Investigation the Origins of Groundwater Salinity in Baghdad City by Using Environmental Isotopes and Hydrochemical Techniques

Abstract


Introduction
Groundwater is a critical source of life and sustainability in arid and semi-arid regions. It is one of the most valuable natural resources in the majority of countries. As the world's population grows, so does the demand for groundwater to support urbanization, industrialization, and agriculture (Su and Wang, 2013). Salinization is major environmental adversity that affects soil and water resources, agriculture, and natural ecosystems. Salinity is a global issue, but it is especially severe in water-stressed arid and semi-arid regions where groundwater is the primary source of water. Increasing groundwater demand causes water table depletion and salinity to rise (Gopal, 2019). High levels of various elements, including sodium, sulfate, boron, fluoride, selenium, arsenic, and radioactivity, have also been linked to increased groundwater salinity (Rusi, 2018). Salinity is commonly expressed as Total Dissolved Solids (TDS) grams of salts per liter, but other proxies such as electrical conductivity (EC-S/cm) or chloride content (mg Cl per liter) are widely used. EC and chloride content can be converted to TDS by multiplying by 0.7 and 1.8, respectively (IAEA, 2004). Iraq is no exception in this regard, particularly in the southern parts, where groundwater demand has increased in recent decades. One of the most common and effective methods for determining the hydrogeochemical characteristics of groundwater are the hydrochemical and isotopic methods. have become essential components of the study of groundwater hydrogeochemical characteristics (Ako et al., 2012, Li et al., 2014Bozdag, 2016). The oxygen and deuterium isotopes ( 18 O and D) have preservative properties and are only affected by atmospheric conditions during recharging (Gat, 1996). In recent years, a large number of studies on the use of water isotopes in hydrological processes and the origin of salinity in groundwater have been published (Al-Paruany 2013; Ajeena, 2014;Ali et al;Nada, 2020). An approach with major elements (Cl -, Na + , Mg 2+ , Ca 2+ , and SO 4 2= ) and multi-isotope ( 2 H, 18 O) is used in this study to solve geochemical variation of salinity in groundwater in Baghdad.

The study Area
The study area is in Baghdad's central business district (Both side Al-Karkh and Russafa) (Fig. 1). The overall area is 1020 km 2 . The research area is located between latitudes 33 10-33 29N and longitude 44 09-44 33 E. Climate data were gathered from meteorological stations in Baghdad. Annual rainfall ranges from 0.04 to 24.6 mm, annual potential evaporation ranges from 66.85 to 530 mm, the annual average minimum temperature is 9.64 degrees Celsius, and annual average maximum temperature is 35.39 degrees Celsius. The Tigris River is the primary source of water supply in the area, stretching about 53 km through the city with a slope of 6.7cm/km, while groundwater is the secondary source, particularly since 2003, and is used for domestic purposes. From oldest to youngest, there are three major geological formations with age of (Holocene, Pleistocene, and Quaternary deposits) (Jassim and Goff, 2006). Sand and gravel aquifers have been determined underground at depths of 8-20 m in the studied area (Al-Hadithi et al., 2019).

Field work
During the wet (March 2021) and dry (August,2021) seasons, a total of 15 samples were collected to determine major cations and anions, as well as stable isotopes of water ( 18 O and 2 H). 12 samples were collected from agricultural and domestic wells, and 3 samples were collected from the Tigris River throughout Baghdad. The samples for stable isotopes analysis were collected in 50mL glassy bottles at each point, while the samples for chemical analysis were collected in one-liter polyethylene bottles. A GPS device was used to obtain information about the location of the well (Trex, Garmin International Inc, USA). Electrical conductivity (EC) and pH were measured in the field using a multi-parameter device (situ). Table 1 shows the coordination, sample number, and sample positions.

Lab Work
Table 2 displays all of the chemical and isotopic analyses of water samples performed according to APHA (2012) and IAEA (2005). Chemical parameters were analyzed in the Environment and Water Directorate's/Ministry of Science and Technology's chemical laboratories, while isotopic analyses were done in the Water Research Isotopes Lab. Table 2. devices, equipment's for water samples analysis.

