Arabian Journal of Earth Sciences (AJES)

Transcription

1 Arabian Journal of Earth Sciences, Vol.3 (2016) - Issue 1: الدورية العربية لعلوم الا رض Arabian Journal of Earth Sciences (AJES) A comparative approach of the vulnerability assessment using methods GOD, SI, DRASTIC and SINTACS study case of Mnasra aquifer (NW, Morocco) Badr Benseddik*, El Mansouri Bouabid Laboratory of Geosciences of Natural Resources, hydroinformatic section, Faculty of Sciences, Ibn Tofail University, Maamora Campus, BP.133, 1400 Kénitra, Morocco, badr.benseddik@gmail.com Keywords Groundwater resources, Mnasra aquifer, Vulnerability, DRASTIC, SI, GOD, SINTACS. ABSTRACT During the last twenty years the groundwater resources of Mnasra experienced their lowest levels relative to domestic demand and agricultural activity that continue to increase. In addition, the water quality becomes increasingly impaired: seawater intrusion fairly high electrical conductivity and nitrate concentration; hence the need for optimal exploitation resolutely turned towards rational planning. In this study we will attempt to shed the light on the vulnerability in Mnasra aquifer using different methods: DRASTIC, SI, GOD, and SINTACS to identify the most vulnerable areas where issue of good management is needed. 1. Introduction The vulnerability term designates the degree of disturbance or stress of a system to an eventual risk. This definition refers us to two types of vulnerability: The intrinsic vulnerability designates all the natural factors of a geological order, hydrogeological, and pedologic that characterize the sensitivity with respect to groundwater contamination that are due to anthropogenic activities (Foster, 1987). In addition to the aforementioned factors, the specific vulnerability relativizes the sensitivity of aquifer to a particular contaminant and allows more analysis of its behavior in relation to the configuration of aquifer, and its process of transfer (COST 620, EU action, 2005). Both types of vulnerability highlight the factors such as the recharge from precipitation,, terms of infiltration, soil properties, characteristics of the saturated and unsaturated zones (lithology, fracturing, thickness, porosity, hydraulic conductivity) and topography can intervene, and then be manipulated, mapped and indexed according to various criteria that each method calls to 14

2 use. However, the particularity of specific vulnerability is the highlighting of contamination modalities relative to a specific pollutant in particular conditions: Seawater intrusion, nitrate pollution, biodegradation, germs transport. In this study, we focus on the elaboration of vulnerability maps for Mnasra aquifer using three methods of intrinsic vulnerability to pollution: DRASTIC, GOD, SINTACS and SI method that is specific for agricultural pollutants especially Nitrates. The simple reading can locate areas where the aquifer present a high sensitivity and therefore areas where protection is needed. The validation of vulnerability maps is based on measurements of nitrate levels contained in the aquifer. Finally, the comparative analysis allowed us to determine the most representative methods. and the "Meseta" in southern margin, its geostructural history has been controlled by various factors such as subsidence of the substratum, eustatic changes, sediment supply and regional tectonics (Combe, 1975; Michard, 1976; Wernli, 1987; Aberkan 1987; Morel, 1988; Flinch, 1993; Cirac, 1985; Chalouan & Michard, 2004). The interaction between tectonic subsidence and sea level has changed the shapes of the overall morphology of sediments (Allouza, 2002). The Lithostratigraphy of Gharb basin can be summarized as follows: Miocene: connected to subsidence of the post-tectonic trench, it is characterized by a thick series of gray marl, pyrite, known as "blue marl" which constitutes the basic substratum of the overlying formations. 2. Description of study area Mnasra aquifer belongs to the coastal area of Gharb basin. Geographically, it covers an area of approximately 600 square kilometers, and along 70 km of the coastal strip between Kenitra in the South and Merja Zerga near Moulay Bouselham in the North. In the East, it is limited by Sebou River extended by parallel line to the line passing through Sidi Allal Tazi in the coast and by the Atlantic Ocean in the west. This aquifer is the only resource of drinking water for the population of four rural municipalities: Bahhara Oulad Ayad, Sidi Mohamed Lahmer, Ben Mansour, and Mnasra (Figure. 1) and for irrigation of agricultural areas in the region. 3. Geological and structural context Mnasra aquifer is the extremity of coastal sedimentary of the Gharb basin which is a hinge between the "Rif" in northern margin Fig. 1. Geographic situation of Mnasra aquifer. 15

