Contribution to the study of groundwater quality and validation of their vulnerabilities to pollution using GIS. Case of the Plio-Pleistocene aquifer of the Essaouira syncline - Morocco

doi:10.21203/rs.3.rs-2196321/v1

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Contribution to the study of groundwater quality and validation of their vulnerabilities to pollution using GIS. Case of the Plio-Pleistocene aquifer of the Essaouira syncline - Morocco

https://doi.org/10.21203/rs.3.rs-2196321/v1

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In coastal areas, groundwater is the main source of consumption for drinking water, industry and agriculture. The Essaouira syncline is part of the Moroccan Atlantic coast, it is an area which is given a heavy responsibility in the socio-economic development of Morocco. The study area is limited to the north by the Jbeb Hadid, to the south by the Tidzi river, to the east by the reliefs of the South Chiadma, the reliefs of the North Haha and the diapir of the Tidzi, and to the West by the 'Atlantic Ocean.

In this article, we have tried to study the quality of groundwater by applying multi-criteria methods and to validate the pollution vulnerability map established by the DRASTIC method. The combination of GIS and multi-criteria analysis confirms the validity of the DRASTIC map with a better correlation between areas with high concentrations of chemical elements in groundwater and areas of very high vulnerability to pollution.

Plio-Pleistocene aquifer

DRASTIC method

ACP

Essaouira

Climate change

Groundwater

Currently, water stress is one of the major challenges for managers of the sector in the Kingdom of Morocco, to guarantee water security and stability of the population. Groundwater remains the main water resources for drinking water consumption, agriculture, industry and tourism in rural and urban areas with arid and semi-arid climates (Sinan et al., 2009; Ouhamdouch et al., 2018a; Ouzerbane et al., 2019; Bahir et al., 2020; Ouzerbane et al., 2021c; CSMD, 2021).

Several factors are involved in the degradation of electrical conductivity and therefore the quality of groundwater, these factors can be of natural origin (leaching, evaporation, dissolution, marine aerosols) or of anthropogenic origin (marine intrusion, overfertilization, water drainage, spreading of wastewater, vehicle emissions), and sometimes a combination of anthropogenic and natural origins (Andreasen and Fleck, 1997; Chamchati et al., 2013; Ouzerbane et al., 2014).

Generally, in nature, groundwater in a relationship with surface water infiltrated and percolated in the unsaturated zone to reach the aquifer, the effective infiltration rate has an influence on the piezometric evolution and therefore the evolution the water quality of the reservoir aquifer (Ouzerbane et al., 2013; Farid et al., 2013; Bahir et al., 2016; Maadid et al., 2017; Besser et al., 2018; Hamad et al., 2018; Kammoun et al., 2018; Mokadem et al., 2018; Ghanem et al., 2019; Saadali et al., 2019; Ncibi et al., 2020). Indeed, a significant effective infiltration causes a rise in the piezometric level, an increase in its flow and leads to the dilution of its mineralization. Conversely, low or absent effective infiltration combined with intensive pumping causes a lowering of the groundwater level and a mineral overconcentration of groundwater (Urish and Frohlich 1990; Sherif et al., 2006 ; Dieng et al., 2017; Ouzerbane et al., 2021a; Ouzerbane et al., 2021b; Ouzerbane et al., 2021c).

These groundwaters are often threatened by contamination by pollutants of different origins which can be biological, chemical or physical in nature (Ouzerbane et al., 2018a ; Chafouq et al., 2018). Their vulnerability to pollution is defined as the intrinsic susceptibility of an aquifer to the modification of its quality and quantity in space and time, due to natural and/or anthropogenic factors and processes (Aller et al., 1987; Civita, 1994 ; Hamza et al, 2008).

The regular water supply is one of the big challenges of humanity worldwide in the coming decades. This water crisis is increased by the dynamic interaction of several processes acting at the local, national and global levels (Bahir et al., 2002; Karroum et al., 2017; Ouzerbane et al., 2014; Ouzerbane et al., 2018b; Bahir et al., 2019). This will emphazise the tension between supply and demand and point to extreme situations in the near future. Due to its geographical location, Morocco is subject to various climatic influences varying from North to South under the effect of latitude and from West to East under the effect of the continentally. Rainfall is characterized by extreme irregularity that manifests itself in one year and from one year to another. This irregularity is much more marked south of the Atlas range. Faced with these problems, it is necessary to put in place mechanisms and actions aimed at the recognition, preservation and safeguarding of water resources.

The syncline of Essaouira is part of the Moroccan Atlantic coastline. It is a space that is given a heavy responsibility in the socio-economic development of Morocco. This development implies a significant increase in water needs in the coming years for both drinking water supply and other active sectors.

In this sense and for the protection of the groundwater in the coastal area of Essaouira, vulnerability to pollution has been addressed through the use of Geographic Information Systems (GIS). This tool makes it possible to highlight problems of overexploitation and degradation of water quality and helps those responsible to make the right decisions to resolve them. GIS are powerful tools par excellence for mapping in time and space the vulnerability of aquifers based on multi-criteria analyzes and updates of the models developed.

