State of Nitric Pollution of the Mnasra Aquifer, Coastal Zone of the Gharb Plain (Morocco)
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State of nitric pollution of the Mnasra aquifer, coastal zone of the Gharb plain (Morocco) Nadia Aziane1, Majdouline Larif2, Achraf Khaddari3, Mohamed Ebn touhami1 , Abdelmjid Zouahri4, Hakima Nassali1, Mohamed Salahdine Elyoubi1 1Ibn Tofail University, Faculty of Sciences, Department of Chemistry, Materials Engineering and Environment Laboratory: Modelling and Application, Kenitra, P. O. Box 133, Morocco 2Ibn Tofail University, Faculty of Sciences, Department of Chemistry, Separation Processes Laboratory, Kenitra, Morocco 3 Ibn Tofail University, Faculty of Sciences, Department of Geology, Kenitra, P. O. Box 133, Morocco 4Institut National de La Recherche Agronomique, CRRAR, Département de l'Environnement et la Conservation des Ressources Naturelles- Rabat, B.P. 6202, Morocco Corresponding author:[email protected] Summary Nitrates are highly soluble and highly mobile chemical elements in water. Their presence in groundwater with high concentrations frequently indicates anthropogenic contamination. They come mainly from the massive use of nitrogen fertilizers or unpurified urban waste. A physicochemical characterization of the nitrate and ammonium contents of the waters of the Mnasra aquifer was carried out following the sampling of 70 wells and boreholes during the months of June and July 2017. The results show high concentrations of nitrates, 82.85% of the wells taken will exceed the potability standard (50 mg / l). This is confirmed by the coarse nature and sandy soil texture that can promote nitrate leaching, the low water table in this region (70% of wells with depths less than 20 m), and the massive use of soil. fertilizers and fertilizers. The analysis of temporal trends in nitrate concentrations observed over 23 years showed a significant degradation of the water quality of wells between 1993 and 2017 with excessive levels reaching five to six times the potability limit. Key words: Nitrate, pollution, groundwater, groundwater, Mnasra, Morocco. I. Introduction Nitrate is the major source of nitrogen in soils [1]. In fact, the clay-humic complex of the soil, which is also negatively charged, does not retain the nitrate, a negatively charged ion. Thus, very soluble, the nitrate brought in surplus in the soil, is driven by leaching and contributes to the pollution of the water tables. Moreover, in the groundwater, the high concentrations of NO3- would be directly related to human activities and more particularly to agricultural activities [2-6]. In addition, several factors influence the mobility of nitrates through the soil leading to groundwater pollution. These factors could be subdivided into intrinsic factors such as the level of the free surface of the groundwater relative to the ground, the depth of the groundwater, the permeability of the soil, its texture and clay content, the direction of flow of the aquifer, and the organic matter and nitrogen content in soils, and dynamic factors that include the processes that govern soil moisture regime and the biogeochemical processes of nitrate transformation and transfer in the unsaturated soil layer. It should be noted that nitrates often participate in diffuse pollutions, which are difficult to identify and therefore to control, because they concern large spaces with imprecise contours and are generated by various practices that mainly include the use of fertilizers and pesticides [7], which interfere in a complex way with the soil and meteorological hazards [8-11]. The aquifer of the M'nasra region, a geographical area of our study, is a very productive and easily accessible aquifer, and is intensively exploited for irrigation by private pumping [12]. The number of wells drilled in the aquifer, exceeding 20 000, shows its agricultural importance, and the high permeability of the soil and subsoil and the shallow depth of the aquifer are highly vulnerable to diffuse agricultural pollution (N-NO3). This has led to an alarming situation that is characterized by the presence of nitrate nitrogen, in high quantities in the soil and its entrainment by draining irrigation [13]. Note that the toxic effects of nitrate ions, on human health and the environment, are mainly methemoglobinemia, the carcinogenicity caused by nitrosamines, about 80% of nitrosamines and almost all nitrosamides studied were carcinogenic in several animal species [14] and eutrophication of aquatic environments [15-17]. II.Material and methods 1. Presentation of the study area The M'nasra region is the most agricultural area of the Gharb Plain forming geographic continuity along the Atlantic Ocean. It extends over an area of 488 km2 between the city of Kenitra in the South, the Oued Sebou extended by the parallel line passing by Sidi Allal Tazi in the East, the Merja Zerga near Moulay Bouselham in the North. The study area is an integral part of the Gharb Subsidiary Sedimentary Basin. This one is