Assessment of groundwater quality using geographical information system (GIS), at north-east Cairo, Egypt

Great attention is paid by the Egyptian authorities to development and new settlements establishment. The north-eastern part of greater Cairo, which is located at the southern part of the eastern Nile Delta, represents a scene for establishment of various investment activities in the form of big agricultural, industrial and urbanization projects. Securing water resources of appropriate quantity and quality is of prime importance for development programs in this important sector. This gives importance to the evaluation of the water resources sustainability, and vulnerability to cope with the prospective development.

The sustainability of groundwater is constrained by renewability of recharge and continuity of flow. The quality of the groundwater is a function of its physical and chemical parameters which depend on the soluble products of weathering, decomposition and the related changes that occur with respect to time and space, as well as anthropogenic impacts (WHO 2011).

The present investigation aims at studying the hydrogeochemical and environmental factors that control the water quality of the groundwater resources to the north-east of Cairo. The study also aims to explore the protectability of the groundwater resources against high pollution threats of the contaminants that diffuse to groundwater through the unsaturated zone. This will help to achieve the sustainable use of groundwater resources and to incorporate water quality constraints into planning and decision making for integrated management. The methodologies of hydrogeochemistry, and a geographical information system (GIS) based protectability index have been used for conducting this investigation.

The study area is mainly covered by Tertiary and Quaternary sediments. This sedimentary succession varies in thickness and shows an increase toward the Nile Delta recording more than l,000 m towards the Mediterranean Sea. The top portion of this section is formed of sand and clay facies, whereas the lower portion is dominated by carbonate facies (El Shazly et al. 1975). The Tertiary carbonates are underlain by Cretaceous limestone and dolomite, which are strongly dissected by a complex system of faults striking mostly E-W (El-Sayed 2005).

In the studied area, groundwater occurs in two main water-bearing formations, namely Quaternary and Miocene. The Quaternary aquifer is mostly found under semi-confined conditions in the Belbeis area due to the presence of an overlying clay bed and is found under unconfined conditions in the Inshas area (Hefny et al. 1983). The depth to groundwater ranges from 21 (NE) to 91 m (SW). The groundwater flow direction is north-eastward with a regional hydraulic gradient ranging between 50 and 60 cm/km (Taha et al. 1997). The Quaternary aquifer is mainly recharged from the Ismailia canal as well as from irrigation water return flow. The Miocene aquifer within the studied area is either found under unconfined or semi-confined conditions depending on the presence or absence of clay overlay (El-Mahmoudi et al. 2006; Shehata & El Sabrouty 2014). The depth to water varies from 160 to 240 m along the Cairo-Ismailia desert road and reaches about 170 m near Cairo city, the direction of groundwater flow in the Heliopolis basin is from SW to NE (Sallouma & Gomaa 1997).

Groundwater samples were collected during summer 2014 from 34 groundwater points tapping the two aquifers (Quaternary and Miocene). These water samples were subjected to both field and laboratory analyses.

The field analyses include electrical conductivity (EC) (μS/cm) which was measured using an EC meter (model: Cole Parmer date meter CON 410 series) and pH which was measured using a pH meter (Jenway, model 3150). The laboratory analyses include the determination of major ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, HCO₃⁻, and SO₄²⁻) and trace elements (Al, B, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sr, V, and Zn). Measurements were carried out by Inductively Coupled Argon Plasma Poma 6500 Spectrophotometer (Unicom, UK). The analyses were carried out according to the standard methods (Rainwater & Thatcher 1960; Fishman & Friedman 1985; American Public Health Association (APHA) 1998; ASTM 2002). The obtained chemical data are expressed in milligram per liter (mg/l). All of the hydrochemical data have been treated and analyzed for calculating water quality index (WQI) and evaluating its suitability for various purposes.

The index and overlay approach (protectability index) combined with the GIS tool have been used to evaluate the groundwater vulnerability against diffuse pollution through the unsaturated zone. The DUPIT index has been calculated for this. The data required for the index calculation have been extracted from 42 boreholes distributed in the study area and indicated in the work of El-Sayed (2005).

The analyzed groundwater samples that were collected from the main aquifers (Quaternary and Miocene) have been used to identify the chemical characteristics and salinization processes of the groundwater system and to evaluate the factors that constrain its quality. Tables 1–4 show the results of the chemical analysis conducted on the groundwater samples, and the average values and ranges of variation of ionic composition are summarized in Table 5.

