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 2/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 2/year.
Rivers play an important role in the transportation of dissolved and solid materials from the mechanical and chemical erosion of continental rocks to the oceans (Livingstone, 1963; Meybeck, 1976; Berner and Berner, 1996) [
These dissolved elements carried by the continental waters can serve as tracers to apprehend the functioning of the fluvial systems. They are influenced by rainfall (Corbonnois, 1996) [
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) [
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) [
The Sebou watershed (
The Sebou watershed has a large lithological diversity at the outcrop. The
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.
- In the South Rifain strait, there are marly and sandstone formations of Cretaceous and Tertiary age overlying Miocene limestone formations.
- 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, 1975; ORMVAG, 1997) [
The average annual rainfall in the Sebou watershed is about 640 mm. It decreases away from the sea towards the Beht valleys and on the upper Sebou with an average of 400 to 550 mm, and increases rapidly on the slopes of the Rif reaching a maximum of 1000 to 1500 mm/year.
The Oued Sebou has a total length of 614 km from its source, it collects from its upstream the waters of the Rifaine (Oued Ouerrha) and those of the ridges (Oued Guigou, Oued Zlouh, Oued Mikkés), and Oued Inaouen which comes from the region of Taza where it borders the middle-atlasic and pre-Rifean regions. After crossing the Pre-Rifean hills, the Sebou opens in the plain of Gharb, where it receives the Oued Beht and Oued R’dom to the South. It joins the Atlantic near Kenitra to Mehdia. Its average flow is 137 m3/s. In winter, the highest average is that of February (350 m3/s), and the peak flow can reach 6000 m3/s during periods of heavy floods.
The basic data used in this study (
The analysis of SO 4 2 − , Cl− is done by ion chromatography, the major cations Ca2+, Mg2+, Na+, K+ by atomic absorption. Since the alkalinity is not measured
Reference | Dates | Q (m3/s) | Ca2+ | Mg2+ | Na− | K+ | Cl− | SO 4 2 − | HCO 3 − | Water level (m) |
---|---|---|---|---|---|---|---|---|---|---|
ICP1 | 10/07/2012 | 41.3 | 2.766 | 1.643 | 4.565 | 0.151 | 4.400 | 1.634 | 2.342 | 172 |
ICP2 | 20/09/2012 | 36 | 2.522 | 1.572 | 3.900 | 0.129 | 3.641 | 1.443 | 2.197 | 158 |
ICP3 | 16/10/2012 | 6 | 5.265 | 3.299 | 14.921 | 0.429 | 14.878 | 3.126 | 4.758 | 60 |
ICP4 | 13/11/2012 | 34.8 | 4.504 | 2.575 | 9.150 | 0.213 | 10.162 | 2.225 | 3.710 | 155 |
ICP5 | 04/12/2012 | 196.5 | 3.840 | 1.457 | 6.551 | 0.155 | 7.517 | 2.364 | 2.291 | 458 |
ICP6 | 14/01/2013 | 18.5 | 3.258 | 1.997 | 6.650 | 0.187 | 5.342 | 1.960 | 3.410 | 108 |
ICP7 | 14/02/2013 | 262.5 | 2.713 | 1.310 | 3.268 | 0.058 | 3.511 | 1.414 | 1.892 | 533 |
ICP8 | 05/03/2013 | 356 | 2.422 | 1.317 | 4.707 | 0.150 | 4.627 | 1.498 | 1.847 | 626 |
ICP9 | 26/03/2013 | 620 | 2.560 | 1.121 | 5.221 | 0.127 | 4.333 | 1.429 | 2.180 | 890 |
ICP10 | 16/04/2013 | 309.6 | 2.927 | 1.885 | 4.369 | 0.125 | 5.672 | 1.492 | 1.960 | 580 |
ICP11 | 14/05/2013 | 109.8 | 2.908 | 1.405 | 4.511 | 0.162 | 3.529 | 1.533 | 2.832 | 327 |
ICP12 | 04/06/2013 | 83.2 | 3.179 | 1.341 | 4.650 | 0.160 | 4.081 | 1.606 | 2.952 | 273 |
