Hydrogeochemical Characterization of the Dissolved Load of the Major Elements Downstream of the Watershed of the Wadi Sebou, Morocco

The aim of this paper was to analyse the dissolved elements in water which were sampled from the Sebou River in Northern Morocco. The sampling was conducted at the Mechra Bel station for two different hydrological cycles (2013/2013 and 2013/2014). The study was to determine the type of water facies present in the area. The piper diagram shows that the dominant water facies in the area are that of Nacl water facies, which contribute to 56% of the total amount of major elements in the water. The study also revealed that during high water levels, there is dilution of water and during low water levels, the concentration of elements increases. The presence of halite and gypsum bearing rocks in the study area gives an indication that the supply of Na and Cl is related to the dissolution of evaporates. The results also revealed that the specific export of the dissolved load amounts to 127.6 t/km/year. Finally, the quantification of the dissolved load, through the determination of dissolved material flows shows that the specific export of material amounts to 127.6 t/km/year.

rainfall (Corbonnois, 1996) [6], flows (Dupraz, 1984;Meybeck, 1985) [7] [8], as well as by the conditions of surface, sub-surface or subterranean water flow (Kattan, 1989;Bakalowicz, 1995) [9] [10]. The problem of the chemical composition of the various rivers and the factors of its variation has always been of interest to hydrologists and geochemists working on the rivers, in particular to quantify the importance of chemical erosion and the loss of materials that are evacuated each year in soluble form from the continent to the oceans. The low intensity chemical weathering phenomena in semi-arid climates are inherent in the very strong seasonal contrast of precipitation between the dry and wet periods. It is therefore interesting to be able to highlight the effects of climatic variations on chemical weathering processes and to assess whether there is an impact related to the construction of dams on water quality. The Sebou watershed, subject to strong anthropic pressure linked to the hydraulic installations that are constantly increasing under the effect of climate change, is a good example for the monitoring of climatic and anthropogenic influences on the geochemical quality of the solid and dissolved flux.
In fact, this Sebou watershed has already been the subject of studies concerning particulate and dissolved transport as well as the reports of mechanical and chemical erosion by Haida (2000) [11], and the resumption of dissolved flux will allow a better appreciation of effects caused by the construction of the Al Wahda dam on Ouerrha one of its main tributaries and climatic variations on the quality of surface waters.
Wadi Sebou is also a source of water for domestic, agricultural and industrial use for the populations of the basin and it is subject to contamination of different orders (ABHS, 2011) [12]. Water pollution has reached critical levels and that affects the economic and social development of the basin (ABHS, 2011).

Area of Study
The Sebou watershed (  main lithological formations whose areas are very important or whose characteristics will play a major role in the erosion process are: -The limestone and dolomitic formations of the Lias, affected by an intense karst erosion which characterizes the upstream part of the basin (Middle Atlas). -The basaltic formations resulting from intense volcanic activity during the Quaternary that appear in the Middle Atlas. -Evaporitic formations corresponding to the salifera and gypsum Triassic diapirs pierce the cover in the Prérif especially at Jbel Tissa and Jorf El Melh.
-In the Gharb plain, alluvia consist of thick Miocene-rich clay-sandy fluviomarine sediments, covered with fine deposits from the Sebou and its tributaries during the Quaternary period (Combe, 1969(Combe, , 1975

Sampling and Analysis
The basic data used in this study (Table 1)

Demonstration of the Mineralization
The dissolution of a mineral in water can be described by a state of equilibrium: when the water is in contact with a mineral, the concentration of the solution increases to reach a maximum for some given physicochemical conditions.
It is said that the solution is saturated with this mineral.

Concentration or Dilution Factor
The factor of concentration or dilution of ions [rN] between two hydrological periods makes it possible to highlight the geochemical behavior of these ions during hydrological cycles. This factor corresponds to the concentration ratio   [26].
In the case of our study the calculations were made using the stochastic method, this method permits the calculation for each period (i), the average of the concentrations weighted by flow rates (C im ), then the flow of dissolved matter which is the product of this concentration (C i ) by the volume of water (Q i ) obtained by integration of the flows for the period (i).