The ordering of the concentration of cationic and anionic species for the studied groundwater samples highlights the stage of mineralogical evolution and the degree of maturity as follows:

• The ionic order HCO₃⁻ > Cl⁻ (SO₄²⁻) > SO₄²⁻ (Cl⁻)/Ca²⁺ (Na⁺) > Na⁺ (Ca²⁺) > Mg²⁺ was recorded for 22% of the Quaternary aquifer samples with bicarbonate-calcium or bicarbonate-sodium water type. This order characterizes the groundwater that has an original fresh water recharge little modified by leaching and cation exchange.

• The ionic order of Cl⁻ > SO₄²⁻ > HCO₃⁻/Na⁺ > Ca²⁺ > Mg²⁺ with chloride-sodium water type is obtained for about 52% of the Quaternary aquifer samples, and all of the Miocene aquifer samples. This ionic order characterizes the advanced stage of mineralization and the potential salinization processes under long rock/water contact and high attack potential of lithofacies of old marine deposits.

• The ionic order SO₄²⁻ > HCO₃⁻ > Cl⁻/Na⁺ > Ca²⁺ > Mg²⁺, i.e. sulfate-sodium water type is characterized for 26% of Quaternary aquifer samples, reflecting the increase of SO₄²⁻ and Na⁺ content that could be related to excessive use of fertilizers and drainage water effects.

The distribution on the Piper diagram of the Quaternary groundwater samples, Figure 3(b), is more dispersed compared to that for the Miocene groundwater samples indicating more interactive salinization processes (natural and anthropogenic) versus freshening by active recharge. The hydrochemical facies of (Na⁺ + K⁺)/(Cl⁻ + SO₄²⁻) and (Ca²⁺ + Mg²⁺)/(Cl⁻ + SO₄²⁻) appear in about 60% of these samples reflecting the effect of cation exchange processes on the surface of clay deposits. Two other hydrochemical facies of HCO₃⁻/(Ca²⁺ + Mg²⁺) and HCO₃⁻/(Na⁺ + K⁺) appear in about 22% of samples reflecting the typical meteoric water with high Na⁺ concentration due to leaching or cationic exchange. The rest of the samples fall in (SO₄²⁻ + Cl⁻)/(Ca²⁺ + Mg²⁺) reflecting the effect of fertilizer application.

Dissolution and precipitation processes (saturation indices). The ‘Solmnique’ computer program has been used to calculate the saturation indices (SI) for the studied groundwater samples in relation to the relevant phases in the system (calcite, dolomite, gypsum, anhydrite and halite). The SI values determine the extent to which a groundwater has reached chemical equilibrium with the minerals within the aquifer matrix (SOLMINEQ.GW 1999). The results in Tables 6 and 7 show that Quaternary and Miocene groundwater samples have –ve saturation index values with respect to Gypsum, Halite and Anhydrite. This reflects the undersaturation and the continual dissolution of these salts on aquifer contact. On the other hand, about 88% of both the Quaternary and Miocene samples have +ve saturation index values with respect to Dolomite, while 68% of the Quaternary and 63% of the Miocene samples have +ve saturation index values with respect to Calcite. These +ve saturation index values reflect the oversaturation with respect to Calcite and Dolomite and the tendency of the groundwater to precipitate them on rock contact.

The WQI is one of the most effective tools to provide feedback on the quality of water to the policy makers and environmentalists. It provides a single number that expresses the overall water quality at a certain location and time, based on several water quality parameters (Poonam & Ashokkumar 2013). Although there have been a variety of attempts to create such a WQI, the most successful attempt to date appears to be the index indicated in the Canadian Environmental Quality Guidelines (Canadian Council of Ministers of the Environment (CCME) 2001). The index is based on a combination of three factors; (scope (F1), frequency (F2), and amplitude (F3)) which are determined for a group of water quality variables chosen in relevance to the prospective evaluation. The maximum permissible values of these variables as indicated in a given standard or guidelines represent the objectives on which the corresponding samples are measured.

Choosing variables and objectives. Sixteen parameters have been selected as variables from The Egyptian Higher Committee of Water guidelines for drinking water quality (1995) to generate the Canadian Council of Ministers of the Environment Water Quality Index (CCME WQI). Those variables (i.e. physicochemical properties of groundwater samples) and objective (i.e. maximum permissible values) are listed in Table 8.

