M and A Physicochemical,Characterization,of,Water,of,Bir,M’Cherga,Reservoir,and,Its,Effluents

　　Yosra Ismail and Abdalah Benalaya 　　Laboratory of Agricultural Production System and Sustainable Development (SPADD), High Institute of Agricultural of Mograne, Mograne 1121, Tunisia
　　Received: February 15, 2011 / Accepted: May 16, 2011 / Published: February 20, 2012.
　　Abstract: This study aims the physico-chemical characterization of surface water of the Bir M’Cherga reservoir and its effluents which are located in Zaghouan governorate of Tunisia. It required measures in situ of physicochemical parameters and chemical analysis at laboratory of major element and nutritive salts. The results of this study revealed the presence of pollutants in the Miliane river at El Fahs area and the eutrophication of water in the Bir M’Cherga reservoir. According to the classification of Stuyfzand and Piper, water in the wadis of Melah, Hdada, Miliane and Boudebbane respectively is brackish, salty, extremely hard, chlorinated, sulphated, calcic, and magnesic. Water in the Bir M’Cherga reservoir is characterized by calcic facies chlorinated sulphated and sodic. Thus, the Bir M’Cherga reservoir presents a worrying situation and a risk of qualitative degradation of its water.
　　Key words: Bir M’Cherga reservoir, rivers, eutofication, pollution, geochemical study.
　　 1. Introduction?
　　In Tunisia, conventional water resources available are limited and it is supposed that they will be exceeded by the demand for water due to the increase in the population and the improvement of the standard of living. To that is added the problem of the degradation of water quality due to the anthropogenic activities. The country is thus confronted with two major challenges: rational exploitation and the safeguarding of water quality.
　　The Bir M’Cherga reservoir was constructed in order to ensure the protection of the downstream areas against floods and the irrigation of the bordering agricultural lands.
　　This reservoir receives, among other things, the surface water of some rivers which include the urban and industrial effluents. The principal problem of water retained in the reservoir is the contamination by chemical and organic compounds of domestic and industrial wastewaters coming from the agglomerations.
　　This problem caused chemical degradation of water as well as fish mortality. This study aims to evaluate the quality of surface water in the Bir M’Cherga reservoir and in the mainly wadis of the catchment area of this reservoir. For this purpose, physicochemical parameters are measured in situ and major elements as well as nutritive salts are determined at laboratory.
　　 2. Study Area
　　The study area is situated in Zaghouan governorate, approximately 47 km to the south-west of Tunis. The Bir M’Cherga reservoir was built in 1971 on Miliane wadi to protect downstream areas (the plains of the south-east of Tunis) against floods and to irrigate the bordering agricultural lands. The rainiest months in this area are November, December, January and February. Annual average of rainfall is about 410 mm. August is hot with mean maximum temperature of 27.1 °C, whereas January is cold with mean minimum of 9.7 °C.
　　The absolute minimum is 3 °C and the absolute maximum can reach 46 °C. Climate in the study area is semi-arid. The geological formations at the site of Bir M’Cherga reservoir form a lithostratigraphic cover dating from the recent Quaternary [1-6].
　　
　　Fig. 1 Location map of the study area showing the measurement points.
　　 3. Materials and Methods
　　Samples of surface water were collected from different points in Bir M’Cherga reservoir and in the mainly wadis of its catchment area (Hdada, Miliane, Boudebbane and Melah wadis) during autumn and spring (October and May) of 2009. The choice of the sampling sites (Fig. 1, Table 1) was made in order to obtain a maximum of information concerning water quality in the Bir M’Cherga reservoir and in the mainly wadis with perennial flows which arrive at this reservoir. It shows, among other things, the zones contaminated by the industrial and domestic wastes coming from the cities located near the Miliane, Hdada and Melah wadis.
　　Sixty (60) representative samples of surface water on various points in the wadis and in the Bir M’Cherga reservoir were collected from monitoring the water chemistry of several ions comprising Na+, Cl–, Ca2+, HCO3–, SO42–, PO43–, NO3–, NO2–, total phosphorus and total nitrogen.
　　The surface water samples were collected and stored in acid-cleaned, high-density polyethylene(HDPE) bottles (1.000 mL), which were carefully rinsed three times before use. The use of HDPE bottles minimizes container pollution and promotes sample preservation [7].
　　In the Bir M’Cherga reservoir, water samples were collected at different depth in seven points numbered from 1 to 7 (Fig. 1, Table 1). The water samples collected from wadis correspond to points numbered from 8 to 12 (Fig. 1, Table 1). Two sets of samples were collected at each point.