Hydrochemical Characterization
The concentrations of ions in the study area vary significantly due to geological conditions, climate, and human activities. The TDS of groundwater in both sides (Al-Karkh and Russafa) ranged from 0.58 to 1.29 mg/L, and from 0.45 mg/L to 1.23 mg/L on both sides, respectively (Table 3 and 4). In general, Cl and Na + had a strong correlation in the groundwater environment; the mean Cl concentration was 25.71 mg/L, which was lower than the Na + concentration. The Ca-SO4 and Ca-Na-Cl-SO4 hydrochemical facies were determined to be the dominant hydrochemical facies of groundwater on the Al-Karkh and Al-Russafa sides, respectively, while the Ca-HCO 3 hydrochemical facies were determined the Tigris river's dominant hydrochemical facies. Human activities and geochemical processes may have caused differences in hydrochemical Facies.  In the month of March 2021, the pH of water ranges from 6.5 to 8, with an average electrical conductivity of 1811 µs/cm and a range of 578 to 3045µs/cm. TDS has an average concentration of 1373 mg/L with a range of 338 to 2582 mg/L. The concentrations of Na and Cl range from 53 to 332 and 65 to 974 mg/L, respectively. HCO3has an average value of 245 mg/L and a range of 104 to 549.46 mg/L. Ca 2+ (48-416 mg/L) and Mg 2+ (15-174 mg/L) concentrations in groundwater samples ranged (from1.9 to 6.8 mg/L) compared to SO4 2-(128-1301 mg/L), K + concentrations in groundwater samples ranged (from1.9 to 6.8 mg/L). The variation of TDS, Cation and Anions in the study area for both periods as shown in Figs. 2 and 3).

Fig. 2. Variation of TDS values and ions concentration for dry period in the study area
In the month of August 2021, The pH of water ranges from 6.7 to 8.2. It has an average electrical conductivity of 1811 µs/cm and a range of 595 to 3507 lS/cm. TDS levels average 1413 mg/L and range from 451 to 2657 mg/L. The concentrations of Na and Cl range from 55 to 342 and 67 to 1002 mg/L, respectively. HCO3 -has an average value of 252 mg/L and a range of 101 to 567 mg/L. Ca 2+ (50-432 mg/L) and Mg 2+ (16-180 mg/L) concentrations were compared to SO4 2-(132-1342 mg/L), and K + concentrations in groundwater samples ranged from 2 to 7 mg/L. Again, the concentrations are very low in comparison to other substances.

Fig. 3. Variation of TDS values and Ions concentration for dry period in the study area
The Gibbs diagram is commonly used to assess the source of dissolved chemical components, such as precipitation dominance, rock dominance, and evaporation. (Gibbs 1970). This graph depicts the Cl -/(Cl -+HCO 3 -) ratio as a function of TDS (Fig. 4). The distribution of sample points indicates that the chemical weathering of rock-forming minerals has an impact on groundwater quality. Except for GW3 and GW4, the majority of the samples are influenced by rock dissolution.

Source of Salinity
In the study area, the chemical analysis of the water samples revealed that the salinity ranges from 0.35 to 2.13 in March and 0.36 to 2.19 in August, 2021. Mansour station has the highest salinity of 2.19 and 2.13 (depth of 10 m), while Bayaa station has the lowest salinity of 0.73 and 0.70. The salinity gradient is increasing from the Russafa side to the Karkh side. However, groundwater salinity increased towards the study area's right side, which is similar to the spatial distribution pattern of electrical conductivity EC and Clconcentrations. The majority of high salinity water is found at depths ranging from 10 to 100 meters. The potential salinization sources in different depth groundwater in the study area are diverse, including natural saline and the presence of wastewater. (Krishnan,1985), As a result, the evaporation process at high temperatures could be a possible source of the different depths of groundwater salinity in the study area. Fig. 5 shows that three distinct groups of water are identified based on EC values and the hydrological setting: (1) Group A refers to well waters with EC values of were 2000µs/cm. (2) Waters collected from wells are assigned to Group B. these waters have EC values ranging from 1000 to 2000 µs/cm. (3) Group C represents water samples collected from the Tigris River. these waters have EC values that are less than 1000µs/cm.