Geographical location of the study area

Essaouira synclinal study area is generally a dune relief area, especially near Essaouira; it is part of the coastal area of the basin with an area of 1418 km2. It is limited to the North by Jbeb Hadid, to the South by Tidzi River, to the East by the reliefs of Chiadma from the South and Haha from the North and by the Tidzi’s Diapir and to the West by the Atlantic Ocean. It is characterized by low hills with altitudes between 0 and 800 m, modeled by a low-density hydrographic network that flows towards the Atlantic Ocean (Ouzerbane et al., 2013; Ouzerbane, 2015). From North to South, two units can be distinguished namely the South Chiadma Unit and the North Haha Unit (Fig. 1).

Geological location of the study area

From south to North the stratigraphic series extends from Triassic to Quaternary (Ouzerbane, 2015). Whereas in the synclinal area of Essaouira, Plio-Pleistocene formations mainly outcrop (Fig. 2).

The Triassic formations outcrop in the southeastern part over a large area in the Tidzi region, along with a NE-SW direction forming the Tidzi diaper (Piqué, 1994; Medina, 1994). While in the extreme, they outcrop in a reduced zone along the Aghbalou River. Saliferous red clays, doleritic basalts and reddish sandstone pelites are mainly know the Triassic formations (Duffaud & Planchot, 1966).

The Upper Jurassic formations, mainly gypsiferous limestones and marly limestones, dated from Jbel Hadid represent the north part of the Essaouira syncline (Suter, 1958). The Cretaceous formations outcrop on the entire Essaouira Basin with thicknesses varying from one place to another. In the study area, they are exposed to the east along Oued Tidzi. They are composed of gray marls and lumachellic limestones of Santonian age, turquoise limestones, dolomitic limestones, limestones, marls and gypsiferous marls Cenomanian in age (Roch, 1931; Rey et al., 1950). To the west, the outcrops of Albian formations correspond to green marl on top of which lay dolomitic limestones of Maastrichtian age (Taj-Eddine, 1991). Lower Cretaceous formations also outcrop northwesterward.

Quaternary and Plio-Pleistocene formations constitute the main outcrops. From the center of the area to the Atlantic coast to the west, the sand dunes and calcarenites become important in space and thickness. The Plio-Pleistocene formations outcrop over the entire area through sandstones, fine sands, yellow limestones and conglomerates.

Hydrogeological context

The Essaouira synclinal study area is a basin with Cretaceous substratum covered by Plio-Pleistocene formations. It is bounded to the north by the anticline of Jbel Hadid where Upper Jurassic formations outcrop, to the east by the outcrops of the Upper Cretaceous and the diapir of Tidzi, to the south by the anticline of Amsittene (Western High Atlas ) and to the West by the Atlantic Ocean. The combination of the effects of tectonics and diapirism have caused the basin to be compartmentalized into several aquifer systems whose functioning and hydraulic relationships are poorly understood. From the north to the south of the zone, the importance of the aquifer system varies according to the nature of the geological formations which constitute the reservoir and its extension. The main aquifers of the basin are the multi-layered Cretaceous aquifer of the plateau and the Plio-Pleistocene aquifer developed at the level of the coastal strip. The most significant aquifer is the Jurassic which is deeper in the center of the study area. They are hydraulically interconnected and interdependent and therefore constitute a single hydraulic system.

The Plio-Pleistocene (Plio-Quaternary) in the study area is a reservoir with a matrix made up of dune sandstones and shell limestones, it extends along the Atlantic coast on a strip 20 km wide and 30 km long. According to studies carried out in the region, the reservoir is only productive when it is in a perched position resting on the impermeable marls of the Cretaceous or Jurassic, in places where the Plio-Pleistocene rests directly on the permeable formations, the water infiltrates vertically to feed the deep aquifer. The saturated thickness of the aquifer varies from 10 to 60 m. The groundwater is fed on the one hand by the infiltration of rainwater and on the other hand by infiltration along the rivers, with a general flow of the groundwater which takes place towards the ocean.

Application of the DRASTIC method to the coastal area of Essaouira

Index mapping methods are most commonly used in mapping vulnerability to groundwater pollution. They are based on the combination of maps of different parameters of the studied area (soil, geology, topography, depth of the water table, isohyetus...), giving a numerical index or a value to each parameter. The purpose of the use of geographic information systems is to structure information and to make the reading of data instantaneous, convivial and clear, and to update the information as it becomes available (Murat, 2000). The DRASTIC method is the multicriterion method chosen to study the vulnerability to pollution of the Plio-Pleistocène aquifer.

The DRASTIC method takes into account seven parameters, which each one is affected by a multiplicative factor (weight), increasing from 1 to 5 depending on the importance of each parameter in the estimating vulnerability. These parameters are as follows:

D : Depth to the Groundwater.
R (Recharge): Net recharge related to precipitation and ETR.
A (Aquifer media) : Nature of the saturated zone.
S (Soil media) : Nature and type of soil.
T (Topography): Topography of the superficial zone with its slope in%.
I (Impact of the vadose zone): Nature of the unsaturated vadose zone.
C (Conductivity): Permeability of the aquifer (saturated zone).

Each parameter is ranked in class at rating ranging from 1 to 10. The smallest rating represents the conditions of lowest vulnerability to pollution. It is applied to the different criteria in order to relativize their respective importance in terms of vulnerability. The final vulnerability index (ID) is the adjusted sum of the seven parameters according to the following formula (Eq. 1):

\(ID = Dp.Dc+Rp.Rc+Ap.Ac+Sp.Sc+Tp.Tc+Ip.Ic+Cp.Cc\) (Eq. 1)

Whither:

D, R, A, S, T, I, and C are the seven parameters of the standard DRASTIC method.

p : is the weight of the parameter.

c: is the associated rating.

