INVESTIGATION OF THE GROUNDWATER SUITABILITY FOR IRRIGATION PRACTICES IN AN ARID REGION : CASE STUDY

Groundwater is an important source of acceptable wa t r for irrigation in the arid regions and in parti cular in the United Arab Emirates (UAE). Demand for groundwa ter is increasing in the UAE due to population growth and significant economic advancement associa ted with political stability. As agricultural pract i es are the main land use in Al Hayer area, southeast U AE, majority of extracted groundwater from major aquifer is used to meet the increasing demands of i rrigation. This study is aimed to assess the suitab ility of groundwater in the study area for irrigation practi ces using classifications of Sodium Adsorption Rati o (SAR), Soluble Sodium Percentage (SSP), Magnesium A dsorption Ratio (MAR) as magnesium hazard and Residual Sodium Carbonate (RSC). In addition to tha t, the quality is assessed using the Total Dissolve d Solids (TDS) and Electrical Conductivity (EC) of co llected groundwater samples. Assessing the quality of groundwater in the region will help the decision ma kers in determining appropriate actions and using t he conventional management tools to protect groundwate r from the possible contamination. The chemical results indicate that the groundwater of the study area contains excess of Na + nd Mg. This is related to the weathering of Oman Mountains which are located to the east of the study area. The data analyses of salinity hazard suggested that about 44% of collect ed groundwater samples from the entire region are grouped as very high salinity and are not suitable for irrigation and about 56% are grouped as high sa linity which is doubtful to be used for irrigation. The ca lculations of SAR reveal that the groundwater of th e study area is good to excellent for crop production. Whil e, the calculated magnesium hazard suggests that al l samples are not suitable for irrigation purposes. T he SAR-EC plot presents two groups of sampling poin ts. The first group of groundwater can be used for irri gation of most crops in almost all soils, however, the second group of groundwater is not suitable for irr igation under normal conditions.


INTRODUCTION
Water is an essential bio-resource for the entire life. The freshwater resources are less than 1% and about 0.01% of all water present on the earth (Hakim et al., 2009 andShiklomanov, 1993). The United Arab Emirates (UAE) is an arid region and one among those countries in the Middle East and North Africa (MENA) facing serious shortages in water resources to meet the demands of rapid developments in the country. Therefore, sustaining water resources in the UAE is a major concern for the decision makers and scientists. Previously, groundwater was considered as a major source of water in the UAE and in particular the investigated area. With the time, the desalinated water became a major source for different domestic, agricultural and industrial uses.
Groundwater is one of the conventional water resources in the world. Human activities and civilizations were concentrated around the sources of water. In UAE, these unplanned human activities and increasing economical developments applied huge stress on groundwater resources. Agricultural practices are the main land use in Al Hayer area, which is situated to the southeast of the UAE. Successful agrictural practices requries high quality irrigated water. More than 70% of the groundwater consumed by agriculture and quality of groundwater is a Science Publications AJES major concern for farmers in the investigated area. Agricultural practices increased over last few decades due to governmental support and this was reflected in changing the size of cultivation in the investigated area. The total irrigated area in the region was 69, 142 and 136 Km 2 in 199669, 142 and 136 Km 2 in , 200069, 142 and 136 Km 2 in and 2004 High salinity groundwater is one of the major constrains in agro-well farming in arid region (Kendaragama, 2000). Irrigation using poor quality water could bring undesirable elements to the soil which might affect the soil fertility and change its physical and chemical properties (Nishanthiny et al., 2010 andIbrahim et al., 2012). Saline water used for irrigation can reduce or even prohibit crop production, while low salinity in irrigation water might reduce water infiltration, which indirectly affects the crop production (Grattan, 2002). In order to avoid associated problems and optimize the crop production, it is essential to understand the quality of irrigation water. This study is designed to assess the toxicity of groundwater and investigate the suitability of it in Al Hayer area for sustainable agricultural practices using different methods.