Isotopic Content
Locally, many researchers were exploring and develop isotopic tools that can be used to determine salinity sources and processes in aquifer systems. (Al-Paruany,2013;Ajeena,2014Al-Paruany, 2017, Al-Kafaghi 2018, Nada, 2020, Ibraheem, 2020. To study the origin and mineralization of groundwater, 12 wells and three sample of Tigris River were subjected of isotopic studies (Table 5, 6). The values of δ 18 O in wells samples range from -6.46 to -3.99‰. and the δ 2 H ranges between -39.9 and 26.6‰, in wet period (March,2018) While The values of δ 18 O in wells samples range from -7.0 to -4.2‰, and the δ 2 H ranges between -40 and 28‰, in wet period (Agust,2018). The isotope values of the Tigris River in March are from -36.7‰ to -34.5 for δ 2 H and -6.62‰ to -6.34 for δ 18 O, while The isotope values of the Tigris River in August are from -39.9‰ to -37.5 for δ 2 H and from -7.2‰ to -6.9 for δ 18 O. Stable isotope values of water are represented graphically in Fig. 6 in comparison with the Global Meteoric Water Line (GMWL) defined by Craig (1961), with δ 2 H = 8 × δ 18 O + 10 and the Iraqi Meteoric Water Line (IMWL) define by Al-Paruany (2013), with δ 2 H = 7.573 × δ 18 O + 13.97.   O diagram, due to mixing with surface water and the effect of sewage water. Group C waters are depleted in March and more enriched in August, and they are influenced by sewage or agricultural water. Isotope results from the study area confirm the presence of at least three salinity sources in the collected samples: old water; dissolution from Holocene, and Pleistocene deposits; and anthropogenic pollution. There is no correlation between the values of 18 O and SO 4 in this study, so we conclude that the values of 18 O sulfate in water depend on the source of sulfate rather than water. The isotopic composition of groundwater from wells and the Tigris River differs insignificantly (Fig.8). The deuterium excess (d-excess) as one of the useful tools for studying water vapor sources, such as vapor source humidity or evaporation effect during rainfall (Clark and Fritz, 1997;Al-Paruany (2013) studied the d-excess in Iraq and found that the precipitation in Iraq has distinct seasonal variation in d-excess values, with high (d 16.9) and low (d >10.4) values. Fig. 9 depicts all of the d-excess sample values in the study area. To determine the source of salinity in water samples from the study area. by discussing the relationship between 18 O and EC, stable isotopes can successfully determine the mechanisms of groundwater salinization and identify the source of salinity. The origin of salinization from salt dissolution is not associated with any significant changes in the stable isotopic composition, but the mixing and/or evaporation processes are invariably associated with sensitive changes in the stable isotopic composition (Blackburn and Mcleod, 1983;IAEA, 2001).
According to Fig 10, The lack of significant differences between 18 O values and EC values may indicate that the source of salinity in the study area is due to mixing and/or dissolution processes. It is clear that some well water in the study area had the same values of 18 O (-6.9-6) with nearby in salinity values during the studied periods. This can be attributed to the influences of dissolution, mixing with sewage, and water sink. Some of these resources are natural, while others are man-made (Al-charideh, 2010). Finally, it is clear that dissolution and mixing processes are the most important factors that affect the source of salinity, as well as the major factor that is related to human activity (mixing with swage water).

Conclusions
 Hydrochemical and stable isotope techniques were used in this study to investigate the source of salinity in some wells in Baghdad. The following are the main findings of this study:  The Ca-SO 4 and Ca-Na-Cl-SO 4 hydrochemical facies were found to be the dominant hydrochemical facies of groundwater on the Al-Karkh and Al-Russafa sides, respectively, while the Ca-HCO3 hydrochemical facies were found to be the dominant hydrochemical facies of the Tigris River.  Human activities and geochemical processes may have caused differences in hydrochemical Facies.  Three distinct groups of water are identified based on EC values and the hydrological setting: (1) Group A refers to well waters with EC values of up to 2000s/cm. (2) Waters collected from wells are assigned to Group B. The EC values of these water range less than 1000µs/cm.  Isotope results from the study area confirm the presence of at least three salinity sources in the collected samples: old water, dissolution from Holocene and Pleistocene deposits, and anthropogenic pollution.  companion the hydrochemical and isotopic techniques to study the origin of salinity in the studied area, it's clear results that, dissolution, mixing process are most factors that affect the source of salinity, as well as the major factor that is related to human activity (mixing with swage water).