ICP13 | 18/06/2013 | 76.1 | 3.410 | 1.656 | 4.152 | 0.121 | 4.658 | 2.127 | 2.450 | 257 |
ICP14 | 16/07/2013 | 54.5 | 3.267 | 1.474 | 3.294 | 0.104 | 3.880 | 1.800 | 2.220 | 205 |
ICP15 | 13/08/2013 | 48.1 | 3.294 | 1.715 | 3.631 | 0.104 | 4.264 | 2.011 | 2.361 | 189 |
ICP16 | 03/09/2013 | 37.9 | 3.597 | 2.078 | 7.775 | 0.200 | 8.570 | 2.431 | 2.399 | 163 |
ICP17 | 24/09/2013 | 28.7 | 4.736 | 2.292 | 10.116 | 0.216 | 10.765 | 2.927 | 3.486 | 138 |
ICP18 | 08/10/2013 | 13.1 | 6.095 | 3.552 | 16.455 | 0.370 | 17.862 | 3.894 | 4.666 | 90 |
ICP19 | 29/10/2013 | 16.6 | 3.872 | 1.797 | 2.153 | 0.232 | 2.963 | 1.768 | 2.166 | 102 |
ICP20 | 12/11/2013 | 92.6 | 3.666 | 1.272 | 5.387 | 0.185 | 6.220 | 1.897 | 2.179 | 293 |
ICP21 | 26/11/2013 | 20.7 | 4.631 | 1.650 | 6.823 | 0.161 | 7.509 | 2.434 | 2.783 | 115 |
ICP22 | 10/12/2013 | 108.3 | 4.639 | 1.642 | 6.006 | 0.206 | 6.442 | 1.868 | 2.958 | 324 |
ICP23 | 19/12/2013 | 205.5 | 4.052 | 1.580 | 6.464 | 0.114 | 6.497 | 2.552 | 2.949 | 469 |
ICP24 | 07/01/2014 | 57.5 | 4.916 | 2.332 | 11.666 | 0.354 | 12.208 | 2.741 | 3.687 | 213 |
ICP25 | 28/01/2014 | 430 | 4.393 | 1.243 | 6.839 | 0.124 | 6.619 | 2.196 | 2.620 | 689 |
ICP26 | 11/02/2014 | 330 | 3.804 | 1.326 | 4.726 | 0.134 | 4.548 | 1.836 | 2.817 | 600 |
ICP27 | 25/02/2014 | 251.1 | 3.535 | 1.414 | 4.696 | 0.106 | 4.864 | 1.897 | 2.511 | 521 |
ICP28 | 11/03/2014 | 348 | 3.492 | 1.453 | 4.562 | 0.113 | 4.608 | 1.929 | 2.539 | 618 |
ICP29 | 25/03/2014 | 535 | 3.519 | 1.252 | 3.428 | 0.086 | 3.584 | 1.566 | 2.441 | 805 |
ICP30 | 08/04/2014 | 510 | 4.093 | 1.424 | 5.288 | 0.115 | 5.384 | 1.853 | 2.907 | 780 |
ICP31 | 29/04/2014 | 333 | 3.407 | 1.313 | 3.511 | 0.082 | 4.117 | 1.785 | 2.177 | 603 |
ICP32 | 13/05/2014 | 153.7 | 3.237 | 1.524 | 4.014 | 0.117 | 4.546 | 1.909 | 2.237 | 401 |
ICP33 | 27/05/2014 | 110.4 | 3.366 | 1.566 | 4.473 | 0.148 | 5.095 | 1.861 | 2.301 | 328 |
ICP34 | 10/06/2014 | 80.5 | 3.714 | 1.497 | 3.237 | 0.087 | 3.814 | 1.765 | 2.534 | 267 |
ICP35 | 24/06/2014 | 69.6 | 3.601 | 1.663 | 3.972 | 0.122 | 4.822 | 2.081 | 2.418 | 242 |
ICP36 | 08/07/2014 | 45.7 | 3.889 | 1.882 | 4.477 | 0.144 | 4.540 | 1.939 | 3.139 | 183 |
in the laboratory, we have estimated it from cations and anions, based on the principle of electroneutrality and assuming that there has been no anionic or cationic deficiency, that is to say that the main anions and cations were dosed (Σ Cations meq/L = Σ Anions meq/L). The validity of the chemical analysis was carried out by the calculation of the ionic balance. The errors of the analysis when the concentrations are measured are lower than ±5%.
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. The saturation state is expressed by the saturation index (CIDU et al., 2009) [
I s = log ( K ) − log (PI)
with K as the constant of the equilibrium and PI the ionic product.
Three cases are possible:
Is < 0 means under saturation of a solution with respect to a mineral;
Is = 0 indicates a state of saturation;
Is > 0 demonstrates a supersaturation.
In our study, the saturation indices for the minerals (halite, gypsum, sylvite, calcite, dolomite and aragonite) and the chemical equilibria were calculated using the PhreeqC program (PARKHURST et al., 1999) [
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 [N] of the chemical element N (meq/l) in flood and in water-weighted by the total dissolved charge (TD).