MATERIALS AND METHODS
The UAE is located in the southeastern part of the Arabian Peninsula between latitudes 22° 40' and 26° 00' N and longitudes 51° 00' and 56° 00' E. The study area namely Al Hayer is situated within the northern part of Al Ain area and northeastern part of Abu Dhabi Emirate ( Fig.  1) and the landmass of the study area is about 20 Km 2 . The study included 25 groundwater wells collected from the entire study area from east where the recharge occurs to the west where the discharge occurs ( Fig. 1).
The physical parameters of pH, temperature, Total Dissolved Solids (TDS) and Electrical Conductivity (EC) for the collected groundwater samples were measured in the field. The collection of groundwater samples was done after the removal of the stagnant water in the pipes and the contaminated water. High density 1 L PVC two bottles were used for sampling in which each sampled bottle was washed with deionized water and then with water from sampled well. Each bottle was filled up to the brim and immediately sealed to avoid exposure to air (Clesceri et al., 1989). Samples used for anion analyses (HCO 3 -& CO 3 -2 ) were stored at a temperature below 4°C before the analyses at the laboratory. However, samples used for cation anlayses (Na + , K + , Ca +2 , Mg +2 ) were acidifed to pH <2 using nitric acid 65% (Appelo and Postma, 1999), . The collected water samples were transported to the Hydrogeology Laboratory at UAE University for further analyses. The samples were analyzed for Sodium (Na + ), Potassium (K + ), Calcium (Ca +2 ), Magnesium (Mg +2 ), Carbonate (CO 3 -2 ) and Bicarbonate (HCO 3 -). Different parameters were calculated based on the results of the chemical analyses of groundwater samples for suitability for irrigation purposes. The classifications of groundwater for irrigation purposes were obtained using the Sodium Adsorption Ratio (SAR), Soluble Sodium Percentage (SSP), Magnesium Adsorption Ratio (MAR) as magnesium hazard and Residual Sodium Carbonate (RSC). The equations for the above parameters are as follows (Domenico and Schwartz, 1990;Eaton, 1950;RSL, 1954;Szabolcs and Darab, 1964;Todd, 1980): All concentrations are expressed in meq/L. In addition to that, correlation analyses between different quality indicators to understand different process that affect the quality of irrigation water was done.

RESULTS
The physical and chemical analyses of collected groundwater samples from the study area are presented in Table 1. The pH of collected groundwater ranges from 7.9 to 8.3 indicating that the groundwater of the investigated area is characterized by alkaline tendency. The temperature of groundwater of the study area varied between 29 to 33°C. The range of EC in groundwater samples spans between 462 to 7155 µ S/cm. The TDS values extend from 206 to 3650 mg L −1 . The chemical results showed that the concentrations of Ca +2 , Mg +2 , Na + and K + in groundwater range at 0.6-9.6, 1.3-17.2, 2.2-43.5 and 0.08-1.02 meq L −1 , respectively. The HCO 3 concentration in groundwater shows a range from 1.6 to 10.6 meq L −1 .
The SAR for groundwater of the study area ranged from 2.6 to 15.7 and the calculated SSP for water samples varied from 52.3 to 78.7. The values of MAR and RSC were in the range of 87.6 to 100 and -6.98 to 2.9, respectively. The RSC values for most of the water samples had negative values suggesting that the dissolved calcium and magnesium are higher than the bicarbonate contents in those groundwater samples. Based on MAR criterion, all samples are unsuitable for irrigation, while based on SAR, all samples are in excellent and good conditions ( Table 2). AquaChem program (V 5.1) is used to construct Wilcox diagram, which is the relationship between salinity hazard expressed in EC (µ S/cm) and sodium hazard expressed in SAR.
The correlation coefficients for the relationships among EC, SAR, MAR, SSP and RSC were determined ( Table 3). It was noticed that EC was correlated with SAR and RSC with correlation coefficient of 0.83 and 0.76, respectively.

DISCUSSION
The pH range of groundwater in the study area is higher than 7 and within the limit of alkaline nature. High pH could be possibly related to the presence of sodium, calcium, magnesium and bicarbonate ions in groundwater. Such water belonging to this range of pH can be used for irrigation. The concentrations of both Ca +2 and K + in groundwater of the study area were far below the maximum concentration compared to 20 and 2 meq L −1 , respectively as the maximum recommended limit. However, the Na + concentration in groundwater exceeded the recommended limit of 40 meq L −1 in three samples. Excessive Na + in the collected samples might be ascribed to evaporation of irrigated water. About 88% of water wells in the investigated area can be used for long term irrigation without any harmful effects on soils and crops. At the same time, 10 out of 25 samples exceeded the allowance limit of Mg +2 of 5 meq L −1 . It is observed that the excess of Na + and Mg +2 are concentrated to the west of the study area as the groundwater moves from east to west. This could be related to weathering of Oman Mountains.
Despite having nil amount of carbonate (CO 3 -2 ) in this groundwater, the concentration of bicarbonate (HCO 3 -) in all samples was within the allowance limit except sample no. 19 which exceeded the recommended limit of 10 meq L −1 . Therefore, groundwater of the study area can be used effectively for irrigation.
Groundwater of the study area was assessed to determine the suitability of groundwater of the region for irrigation. High salinity in irrigation water is toxic to plants and may pose a salinity hazard and soils with high concentrations of salts may induce physiological drought conditions (Nishanthiny et al., 2010). The salinity of water is measured and classified according to Sadashivaiah et al. (2008). Based on EC classification, about 44% of the collected samples (11 samples out of 25) are classified as very high salinity which is unsuitable for irrigation, but the remaining 56% (14 samples) is characterized by high salinity which is doubtful to be used for irrigation. Such high salinity in irrigation water might affect the osmotic pressure of the soil solution. It is noticed that, the salinity of groundwater increases towards the west of the study area away from the recharge area suggesting that salinity accumulates as the groundwater travel from recharge to discharge area.

