The Hydrochemical Characteristics of Drinking Water in Central Settlements of Sukhbaatar Province, Eastern Mongolia

: A focus of this study was to investigate water quality in the central settlement area of Sukhbaatar province, which is characterized by semi-arid climate. In total, 47 water samples were collected from 13 soums in 2021, and major ions and trace elements were analyzed to evaluate the suitability of groundwater for drinking purposes. The dominant hydro-chemical facies of groundwater were the Na-HCO 3 type, which represents 46.8% of the total analyzed samples. The water supply wells of Baruun-Urt soum, Asgat, Khalzan, and Erdenetsagaan soums do not meet the requirements of drinking water standards due to the content of magnesium, ﬂuoride, and uranium ions. Additionally, the ﬂuoride ion content in most wells exceeds the drinking water standard, while the ﬂuoride content of Dariganga soum water was less than the drinking water standard; however, the Munkhkhaan soum was suitable.


Introduction
One of the main factors affecting the development of any country is clean and fresh water. Groundwater quality issues are crucial for sustainable water resources management in many countries worldwide, especially in arid and semi-arid regions. In the region, drinking water supply mainly depends on the groundwater, with no surface water resources due to natural moisture supply conditions. There is a natural pattern of poor water quality due to poor groundwater recharge in the region. Chemical elements in drinking water could possibly affect human health [1].
The main objective of this work was to study the drinking water quality of the soum centers in the Sukhbaatar province.

Materials and Methods
The Sukhbaatar province has a total area of 82.3 thousand km 2 and is included in the steppe zone. In terms of climate, it is dry, with cool summers and cold winters, while the average temperature in July is +21.3 • C, and the average temperature in January is −19.9 • C [2].
We collected 43 groundwater (GW) and 4 spring water samples from soum centers in June (summer) 2021 ( Figure 1). All sampling sites were used for drinking water supply. Electrical conductivity (EC), Total Dissolved Solid (TDS), pH, ORP, and water temperature were measured on-site using Multiparameter Hanna HI98195. The sampling site locations were verified using a portable GPS meter (GPSMAP 76S; Garmin Ltd.). All water samples were sealed in 1000 mL (anion and cation) and 100 mL (heavy metal) polyethylene bottles for chemical analysis. We measured Ca 2+ , Mg 2+ , Na + , K + , HCO 3 − , and Cl − via titration. water samples were sealed in 1000 mL (anion and cation) and 100 mL (heavy metal) polyethylene bottles for chemical analysis. We measured Ca 2+ , Mg 2+ , Na + , K + , HCO3 − , and Cl − via titration. NH4 + , NO2 − , NO3 − , SO4 2− , F, and Fe were analyzed using a Т-60 U spectrophotometer. The heavy metals were analyzed via ICP.
As a quality check on the analysis, the charge balance error (CBE ± 10%) was calculated using the following equation, and the calculated error did not exceed ±10%. The data were compared to the Mongolian standard (MNS0900:2018) [3].

CBE =
∑ cations eq − |∑ anions| ∑ cations eq + |∑ anions| × 100% (1) Figure 1. Study area. Figure 2 shows the classification of the hydro-chemical characteristics of water samples based on the main ion concentrations. The dominant hydro-chemical facies of groundwater were the Na-HCO3 type, which represents 46.8% of the total analyzed samples, while Ca-HCO3 and HCO3-mixed types each represent 17%, the HCO3-Na-Mg type and HCO3-Mg-Na type each represent 8.5%, and the mixed-Na-Mg type represents 2.1% of the total samples ( Figure 2). The water supply wells of Baruun-Urt soum and Asgat, Khalzan, and Erdenetsagaan soums do not meet the requirements of drinking water standards due to their content of magnesium, fluorine, and uranium ions. Additionally, the fluoride ion content in most wells exceeds the drinking water standard, while the fluoride content of Dariganga soum water was less than the drinking water standard; however, Munkhkhaan soum was suitable. Almost 60% of the water samples exceeds the standard by magnesium.

Results and Discussion
In total, 21 or 44.7% of all samples do not meet drinking water standards due to uranium content, and 11 or 23.4% of all samples have nitrate pollution. These findings As a quality check on the analysis, the charge balance error (CBE ± 10%) was calculated using the following equation, and the calculated error did not exceed ±10%. The data were compared to the Mongolian standard (MNS0900:2018) [3]. CBE = ∑ cations(eq) − |∑ anions| ∑ cations(eq) + |∑ anions| × 100% (1) Figure 2 shows the classification of the hydro-chemical characteristics of water samples based on the main ion concentrations. The dominant hydro-chemical facies of groundwater were the Na-HCO 3 type, which represents 46.8% of the total analyzed samples, while Ca-HCO 3 and HCO 3 -mixed types each represent 17%, the HCO 3 -Na-Mg type and HCO 3 -Mg-Na type each represent 8.5%, and the mixed-Na-Mg type represents 2.1% of the total samples ( Figure 2). The water supply wells of Baruun-Urt soum and Asgat, Khalzan, and Erdenetsagaan soums do not meet the requirements of drinking water standards due to their content of magnesium, fluorine, and uranium ions. Additionally, the fluoride ion content in most wells exceeds the drinking water standard, while the fluoride content of Dariganga soum water was less than the drinking water standard; however, Munkhkhaan soum was suitable. Almost 60% of the water samples exceeds the standard by magnesium.

Results and Discussion
In total, 21 or 44.7% of all samples do not meet drinking water standards due to uranium content, and 11 or 23.4% of all samples have nitrate pollution. These findings suggest that appropriate actions have to be taken to improve groundwater quality and the protection of public health in the Sukhbaatar province. suggest that appropriate actions have to be taken to improve groundwater quality and the protection of public health in the Sukhbaatar province. A Gibbs diagram was used for the geochemical evolution of groundwater, as well as evaluating evaporation, weathering, and precipitation in arid and semi-arid regions [4]. We can see in Figure 3 that most of the samples are plotted in the field of the rock dominance area, indicating that the main geochemical process is rock-water interaction in the study area.  A Gibbs diagram was used for the geochemical evolution of groundwater, as well as evaluating evaporation, weathering, and precipitation in arid and semi-arid regions [4]. We can see in Figure 3 that most of the samples are plotted in the field of the rock dominance area, indicating that the main geochemical process is rock-water interaction in the study area. suggest that appropriate actions have to be taken to improve groundwater quality and the protection of public health in the Sukhbaatar province. A Gibbs diagram was used for the geochemical evolution of groundwater, as well as evaluating evaporation, weathering, and precipitation in arid and semi-arid regions [4]. We can see in Figure 3 that most of the samples are plotted in the field of the rock dominance area, indicating that the main geochemical process is rock-water interaction in the study area.