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Evaluation of the Hydrochemical Characteristics of Groundwater and its Suitability for Desalination to Product Potable Water in Tolul Al-Baj, Salahaldin, Iraq

The study area, Tlul Al-Baj, suffers from a shortage of fresh water and most people depend on groundwater for different uses (drinking, domestic, irrigation, etc.). The present research aims to select the most suitable wells for desalination and production of potable water in Tlul Al-Baj area. Twenty-two samples of groundwater were collected to evaluate the hydrochemical properties of groundwater in the shallow aquifer in the area and to determine their suitability for desalination purposes. The study included measuring the physicochemical characteristics of groundwater, such as total hardness (TH), total dissolved solids(TDS), sodium adsorption ratio (SAR), sodium ratio (Na%), turbidity (Tur), pH...etc. Chemical analyses for the main components of water samples were also conducted, including cations such as sodium (Na +) , potassium (K +) , calcium (Ca ++) , and magnesium (Mg ++) , as well as anions such as chloride (Cl) , sulfate (SO4 = ), bicarbonates (HCO3 ), and nitrate (NO3 ). In addition, concentrations of trace elements such as iron (Fe), copper (Cu), zinc (Zn), lead (Pb), nickel (Ni),cobalt (CO), chromium(Cr) and cadmium (Cd) were determined. The results of the physical and chemical analyses for the groundwater of the study area were compared with the international and local standards to determine their suitability for drinking uses and to select the most suitable wells for the production of drinking water by desalination. The results indicated that the most suitable wells for desalination were wells numbered 4, 8, 9, 17, and 19.


Introduction
Water is one of the most important materials for life. No internal biological process in the body of any organism takes place without water, as water represents the most widely distributed fluid in nature and plays a basic role in many of the vital processes in the body of the organisms; for example, water is found in human cells by in a proportion of 50-60% [1].
Drinking water is the water free of chemical and biological contaminants and toxic material [2]. Many of the water sources used by humans contain some vectors of disease that may cause long-term problems. The World Health Organization (WHO) and other national and international organizations have set a number of standards for potable water. Most of these specifications agreed that a level of 500 ppm is the acceptable limit for TDS in drinking water. Dissolved salts vary in their effects on human health; for example, calcium carbonate has no physiological effects, alkaline carbonate causes severe damages, while alkaline sulphates are less harmful. Alkaline chlorides, such as table salt exert moderate effects, magnesium sulfates give a bitter taste to water, whereas iron at 0.5ppm makes the taste of water unpalatable [3].
The main problem in most parts of the world is the supply of drinking water to the population as a result of the increase in human population. As the gap between drinking water supply and requirements can expand and reach dangerous levels in most parts of the world, it can be a threat to human existence [4], with the reports that the coming wars will be because of water [3].Therefore, the scarcity of fresh water can cause a growing problem around the world, especially in areas of dry climates with less than 100 mm of rain [5].
Iraq suffers from a shortage of fresh water resources as a result of its geographical location within the arid region. Moreover, the headwaters of the Tigris and Euphrates rivers are located outside Iraq's administrative boundaries. The continuous increase in population growth and the increasing demand rates made it necessary to find alternatives to fresh water resources. This led many researchers in this field to study groundwater as an alternative source of fresh water. One possible solution is treating groundwater for producing fresh water that can be used for human drinking purposes, as well as other domestic uses [6]. Groundwater desalination is an essential approach for the provision of drinking water in the dry areas and away from any freshwater source. Groundwater desalination is desirable for several factors that include high recovery rate, working with desirable energy sources such as solar energy, amount of local water produced, percentage of concentrated return water, and level of energy consumed [7].
One of the main reasons for this study is the lack of fresh water in the study area and the high number of people depending on groundwater for drinking, domestic and other uses. The population of the study area exceeds 10,000, according to the census of the district council.
Desalination can be defined as any process that removes salts from water, whether it is groundwater or marine water. With improvements in technology, desalination processes are becoming cost-competitive with other methods of producing usable water for our growing needs [8]. Brackish Water is the water whose salinity is less than the salinity of sea water, in which the concentration of salts is between, 5000 and 20000 ppm, while that of sea water is higher than 20000 ppm [9].
A desalination process essentially separates saline water into two parts, one with a low concentration of salt, known as treated water or product water and the other with a much higher concentration than the original feed water, usually referred to as brine concentrate or simply as concentrate.
The major types of technologies that are used around the world for desalination can be broadly classified into either thermal or membrane. Both technologies need energy to operate and produce fresh water. Within those two broad types, there are sub-categories (processes) using different techniques. Membrane technologies can be subdivided into two broad categories; Electro-dialysis/ Electro-dialysis Reversal (ED/EDR), and Reverse Osmosis (RO) [10].
Thermal technologies, as the name implies, involve heating saline water and collecting the condensed vapor (distillate) to produce pure water. Thermal technologies have rarely been used for brackish water desalination, because of the high costs involved. They have however been used for seawater desalination and can be sub-divided into three groups; Multi-Stage Flash Distillation (MSF), Multi-Effect Distillation (MED), and Vapor Compression Distillation (VCD). The choice of desalination method depends on cost, water quality and quantity of water produced, [10].
Many countries in the world are trying to provide fresh water by developing their natural resources, rationalizing consumption, especially in agriculture, and treatment and reuse of industrial, agricultural and sanitary drainage water, as well as desalination of salt water. Desalination is one of the alternatives to obtain fresh water in the world and is a strategic alternative to many countries, especially in the arid and semi-arid regions [11].
In recent years, RO is the best desalination technology for saline surface and groundwater to provide potable water. It is the most wide spread method that consumes less energy than other methods [8].RO is used in the production of80% of the desalinized water worldwide, while thermal methods represent only 20%.
The RO process is relatively new in comparison to other technologies. The cost of water desalination in membrane processes varies according to the type and composition of the feed water. Large-scale RO plants can use brackish water containing TDS of 2000 to 10 000 ppm, but, as TDS concentration increases, the unit cost of the desalinated water also increases [12]. The study area is located in the far north of Salahaldin governorate, northern Iraq, and represents the boundary between Salahaldin and Mosul governorates. The area covers about 240 Km 2 (Figure-1).