3 Pliocene: represented by the thin regressive deposits. This is sandy limestone, sands, sandstones and conglomerates. This facies is characterized by a strong accumulation of shellfish tests and includes pyritisation indices. Bousselham to Rabat) is a flat topography, interrupted only by the divagations of Sebou River and former coastal cordons parallel to the shores, and which prolong from the coast to the West (Guilcher 1954; Combe 1975 Aberkan, 1989; Akil, 1990). This sector is regular, monotonous, and constituted by long stretches of straight sand, interrupted by a few outcrops of sandstone dune and the mouths of the Sebou River and Merja Zerga (Moulay Bousselham lagoon). The coastal area is a strip (parallel to the coast) between the totality of alluvial plains and the ocean. It is a complex set of dune systems and interdunal depressions, in which one can distinguish three main homogeneous units from the west to the east: coastal dunes, interior dunes, and alluvial plains. Fig. 2. Geology of Mnasra aquifer. Quaternary: in the coastal area, it is made of marine influence sediment associated with Quaternary transgressions. This is sandy limestone, sandstone, sand and consolidated sands dune ridges, which can exceed 200 m thick. 4. Geomorphology The coastline of Gharb consists of consolidated dunes that give alignments of ridges and furrows parallel to the shore. The coastline of Gharb plain (from Moulay 5. Hydrogeological context The analysis of geological cuts, boreholes and polls (ABHS, 2007) show that the roof of the aquifer corresponds to the topographic surface. The wall of the aquifer is represented by deep levels marl dominance of Mio- Pliocene. Indeed, training conceals a shallow water table, which covers the area of outcrop of sandy sandstone deposits which is fed mainly by rainwater. The piezometric map of Mnasra was established in December 2007 (ABHS, 2009), and is developed entirely on the basis of leveled wells; according to this map, the flows of groundwater diverge from the east side toward the Atlantic Ocean that is a Release boundary. The northern boundary coincides with the Mio-Pliocene outcrops forming a tight limit. Regarding the Gharb plain, the flow direction is E-W. In the south limit the river drain the water table along this stretch. Finally, the piezometric map of 2007 (ABHS, 16

4 Fig. 3. 3D view of Mnasra aquifer. 2009) will be useful for determining the depth of water used for mapping vulnerability in this work. 6. Vulnerability methods Simulation model: is an Analytical/numerical solution of a mathematical model describing the physical process of the contaminants transfer from the soil surface. The observation techniques in field are used to validate and evaluate the convergence of solution, and finally identify the most vulnerable areas in the aquifer (Focazio et al, 2002; Frind et al, 2006). Statistical Methods: tend to predict factors that control the vulnerability of an aquifer to a specific pollutant, ranging from simple descriptive statistics (Welch et al., 2000), or correlation methods (Eckardt and Stackelberg, 1995) to the analysis of more complex conditional probability that highlight the contaminant concentration at the spatial scale (Muller et al., 1997; Greene et al., 2004; Masetti et al., 2009). Mapping methods with index: based on mapping of different parameters affecting a theoretical value for each parameter. Generally, this combination is realized using a geographic information system (GIS). For each mapping methods with index, there are many criteria which determine the vulnerability aquifer, including: Depth of the vadose zone: thickness of vadose zone determines the flow of pollutants from surface to the water table and his eventual degradation during contamination. 17