Chemical Facies of Water
Sebou waters at the Mechra Bel Ksiri station are highly mineralized. The con-    The comparison between the curves representing the flow-concentration relationship (solid line) and the theoretical dilution curve (dashed lines) reveals that during periods of low water (low water), there is a high concentration of chemical elements, and that as the flow rates increases the concentrations decreases but remain higher than the values corresponding to the theoretical dilution curve. The concentrations for the theoretical dilution curve tends to approach zero, while the concentrations for the flow-concentration curves of the different chemical elements of the river water decreases but deviates significantly and remains above the theoretical dilution curve. The comparison of the theoretical dilution curves and the flow-concentration curves reveals the contribution of two sources of inputs of chemical elements: groundwater during periods of low water and runoff water during high-water periods. The Na + and Cl − and K + elements are well connected to the flows, they have a very marked deviation of the concentrations during the high-water period.

Origin of Dissolved Elements
The complex interactions between the lithosphere, the atmosphere, the hydros-

Demonstration of the Mineralization of the Waters
On the basis of the thermodynamic equilibrium, it is possible to define a chemical evolution of Sebou water in Mechra Bel Ksiri; as a result the trend of the chemical forms between the dissolved phase and the mineral phase was realized.
The dissolution of a mineral in water can be described by a state of equilibrium. When the water is in contact with a mineral, the concentration of the solution increases to reach a maximum for given physicochemical conditions. It is said that the solution is saturated with this mineral.
The saturation index expresses the degree of chemical equilibrium between water and the mineral in the rock and can be considered as a measure of the dissolution and/or precipitation process concerning the water-rock interaction.  CaSO Ca SO Calcite: CaCO CO HO O Ca 2HCO Dolomite:     HCO − during the low-water period can be attributed to a precipitation of calcium carbonates in the alluvial plain in accordance with the process called "Ca 2+ dynamics" by soil scientists in vertisols (Bryssine, 1968) [31]. Cl − and Na + remain diluted for any hydrological period.

Flow of Major Elements
The flow of suspended matter from a watercourse is defined as the amount of SS mass transiting through this section for one unit of time (Vanoni, 1975;Droux et al., 2003) [32] [33]. Indeed the surface of the continents is continually attacked by rainwater through two major and competitive geodynamic processes.
Chemical alteration partially dissolves continental rocks to form soils, then these soils and their "mother rocks" are physically attacked by mechanical erosion.
The products that result from these erosion and weathering processes are carried by the rivers and are the main stream of materials that feed the oceans.  Table 5 shows that the quantity of chloride exported to Mechra Bel Ksiri during the hydrological cycles is on average 101, 4.10 4 tonnes, is 30% of total transport in solution. The tonnages of bicarbonates, sodium and sulphates are also high, accounting respectively for 23%, 19% and 14% of total transport. The four elements Cl − , 3 HCO − , Na + and 2 4 SO − are thus responsible for 86% of the transport in solution downstream from Sebou to Mechra Bel Ksiri. The calcium flux represents 10% of the total dissolved intake.
The comparison of the dissolved specific transport at Mechra Bel ksiri between the 1996/1998 (Haida, 2000) and 2012/2014 (Table 6) hydrological cycles shows a 23.6% decrease in dissolved transport downstream of the Sebou basin.
The specific tonnage increases from 167 t/km 2 /year to 127.6 t/km 2 /year. In general, these contributions in solution are mainly controlled by natural factors: lithology, relief and climate (Meybeck, 1984;Bluth and Kump, 1994)

Conclusions
The chemical composition of the waters downstream of Sebou shows very high dissolved loads ranging between 400 mg/l and 1400 mg/l, with predominant proportions of chlorides and sodium (52%) making the waters of the wadi sodium-chloride facies, followed by calcium (18%), bicarbonates (13%) and sulphates (9%).
The evolution of the concentrations of the chemical elements measured during the hydrological cycles 2012/2013 and 2013/2014 showed a decrease in concentrations during the high-water period and the opposite during the period of low water, with a marked decrease in period of high water for Na + , Cl − and K + .
This behavior is attributed to a dilution of river waters more concentrated in chemical elements by much less concentrated meteoric waters. The HCO − from 4.5 and 3.5 to below the two elements, while the Na + and Cl − remained diluted regardless of the hydrological period.
The annual tonnage of materials exported during the study period shows that the total mass of dissolved elements exported by the waters downstream of Sebou to Mechra Bel Ksiri is dominated by 53% chloride and sodium, while 82% of the total material transported to the oceans is during periods of high water. The analysis of the fluxes calculated for the different elements can lead us to say that the main origin of chloride and sodium is the leaching of soils. This high tonnage of chloride and sodium can be attributed to the lithology which is characterized by an abundance of evaporitic and carbonate rocks with the influence of irregular hydrological regimes.