Material and Methods
Geological, geomorphological and hydrological background of the study area was reviewed and extracted, then the topographic maps were prepared with the scales of 1: 100000 and 1: 2500000. The accuracy of field measurements and sampling is usually reflected in the accuracy of laboratory results Therefore, samples with a possible error might indicate a lack of intensive care in the sampling, and these are secondary processes that occur on samples from the beginning of sampling to the end (analyses in the laboratory). Furthermore, in some samples the error may be above the permissible limits. This in turn is due to several reasons such as that sampling is inaccurate. Therefore, safe collection of water samples is as important as the test results. It is not possible to standardize a particular method of sampling, because of different source conditions and variety of tests. However, the following conditions were adopted in the sampling processes: 1. Water was pumped from the well for at least half an hour to ensure that the sample significantly represents the quality of the groundwater aquifer. 2. The amount of the sample was sufficient for all required analyses. 3. The temperature was recorded in the field because it has important significance. 4. The samples were labeled with information about the day and hour at which the sample was collected. 5. The sample was maintained so that there was no change in the hydrochemical properties of the water before testing. 3767 Field work included: 1. A first field trip to the study area to identify the geomorphological effect and the rock outcrops of the geological formations, and to determine the different field measurements. 2. A second field trip that included a field survey of the study area to determine the locations and coordinates of the wells using GPS, and to collect 22 water samples taken from wells distributed in the study area, as homogeneously as possible. Physiochemical measurements and chemical analyses of water samples were performed in the laboratories of the Water and Soil Science Center /Ministry of Science and Technology / Baghdad. Accuracy, which is a measure of the appropriateness and proximity of the results to their true values, was calculated in epm using the ionic balance method. This was performed by measuring the relative difference, which is the difference among sum of major ions divided by the total ions [13].Ionic balance was used to verify the accuracy of the results and , (Table 1)

A:Accuracy
The causes of the error in chemical analyses include the method of analyses, the method of preparation of the samples, the quality of chemicals materials, the efficiency of the devices, among others. Therefore, the error percentage (E%)must be calculated to ensure the accuracy of the results. When comparing the error percentage of chemical analyses of the water samples (Table 1), it was found to be within the permissible limits for the analyses accuracy, except for the results from two wells (W4, W21), which had an increase by a very small percentage that can be neglected. Uncertain A < 90% U > 10% Arelevant software (Arc GIS, Surfer, 13) was used to draw and prepare the maps.