located in the zone of contact between the two large structural units of Morocco: the primary coastal meseta plunges progressively on the south towards the north and the Domain rifain which belongs to the alpine domain circum mediterranean. The geological structure is formed at the base by a primary base, consisting of schists and quartzites, located more than 2500 m deep in the center of the plain. A powerful marly series of up to 2000 m settles discordance on primary formations. This series of '' blue marls '' is the impervious floor of the Gharb aquifer system. From the Pliocene, sedimentation is regressive, consisting of lumachels, sandstones and conglomerates. On these formations are deposited on 200 m thick, Pliocene sediments of marine origin. They are located in the coastal zone and are formed of sand, sandstone and limestone sandstone, in which are interspersed with thin clay-limonous levels. These formations constitute an aquifer reservoir of prime importance (Figure 1). Figure 1: NW-SE geological section of the Gharb coastal zone [18] The aquifer system of the coastal zone of Gharb can be presented by the succession of two different layers: a sandy-sandstone superficial layer which has a thickness of between 5 and 10 m in the inner dunes, and between 20 and 30 m in the berm dune. Within this layer, we find the water table characterized by a shallow depth in the interior dunes (2 to 10 m), and a fairly large depth in the berm (10 to 40 m). The second layer which contains the deep sheet is very thick compared to the first, and it consists essentially of sandstone. Hydraulic communication between these two levels is achieved through a red clay-sandy screen, whose thickness varies from 10 to 20 m [19]. It should also be noted that the pedology of the region is occupied by heavy soils (vertisols and fluvisols). The sandy soils of the coastal zone cover about 39 000 ha or 15% [20]. The climate is Mediterranean with oceanic influence. It receives annually average rainfall, about 551 mm. The rainfall period extends from October to the end of April, with a maximum for November, December, and January. Temperatures range from 12 °C during the winter to 23°C during the summer. Potential evaporation exceeds 150 mm during the dry months from June to September and is less than 80 mm in the months of December to February. 2. Sampling and analysis In order to help identify the problem of nitrate pollution of the water table of M'nasra, the waters of 63 and 7 boreholes. The set of sampling points covers the studied area in order to inquire about the situation of the quality of its water table (Figure 2). The choice of these wells was essentially based on two criteria: the intensification of the crops and the depth of the aquifer. For sampling, water is taken directly from the irrigation network for wells that are equipped with discharge pumps. For those who are not equipped, the sampling is done using of metal ballast. All the samples were taken in bottles made of polyethylene or borosilicate glass. The samples were carefully labeled and stored at +4 °C, until they arrived at the laboratory according to Afnor standards or cited by Rodier (1984) [21]. The soils for the 70 sampling points were selected at depths of 20 cm from the soil surface using an auger and stored in plastic bags, the soil samples were analyzed after adequate treatment. The nitrates, ammonium ions and chlorides in the groundwater of the Mnasra region were determined by titrimetry and the sulphates were carried out by spectrophotometry. The soil particle size is determined using Robinson pipette, after destruction of organic matter with hydrogen peroxide, total limestone with 1N sodium acetate, dispersion with sodium hexametphosphate and the organic matter of the soil is determined by the Walkley-Black method, which consists of a cold oxidation of the organic fraction of the carbon by potassium dichromate (K2Cr2O7 at 1N) in acidic medium and a return titration by Mohr salt. Figure 2: Location of wells and boreholes studied in the Mnasra area III. Results and discussion 1. Current situation of nitric pollution of Mnasra International standards stipulate that the nitrate content of drinking water must not exceed the concentration of 50 mg / L [22]. The nitrate and ammonium values found in the waters of the groundwater of the study area (M'nasra) taken in the period June/July of 2017. A classification of these contents wars established according to the content of the water in nitrates and ammonium summarized in table 1. Table 1: Simplified grid for assessing the overall quality of groundwater [23] Nitrates (mg/l) Percentage of Ammonium Percentage of analyzed wells (mg/l) analyzed wells Excellent ˂5 0 ˂ 0 ,1 0 Good 5-25 7.15 0,1-0,5 0 Average 25-50 11.42 0,5-2 2,85 Bad 50-100 32.85 2-8 55,71 Very bad >100 48.58 > 8 41,44 The histogram of nitrate contents (Figure 3) shows maximum content observed at the P23 wells with 322.4 mg / l and the minimum content is observed at the F4 drill with 12.4 mg / l.