Water Resources Sustainability of Ulaanbaatar City, Mongolia

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Article Water Resources Sustainability of Ulaanbaatar City, Mongolia

Naranchimeg Batsaikhan 1,2, Jae Min Lee 3 ID , Buyankhishig Nemer 2 and Nam C. Woo 1,* ID 1 Department of Earth System Sciences, Yonsei University, Seoul 03722, Korea; [email protected] 2 School of Geology and Mining Engineering, Mongolian University of Science and Technology, Ulaanbaatar 216046, Mongolia; [email protected] 3 Institute of Natural Sciences, Yonsei University, Seoul 03722, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-2123-2674

Received: 17 May 2018; Accepted: 5 June 2018; Published: 8 June 2018

Abstract: Ulaanbaatar (UB), the capital of Mongolia, is one of the fastest-growing cities in the developing world. Due to increasing demand driven by rapid population and industrial growth, sustainable water resource management is required. Therefore, we investigated sustainability in UB from the perspective of water quality. During five sampling campaigns, we collected 135 water samples (58 from bedrock wells, 44 from shallow wells tapped into the alluvial aquifer, 24 from rivers, and 9 from springs). The hydrochemistry of the water samples was controlled by two major processes: NO3 contamination, and silicate and carbonate mineral weathering. The groundwater samples could be classified into three groups based on their NO3 levels and spatial distribution. Group 1 had natural background NO3 levels (median: 1.7 mg/L) and silicate weathering–dominant water–rock interactions and was distributed in the alluvial aquifer along the floodplain. Group 2 was dominated by carbonate weathering processes, had a maximum NO3 concentration of 47.4 mg/L, and was distributed between the riverbank and upslope area; overall, it reflected ongoing contamination. Group 3 was distributed in the upslope Ger districts and showed significant NO3 contamination (range: 64.0–305.4 mg/L) due to dense and poor living conditions. The stable isotope signatures indicated that the city’s major water supply from riverbank filtration (i.e., Group 1 wells) mixed dynamically with the river; therefore, it showed no sign of NO3 contamination. However, the isotope values and bedrock groundwater quality of wells in Groups 2 and 3 implied that they were closely connected, with the same water source, and showed a strong potential for expanding NO3 contamination toward Group 1 wells. To support sustainable development in UB, the implementation of appropriate institutional measures to protect and preserve water resources, with systematic spatio-temporal monitoring and a focus on Ger districts, is crucial.

Keywords: monitoring; nitrate; sustainability; Ulaanbaatar; water resources

1. Introduction Mongolia, a landlocked country in central Asia, has a total area of about 1.5 million km2 and an average altitude of 1580 m. Its climate is semi-arid to arid, with the mean annual precipitation ranging from 400 mm in the north to less than 50 mm in the southern Gobi region [1]. Due to global warming, the temperature in Mongolia has increased at least 2.14 ◦C since 1940 and is projected to increase by up to 5 ◦C by the end of the 21st century [2]. As the political, industrial, and economic center of Mongolia, Ulaanbaatar (UB) is one of the fastest-growing cities in the developing world [3]. Rapid industrial development, including that in textiles, tanning, food processing, energy production, communications, and construction, is concentrated primarily in UB and has attracted people from rural areas, resulting in signiﬁcant

