Hydrochemical and multivariate analysis of groundwater quality in the northwest of Sinai, Egypt

Securing water resources of appropriate quantity and quality is of prime importance for development programs in this important sector. This emphasises the importance of evaluation of the sustainability, potentiality and vulnerability of water resources from a development perspective.

The annual rainfall in the study area is limited; it varies between 26 and 54.8 mm and the total quantity of rainfall generally increases northward (El-Sheikh 2008). The national project of the El-Salam Canal has been started and River Nile water is being carried to Sinai. According to the Ministry of Water Resources and Irrigation (1991, 2009) and Hafez (2005), it is expected that the Nile water supply to this area will not fulfill the requirements of all the planned projects. Therefore, the demand for freshwater supplies has accordingly increased and attention is focused on groundwater utilization as an alternative or additive to rainfall.

The quality of groundwater resources in coastal zones is affected by many constraints (natural and/or anthropogenic). These include seawater intrusion, rock/water interaction, evaporation, irrigation return and drainage water effects, etc. Identifying the principal factors that control the salinity evolution and water quality aspects helps to achieve sustainable use of coastal resources.

This paper describes the prevailing groundwater conditions in the northwestern coastal zone of Sinai and explores the attributes of water quality and the controlling processes. The chemometric multivariate analysis has been performed to define the principal water quality components that account for much of the variability in the system under study.

Hydrogeologically, the Quaternary deposits constitute the major water-bearing formations in northwestern Sinai (Geological Survey of Egypt 1992). These deposits consist mainly of loose sands with a few clay intercalations. The groundwater exists under a free water table condition with the depth to water varying from 0.5 m in the northwest to 9.1 m in the southeast. The water table ranges from −3.7 to 10 mas (Embaby & El-Barbary 2011).

Thirty-nine groundwater samples tapping the Quaternary aquifer were collected from the study area, as shown in Figure 1. These water samples were subjected to both field and laboratory analyses. The field analyses include electrical conductivity (EC) (μS/cm) and pH, which has been measured using an EC meter and a pH meter (Jenway, model 3150).

The laboratory analyses include the determination of major ions (Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, ⁠, ⁠, and ⁠). Chloride, calcium, carbonate and bicarbonate were determined using a titrimetric method. Sulfate ion concentration was determined calorimetrically using the turbidity method (USEPA 1979) by UV/visible spectrophotometer. Sodium and potassium content were measured by flame photometer, according to Rhoades (1982).

The results of the chemical analysis are expressed in milligram per liter (mg/l). The multivariate statistical analyses of the chemical parameters were conducted using SPSS (software 22 version).

The GALDIT index has been combined with geographic information system (GIS) tool to evaluate the vulnerability of groundwater aquifer to seawater intrusion.

The results of hydrochemical analyses of the studied groundwater samples are shown in Table 1. These results have been statistically treated and their coefficient of variation are calculated. The coefficients of variation of the six hydrochemical parameters, Na⁺, K⁺, Cl⁻, Ca²⁺, Mg²⁺ and ⁠, are equal to 0.63, 0.69, 0.53, 0.68, 0.73 and 0.49, respectively; these values are relatively high indicating heterogeneity in aquifer recharge and salinization processes.

The results of chemical analysis have been used to identify the hydrochemical characteristics and salinization processes of the groundwater under study and to evaluate its quality aspects. The water types of the studied samples are classified into six major groups as follows.

Samples of Cl-Na, Cl-Ca, and Cl-Mg water types dominate 72%, 15.4% and 2.5%, respectively, of all samples and represent the highest mineralized water which may develop through leaching and dissolution of marine sediments, cation exchange and/or seawater intrusion.

Samples of HCO₃-Ca, HCO₃-Na, and SO₄-Na water types dominate a total of 10.1% of all samples; these are located in the renewable recharge area close to the irrigation canals.

Real hydrochemical data often contain some less important parameters besides the ones which encode important information about the quality (Malinowski & Howery 1980; Malinowski 1991; Lavine 2000; Jolliffe 2002; Praus 2005). Multivariate statistical analysis of the chemical analysis data has been conducted using SPSS program Version 22.0. This helps to infer the principal parameters that control the salinity and the quality of the groundwater under study.