The maps obtained provide a visualization of the relative degree of vulnerability of a sector of the study area. The pollution potential increases in the same direction as the calculated vulnerability index (ID). The values obtained from this index represent the measurement of the vulnerability to pollution of the aquifer. Their possible range is between 23 and 226 in the case of the standard version. This paragraph is the result of the work of Ouzerbane (Ouzerbane et al., 2021c).

Analyse en composantes principales ACP

The objective of this methodology is to determine the factors that influence the variability of the parameters of the Essaouira basin. The ACP method used for this study is based on the interpretation of the correlation matrix as well as the various factors obtained as a result of data processing. The choice of the main axes takes into account the reduction in the number of factors. This number is such that the cumulative sum of the contributions is significant (75% which represents three quarters of the total inertia). Indeed, two variables are correlated when their correlation coefficient is greater than or equal to 0.7.

In addition, at the factorial design level, variables are only representative when they are close to the end of these factors. When two variables are correlated, the variation of one results in the variation of the other. Analyzes of variance were used to judge the significance of the relationships highlighted by the factor analysis.

The water samples were taken from the plio-quaternary water table in the coastal zone of Essaouira between Qsob river and Tidzi river at the level of 1 to 35 wells (Table 1, 2) during 1995 (Mennani 2001) and at the level of 36 to 47 wells (Table 1, 2) in 2009 (Chamchati 2014). The physical parameters relate to the piezometric level (PL), the temperature (T) and the electrical conductivity (CE).The chemical parameters concern on the one hand the major cations (calcium, magnesium, sodium and potassium) and on the other hand the major anions (chlorine, nitrates, sulphates and HCO3-). The data has been processed by advanced statistical analysis techniques. The Principal component analysis (ACP) method was used to understand and visualize the spatial and temporal distribution of the samples. In addition, this paragraph is the result of the work of Ouzerbane (Ouzerbane et al., 2021b).

Table 1

Classification data and visualization of the spatial and temporal distribution of samples (Classes 1, 2).
Classes	Stations	X	Y	Z	PL (m)	T (°C)	CE 25° (µs/cm)	HCO₃⁻ (mg/l)	Cl⁻ (mg/l)	SO₄²⁻ (mg/l)	Ca²⁺ (mg/l)	Mg²⁺ (mg/l)	Na⁺ (mg/l)	K⁺ (mg/l)	rMg/rCa	i.e.b
1	36	85500	105620	22	22,00	20,00	2440,00	146,40	475,70	128,60	40,00	38,64	266,80	5,46	0,97	0,43
1	45	87800	98800	70	32,00	22,00	2550,00	146,40	442,33	96,00	40,00	42,36	236,44	3,90	1,06	0,46
1	47	84980	111080	23	18,00	23,00	3060,00	207,40	372,75	168,48	66,00	59,28	219,42	19,50	0,90	0,36
1	39	90290	102260	102	54,00	19,00	2249,00	176,90	390,50	102,72	42,00	39,84	207,69	10,53	0,95	0,44
1	44	97170	100760	105.5	105,50	20,00	2180,00	189,10	355,00	129,60	42,00	38,64	212,98	4,29	0,92	0,39
1	46	81400	93400	18	14,00	19,00	2130,00	195,20	344,35	44,64	34,00	30,24	254,38	5,85	0,89	0,24
1	38	91150	102300	78	49,00	20,00	2040,00	207,40	347,90	104,64	40,00	30,24	221,95	13,65	0,76	0,32
1	6	88800	88800	130	24,00	21,50	1870,00	162,30	422,50	78,60	92,00	27,80	253,00	5,10	0,30	0,39
1	19	86550	102700	60	29,00	22,00	1590,00	205,00	429,60	100,10	112,80	55,20	200,10	12,10	0,49	0,51
1	40	95150	104490	97	69,20	20,00	770,00	67,10	74,55	27,36	30,00	24,12	36,80	3,12	0,80	0,46
1	41	93410	102680	114	67,00	19,00	1763,00	219,60	241,40	121,44	44,00	35,04	138,69	4,68	0,80	0,41
1	42	91200	100750	90	56,00	23,00	1720,00	195,20	255,60	119,52	46,00	41,04	127,42	5,46	0,89	0,48
1	43	94820	102170	99	76,50	21,00	1671,00	176,90	217,97	111,84	38,00	31,44	135,70	4,68	0,83	0,36
1	8	91150	102300	78	17,20	20,50	1890,00	346,50	436,70	187,10	107,20	73,00	167,90	1,20	0,68	0,61
1	17	96420	99250	124	44,00	23,00	2000,00	266,00	436,70	178,40	146,80	48,70	241,50	2,30	0,33	0,44
1	25	93650	94450	148	62,00	23,00	1860,00	241,60	586,80	45,50	140,40	67,90	218,50	2,00	0,48	0,62
2	30	90950	102700	66	15,00	20,50	2340,00	369,70	532,50	244,60	102,00	104,40	287,50	1,60	1,02	0,46
2	34	97500	99650	120	14,20	21,00	2380,00	342,80	582,20	241,40	120,00	118,10	264,50	2,00	0,98	0,54
2	1	87300	103600	60	10,00	19,00	2700,00	363,60	738,40	177,80	179,60	56,60	333,50	3,10	0,32	0,54
2	18	85100	105800	40	38,00	22,00	3580,00	353,80	1075,70	216,30	104,80	101,30	655,50	4,70	0,97	0,39
2	37	87850	92830	109	104,00	21,00	3520,00	109,80	766,80	124,80	38,00	45,96	438,15	10,53	1,21	0,41
2	29	91350	101800	85	20,38	22,00	2170,00	324,50	500,60	205,90	112,00	77,30	299,00	1,20	0,69	0,40
2	31	92370	101900	98	29,58	21,50	2150,00	339,20	489,90	202,90	111,20	128,60	230,00	1,60	1,16	0,53
2	12	81400	93400	18	5,00	19,00	2550,00	412,40	646,10	163,40	117,20	52,60	391,00	4,70	0,45	0,39