The calculated
SSP for groundwater in the investigated area showed that about 44% of the samples (11 samples out of 25) are grouped under permissible class (SSP from 40 to 60%) and 56% of the samples (14 samples) are grouped under doubtful class (SSP from 60 to 80%). Most of the samples are problematic for irrigation purposes in the study area. However, most of the groundwater in the study area based on SAR calcuation is classified as good (24%) to excellent (76%) for crop production. On the other hand, the calculated magnesium hazard indicated that groundwater are not suitable for irrigation purposes as MAR exceeded 50% and could cause harmful effect to the soil.
There is a tendency for Ca +2 and Mg +2 to precipitate in water having high concentration of bicarbonate. Consequently, the proportion of sodium in water is increased in the form of sodium bicarbonate (Sadashivaiah et al., 2008). All samples have RSC less than 1.25 except three samples (samples no. 3, 10 and 14) and are considered safe for irrigation uses, but one sample (sample no. 10) is unsuitable for irrigation purposes as RSC is 2.9. Groundwater from wells no. 3 and 14 can be used with good management and salinity monitoring.
Relationships between different quality parameters were determined (Table 3). It is observed that a slight good and positive relationship of EC with SAR and RSC with r 2 of 0.83 and 0.76, respectively. This reveals that sodium accumulates in groundwater due to heavy use of agricultural practices in the area.
The relationship between SAR (meq/L) and EC (µS/cm) is plotted with R 2 of 0.8 (Fig. 2). The samples distribution in the plot indicates that there are two groupings of the samples. The first group, located to the east of the study area, is characterized by low salinity and low SAR which is equivalent to C3-S1 class in Wilcox diagram. Such class is of high salinity and low sodium content. Consequently, special management for salinity control is required and plants with good salt tolerance should be selected. The water of this class is acceptable for irrigation of most crops in most soils. The second group of the samples, located to the west of the study area, is characterized by high salinity and high SAR which is equivalent to C4-S3 class of Wilcox diagram implying that the groundwater is not suitable for irrigation under ordinary conditions, but may be used in soils that are permeable with adequate drainage and application of excess irrigation water in order to allow leaching. Such water can be considered for very-salt tolerant crops.

CONCLUSION
Investigating the suitability of groundwater in the study area was tested based on salinity hazard (EC), sodium hazard (SAR and SSP), magnesium hazard (MAR) and bicarbonate hazard (RSC). Excess of Na + and Mg +2 may be ascribed to the weathering of Oman Mountains which is located to the east of the study area. The results showed that about 88% of groundwater from the study area can be used for long-term irrigation without any effects on the crop production.
The analyses based on salinity hazard showed that about 44% of collected groundwater samples from the entire region are grouped as very high salinity and are not suitable for irrigation and about 56% are grouped as high salinity which are doubtful to be used for irrigation. Such high salinity in irrigation water might affect the osmotic pressure of the soil solution. However, using SSP classification, about 44% of collected samples are permissible for the irrigation uses and about 56% of the collected groundwater samples are doubt to be used for irrigation practices. The SAR classification indicates that the groundwater of the study area ranges from good to excellent for crop production, while the calculated magnesium hazard indicates that all samples are not suitable for irrigation purposes.
There are two groups of samples based on SAR-EC relationship. The first group is mainly of high salinity and low sodium content. This type of water requires special attention with selection of plants of good salt tolerance and the water can be accepted for irrigation of most crops in most soils. The second group of groundwater is not suitable for irrigation under ordinary conditions, but may be used under special conditions.
In conclusion, it is highly recommended to monitor the quality of soil and irrigation water based on regular basis to avoid problems associated with crop production. Future research must focus on understanding the effect of agriculture practices on the quality of groundwater in neighboring areas. Remote sensing and Geographic Information System (GIS) may use as a tool to identify the extension of irrigated areas and can help to plan future research. Appelo, C.A.J. and D. Postma, 1999. Geochemistry, groundwater and pollution. Fourth Print, A. A. Balkema, Rotterdam, Netherlands, pp: 536.