Hydrochemical Characteristics of Groundwater in Siwa Oasis, Egypt. Part I. Significance of the Situation Groundwater Resources and Future Outlook
Moustafa M. Abo EL-Fadl1
, Magdy A. Wassel2
, Ahmed Z. Sayed3
, Ammar M. Mahmod4
1Water Chemistry Department, Desert Research Center (DRC), Cairo, Egypt
2, 3, 4Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City, P.O. 11884, Cairo, Egypt
International Journal of Science and Research (IJSR) ISSN (Online): 2319-7064 , Volume 4 Issue 10, October 2015, PP 447 - 458 :
Abstract:
This research, carried out with the objective of developing water management strategies in Siwa Oasis to ensure its sustainable development. Siwa Oasis (1000 km2 ) is today suffering an excessive rise in the subsoil water levels. The rate of groundwater rises from 1.33 to 4.6 cm/ year. Consequently, the fertile soils are subjected to deterioration and salinization. This is proposing and testing chain water management scenarios that makes use of suitable water (multiple reuses) before it reaches the last disposal point to increase the cultivated area (17000 feddans). So, this paper carried out the hydrogeochemical groundwater study based on the results of the chemical analysis of groundwater samples collected. To determine the evolution of groundwater resources through different analysis of water, such as total dissolved salts (TDS) and total hardness in two aquifers, Nubian sandstone and fractured dolomite limestone. The results show, the salinity of groundwater representing the area changed from fresh to saline groundwater, 168.8 to 7472.8 mg/l and total hardness from soft to hard, 30.7 to 1829.7mg/l as CaCO3 .
Keywords: Groundwater, Hydrochemical, salinity, Siwa Oasis, water resources
1. Introduction
Groundwater is a very important source of water, especially in arid and semi-arid regions. It is considered as the only source of water in the Eastern and Western Deserts of Egypt. In Africa, groundwater is a vital resource with nearly 80% of the continent’s population relying on it as a main source of drinking water. However, in many parts of the continent, reaching basic health requirements is still a concern [1].
Now, the drinking and irrigation water in Egypt is one of the biggest problems. Egypt is endowed with numerous water resources, but an assessment of these resources, including their reliability, quality and sustainability has not been systematically carried out. With the increasing demands for water due to increasing population, urbanization and agricultural expansion, groundwater resources are gaining much attention, particularly in arid and semi-arid regions.
In Egypt, in the recent years a great deal of attention has been directed toward expanding agricultural project developed on groundwater. One of the major regions qualified for this activity was west of the Nile Delta region and the New Valley including Siwa Oasis due to its soil and groundwater potentialities [2]. Siwa Oasis is a natural depression about 18m below sea level. It is located in the northern part of the Western Desert of Egypt (about 90 kilometers east of the Libyan border) and 300 kilometers south of the Mediterranean Sea and occur mostly to the east of Agormy [3]. Groundwater is the only source of drinking and irrigation water in Siwa Oasis. Old artesian wells originated from the top shallow aquifer are the traditional source of irrigation water in Siwa Oasis; the numbers of these wells are 220. As the need for more land aired, the upper limestone aquifer has been tapped by hand-drilled and boreholes as a new source of irrigation water.
For historical, demographic, and economic reasons as well as its high potential for agricultural development, the Siwa Oasis is considered the most important oasis in the western desert of Egypt. In addition to the existing cultivated area, there are more than 17,000 feddans that were determined to be suitable for agricultural development. The area under cultivation has been gradually increased in recent years as the population of the oasis is on the rise. New land reclamation development projects aimed at the exploitation of water resources have been in progress. Therefore, there is an urgent need for monitoring water quality in Siwa Oasis [4,5].