Table 2

Classification data and visualization of the spatial and temporal distribution of samples (Classes 2, 3).
Classes	Stations	X	Y	Z	PL (m)	T (°C)	CE 25° (µs/cm)	HCO3- (mg/l)	Cl- (mg/l)	SO42- (mg/l)	Ca2+ (mg/l)	Mg2+ (mg/l)	Na+ (mg/l)	K+ (mg/l)	rMg/rCa	i.e.b
2	2	88150	96050	92	26,00	22,00	3250,00	209,80	1029,50	221,20	159,20	76,30	471,50	7,00	0,48	0,54
2	13	85590	93700	85	80,00	23,00	3130,00	383,10	972,70	163,90	164,00	61,90	414,00	9,80	0,38	0,56
2	9	94820	102175	99	26,00	21,00	2100,00	314,80	532,50	213,30	112,20	85,20	207,00	2,70	0,76	0,61
2	32	96550	100800	106	35,80	20,50	2090,00	287,90	539,60	238,90	109,60	71,30	218,50	2,30	0,65	0,59
2	33	95500	91300	208	34,30	21,00	2000,00	358,70	433,10	229,40	108,00	106,10	165,60	2,30	0,98	0,61
2	10	97170	100760	105.5	28,00	20,50	2280,00	295,20	600,00	243,30	108,00	98,40	253,00	2,70	0,91	0,57
2	3	92450	97900	110	42,00	22,00	2500,00	219,60	788,00	164,80	155,20	73,70	287,50	2,30	0,47	0,63
2	4	100650	96000	200	21,50	22,00	2610,00	285,50	671,00	143,50	194,40	69,10	299,00	1,90	0,36	0,55
2	22	89750	98500	99	38,50	15,50	2580,00	226,90	830,70	163,40	185,60	75,10	322,00	2,30	0,40	0,61
2	14	97200	91800	225	49,00	23,00	3360,00	339,20	1136,00	137,50	227,20	109,40	333,50	3,50	0,48	0,70
2	15	93250	94100	155	60,00	24,00	3330,00	226,90	1228,30	106,40	279,60	63,10	368,00	3,10	0,23	0,70
2	26	94800	96950	137	58,00	24,50	3240,00	222,00	1189,30	132,00	306,00	64,10	356,50	3,10	0,21	0,70
2	28	92900	97300	125	53,00	23,00	3220,00	102,50	1221,20	147,90	291,20	121,70	368,00	2,70	0,42	0,70
2	16	97400	99750	115	36,40	22,00	2100,00	268,40	546,70	168,30	152,00	53,30	287,50	3,50	0,35	0,47
2	5	96800	96750	164	68,00	21,00	2340,00	285,50	646,10	84,30	172,00	43,20	299,00	2,30	0,25	0,53
2	20	81250	92800	12	8,30	19,00	2700,00	390,40	777,50	131,80	184,40	19,40	402,50	4,70	0,11	0,48
3	11	87800	98800	70	26,00	22,00	4710,00	307,40	1462,60	402,20	295,20	150,00	678,50	3,50	0,51	0,53
3	21	87800	99200	78	18,00	21,00	5040,00	302,60	1693,40	418,30	320,80	76,80	713,00	3,10	0,24	0,58
3	35	80450	96450	8	2,70	18,00	4830,00	263,50	1654,30	290,70	172,40	101,00	736,00	35,10	0,59	0,53
3	24	89400	91400	89.6	27,70	20,00	4460,00	207,40	1728,90	162,00	376,00	61,40	586,50	2,30	0,16	0,66
3	7	87850	92830	109	25,00	21,00	3960,00	192,80	1487,50	166,40	298,40	104,60	517,50	3,90	0,35	0,65
3	23	86750	94450	87	18,50	21,00	3490,00	175,70	1320,60	129,30	248,00	91,20	471,50	3,90	0,37	0,64
3	27	94400	97800	125	52,00	19,00	3570,00	180,60	1359,70	131,50	325,60	106,30	391,00	2,70	0,33	0,71

Piezometry of the Plio-Pleistocene aquifer

The Plio-Pleistocene aquifer is the most exploited in the region by a very large number of wells capturing this aquifer. As part of this work, a mission was carried out in the study area to measure the physicochemical parameters of the Plio-Pleistocene aquifer in September 2013 (Table 3, 4).

The piezometric map was established based on data from field measurement campaigns, from around 74 wells and some springs sampled (Fig. 3 and Table 3, 4). Its interpretation is done by taking into account the parameters that can influence the piezometry of the Plio-Pleistocene aquifer, namely the geometry of the wall and the precipitation (Ouzerbane, 2015; Ouzerbane et al., 2021c).