　　Temperature, pH, dissolved oxygen and electric conductivity were measured in the field by multi-parameter apparatus at each depth of each point in the reservoir and at each point of the wadis and the same time of collecting water samples.
　　Samples were stored in a refrigerator at 4 °C prior to analysis.
　　The analysis of the major elements was made after samples filtration. The chlorides were determined according to the method described by Mohr, sulphates by gravimetry and bicarbonates by the volumetric method. The ions Na+ and K+ were accounted by spectrometer with standard Gallenhamp flame according to the method of atomic absorption in emission by using a flame with air and acetylene. The ions Ca2+ and Mg2+ were accounted by a spectrometer of atomic absorption, a standard Philips PU 9100. Nitrates, nitrites, orthophosphates, total phosphorus, total nitrogen and chlorophylls were analyzed by colorimetry using a molecular spectrophotometer [8]. Moreover, geochemical characterisation of the reservoir water was made by using the diagrams of piper and Shoeller-Berkaloff.
　　Table 1 Geographical coordinates of the sampling points.
　　
　　 4. Results and Discussion
　　4.1 Physicochemical Parameters
　　As indicated above, the physicochemical parameters of water measured in situ are temperature, pH, electric conductivity, salinity and dissolved oxygen.
　　The temperature of surface waters is influenced directly by climatic conditions:
　　It is more important in May than in October;
　　It varies between 21.7 °C and 17.36 °C in May and between 36.20 °C and 28 °C in October.
　　Thermal stratification: At the same depth and at the same date of measurement, the temperature has almost the same values in the various measurement points. According to the classification of Benmoussa [9], the water of Bir M’Cherga reservoir is relatively warm, like those in the other reservoirs of the Mediterranean region. The temperature values of water in the wadis show a little increase compared to those measured in the Bir M’Cherga reservoir considering the depth of water which is not important.
　　The water quality data for the collected samples are summarized in Table 2 and 3. A range of variation exists in the parameters of water. The pH values measured at various point of the Bir M’Cherga reservoir draw attention to the following:
　　Table 2 Chemical data of water samples collected in May 2009.
　　
　　Table 3 Chemical data of water samples collected in October 2009.
　　
　　At the same depth of each measurement point, the value of pH is lower in October than on May;
　　The pH varies from 9.04 to 8.12 during May and from 8.5 to 7.67 during October. Moreover, the lowest value which is of 7.45 was recorded in the Melah wadi. The maximum value was recorded in the Bir M’Cherga reservoir.
　　In almost measurement points of the reservoir, the dissolved oxygen concentrations of water vary between 5.1 and 7 mg/L during May and between 8.1 and 8.45 mg/L during October. Broadly the oxygen contents dissolved in the reservoir are higher on May than on October and almost do not vary with the depth of water. On the wadis, the values of the dissolved oxygen concentration were found ranging between 5.7 and 7.2 mg/L during May and between 7.1 and 8.7 mg/L during October. The Melah wadi presents the lowest values during the two measurement periods.
　　The values of the electric conductivity of water were found more important on October than on May. On May, electric conductivity varies from 2.38 to 4.1 mS/cm in the Bir M’Cherga reservoir (points 1 to 7) and from 2 to 8.83 mS/cm in the wadis. The most important value is noted at point 10 in Melah wadi(8.83 mS/cm). During October, electric conductivity varies from 3.9 to 5.05 mS/cm in Bir M’Cherga reservoir and it varies from 4.5 to 20.17 mS/cm in the wadis. The values recorded on the wadis are more important than those of the Bir M’herga reservoir. It varies from 0.132 to 1.75 mg/L in October and from 0.101 to 0.598 mg/L in May. As the values of the pH vary from 7.45 to 9.04, the principal shape of dissolved ionic phosphorus is HPO42-, it is the most stable form for pH values ranging between 7.2 and 12.3 [10]. In almost wadis, the values of the total phosphorus concentration were found higher than the Tunisian standard one which is 0.5 mg/L. On the other hand, the values of the orthophosphate concentration in these wadis are lower than the standard one of 0.05 mg/L. The total nitrogen content in water is generally higher in the wadis than in the reservoir. It exceeds the Tunisian standard value which is 1 mg/L in almost measurement points. Moreover, it does not vary much from May to October.