5 Aquifer type: free, captive or semi captive, the absence or the presence of an impermeable horizon gives to water table the character and degree of disturbance. Aquifer facies: facies variations, thickness of their discontinuity, their position relative to the vertical profile, texture, organic matter content, are all factors that influence the migration toward water table. Recharge: This is the amount of water that seeps into the saturated zone in a time unity; it mainly depends on the effective infiltration and its distribution in space and time. Soil Type: nature, texture, clay content, thickness and vertical permeability of the soil regulate the nature of exchange between the surface and the saturated zone. Hydraulic conductivity: this is a hydrodynamic parameter in determining the velocity of migration of the contaminants toward water table. Depth of water: it depends on the geometry of the aquifer and its hydrodynamic behavior that can change in a hydrological cycle, and between wet and dry periods. Topography: topographic nature is a conditional factor that affects the speed and direction of flow in addition to the nature of training and the hydrodynamic parameters. 7. Materials and methodology 7.1. Intrinsic Methods: GOD, DRASTIC and SINTACS GOD This method of rating system considers the vulnerability aquifer relatively to the vertical percolation of pollutant, and do not consider the lateral migration of pollutant toward the saturated zone (Foster, 1987). It is based on the identification of three parameters: Groundwater occurrence, Overall aquifer, depth of water table. Each parameter is assigned a score reflecting a relative sensitivity to the contamination; the final vulnerability index is calculated using the formula: GOD Index = C l C a C d Where C a : Aquifer type, C l : Lithology of the vadose zone, C d : Depth on the water table. The resulting index is reclassified in four classes ranging from 0.1 to 1 (from low to very high vulnerability). Vulnerability degree Low Medium High Very high Vulnerability index Tab. 1. Criteria for evaluation of vulnerability in the God method (FOSTER, 1987) DRASTIC Developed in Environmental Protection Agency by (Aller and al, 1987) in order to estimate the vulnerability of aquifers based on seven parameters: Depth of water (D), Recharge (R), Aquifer media (A), Soil media (S) Topography (T), Impact of vadose zone (I), and Conductivity (C). It is a method Point Count System Model that estimates relative importance of each parameter through a weight which directly influences the overall vulnerability aquifers. The final vulnerability index is the weighted sum of the following parameters: 18

6 DRASTIC INDEX= D n D w +R n R w + A n A w +S n S w +T n T w + I n I w +C n C w Where D, R, A, S, T, I, C are the above mentioned parameters. n: score for each parameter. p: weighting factor given to each parameter. There are two weighting systems that depend on the conditions of occupation: Normal or standard, and Pesticide where the agricultural activity is intensive (Tab. 2). Parameters DRASTIC standard DRASTIC pesticides D : Depth to water 5 5 R : Recharge 4 4 A : Aquifer media 3 3 S : Soil media 2 5 T : Topography 1 3 I : Impact of vadose zone 5 4 C :Hydraulic Conductivity 3 2 Tab. 2. Assigned Weight to the DRASTIC method's parameters. The resulting maps tend to visualize the relative degree of the vulnerability aquifer based on following criteria: Vulnerability degree Very Low Low Moderate High Very high Vulnerability index < >200 Table.3: Criteria for evaluation of vulnerability in the DRASTIC method (ALLER and al, 1987) SINTACS SINTACS method is suitable version for the hydrogeological diversity in Mediterranean areas (Civita, 1994), it highlights the same parameters of DRASTIC method. The vulnerability index is expressed as: SINTACS INDEX= S n S p +I n I p + N n N p +T n T p +A n A p +C n C p +S n S p Where S: Depth to groundwater, I: effective infiltration, N: unsaturated zone, T: Soil type, A: Aquifer media, C: hydraulic conductivity, S: Topography. n: score for each parameter. p: weighting factor given to each parameter. The weighting system of this method is more flexible than DRASTIC, because it offers a wide variety of scenarios that relativize the vulnerability to the aquifer nature (karst, fissured) or to the intensity of human activities (severe impact, drainage from surficial network). Finally, the resulting index is reclassified in six classes (table.5). 19