The extent of the reservoir rock and the geometry of the wall, consisting of Cretaceous marl in the study area, show a Plio-Pleistocene aquifer formed of three sectors separated by watershed lines and whose direction geological layers of Cretaceous age, with a general SE-NW direction of flow:

The sector located north of Qsob river and south of the Jbel Hadid anticline (Fig. 3), with streamlines oriented in the general direction of flow towards the Atlantic Ocean in the direction of groundwater diverge towards the NW to join the sources along the Ait Tahria wadi (exp: Source of Ain Lahjar). The altitude of the piezometric surface is between 180 m in the East and 35 m in the West, with a gradient of about 1%.

Table 3

The physico-chemical parameters of the Plio-Pleistocene aquifer in September 2013.
N° of wells	Location	X	Y	Z	H (m)	PL (m)	T (°C)	pH	CE 25° (µs/cm)
P1	LGOUTRA	113301	111587	273	17.5	255.5	22.6	4.18	3982
P2	AIT ABDESSALAM	114002	111622	269	12.0	257.0	22.2	8.60	3604
P3	AIT LOUAFI	114256	111258	273	33.4	239.6	23.2	6.43	3772
P4	AIT HIDA	113358	112545	265	36.0	229.0	22.5	6.61	4000
P5	BOUJLAKH	108482	113349	242	37.5	204.5	22.5	7.23	3333
P6	BOUJLAKH	108291	113042	244	37.0	207.0	23.8	6.37	3361
P7	AIT HMAD	106416	112420	223	30.6	192.4	23.5	7.00	3298
P8	AIT HMAD	106785	112222	225	28.0	197.0	22.9	8.23	2838
P9	LAHRARTA	99631	110099	210	53.3	156.7	23.7	6.45	3376
P10	LAHRARTA	99488	110225	217	65.5	151.5	25.9	7.80	2703
P11	LAHRARTA	98840	110264	213	80.0	133.0	25.4	7.60	1969
P12	LAHRARTA	98474	110964	203	63.5	139.5	26.1	8.72	2586
P13	LAARAB	91904	108273	109	57.4	51.6	23.0	8.13	1522
P14	LAARAB	91464	108577	108	51.0	57.0	22.3	7.08	3027
P15	LOUBANE	91720	111175	121	74.3	46.7	23.9	6.40	2510
P16	LAARAB	91559	109501	108	87.0	21.0	22.8	6.70	2083
P17	LAARAYCH	92800	114626	129	87.0	42.0	27.2	7.90	4320
P18	LAARAYCH	93035	114559	123	61.0	62.0	23.0	7.93	2391
P19	LAARAYCH	92724	112184	125	72.0	53.0	24.3	8.50	1955
P20	LAARAYCH	92024	112283	106	60.0	46.0	23.4	8.39	2885
P21	EL KOUACH	98761	114362	156	39.3	116.7	26.0	8.20	1346
P22	EL KOUACH	98856	114421	152	33.0	119.0	23.9	7.23	2510
P23	OUGDILA	100092	114900	168	47.0	121.0	23.8	7.90	1261
P24	TIGLAT	100292	114523	183	53.2	129.8	24.0	7.80	1875
P25	TIGLAT	100598	113599	197	49.0	148.0	23.6	7.08	1483
P26	TABYA	108415	116287	216	15.3	200.7	22.8	7.56	2961
P27	TABYA	108463	116160	221	17.4	203.6	22.7	7.02	2974
P28	TABYA	108602	115959	218	18.5	199.5	22.8	6.74	2632
P29	CHEBABTA	109245	115676	219	20.0	199.0	22.7	8.80	3304
P30	MATRAZZA	109254	121141	216	15.5	200.5	22.7	8.30	6278
P31	MATRAZZA	110259	120150	214	20.5	193.5	22.6	7.70	5310
P32	NGUIAA	107559	121921	211	27.0	184.0	21.8	8.40	3555
P33	NGUIAA	107548	122023	217	31.0	186.0	22.1	7.80	5656
P34	NGUIAA	107636	121905	203	25.0	178.0	22.0	6.90	6818
P35	AMARAT	106947	119576	203	16.0	187.0	22.5	8.30	7111
P36	IMZOUGHAR	101538	119052	187	68.0	119.0	24.2	7.60	3202
P37	IMZOUGHAR	101139	118552	178	54.8	123.2	23.5	6.80	3830
P38	IMZOUGHAR	100975	118189	176	52.5	123.5	23.6	8.60	1695
P39	BOUCHAAIB	99897	116598	163	49.0	114.0	23.4	6.40	2030