It is noted that the water quality in Siwa Oasis is deteriorating over time, and there is an urgent need for long term monitoring of water quality of the available water resources in the Siwa Oasis. The current research study was set to do just that. The present paper presents the results of this investigation and the future outlook for the situation of the limited water resources of the oasis. Scientific study concluded that the Siwa Oasis is suffering from high salinity, soil as a result of the exchange, which will reflect negatively on agricultural areas where irrigation water is misused, leading to lower productivity of crops [6,7]. In order to achieve sustainable development in these regions an integrated, complete, and accurate monitoring information system regarding the status of the groundwater quality is indispensable. Groundwater and surface water geochemical studies can provide a better understanding of potential water quality variations due to geology and land use practices [8,9].
In Siwa Oasis the strata of hydrogeological interest are composed of 450-600 m thick of fractured dolomite limestone overlying the Nubian sandstone aquifer. The fractured carbonate zones are separated from the underlying Nubian sandstone by the low permeability shale and clay layer. This layer acts as acting and its thickness varies between 60 in the west and 250 m in the east [10].
The Nubian aquifer has a thickness of about 2600 m and belongs to the Mesozoic and Paleozoic age. The upper zone of the Nubian Sandstone aquifer in Siwa Oasis has a thickness of about 500m and is saturated with fresh water with a salinity less than 500 ppm. The net sand thickness of this aquifer decreases northward and westward. The water pressure head of the Nubian Sandstone aquifer ranges between 80m in the western part of the Siwa area (where shallow wells and springs exist) and 120 meters in the east [11].
The aim of the present work is to study the hydrochemical assessment of the aquifer in Siwa Oasis, Northwestern Desert, Egypt. In continuation of our previous work [12,13] were studied of different localities for water resources quality with monitor overall objectives. In the Siwa Oasis study way to remove unwanted ions by safe and environmentally friendly methods in the second stage in our searches.
2. Experimental Method
Groundwater samples were collected from 24 different locations that cover Siwa Oasis water resources through September 2014. These water samples were collected in clean polyethylene bottles. At the time of sampling, bottles were thoroughly rinsed 2–3 times with groundwater to be sampled. In the case of bore holes and hand pumps, the water samples were collected after pumping for 10 min. This was done to remove groundwater stored in the well. These water samples carried out chemical analysis to assess the water quality. These samples were analyzed at Water Chemistry Department, Desert Research Center (DRC). Laboratory measurements were carried out by an EC meter (Orion model 150A+), pH meter (Jenway model 3510), flame photometer (Jenway model PFP7) and UV/Visible spectrophotometer (Thermo-Spectronic model 300). The analysis includes the determination of the different properties of water, such as TDS (Total Dissolved Salts), Major ions as Na+ , K+ , Ca2+ , Mg2+, CO3 2- , HCO3 - , SO4 2- and Cl- , Table (1). The concentrations of these constituents are expressed in milligrams per liter (mg/l, ppm), milli equivalents per liter (me/l) and percentage (%), [14].
3. Results and Discussion
The study is mainly based on the results of the chemical analysis for the collected groundwater samples (24 samples) in the Siwa Oasis. Most of the samples (79% of total samples) are located in the fracture dolomite limestone aquifer and the rest of the samples (21% of total samples) are in the Nubian sandstone aquifer, Fig. (1).
The study of the groundwater chemistry in Siwa Oasis and its relation to the prevailing hydrogeological and environmental conditions are the main target of the present study. The chemical composition of groundwater in Siwa Oasis is the combined result of the water constituents that passes into the groundwater and reacts with main channels that may modify the water composition, Table (1).
3.1 Chemistry of Ground Water
The chemistry of the groundwater includes the following topics such as:
3.1.1. Water salinity (TDS, Total dissolved solids)
Water salinity is detected from the chemical analysis can be expressed by TDS which is widely used in evaluating water quality and convenient mean used for comparing water sample with another one. Total dissolved solid (TDS) is the sum of all cations and anions which is widely used in evaluating water quality and convenient mean used for comparing water samples Twenty four groundwater samples were collected from the Siwa Oasis area to interpret the variations in total salinity and ion behaviors. The chemical data of groundwater reveal that water salinity of the Siwa Oasis area varies from 168.8 mg/l to 7472.8 mg/l as in Table (2).