Results and discussion Physiochemical characteristics
The results of the physiochemical characteristics are tabulated in table 3. 1. Temperature: The temperature of water directly affects many of its physical and chemical characteristics. Because groundwater is stored underground, it has a relatively constant temperature throughout the year [14]. The temperature values of water wells in the study area ranged between 23 and 25 Cº. The slight variation indicates the absence of thermal pollution, with the values being within the permissible limits (8.88-33.8 Cº). 2. pH: The pH is an important variable in water quality assessment as it influences many biological and chemical processes within a water body and all processes associated with water supply and treatment. pH value typically represents the result of the equation, pH=-Log [H+] [15]. The pH values were between 7-8, which were within the permissible limits.  [17], Their values ranged 0.9 -21.5 NTU. 5. Alkalinity: Alk. is an index of the buffering capacity of water-produced anions of weak acids, such as hydroxides, bicarbonates and carbonates [17]. Their values showed a range of 61 -244.
6.Dissolved oxygen: The level of dissolved oxygen in water is used as an indication of pollution and its potability [18]. DO%, values for samples showed a range of 28.2 -46.6%. 7.Total dissolved solids: TDS are the total dissolved salts in a solution, whether ionized or nonionized, and do not include suspended solids and dissolved gases in that solution. TDS consists mainly of the sum of cations and anions [19]. When comparing TDS values of groundwater samples in the study area using previously adopted classifications of Todd [20] and Klimentove [21],the results indicated that the groundwater of the studied area was Slightly Water and Brackish Water, with the values ranging between 1934 and 7910 ppm (Figure-2).  [22], dependent on dissolved ions in water. Values of EC for water samples ranged between 2720 to 11340 ms (Figure-2).  The above figure shows a high compatibility between TDS and EC results, which is an important indicator that reflects the accuracy of the analyses results, since the relationship between them is direct [20]. Salt content and electrical conductivity were minimum in the eastern and southeastern parts of the study area.

Chemical analyses
The results of the chemical analyses are tabulated in table 4. Cations 1.Calcium (Ca +2 ): The main source of calcium ion in aquifers is the dissolution of some sedimentary minerals, such as calcite, dolomite, anhydrite, and gypsum [22]. Calcium ion values in water samples of the study area ranged 300-654 ppm. 2. Magnesium (Mg +2 ): Gypsum and clay minerals are the most important sources of magnesium ions in water [23]. Its concentrations in the present study ranged between 60 and 334 ppm. 3. Sodium (Na +1 ): Weathering the evaporative rocks is the most important source of sodium ion [24]. Its concentrations ranged between 222 -533 ppm. 4. Potassium (K +1 ): The concentration of potassium in natural water is much lower than that of sodium [23]. Its concentrations in the water wells of the study area were within the permissible limits, and ranged 8-16 ppm. Maps of Ca +2 , Mg +2 and Na +1 distribution (ppm) in groundwater samples are shown in Figure-

Trace elements
The concentrations of the trace elements in the groundwater samples are tabulated in Table-5

* WHO 2006
The results of physiochemical tests and chemical analyses of groundwater samples were compared with the international and local specifications of drinking water (WHO 2017, Canada 2017, India 2012, IQS 2009), table 6, it was showed that this water is not suitable for drinking purposes, because most of the parameters' concentrations were higher than the permissible limits for drinking purposes, except those of carbonate and nitrate, which were within the permissible limits.
The general principle of desalination is that productivity and efficiency are inversely proportional to the salt content of raw water. Therefore, it is necessary to find water sources with minimal dissolved salt content, so that the desalination process can be more economic, with high quality, efficiency, and productivity, and with reduction of rejected water.
Depending on the values of TDS and EC, which are used as a key measurements of water salt content, and the results of chemical analyses, the results show that wells with closest values to the applied specifications are those with numbers 4, 8, 9, 17, and 19. Depending on the TDS value, RO is the most suitable method for desalination. It is also the most widely used method of desalination of groundwater in the recent time recently.