7 Parameter Scenario Normal Severe Important Drainage Karst Fissured S I N T A C S Tab. 4. Weights attributed to parameters in the different scenarios of the SINTACS method (Civita, 1994). Vulnerability degree Low Moderate High Very High Vulnerability index < > 210 Tab. 5. Criteria for evaluation of vulnerability in the SINTACS method (Civita, 1994) Specific method: SI (Susceptibility Index) The SI (Susceptibility Index) is a method of the specific vulnerability that takes into account the behavior of agricultural pollutants, especially nitrates. This method highlights the parameters: Depth to water (D), net recharge (R), Aquifer media (A), Topography (T), and Land Use (LU); the last is based on the classification of "Corine Land Cover". According to Ribeiro (2000), the hydraulic conductivity of an aquifer is difficult to assess, although it is indirectly included in Aquifer media' parameter (A). In addition, the lithology of vadose zone and soil type does not have a "great impact" on the transfer of pollutants to the water table (Foster et al., 1987; Vrba and Zoporozec, 1994). The weights assigned to SI parameters vary from 0 to 1, relatively to vulnerability degrees (Tab. 6). Parameters D R A T LU Weights 0,186 0,212 0,259 0,121 0,222 Tab. 6. Weights attributed to parameters in the different scenarios of the SI method (Ribeiro, 2000). The final index of vulnerability is expressed as: SI INDEX= D n D p +R n R p + A n A p +T n T p +LU n LU p Where D, R, A, T, LU are the above mentioned parameters. n: score for each parameter. 20

8 p: weighting factor given to each parameter. The resulting map is classified into four criteria: Vulnerability degree Low Moderate High Very High Vulnerability index < > 85 Tab. 7. Criteria for evaluation of vulnerability in the SI method (RIBEIRO, 2000). 9. Vulnerability maps The works previously performed were useful for the knowledge of geology (Combe, 1975; Michard, 1976; Aberkan 1987), hydrogeology (Kili and all, 2007; DRPE 1994; ABHS, 2007), soils and hydroclimate (Kili and all, 2006; ORMVAG, 1998). The Parameters: hydraulic conductivity and effective recharge have been the result of the hydrodynamic model calibrated on piezometric heads observed into the field, and based on spatial/temporal variability of precipitations (steady and transient); This allowed a better understanding of the hydric behavior of aquifer system according to modeling approach; and evaluate its response to the socio-economic actions: increasing pumping or climate risks. For the weighting system methods: DRASTIC (Aller and al, 1987) and SINTACS (Civita, 1994) the scenarios considered are respectively: "Severe impact" and "Pesticides", because the agricultural activity is very intensive in the aquifer perimeter. To clarify concordance and/or difference between the maps realized, we propose for each method a table gives percentage of area occupied by the different vulnerability degrees. 10. Results and Discussion GOD This method of intrinsic vulnerability of multiplicative index highlights three factors: Groundwater occurrence, Overall aquifer, Depth of water table. The maps in Figure 4 show the partial index of each parameter. Fig. 4. Partial index of GOD parameters. 21

9 According to the below map, we note that the vulnerability is high to very high in the center of aquifer especially in the interior dunes and in north contrary to alluvial plains and coastal dunes where it is not alarming. Degree Percentage of surface occupied (%) Low 2.56 Moderate High Very high Tab. 8. Percentage of surface occupied by the different vulnerability degrees DRASTIC This is a method developed count system, which highlights seven parameters like Depth of water (D) Recharge (R), Aquifer media (A), Soil media (S) Topography (T), Impact of vadose zone (I), Conductivity (C). Each of these has its relative importance compared to the developed scenario. For this study, the agriculture of Mnasra region is quite intensive hence the use of the weighting system "Pesticide". We record a high to very high vulnerability which occupies more than 60% of the water table. It focuses particularly in the center, in north and decreases increasingly towards coastal areas, particularly in south. Fig. 5. GOD vulnerability index SINTACS This mapping is an adapted version of the DRASTIC method in the Mediterranean regions, and takes into account the same parameters and the context of a scenario that puts the vulnerability to the system configuration. In this work we will use the "severe" weighting because the Mnasra region is agricultural. 22