According to Chebotarev classification [15], the groundwater water can be classified into three main categories, as follows:
1)Fresh groundwater type: fresh groundwater samples lie in the Nubian sandstone aquifer. The low salinity is attributed to low leaching and dissolution processes due to direct recharge from the local rainfall in the catchment area, low evaporation rate where the depth to water (1000 m).
2)Brackish groundwater type: the brackish groundwater samples are located in the fractured dolomite limestone aquifer
3)Saline groundwater type: the saline groundwater samples are located in the fractured dolomite limestone aquifer
The two types of groundwater brackish and saline are due to the stagnation of the groundwater due to the well abandonment, the shallow depth of the wells which activates the role of evaporation and hence, the concentration of salts, the scarcity of rainfall (45 mm/year) which is considered the main source of the aquifer recharge, the marine origin of the exposed leached sediments dominating the recharge area which are mostly composed of limestone and shale [16].
Figure 1: Location map of water samples in Siwa Oasis
Table 1: The hydrochemical results of groundwater samples of Siwa Oasis
Table 2: Frequency distribution of water salinity of the groundwater samples of the studied aquifers in Siwa Oasis
3.1.2. Distribution of total water hardness
According to the chemical analysis of the groundwater samples of Nubian sand stone aquifer(fresh water) and Fractured dolomite limestone aquifer (brackish and saline water) it is clear that the mean value of total, temporary and permanent hardness reaches (61.9, 126.1 ,0), and (1080.7, 136.2, 944.5) mg/l as CaCO3, respectively. Noteworthy to mention that the groundwater samples nearly have the same values of total, temporary and permanent hardness also, the temporary hardness relative to total hardness in the fresh water (Nubian sandstone aquifer) is more than that brackish and saline water (fractured dolomite limestone aquifer) and vice versa in permanent hardness relative to total hardness.
The hardness data indicate an increase in total, temporary and permanent hardness with increasing water salinity in all groundwater, according to the change of water type from fresh to saline water as in Table (3). This is mainly attributed to the effect of leaching and dissolution of soluble salts leading to the increase of hardness with particular importance to the effect of NaCl concentration (effect of ionic strength) on increasing solubility of Ca2+ and Mg2+ in water [17,18]. This does not exclude the contribution of CO2, longer residence time, influence of salty water and cation exchange process.
With regard to total, permanent and temporary hardness relative to water salinity (TH/TDS, NCH/TDS and CH/TDS %) in the Nubian sandstone aquifer (fresh water) and Fractured dolomite limestone aquifer (brackish and saline water) samples, the obtained ratios are (25%, 0% and 53%) and (27%, 23% and 4%), respectively in the fresh to highly saline water type.
Table 3: Average and relative values of total, temporary and permanent hardness compared to the water salinity of groundwater.
3.1.3. Frequency distribution of major ion concentration.
The major ions Na+ , Ca2+, Mg2+, HCO3 - , Cl- and SO4 2- constitute about 98% of total mineralization of most groundwater. All samples of groundwater chemistry in Siwa Oasis can distribute into histograms as cations and anions for fresh, brackish and saline as in Table (4) and Fig. (2):
Table 4: Range and mean values of the major dissolved ions of groundwater, mg/l.
3.2 Nubian sandstone aquifer (Fresh water type, 21% of the total samples)
The frequency distribution of Ca2+ patterns shows three unequal categories; where the majority of fresh groundwater samples (40%) has Ca2+ within the range 5-10 mg/l and 20- 25 mg/l respectively. However, the mean value for Ca2+ of fresh groundwater samples is 12.9 mg/l. The frequency distribution of Mg2+ patterns shows three unequal categories; where the majority of fresh groundwater samples (40%) has Mg2+ within the range 3-6 mg/l and 6–9 mg/l respectively. However, the mean value for Mg2+ of fresh groundwater samples is 7.21 mg/l. The frequency distribution of (Na+ and K + ) patterns shows three categories; where the majority of fresh groundwater samples (40%) has (Na+ and K+ ) within the range 40-60 mg/l and 80-100 mg/l. However, the mean value for (Na+ and K+ ) of groundwater samples is 76 mg/l.