Thus: rN = ([N]/[TD]) raw/([N]/[TD]) low water
If rN < 1 there is a dilution in flood
If rN > 1 we have a concentration in flood
To monitor the temporal variation in concentrations and flows, we will try to compare the results obtained for the period of 2012-2014 with the results of monitoring carried out by HAIDA (2000) for the period of 1996-1998. Since changes in the concentration of dissolved elements in river waters are mainly related to fluctuations in the flow of these rivers (MANCZAK and FLORCZYK, 1971, PROBST, 1983, KATTAN, 1989) [
The flow is expressed as follows: Φ i = C i × Q i
Two methods exist to perform the calculation of the flow of suspended matter, stochastic methods based on the use of means and deterministic methods using flow-concentration relationships. In fact, the quantitative and qualitative studies of these inputs make it possible to calculate the denudation rates of continental rocks (Meybeck, 1979; Stallard and Edmond, 1983; Probst, 1990; Amiotte-Suchet, 1995) [
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 (Cim), then the flow of dissolved matter which is the product of this concentration (Ci) by the volume of water (Qi) obtained by integration of the flows for the period (i).
C i m = 1 / ∑ i = 1 n ( Q i ) × ∑ i = 1 n ( C i ) × (Qi)
Φ i = ∑ i = 1 n ( C i m ) × ∫ t 1 t 2 Q d t
The Sebou watershed is controlled downstream by the Mechra Bel Ksiri station, which is located after the confluence of the Wadi Ouerrha (its main tributary) and the middle course of Sebou. At this station, the water coming from upstream of Sebou is managed by the Idriss 1ier dam impounded in 1973, and the waters which come from Ouerrha which are governed by the Al Wahda dam impounded in 1996. The Mechra Bel Ksiri station is located upstream of the Gharb flood plain, 90% of whose flood problems are caused by inputs from Wadi Ouerrha.
Before 1973, the date of impoundment of the Idriss 1ier, the average liquid inputs from the Sebou to the Mechra Bel Ksiri station were approximately 5.109 m3/year. These contributions are slightly different from the combined contributions from Azib Es Soltane station (Middle Sebou) and that of M’jara (Ouerrha), which amount to 5.48 109 m3/year. The difference between these two values can be attributed to water losses between upstream and downstream of the basin and more particularly to the phenomenon of overflow of water during floods. From 1973, the contributions from the Middle Sebou lowered by 55% (effect of the dam Idriss 1ier), only the Ouerrha continues to bring water volumes little different from the previous period (2.03 109 m3/year) which accounts for nearly 70% of the inflows arriving at Mechra Bel Ksiri. With the commissioning of the Al Wahda dam, the reduction of the liquid inputs of oued Ouerrha can be estimated for the year 1996/1997 to about 60%, thus in November 1996 about 90% of the volume of water that overflows in the plain could be avoided despite heavy rainfall experienced in this plain in December 1996.
Thus, Bahin’s (2017) [
Sebou waters at the Mechra Bel Ksiri station are highly mineralized. The conductivity varies between 712 μS/cm and 2150 μS/cm, with an average of 980 μS/cm. It is high during periods of low water and gradually decreases during floods. This high conductivity shows the important dissolution or hydrolysis of the various minerals that come from the rocks sources of the basin. The temperature of the water varies between 11.5˚C and 18.5˚C during the high-water period, and between 20.5˚C and 28.5˚C during the low water period, with an annual average of 18˚C. The measured pH is 7.45 in January (flood period) and 8.89 in July (low water period) and indicates the basic character of Sebou waters in Mechra Bel Ksiri throughout the year.
The proportions of anions and cations in Sebou water in Mechra Bel Ksiri (
Indeed, the order of abundance of the major elements in Sebou waters at Mechra Bel Ksiri is not affected by seasonal or annual variations in flows. It depends mainly on the contribution of the evaporitic and carbonate formations of the basin and the importance of the drainage of the superficial aquifer by the watercourse.
The projection of surface water analysis from Sebou to Mechra Bel Ksiri (
An examination of the flow-concentration relationship (
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.
The complex interactions between the lithosphere, the atmosphere, the hydrosphere and the biosphere constitute the factors governing chemical weathering (Chetsworth, 1992) [
The chemical composition of suspended matter carried by river waters reflects the lithological nature of the rocks and soils of the drainage catchments. The correlations between the different chemical elements (
Thus, from the correlations (
The correlation between Sodium and chloride (
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. Saturation indices for minerals (halite, gypsum, sylvite, calcite, dolomite and aragonite) and chemical equilibria were calculated using the PhreeqC program, which allows chemical alteration simulations much closer to natural conditions (PARKHURST et al., 1999).