Table 4

The physico-chemical parameters of the Plio-Pleistocene aquifer in September 2013.
N° of wells	Location	X	Y	Z	H (m)	PL (m)	T (°C)	pH	CE 25° (µs/cm)
P40	Plage Sidi Bouzakri	94546	124969	19	10.0	9.0	21.5	8.07	4395
P41	AIT TAHRIYA	98704	125749	43	11.5	31.5	21.8	8.18	3589
P42	AIT TAHRIYA	99113	125844	48	14.0	34.0	23.9	8.13	4916
P43	AGADIR	102135	125566	115	100.0	15.0	25.0	8.30	2220
S1	AIN LAHJAR	103132	124571	100	0.0	100.0	22.3	8.40	4641
P44	SIDI YAHIA	105544	126217	145	4.3	140.7	23.3	8.30	3938
S2	AIN LKHATARAT	105167	126407	127	0.0	127.0	24.4	8.30	4232
P45	AIT HIMOUCH	113980	126990	284	120.0	164.0	26.2	8.34	3015
P46	AIT LAARBI	114081	126486	279	70.0	209.0	26.2	8.31	4103
P47	TAKKAT	114415	124547	282	5.6	276.4	21.5	8.22	3186
P48	Imi NTAGANT	86914	101829	80	20.0	60.0	22.2	8.30	3536
P49	Imi NTAGANT	86911	101696	72	20.0	52.0	22.5	8.30	3478
P50	AIT ACHOUAYAKH	90738	101920	107	36.0	71.0	22.3	8.30	2859
P51	IT ACHOUAYAKH	90440	101980	107	46.0	61.0	22.8	8.30	2950
P52	ZAOUIT SIDI BRAHIM	93633	103430	90	18.0	72.0	21.6	8.40	2326
P53	TAZITOUNTE	86047	97636	66	7.0	59.0	21.0	8.40	6536
P54	ID ZAOUIT	95472	88396	263	46.0	217.0	23.0	8.40	1924
P55	IKAABAR	95527	90821	200	29.0	171.0	22.3	8.08	1009
S3	AIN TASSAKRA	83872	80561	175	0.0	175.0	23.1	8.12	2413
S4	AIN AGHBALOU	83380	80749	89	0.0	89.0	23.4	8.35	3376
P56	ID AMAR	99626	109044	188	17.3	170.7	23.4	8.21	611
P57	HRARTA	99027	107695	162	8.0	154.0	23.7	8.07	1973
P58	ICHBAK 1	98547	106176	144	5.5	138.5	21.1	8.34	3957
P59	ICHBAK 2	97048	104906	135	34.3	100.7	22.7	8.62	2985
P60	SAHLI	95736	104124	113	30.5	82.5	23.0	8.04	6293
P61	-	90185	100586	104	40.0	64.0	22.5	8.30	1883
P62	TIDZI	89380	88397	132	24.0	108.0	23.1	8.47	3333
P63	SMIMOU	89655	77479	259	5.0	254.0	22.2	8.63	1417
P64	DIABAT	85109	105776	43	37.2	5.8	23.4	6.36	3160
P65	AIT MOUMEN	89298	98612	94	38.0	56.0	22.7	6.51	2510
P66	ZAOUIT SIDI BRAHIM	93416	102734	103	47.0	56.0	20.8	6.60	1680
P67	SIDI ALI BAZOUR	93656	100268	100	34.0	66.0	23.6	6.58	1680
P68	ZAOUIT SIDI BRAHIM	94534	102323	98	22.5	75.5	21.4	6.56	1610
P69	SIDI KAWKI	82006	93200	14	4.0	10.0	23.3	6.52	2120
P70	ECHAABAT	90156	95296	119	61.0	58.0	21.3	6.60	1920
P71	SIDI HMAD OU HMAD	89402	91160	130	28.0	102.0	23.4	6.27	5780
P72	ID KADDOUR	87980	79808	240	39.0	201.0	22.7	6.33	3350
P73	SMIMOU	89678	76643	257	6.0	251.0	22.4	6.30	1610
P74	BOU TAZERT	87745	95875	90	5.0	85.0	23.2	6.50	3530

- The central sector between Qsob river and Tidzi river where Albian-Aptian age marls outcrop, shows an altitude of the piezometric surface of about 180 m to the east where the hydraulic gradient is strong, of the order of 2% due to the rise of the Tidzi diapir, 0.27% going towards the center, then tends to increase downstream to reach 2%. This increase is explained by the rise of hidden diapir detected by oil geophysics south of the city of Essaouira (Mennani, 2001). This sector is separated in two, one to the North where the water circulates according to the global hydraulic gradient, the other to the South with lines of currents directed from East to West to reach the sea.

- The third sector in the South between the Tidzi river and the Aghbalou river, presents an altitude of the piezometric surface of approximately 230 m in the East and 80 m in the West where the formations of Albian-Aptian age outcrop, with a hydraulic gradient of the order of 1%. Groundwater flows from East to West to diverge towards stratification planes (eg: Source Aghbalou).

Electrical conductivity of groundwater

The electrical conductivity gives an idea of the mineralization of the waters, therefore informs about its salinity and allows its quality to be assessed. The level of dissolved and ionized salts in water contributes to their mineralization (Bermond and Perrodon, 1979 in Kaid Rassou, 2009).

The first interpretations of the map established by measurements of electrical conductivity (Fig. 4), carried out during surveys of water points (wells and sources) in the study area, show a spatial variety of values in one direction. NE-SW, with minimum values of around 1334 µS/cm (Wells P25 and P56) in the center of the zone and towards the South-East at the level of the axis of wells P25 and P65. Going towards the SW and towards the NE of the study area, the values increase to reach the maximum of around 6524 µS/cm (well P34).

The waters of the plio-quaternary reservoir have a very varied electrical conductivity going from the center of the study area where the values are minimum (1334 µS/cm) towards the East and towards the North West where the values are maximum (6524 µS/cm).