10 Fig. 6. Partial index of DRASTIC parameters Degree Percentage of surface occupied (%) Low 14,14 Moderate 18,85 High 34,22 Very high 33,08 Fig. 7. DRASTIC vulnerability index. Tab. 9. Percentage of surface occupied by the different vulnerability degrees. 23

11 Fig. 8. Partial index of SINTACS parameters Degree Percentage Of Surface Occupied (%) Low Moderate High Very high Fig. 10. SINTACS vulnerability index. Tab. 10. Percentage of surface occupied by the different vulnerability degrees. 29

12 As shown in the above map, the vulnerability is localized mainly in the center, a few regions in the north and in small limited perimeters in alluvial plains. However, the interior dunes are less exposed to pollution SI This intrinsic mapping studies the vulnerability to pollution in relation to agricultural pollution specially: Nitrates which has the distinction of highlighting the land use parameter; it reflects in detail the agricultural or urbanistic vocation in the study area. Based on the map of SI index (figure.11), the aquifer is vulnerable to highly vulnerable in interior dunes, from north toward the alluvial plains in south; in fact, over 70% of the surface of aquifer is vulnerable. In addition, the coastal areas seem less exposed to agricultural pollution. There are four vulnerability degrees : Low Moderate, High and Very High spread over three geomorphological features in the aquifer: coastal dunes, interior dunes, alluvial plains. A first reading of four maps allows seeing a high vulnerability Fig. 9. Partial index of SI parameters. 30

13 concentrated in center of the aquifer: interior dunes. The vulnerability varies between "Low" and "Moderate" in coastal dunes and between "Moderate", "High" and Very High" in alluvial plains. For comparison, the most homogeneous maps are those of DRASTIC and SI since the degree of vulnerability is approximately concentrated in space fairly, there is also a good similarity between SINTACS and DRASTIC despite the difference in southern alluvial plains, however, GOD map gives relatively different results. Degree Percentage of surface occupied (%) Low Moderate High Very high Tab. 11. Percentage of surface occupied by the different vulnerability degrees. According to the tables of vulnerability degree there is a great analogy between DRASTIC, SI and SINTACS for vulnerability class "Very High". There is also an almost complete concordance between the different classes of DRASTIC and SI vulnerability maps. Except, GOD map gives different results. 11. Remark Fig. 11. SI vulnerability index The GOD method is a multiplicative rating system where the overall index is based on fairly limited factors and does not expresses aptly the terms of pollutants migration like point count systems such as: DRASTIC, SINTACS and SI. The latter methods are relatively more representative and more refined. Since they highlight the parameters that inevitably occur at all contamination processes (hydrodynamics, soil and nature of vadose zone), each of which has a degree of importance in relation to the configuration system. In fact this explains this difference with respect to other obtained maps. 31