Water 2018, 10, 750; doi:10.3390/w10060750 www.mdpi.com/journal/water Water 2018, 10, 750 2 of 18 growth in the city’s population from about 0.5 million in 1989 to 1.2 million in 2016 [4,5]. The rapid introduction of rural people into the city and the residential areas along the hillslope surrounding it (Ger districts) has caused many issues, including those related to water supply and waste treatment infrastructure. Urban development in UB during the last several decades has caused a dramatic increase in water demand for drinking, domestic, and industrial use. As the city is located along the Tuul River, the central urban area receives water predominantly from wells tapped into the alluvial aquifer, a system that relies on bank filtration [6]. However, the expansion of the city has outpaced that of its infrastructure. Pipeline water distribution is limited to the urban center, and most residents in the outskirts purchase water for drinking and daily domestic use from kiosks. Most kiosks are associated with wells tapped into the bedrock aquifer, through which groundwater eventually discharges and feeds the river [7]. Consequently, the water level in the source zone, the alluvial aquifer, declined from 1.6 m in 1948 to 3.1 m in 1998 [8]. Seasonal water shortages have become increasingly common, and researchers have warned that the city will face critical water shortages in the near future [9,10]. Industrial activities have become the principal sources of air, water, and soil contamination in and around UB. The quality of its water sources meets the standards for drinking and industrial uses, with concentrations of total dissolved solids (TDS) ranging from 60 to 100 mg/L, remaining unaffected by sources of pollution in the city [10]. However, since the late 1990s, water quality in the downstream area of the Tuul River has deteriorated, due mainly to urban and industrial pollution [11–15]. Moreover, the unplanned expansion of Ger districts around the city has created a high potential for water contamination due to a lack of appropriate sanitation and sewage systems, which are based on pit latrines. By consequence, the area around UB has changed dramatically with the rapid growth of the city, which could have severe environmental consequences [16–18]. Water resource security, which is considered to reflect rapid development in developing countries, has worsened in terms of quantity and quality, becoming a critical issue in UB. This issue raises the question of whether development in UB could be sustainable in the face of limited water resources and degrading environmental quality [19–22]. Studies of water resources in UB have focused mainly on groundwater availability [23–28], and limited information is available on water quality in this city. High levels of U in groundwater (mean concentration: 4.6 µg/L) originating from local geogenic sources have been reported [29]. In addition, the influence of land use on groundwater quality in UB was recently assessed [30]. In this study, we evaluated sustainability in UB from the perspective of water quality issues by gaining a scientific understanding of the interactions between the Tuul River and aquifer groundwater and determining the spatio-temporal variation in the concentration of NO3, the most common pollutant in areas without proper waste treatment systems, in the water system.

2. Methods

2.1. Study Area UB is located in central Mongolia (47◦55012 N, 106◦55012 E), which is a continental country with a cold, semi-arid climate (Figure1). The mean altitude is about 1350 m a.s.l. [ 31]. UB relies on groundwater drawn from the alluvial aquifer along the Tuul River, where a bank ﬁltration system is operated [7]. The city pumps groundwater at an average rate of 150,000 m3/day to supply inner-city residences and industries [4]. By contrast, residents living on the outskirts of the city in Ger districts use hand-dug wells as independent sources of water or purchase water at kiosks that function as local distribution centers for water pumped from the alluvial aquifer [7]. Water 2018, 10, 750 3 of 18 Water 2018, 10, x FOR PEER REVIEW 3 of 18

FigureFigure 1. 1.Hydrogeological Hydrogeological map map of Ulaanbaatar (UB) (UB) and and the the sampling sampling locations locations (modified (modiﬁed from from a a 1:200,0001:200,000 hydrogeologicalhydrogeological map map [10]). [10]).