The frequency distribution of (Cl- ) patterns show four categories; where the majority of fresh groundwater samples (40%) has (Cl- ) within the range 20-40, 60-80 mg/l. However, the mean value for (Cl- ) in fresh groundwater samples is 51.7 mg/l.
The (HCO3 - and CO3 2- ) distributions show four unequal categories. The majority of samples (40%) have (HCO3 - and CO3 2- ) concentration ranges between 120 to 160 mg/l, the mean value for (HCO3 - and CO3 2- ) in fresh groundwater samples is 153.72mg/l. The (SO4 2- ) distributions show two unequal categories from 13.54 to 20.5 mg/l. The majority of samples have the concentration range between 20-25mg/l (60% of the total fresh samples), the mean value for (SO4 2- ) in fresh groundwater samples is 18mg/l.
3.3 Fractured dolomite limestone aquifer (brackish and saline water, 79% of the total samples)
From the distribution histograms as in Table (5) and Fig. (3), the following are the main findings. The frequency distribution of Ca2+ patterns shows three unequal categories; where the majority of groundwater samples (47%) have Ca2+ within the range 150-300 mg/l. However, the mean value for Ca2+ of groundwater samples is 210.9 mg/l. The frequency distribution of Mg2+ patterns shows four unequal categories; where the majority of groundwater samples (53%) have Mg2+ within the range 50–100 mg/l. However, the mean value for Mg2+ of groundwater samples is 134.6 mg/l. The frequency distribution of (Na+ and K+ ) patterns shows five categories; where the majority of groundwater samples (47%) have (Na+ and K+ ) within the range 400-800 mg/l. However, the mean value for (Na+ and K+ ) of groundwater samples is 1101 mg/l.
The frequency distribution of (Cl- ) patterns show five categories; where the majority of groundwater samples (42%) has (Cl- ) within the range 750-15000mg/l, (21.37-34.4 me/l). However, the mean value for (Cl- ) of groundwater samples is 1961.28 mg/l. The (HCO3 - and CO3 2- ) distributions show four unequal categories, the majority of samples (53%) have (HCO3 - and CO3 2- ) concentration ranges between 120 to 160 mg/l, the mean value for (HCO3 - and CO3 2- ) of groundwater samples is 165.98 mg/l. The (SO4 2- ) distributions show four unequal categories from 295.5 to 1211 mg/l. The majority of samples has the concentration range between 350-700 mg/l (47% of the samples), the mean value for (SO4 2- ) in brackish groundwater samples is 561.1mg/l.
4. Conclusion
The groundwater samples in Siwa Oasis lie in two different aquifers, Nubian sandstone and fractured dolomite limestone. The fresh groundwater samples and brackish & saline are lie in Nubian sandstone and fractured dolomite limestone, respectively. The obtained data show that the TDS changed from fresh to saline 168.8 to 7472 mg/l, total hardness ranges from soft to hard, 30.7 to 1829.7mg/l as CaCO3. The effective ions that cause an increase of water salinity are in descending order: As cations; Na+ > Mg2+ > Ca2+ while the anions Cl- > SO4 2- > HCO3 - in both brackish and saline groundwater samples and HCO3 - > Cl- > SO4 2- in fresh. Also, the ion ratios indicated that, the groundwater samples have a mixed mineralization that is possibly pure meteoric water affected by leaching and dissolution and cation exchange of both terrestrial and marine salts. Besides that, the hypothetical salts of groundwater samples show three main groups. With continued research, we will evaluate the groundwater samples for drinking, domestic and irrigation uses with solving problem of water samples to be valid for drinking and agriculture uses.
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