　　4.2 Geochemical Types of Water Classification According to the Piper Diagram
　　Results of chemical analysis of water samples collected from the Bir M’Cherga reservoir in May and in October are used on the Piper diagram (Figs. 2 and 3). These figures show that the predominance of the chlorides ions is clear on sulphates and bicarbonates. Cl- (from 55% to 82%) > SO42- (from 18% to 45%) > HCO3- (from 0% to 20%). According to the diagram of the cations, one notices a relative prevalence of the magnesium ions: Mg2+(60%-80%) > Na+ (40%-60%) > Ca2+ (20%-35%). The results of the chemical analysis of water in the wadis related to the two measurement dates (May and October) are presented on the Piper diagrams (Figs. 4 and 5).
　　 variation between October and May. In general, water of the hydraulic system is of type:
　　
　　 Fig. 2 Piper diagrams illustrating variability in major ion composition in the Bir M’Cherga reservoir (May 2009).
　　
　　 Fig. 3 Piper diagrams illustrating variability in major ion composition in the Bir M’Cherga reservoir (October 2009).
　　
　　 Fig. 4 Piper diagrams illustrating variability in major ion composition in the wadis in May.
　　The diagram of the anions shows a group of dots close to the chlorinated pole. The predominance of the ions chlorides is clear on sulphates and bicarbonates. Cl- (from 90% to 100%) > SO42- (from 15% to 55%) > HCO3- (from 0% to 10%). In the cations diagram (Fig. 5), one notices a relative prevalence of the magnesium ions: Mg2+ (70%-90%) > Na+(55%-70%) > Ca2+ (20%-50%). One didn’t notice a
　　Chlorinated and sulphated calcic and magnesian;
　　Chlorinated sodic and potassic or sulphated sodic.
　　Thus, the geochemical processes which can take place in the Bir M’Cherga reservoir are:
　　Gypsum dissolution (CaSO4·2H2O);? Halite dissolution (NaCl);? Dolomite dissolution;? Salt water intrusion.
　　Type of water in the study area according to the Stuyfzand classification Water in the wadis of Melah, Hdada, Miliane and Boudebbane is salted, extremely hard, chlorinated and sulphated calcic and magnesian and with a surplus of (Na + K + Mg). Water in the Bir M’Cherga reservoir is salted, very hard, chlorinated and sulphated calcic with sodic and with a surplus of(Na + K + Mg).
　　4.3 Eutrophication of Water
　　The eutrophication state was given by using the median values of measurements of phosphorus, Secchi transparency and chlorophyll (A). The total phosphorus concentration values from 32 to 170 μg/L in the measurement points of the Bir M’Cherga reservoir. The chlorophyll (A) content values recorded in May and October lie between 32.9 and 76 μg/L. The transparency of water in the Bir M’Cherga reservoir varies between 0.4 and 0.6 m. The low value was recorded in the Hdada wadi and the most important in the Bir M’Cherga reservoir near the water intake tower. According to the OCDE [11] model, the total phosphorus and chlorophyll (A) concentrations indicate that water of the Bir M’Cherga reservoir is in a eutrophic state. Those of the transparency indicate that this water is in hypereutrophic state.
　　
　　 Fig. 5 Piper diagrams illustrating variability in major ion composition in the wadis in October.
　　 5. Conclusion
　　Throughout this study, the authors were able to confirm the qualitative degradation of water in the Bir M’Cherga reservoir and in the mainly wadis of its catchment area. The contents of major elements which control the salinity (Na+, Cl-, Ca2+ and SO42-) were found higher in the wadis than in the reservoir. The strongest contents correspond to the Melah wadi. In the Bir M’Cherga reservoir, the values of salinity do not show significant variations with to the depth. The variation in salinity between the two periods of sampling (May and October) can be explained by the cumulative effects in water remaining more than one year in the reservoir, the lack of rain and the strong evaporation. In the same way, the contents in nutritive salts (PO43-, NO3-, NO2-, P, NT) are generally higher in the wadis than in the reservoir. The total nitrogen (NT) and total phosphorus (P) concentrations exceed the Tunisian standards of the rejections in public waterways. The nutritive salts concentrations in water of the reservoir were relatively high and caused the eutrophication and consequently the development of algae. According to Stuyfzand and Piper classifications, water of the wadis (Melah, Hdada, Miliane and Boudebbane wadis) is salted, extremely hard, chlorinated and sulphated calcic and magnesic. Water of the Bir M’Cherga reservoir is salted, very hard, chlorinated sodium and potassic.
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