Two principal components have been defined as best descriptors of the variability of chemical composition of the groundwater samples under study. These express about 72% of the variance as indicated in Table 2.

The first component represents about 56% of the variance and combines the chemical variables TDS, Cl, SO₄, Na and Ca; the second component represents about 15.8% of the variance and combines K, Na and HCO₃. The weighting values of the specific parameters on the corresponding principal component are indicated in Table 3. These values represent their effective role on the ionic composition and water quality. The salinization processes that could contribute to the distribution of the first component parameters are evaporation, leaching/dissolution of marine salts and seawater intrusion, while dilution with rain water and the El-Salam Canal could be responsible for the second component.

The general geochemical features of the studied groundwater samples and the chemometric multivariate analysis highlighted the possible salinization processes that might contribute to salt composition and water quality. To put more emphasis on this subject, three mechanisms of salinization have been checked as outlined below.

To determine the possibility of dissolution/precipitation on salinization processes in the study area, the saturation index of the groundwater samples with respect to the relevant salts in the system (calcite, dolomite, anhydrite, halite and gypsum) has been calculated using the SOLMINEQ program (SOLMINEQ.GW 1999).

Table 4 reveals that about 90% of the groundwater samples are oversaturated with respect to dolomite and calcite, reflecting a tendency for precipitation. On the other hand, the groundwater samples are undersaturated with respect to gypsum, anhydrite and halite, reflecting a tendency for continuing dissolution of these salts from the aquifer matrix.

To check the potential of seawater intrusion as a salinization process acting on the groundwater under study, the GALDIT index (Chachadi & Lobo-Ferreira 2001), one of the weighting/rating driven indicators, has been used. It is determined based on six hydrogeochemical, hydrogeological and physical parameters inherent in the groundwater system. These parameters are as follows: (i) groundwater occurrence (aquifer type; unconfined, confined and semi-confined); (ii) aquifer hydraulic conductivity; (iii) the level of groundwater relative to sea level; (iv) distance from the shore (distance inland perpendicular from the shoreline); (v) impact on existing status of seawater intrusion in the area; and (vi) the thickness of the aquifer being mapped.

The overall GALDIT index has been calculated using Equation (1). It has been used to classify the study area according to its vulnerability to seawater intrusion. The computed GALDIT index of the studied groundwater samples has values in the range 2.5–7.5 (Figure 9). Based on Table 6, about 71% of the study area is of low seawater intrusion vulnerability and about 29% is of medium seawater vulnerability as a result of marine water associating Sabkha.

The evaluation of groundwater quality for various uses (drinking by both humans and livestock, as well as irrigation) is mainly based on TDS and major ions concentration in comparison with the recommended limits given in the standards for the different uses.

According to the Egyptian standards for drinking and domestic uses adapted from Higher Committee for Water (1995) (Table 7), about only 10% of the collected groundwater samples can be used for human drinking and the rest can not be used because their TDS and major ion concentrations exceed the permissible limits.

Water to be used for livestock and poultry is subject to quality limitations like those for human consumption. According to the upper limits of concentration for stock and poultry water, shown in Table 8, it appears that nearly all the groundwater samples in the studied area are suitable for the drinking by cattle (dairy) and about 69% and 15% are suitable for horses and poultry, respectively.

Water quality index (WQI) is a valuable and unique rating to depict the overall water quality status in a single term that is helpful for the selection of an appropriate treatment technique to meet the issues concerned (Chowdhury et al. 2012). An attempt has been made to use the calculated WQI values for irrigation suitability. In this study, WQI input on overall water quality deteriorations was used to achieve the pre-planned goals of this study (Rao et al. 2010; Balan et al. 2012).

The quality of groundwater resources in the study area is stressed by both natural and anthropogenic constraints. The hydrogeochemical characteristics of the system, and multivariate statistical analysis have been used to explore the water quality and salinization processes of groundwater resources in the northwest of Sinai. It has been concluded that the the water with the highest mineralization is primarily developed through leaching and dissolution of marine sediments, cation exchange with a small contribution from marine water associating Sabkha. The water quality of the study area is generally not suitable for human drinking, although this water can be used for livestock and poultry drinking and for irrigation purposes with some limitations.