Groundwater temperature

The analysis of the map of spatial distribution of groundwater temperature (Fig. 5), shows an increase in temperature according to the hydraulic gradient in the northern part of Qsob river. On the other hand in the south of Qsob river, the temperature of Waters of the Plio-Pleistocene aquifer increase as they move away from the river towards the south (Tidzi river).

This variation in temperature values, which fluctuates between 22.1°C and 24.7°C, is most likely due to the increase in depth in the northern part Qsob, and for the southern zone Qsob this increase in direction North-South is probably due to the feeding of the aquifer (Plio-Pleistocene) by the cool waters of Wadi Qsob.

Groundwater pH of groundwater

The analysis of the distribution map of the pH of the waters of the Plio-Pleistocene aquifer shows pH water values which vary between 7.03 and 8.19 and the high values are generally located in the northern part (South of Jbel Hadid) .

The basicity is explained by the salinity of the waters in this region (Fig. 6), on the other hand the pH is stable in the zone located between Qsob river and Tidzi river. This stability is explained by the recharge of the aquifer from runoff water despite the high values of electrical conductivity recorded northwest of Tidzi river.

Map of vulnerability to pollution of the Plio-Pleistocene aquifer

The calculation of the DRASTIC index by the combination of the thematic maps resulting from the product of the seven parameters constitute the DRASTIC method by the GIS tool (Ouzerbane et al., 2021c), the sum of the maps finally gives a map of vulnerability to pollution following the model of the steps of the DRASTIC method (Fig. 7).

The analysis of the pollution vulnerability map of the Plio-Pleistocene aquifer of the Essaouira coastal zone shows a heterogeneous variation in vulnerability throughout the area, with values of the DRASTIC index (ID) vary from 45 to 149 distributed spatially in the study area according to four classes when determined by the reclassification of the results (Fig. 7).

This vulnerability to pollution calculated by the standard DRASTIC method is essentially dependent on the depth of the aquifer and the unsaturated zone (Ouzerbane et al., 2021c). These two parameters have a very important weight compared to the other parameters constitute this method.

The class of vulnerability to medium degree pollution (78–92) presents a higher spatial distribution in the region with a covering of 53% of the surface of the study area (Fig. 8). Followed by the high degree vulnerability class (93–112) with an overlap of 22% of the total area of the region studied, this class is represented by the area between Qsob river and Tidzi river, the area of Ain Lahjar and the zone between Cap Sim and Aghbalou river. Thereafter, the class of vulnerability to pollution of low degree (45–77) represents a spatial distribution of about 18% of the total area of the coastal zone of Essaouira, it is located mainly in the North- East (Chiadma Sud) and South-East (Tidzi-Smimou). Followed by the very high degree vulnerability class (113–149) presents a very low spatial distribution of around 7% of the surface area of the Essaouira coastal zone, it is represented by the alluvial zone of Adamna (Qsob river) and the Atlantic coast between Sidi Kawki and Ain Aghbalou. The high degree in these areas can be explained by the presence of permeable formations (alluvium, gravel, sand and sandstone).

Principal component analysis (ACP)

Study of the variables in the correlation circle

The correlation circle corresponds to a projection of the variables on a two-dimensional plane made up of two factors. Only variables located far from the center of the circle are taken into account. If they are close to each other, then they are significantly positively correlated (R close to 1). If they are orthogonal with respect to each other, then they are significantly uncorrelated (R close to 0). If they are symmetrically opposed with respect to the center, then they are relatively negatively correlated (R close to -1).

The analysis in the space of the variables was carried out in the factorial designs PC1- PC2 (Fig. 8) and PC1- PC3 (Fig. 9) which give a satisfactory account of the data structure and explain 71.5% some information.

The first principal component (PC1): A first grouping located in the positive values includes the elements Ca²⁺, Cl⁻, CE 25°C, Na⁺, Mg²⁺ and to a lesser degree i.e.b.

A second grouping located in the negative values includes PL (m) and rMg / rCa. The second principal component (PC2) correlates slightly with T (°C)in the positive part and with K⁺ and HCO3⁻ in the negative part.

The factors PC1 and PC3: A group of elements (Na⁺, Cl⁻, SO4²⁻, Ca²⁺, i.e.b.) are closer and therefore show a positive correlation.

Projection of individuals

The axes express a significant percentage of the information with respectively 42% by PC1 and 16.1% by PC2; which gives a total of 58.1% of the information (Fig. 10, 11)

The analysis of the descriptive statistical characteristics, the physicochemical variables used, reveals that the groundwater in the study area has temperatures ranging between 15.5 and 24.5 ° C, with an average of 21.02°C. The electrical conductivity oscillates between 770 and 5040 µs / cm with an average of 2682.19 µs / cm (Table 5).

Class 1 shows the highest Piezometric Level (PL), while class 3 shows the highest concentrations of Electrical Conductivity (CE), Cl⁻ and Na⁺ compared to the other two classes (Table 5).