14 Quality Excellent Good Passable Bad Very Bad NO3 (mg/l) < >100 Concentration degree Very low Low Moderate High Very High Tab. 12. Status grid of groundwater quality assigned to the nitrate concentrations (SEEE, 2011). Water Quality Good Passable Bad Very Bad Percentage of surface (%) Tab. 13. Percentage of surface occupied by the water quality degrees. 12. Results validation Many methods for the vulnerability validation maps: bacteriological analysis, chemical tracers, numerical models. For this study we used a map of nitrate concentration recorded during the field mission (ABHS, 2009); each concentration reflects an approximate water quality according to the status grid (SEEE, 2011) which assigns to a measured concentration one theoretical quality. The monitoring results showed that High or Very High nitrate contents are found in Mnasra groundwater, in the area of interior dunes completely sandy, the levels are generally Very High (> 100 mg / l); in coastal dunes the contents become Medium to High, towards the alluvial plains where concentrations are occasionally High, the quality becomes completely Bad. As shown in the below map, the areas where water quality is Bad or Very Bad conform generally to a High or Very High" vulnerability ", which indeed confirms the obtained results. Conclusion The GOD method expresses the intrinsic vulnerability of multiplicative index that takes into account the limited parameters that do not Fig. 12. Nitrate concentration degree. actually describe the terms of the contamination. Finally, the obtained maps designate most vulnerable areas located mainly in interior dunes and in the north where the water quality is impaired. The methods DRASTIC and SINTACS express the vulnerability to pollution through a scenario on the system configuration and not a particular pollutant such as nitrates, these two indices show the factors that are not 32

15 obvious to identify in field scale (hydraulic conductivity, and the type of soil). As to the SI method, these two parameters are replaced by "Land Use" is more apparent and clearly reflects the urbanistic or agricultural vocation. However, the comparison with field results reveals that in the SI cartography: the areas where the vulnerability is intense correspond to high nitrate concentrations, which explains the representativeness of this method. Also, the mapping SINTACS underestimates the vulnerability in some areas: especially in alluvial plains in the south and near the Sebou river. There is also a good conformity between SI and DRASTIC that remains alarmist in some areas. Finally, among the four methods SI card is more realistic in terms of vulnerability and may be useful to delineate the perimeters of protection of groundwater in Mnasra region. Acknowledgment The authors would like to thank editors Abdelouahed Lagnaoui, Ibrahim Babikir and Hinde Cherkaoui for their constructive reviews. References Aberkan, M., Etude des formations quaternaires des marges du bassin du Gharb (Maroc noroccidental). Thèse es Sciences, Université Bordeaux I, France, 290 Akil, M. (1990). Les dépôts quaternaires littoraux (Méséta côtière marocaine): études géomorphologiques et sédimentologiques. Thèse Doct. d Etat, Univ. Mohammed V, Fac. Sci. Rabat, 417 p. Aller J.R., Bennet, T., Feher J.H., Petty R.J. & Hackett G. (1987). DRASTIC: a standardized system for evaluating groundwater pollution potential using hydrogeological settings, US Environmental Protection Agency, EPA/600/ , 455 p. Allouza, M. (2002). Evolution morphologique et sédimentologique de la frange littorale de la région de Kenitra. Bilan sédimentaire. Mémoire de fin d étude (DESA). Fac. Sc. Univ. Kenitra Ibn Tofail. Option géoscience de l environnement. 102 p. Chalouan, A., Michard, A., The Alpine Rif Belt (Morocco): a case of mountain building in a subduction-subduction-transform fault triple junction. Pure and Applied Geophysics, 161, Cirac, P., Le bassin sud-rifain occidental au Néogène supérieur. Évolution de la dynamique sédimentaire et de la paléogéographie au cours d une phase de comblement. Thèse es Sciences, Université de Bordeaux I, France, 283. Civita M. & De Malo M. (1994). Mapping groundwater vulnerability by the point count system model SINTACS, In Managing Hydrogeological Disasters in a vulnerable Environment (IHP Unesco), GNDCI 1900, pp Civita M. & De Malo M. (1998). Mapping groundwater vulnerability by the point count system model SINTACS, In Managing Hydrogeological Disasters in a vulnerable Environment (IHP Unesco), GNDCI 1900, pp Combe, M., Le bassin Gharb-Maamora et les petits bassins septentrionaux des oueds Dradère et Souieire. Ressources en eau du Maroc, Tome 2, plaines et bassin du Maroc Atlantique. Notes et Mem. Serv. Géol. Maroc, 231, COST-Action 620 (2005). Vulnerability and Risk Mapping for the Protection of Carbonate (Karst) Aquifers. Final report. EUR, European Commission, Bruxelles. DRPE, Direction de la Recherche et de la Planification des Eaux Etude de modélisation de la nappe côtière du Gharb (Région d El Mnasra). Ministère des Travaux Publics de la Formation Professionnelle et de la Formation des Cadres, Administration de l Hydraulique, Rabat, Maroc. Rapport interne. Eckardt D.A., and Stackelberg P.E., Relation of groundwater quality to land use on Long Island, New York. Ground Water, vol. 33: pp