Halite: NaCl = Na + + Cl − (1) |
---|
Sylvite: KCl = K + + Cl − (2) |
Gypsum: CaSO 4 = Ca 2 + + SO 4 2 − (3) |
Calcite: CaCO 3 + CO 2 + HO 2 O = Ca 2 + + 2 HCO 3 − (4) |
Dolomite: CaMgCO 3 + CO 2 + H 2 O = Ca 2 + + Mg 2 + + 4 HCO 3 − (5) |
The average values of the water saturation indices in relation to the minerals at Mechra Bel Ksiri (
On the other hand, there is rather under saturation of the water vis-à-vis the minerals of halite, gypsum and sylvite, their indices of saturation vary between −8 and −1. These minerals are dissolved and strongly contribute to the supply of Na and Cl in the water.
Parameter | Average | Minimum | Maximum | standard deviation |
---|---|---|---|---|
Aragonite | 0.43 | −0.16 | 1.18 | 0.37 |
Calcite | 0.58 | −0.01 | 1.32 | 0.36 |
Dolomite | 0.85 | −0.37 | 2.47 | 0.83 |
Gypsium | −1.70 | −1.99 | −1.38 | 0.15 |
Halite | −6.15 | −6.78 | −5.21 | 0.37 |
Sylvite | −7 | −7.61 | −6.18 | 0.36 |
The concentration and dilution factors of the 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) [
The balance of the main elements in solution exported by Sebou to Mechra Bel ksiri is calculated for the hydrological cycles 2012/2013 and 2013/2014. During the two cycles studied, the Sebou exported on average 333.4 tonnes of elements in solution, i.e. a specific transport of 127.33 t/km2/year. In this transport of material, it is mainly floods that are responsible for most of these inputs (82%) (
The comparison of the dissolved specific transport at Mechra Bel ksiri between the 1996/1998 (Haida, 2000) and 2012/2014 (
rCa2+ | rMg2+ | rNa+ | rK+ | rCl− | r SO 4 2 − | HCO 3 − | |
---|---|---|---|---|---|---|---|
1962/70 | 4.5 | 0.81 | 0.63 | 1 | 0.62 | 0.9 | 3.5 |
1993/95 | 1.38 | 1.25 | 0.76 | 1 | 0.77 | 1.4 | 1.38 |
1996/97 | 1.18 | 1 | 1 | 1 | 0.97 | 0.66 | 1.18 |
1997/98 | 1.31 | 1 | 0.86 | 1 | 0.87 | 1.12 | 1.27 |
2012/13 | 1.06 | 0.94 | 1 | 1 | 0.96 | 1.04 | 1.19 |
2013/14 | 1.20 | 0.97 | 0.92 | 1 | 0.88 | 1.10 | 1.29 |
Flux of major elements | 2012/2013 | 2013/2014 | |||||
---|---|---|---|---|---|---|---|
Concentration (mg/l) | Dissolved flux (Tonnes) | Specific flux (T/km2/year) | Concentration (mg/l) | Dissolved flux (Tonnes) | Specific flux (T/km2/year) | ||
Sebou at Mechra Bel Ksiri | Ca2+ Mg2− Na+ K+ Cl− SO 4 − HCO 3 − | 65.03 21.10 135.55 6.51 208.5 88.06 163.41 | 28.5 × 104 9.3 × 104 59.1 × 104 2.8 × 104 91.6 × 104 38.5 × 104 71.8 × 104 | 10.9 3.56 22.6 1.1 34.9 14.72 27.4 | 78.97 20.57 132.99 6.15 221.82 102.13 165.77 | 39.7 × 104 10.3 × 104 66.2 × 104 3 × 104 111.2 × 104 51.8 × 104 83 × 104 | 15.17 3.97 25.28 1.17 42.44 19.8 31.65 |
Specific dissolved fluxes (t/km2/year) at Mechra Bel Ksiri | ||
---|---|---|
1996-1998 | 2012-2014 | |
Ca2+ Mg2− Na+ K+ Cl− SO 4 − HCO 3 − | 14.5 5 32 0.7 53 18 40.6 | 13 3.76 24 1.4 38.67 17.26 29.52 |
Total | 167 | 127.6 |
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 “flow-concentrations” relationships enabled us to identify the deep or superficial origin of the different chemical elements measured in solution in the water, with either a chemical element input of the highly concentrated groundwater during periods of low water, or a contribution of dissolved element by the superficial waters through the leaching of soils. Concentration or dilution factors showed that concentrations of chemical elements decreased significantly between the wet period and the dry period for Ca2+ and HCO 3 − 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.
Bahin, E.Y.B. and Haida, S. (2018) Hydrogeochemical Characterization of the Dissolved Load of the Major Elements Downstream of the Watershed of the Wadi Sebou, Morocco. Journal of Geoscience and Environment Protection, 6, 159-177. https://doi.org/10.4236/gep.2018.67011