the majority of the points are scattered around the GMWL and LMWL re ecting a recharge by in ltration of oceanic rainwater (Group1). This supply of the shallow aquifer by rainwater, which is at the origin of the reduction in the salinity of the waters in these wells, is in perfect agreement with the hydrogeochemical data, in particular, well 27/51 which has low salinity. This well is the closest to the freshwater pole. Other local recharge points for rainwater have been identi ed in the bowl of the Essaouira basin (example O6, 15/51). This recharge is probably favored by the lithological nature and the small thickness of the unsaturated zone. This group contains both the majority of the samples representing the shallow Plio-Quaternary aquifer and all the water points representing the deep Turonian aquifer. This suggests the existence of a connection between these two aquifer systems.
Other water points are distinguished by their position below the GMWL (Group 2), they line up along a line with a slope less than 8 characteristics of evaporation phenomenon. This last process mainly concerns surface waters (O98 and O99) and wells 105/51 and 327/51 located respectively in the northeast and south part of the aquifer (Fig.18). Evaporation can probably take place before water in ltration, in the unsaturated zone or during sampling. In the same diagram, well 11/51 is aligned on the freshwaterseawater mixture line. This con rms that the increase in mineralization in this well is caused by the phenomenon of marine intrusion.
Following the availability of Tritium data, only the Cenomanian-Turonian aquifer was the subject of the groundwater dating in the study area. However, the tritium contents vary between 0 and 2.1TU. The highest values were recorded in the Et Tleta Hanchane region and the Kourimat region. This con rms that the recharge of the aquifer through rainwater is low and limited to a few regions (Fig.19). According to Mazor (1991), a tritium content greater than 1 TU indicates a post-nuclear recharge and content less than 1 TU represents a pre-nuclear recharge or a mixture between recent and old waters. The high tritium levels have been observed in the Kourimat and Et Tleta Hanchane region (recharge zone), and they can be attributed to the recent in ltration of precipitation.
The projection of the samples from the two 2016 and 2007 campaigns (Fig.19) shows that certain points are located above the line 1 TU re ecting a recent recharge of the Cenomanian-Turonian aquifer, while the other points are located above below this line con rming a pre-nuclear recharge.  Figure 21 where the electrical conductivity is around 10 mS/cm. According to Figure 21 we see that the increase in electrical conductivity (salinity) is accompanied by a very small increase in the 18-oxygen contents. This slight enrichment in 18 O (1 to 1.5 ‰), may be due to the effect of evaporation caused by the increase in air temperature under the effect of climate change. This suggests that the isotopic content of the study area is impacted by climatic variations and therefore it can be concluded that global warming has an effect on the isotopic signature of groundwater within the Essaouira basin.

Conclusion
The water resource within the Essaouira basin is limited and unevenly distributed in space and time. This problem could limit water supply, which will be aggravated by the depletion of this resource due to the climate change impact which has become an ambiguous reality and whose effects on the environment are already visible.
The combination of hydroclimatic, piezometric, hydrogeochemical and isotopic approaches in the study of the groundwater resource within the Essaouira basin led to the following conclusions: The analysis results of the annual precipitation time series using the statistical tests, in particular, that of Pettitt and that of Mann-Kendall, made it possible to detect a decrease in precipitation in the whole basin of 12 to 16%. This decrease in precipitation is accompanied by an increase in temperatures with a signi cant extent of warming of 2.3 °C. Based on the Gaussen diagram, the comparison of the duration of the dry period for the two periods 1987-2000 and 2001-2014 shows an extension of one month. This will no doubt have a negative effect on the groundwater recharge.
The piezometric approach has shown that the Cenomanian-Turonian, Plio-Quaternary, Barremian-Aptian and Hauterivian aquifers have retained the general direction of ow of their groundwater, during the study period. Monitoring the piezometry over a period of 24 years ( The general decline in the piezometric level could be explained by the decrease in precipitation following the harmful effect of climate change. This drawdown would probably cause a qualitative degradation of groundwater.
The hydrogeochemical study showed that the groundwater of the Cenomanian-Turonian aquifer presents the Cl-Ca-Mg, Cl-Ca, Cl-Na, and HCO 3 -Ca mix facies with the dominance of the Cl-Ca-Mg mix facies, and Cl-Ca. The study of the temporal evolution of these facies shows that there has been no remarkable change. The groundwater of the Plio-Quaternary and Turonian aquifers are of mixed type between Cl-Na and Cl-Ca-Mg. The chemical facies experienced a slight evolution from the Cl-Na facies to the Cl-Na and Cl-Ca-Mg facies for the Plio-Quaternary aquifer and from the Cl-Na facies to the Cl-Ca-Mg facies for the Turonian aquifer. As for the Barremian-Aptian and Hauterivian aquifers, they generally have three types of chemical facies: Cl-Na, Cl-Ca-Mg, and HCO 3 -Ca-Mg, with the dominance of the Cl-Ca-Mg facies. For the study period, a remarkable evolution of the facies was observed; from the Cl-Na facies to the Cl-Ca-Mg facies.
Examination of the correlations established between the concentrations of major elements has shown that the mineralization of groundwater is controlled by the phenomenon of the dissolution of the evaporitic minerals (halite, gypsum and/or anhydrites) and carbonates (dolomite), by the reverse ion exchange phenomenon and by the marine intrusion, especially at the Plio-Quaternary aquifer. The study of the spatio-temporal evolution of the groundwater quality in the study area shows a gradual deterioration in time and space.
The tracing of the groundwater in the Essaouira basin by stable isotopes has shown that the groundwater recharge in the upstream part of the basin studied is ensured by precipitation of Atlantic origin without signi cant evaporation. The same method of recharging is marked at the downstream part, with this time the presence of contamination by seawater.

Figure 20
Relation oxygen-18 and tritium content of Cenomanian-Turonian groundwater Figure 21 Isotopic contents evolution of the groundwater of the Plio-Quaternary and Turonian aquifers