F3 (Amplitude) represents the amount by which failed test values do not meet their objectives. F3 is calculated in three steps:

• (i) The number of times by which an individual concentration is greater than (or less than, when the objective is a minimum) the objective is termed an ‘excursion’ and is expressed as follows. When the test value must not exceed the objective:

  

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• (ii) The collective amount by which individual tests are out of compliance is calculated by summing the excursions of individual tests from their objectives and dividing by the total number of tests (both those meeting objectives and those not meeting objectives). This variable, referred to as the normalized sum of excursions (nse), is calculated as:

  

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• (iii) F3 is then calculated by an asymptotic function that scales the normalized sum of the excursions from objectives (nse) to yield a range between 0 and 100.

  

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Comparing the hydrochemical data of the different aquifers in the studied area with the upper limits of concentration for stock and poultry drinking (McKee & Wolf 1963), it appears that nearly all the groundwater samples of the Miocene aquifer are suitable for drinking by livestock, but they are not suitable for poultry. On the other hand, about 12% of the Quaternary groundwater samples are not suitable for poultry and the rest are suitable for livestock and poultry.

Evaluation of the water quality for irrigation will be discussed through the following items.

Total dissolved solids. By comparing the results of chemical analyses of the studied samples with the classification of irrigation water according to TDS (Willcox 1955), Table 10 is obtained as an evaluation of the groundwater for irrigation purposes.

The studied area is considered as one of the most promising localities for industrial activities. Industrial water is quite diverse and the required water quality is dependent on the type of industry. Comparing the results of the chemical analyses of the collected groundwater samples with the suggested limits for some selected industries (National Academy of Science 1972), Table 11, we can conclude that about 56% of the Quaternary samples are suitable for most industrial usage but the rest of them have higher salinities and all of the Miocene samples can be used for the petroleum industry only.

Water which can be used for building purposes in concrete must be free from some factors that cause concrete degradation. The data of the chemical analyses of the groundwater samples in the studied area were compared with the standard limits (National Academy of Science 1972) presented in Table 12, showing that about 76% of the groundwater of the Quaternary aquifer and about 25% of that of the Miocene one can be used for building purposes.

Mapping the protectability of the groundwater system against diffusion through the unsaturated zone is a mandatory goal, hence, industrial development as well as new settlements establishment in the study area threaten the groundwater system. On the other hand, those prospective developments are constrained by the availability of water resources which should be secured with active recurrence and suitable quality.

where Dr and Dw, Ur and Uw, Pr and Pw, Ir and Iw, and Tr and Tw are the rating factors and weighting coefficients for depth to groundwater, upper layer lithology, unsaturated zone permeability, impermeable layer thickness, and topography slope factor, respectively.

Based on the thematic map of the DUPIT index for the studied area (having an index values range from 37 to 52) the area can be ranked into three categories specifying areas of different protectability against diffuse pollutants, based on Table 14 as follows:

• The higher protectability (less vulnerable areas), having an overall index in the range from 52 to 67, are located in the middle and eastern parts of the domain of interest, especially at Tenth of Ramadan city, El-Obour city, Inshas area and some desert areas near the Cairo-Ismailia desert road.

• The moderately protectable (moderately vulnerable areas) have an overall index in the range from 37 to 51 and are located in the northern sectors of the study area.

• The lower protectability areas are located in some parts of Belbis Al-Asher desert road and have a DUPIT index in the range from 22 to 36.

Hydrochemical analyses of the studied groundwater samples (Quaternary and Miocene) have been used to identify the chemical characteristics and salinization processes of the groundwater system. It is revealed that the Ismailia canal is the main recharge source for Quaternary aquifer and that solute content gradually increases along the flow path in the north-west direction under the effects of rock/water interaction processes as well as extra use for irrigation and irrigation return. On the other hand, the Miocene aquifer is less rechargeable and has a retarded flow character. The GIS ground WQI map of the study area delineates the zones of the two determined categories: fair (CCME WQI Value 65–79) for all of the Miocene aquifer groundwater, and also for some Quaternary groundwater samples and good (CCME WQI Value 80–94) for the Quaternary aquifer groundwater samples adjacent to Ismailia Canal.

The GIS based DUPIT index has been used to delineate the zones of different vulnerability against diffuse pollutants in the studied area. It separated the aquifers into two main classes and mapped their vulnerability as follows: more protectable less vulnerable areas (DUPIT index values 52 to 67) are located to the middle and eastern part of the domain of interest and moderately protectable moderately vulnerable areas (DUPIT index values 37 to 51) are located in the northern sectors of the study area.

This publication was made possible by the support of the Egyptian Nuclear and Radiological Regulatory Authority (ENRRA).