16 Flinch, J. F., Vail, P. R., Plio-Pleistocene sequence stratigraphy and tectonics of the Gibraltar arc,. Mesozoic and Cenozoic Sequence Stratigraphy of European Basins, SEMP Spec. Publ., 60, Focazio M.J., Reilly T.E., Rupert M.G., and Helsel D.R., Assessing Ground-Water Vulnerability to Contamination: Providing Scientifically Defensible Information for Decision Makers. U.S. Geological Survey Circular ISBN: Foster S.D.D., (1987). Fundamental concepts in aquifer vulnerability, pollution risk and protection strategy, Vulnerability of soil and groundwater to pollutants ed. Processing and information committee for hydrological research, pp Frind E.O., Molson J.W., and Rudolph D.L., Ground Water, vol. 44, no. 5, pp doi: /j x. Gogu R. & Dassargues A. (2000). Sensitivity analysis for the EPIK vulnerability assessment in a local karstic aquifer, Hydrogeology Journal, v. 8, n 3, pp Greene E.A., LaMotte A.E., and Cullinan K.A., Ground-water vulnerability to nitrate contamination at multiple thresholds in the Mid- Atlantic Region using spatial probability models. US Geological Survey Scientific Investigations Report , pp. 24. Guilcher, A. (1954). Morphologie littorale et sousmarine. Paris- Presse Univ.Fr., Coll. «Orbis», 216 p. Kili M., El Mansouri, B., Chao, J., Ait Fora, A., De nouveaux éléments structuraux du complexeaquifère profond du bassin du Rharb (Maroc): implications hydrogéologiques. C. R. Geoscience, 338, Killi, M., El Mansouri, B., Chao, J., et Ait Fora, A. (2007). Bilan hydrique des sols et recharge de la nappe profonde de la plaine du Gharb (Maroc). Article scientifique, Sécheresse 2008 ; vol. 19, n 2, Masetti M., Sterlacchini S., Ballabio C., Sorichetta A., and Poli S., Influence of threshold value in the use of statistical methods for groundwater vulnerability assessment. Science of the Total Environment, vol. 407, pp doi: /j.scitotenv Morel, J. L., Évolution récente del orogène rifaine et de son avant-pays depuis la fin de la mise en place des nappes, Mém. Géodifusion, Paris, France, 4, 584. Mueller D.K., Ruddy B.C., and Battaglin W.A.,1997. Logistic model of nitrate in streams of the upper-midwestern United States. Journal of Environmental Quality, vol. 26, no. 5, pp ORMVAG, Office Régional de Mise en Valeur Agricole du Gharb Aménagement hydro-agricole de la troisième tranche d irrigation de la plaine du Gharb, zone Mnasra (zone côtière) Maroc. Rapport interne, Kénitra, Maroc, 452. Ribeiro L (2000) SI : new index of aquifer susceptibility to agricultural pollution]. Internal report, ER-SHA/CVRM, Instituto Superior Tcnico, Lisbon, Portugal, 12 pp. septentrional et description systématique des Foraminifères plancto-niques. Notes et Mem. Serv. Géol. Maroc, 331, 265. Welch A.H., Westjohn D.B., Helsel D.R., and Wanty R.B., Arsenic in ground water of the United States Occurrence and geochemistry. Ground Water, vol. 38, no. 4, pp. 589 Wernli, R., Micropaléontologie du Néogène post-nappes du Maroc. 34