Table 5

Basic statistical quantities (Min, avg, max) for the physico-chemical parameters of different classes.
		PL	T	CE 25°	HCO3-	Cl-	SO42-	Ca2+	Mg2+	Na+ K+ rMg/rCa i.e.b
		(m)	(°C)	(µs/cm) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l)
Total base	Min	2.7	15.5	770	67.1	74.55	27.36	30	19.4	36.8 1.2 0.11 0.24
	Avg	38.52	21.02	2682.19	251.30	744.93	164.08	146.62	69.17	323.10 5.19 0.61 0.52
	Max	105.5	24.5	5040	412.4	1728.9	418.3	376	150	736 35.1 1.21 0.71
Classe I	Min	14	19	770	67.1	74.55	27.36	30	24.12	36.8 1.2 0.30 0.24
	Avg	46.21	21	1986.44	196.81	364.40	109.03	66.32	42.72	196.20 6.49 0.75 0.43
	Max	105.5	23	3060	346.5	586.8	187.1	146.8	73	266.8 19.5 1.06 0.62
Classe II	Min	5	15.5	2000	102.5	433.1	84.3	28	19.4	165.6 1.2 0.10 0.39
	Avg	37.54	21.25	2675.83	293.01	769.77	177.79	158.06	78.17	331.34 3.57 0.59 0.55
	Max	104	24.5	3580	412.4	1228.3	244.6	306	128.6	655.5 10.53 1.21 0.70
Classe III	Min	2.7	18	3490	175.7	1320.6	129.3	172.4	61.4	391 2.3 0.16 0.53
	Avg	24.27	20.29	4294.29	232.86	1529.57	242.91	290.91	98.76	584.86 7.79 0.36 0.61
	Max	52	22	5040	307.4	1728.9	418.3	376	150	736 35.1 0.59 0.71

Validation of the pollution vulnerability map of the Plio-Pleistocene aquifer

The projection of the three classes obtained by principal component analysis (PCA) on the pollution vulnerability map shows that the zone with very high vulnerability essentially groups together the samples of class 1, which has low values of the physico-chemical parameters.

While the high vulnerability zone mainly includes samples from class 2, which has an increase in values compared to class 1.

Class 3 is located to the West of the study area where the vulnerability is very high, and at the same time the highest values of the physico-chemical parameters, which would be due to the proximity to the Atlantic Ocean (Fig. 12).

The study of the physico-chemical parameters of the Plio-Pleistocene aquifer shows that its hydraulic gradient is strong in the east and west of the region. It is around 2% and becomes weak in the center (0.27%). Its groundwater temperature varies between 22 and 25°C. On the other hand, its electrical conductivity oscillates between 1300 in the center and 6500 µS /cm in the North-East and South-West of the coastal zone of Essaouira.

The analysis of the pollution vulnerability map of the study area shows that the central zone (Qsob river) and the South-West zone (Sidi Kawki-Ain Aghbalou) have high to very high pollution vulnerabilities. This sensitivity to pollution is generally due to the low slope of the land at the level of the Adamna zone crossed by Qsob river with alluvial deposits of Quaternary age and sands and sandy clays of Plio-Pleistocene age.

Towards the South-West, the very high vulnerability along the coast up to the Ain Aghbalou area is mainly due to the low or even zero value of the depth of the Plio-Pleistocene aquifer and the nature of the geological formations permeable (Sandstone, sand and consolidated sand) of Quaternary and/or Plio-Pleistocene age.

The statistical study by the ACP of the hydro-chemical data for the years 1995 (Mennani, 2001) and 2009 (Chamchati, 2014), showed an evolution of the values of the physico-chemical parameters as a function of time (between 1995 and 2009), and of the space with an increase in the values of the parameters going from the center of the study area (Qsob river) towards the South-West (Atlantic Oceans). This increase is generally due to the influence of the Qsob river (dilution of groundwater), the diapiritic influence and the influence of the Atlantic Ocean.

Finally, the results of this statistical study confirm and validate the map of the intrinsic vulnerability to pollution of the coastal zone of Essaouira.

Acknowledgements

The logistic support during exploration and sampling was funded by the Faculty of Science and Technology, Sultan Moulay Slimane, University - Béni Mellal (Morocco). We are greatful to an anonymous reviewer for the fruitful remarks that helped to improve the manuscript.

Ethical Approval (Not applicable)

Consent to Participate (Not applicable)

Consent to Publish (Not applicable)

Authors Contributions

All authors have made a significant contribution to this research. Zakaria Ouzerbane, Abdellah El Hmaidi and Ali Essahlaoui conceived the research idea and designed the framework of the research. Anas El Ouali, Soumia Loulida and Abdellah El Hmaidi participate in the fieldwork. Zakaria Ouzerbane and Ali Essahlaoui wrote the paper. Zakaria Ouzerbane and Abdessamad Najine contributed Data processing and model creation.

Funding

This work is a part of a PhD thesis funded by Faculty of Science-Meknes, Moulay Ismail University Morocco.

Competing Interests

The authors declare that they have no competing interests.

Availability of data and materials (Not applicable)

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No competing interests reported.

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Contribution to the study of groundwater quality and validation of their vulnerabilities to pollution using GIS. Case of the Plio-Pleistocene aquifer of the Essaouira syncline - Morocco

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Abstract

Figures

Introduction

Material And Methods

Geographical location of the study area

Geological location of the study area

Hydrogeological context

Application of the DRASTIC method to the coastal area of Essaouira

Analyse en composantes principales ACP

Results And Discussion

Piezometry of the Plio-Pleistocene aquifer

Electrical conductivity of groundwater

Groundwater temperature

Groundwater pH of groundwater

Map of vulnerability to pollution of the Plio-Pleistocene aquifer

Principal component analysis (ACP)

Study of the variables in the correlation circle

Projection of individuals

Validation of the pollution vulnerability map of the Plio-Pleistocene aquifer

Conclusion

Declarations

References

Additional Declarations

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