The Tuul River originates in the Khentii Mountains and generally flows from north-east to The Tuul River originates in the Khentii Mountains and generally ﬂows from north-east to south-west in a variably meandering channel. Its total length is 704 km, with a total catchment area south-west in a variably meandering channel. Its total length is 704 km, with a total catchment of 49,840 km2. The continental climatic features are characterized by wide variation in annual, area of 49,840 km2. The continental climatic features are characterized by wide variation in annual, monthly, and daily temperatures with cold, long winters and warm summers. About 74% of the annual monthly, and daily temperatures with cold, long winters and warm summers. About 74% of the annual precipitation falls during summer, from June to August. During winter (November–March), the ground precipitationis frozen, and falls precipitation during summer, accumulates from June as tosnow. August. Snow During usually winter falls between (November–March), mid-October theand ground mid- isApril. frozen, In andwinter precipitation months, thick accumulates snow covers as the snow. surrounding Snow usually mountains falls, which between remain mid-October covered until and mid-April.early April. In The winter annual months, average thick air snow temperature covers theis −1.2 surrounding °C; the temperature mountains, reaches which the remain minimum covered of ◦ until−39.6 early °C April.in January The annualand the average maximum air temperatureof 34.5 °C during is −1.2 summerC; the [13]. temperature The width reaches of the the Tuul minimum River ◦ ◦ of around−39.6 UBC in is Januaryabout 35 and m; however, the maximum it is reduced of 34.5 to C5−18 during m in summerthe dry season. [13]. The Average width depth of the of Tuul the river River aroundis about UB 1.3 is m about from 35May m; to however, Nov., with it the is reduced most shall toow 5− during18 m inMarch the dryto April season. (about Average 0.6 m) and depth the of thedeepest river is of about 2.1 m 1.3 in July m from 1993 May. The tosurface Nov., water with thein UB most is partially shallow to during completely March frozen to April from (about December 0.6 m) andto the February. deepest The of 2.1 flow m begins in July gradually 1993. The in surface spring water (March), in UB and is dischargepartially to increases completely gradually frozen to from a Decembermaximum to in February. the rainy The season ﬂow (July begins–August) gradually [6,7,10,28 in spring]. (March), and discharge increases gradually to a maximumThe geological in the rainy features season of (July–August) the study area [ 6 consist,7,10,28 ]. mainly of Devonian and Carboniferous sedimentaryThe geological rocks of features very low of permeability the study areaintruded consist by granite, mainly characteri of Devonianzed as andthe basement Carboniferous rock sedimentary(Figure 2). This rocks basement of very rock low is permeability overlain by Cretaceous intruded bysandstone granite, and characterized mudstone, clay as theand basement sand of rockNeogene (Figure deposits,2). This basement and Quaternary rock is overlain sand and by gravel Cretaceous deposi sandstonets. Quaternary and mudstone, deposits are clay also and sanddistributed of Neogene widely deposits, throughout and Quaternarythe Tuul River sand Basin and [6,32]. gravel deposits. Quaternary deposits are also distributed widely throughout the Tuul River Basin [6,32].

Water 2018, 10, 750 4 of 18 Water 2018, 10, x FOR PEER REVIEW 4 of 18

FigureFigure 2. 2.Hydrogeological Hydrogeological cross-sectioncross-section of of the the alluvial alluvial aquifer aquifer around around Ulaanbaatar Ulaanbaatar (modified (modiﬁed from from [24]). [24 ]).

The alluvial aquifer is composed of sediments forming two unconfined layers, which differ primarilyThe alluvial in composition, aquifer is porosity, composed and of water sediments saturation forming (Figure two 2). The unconfined upper layer layers, consists which mainly differ primarilyof unconsolidated in composition, gravel porosity,and boulders, and water and coarse saturation sand (Figurefills the2 voids,). The representing upper layer consists15–30% mainlyof the ofsediment unconsolidated by volume. gravel Theand thickness boulders, ranges and from coarse 2 to 30 sand m (average: fills the voids,25 m) and representing generally increases 15–30% of thetoward sediment the by cente volume.r of the The river thickness basin ranges [6,33]. The from lower 2 to 30 layer m (average: extends 25 down m) and to the generally Neogene increases and towardCarboniferous the center deposits, of the with river a thickness basin [6,33 ranging]. The from lower 3 to layer 20 m. extends Its composition down to is thesimila Neogener to that andof Carboniferousthe upper layer, deposits, with occasional with a thickness clay-dominant ranging lenses. from Fine 3 to 20silty m. sand Its compositionand clay fill the is similar voids between to that of thegravel upper and layer, boulder with particles occasional and, clay-dominant in some areas, lenses. create Fineclay layers silty sand ranging and from clay filla few the centimete voids betweenrs to gravel5–8 m and thick. boulder Although particles the composition and, in some of areas,the alluvial create aquifer clay layers is generally ranging uniform, from a the few presence centimeters of toclay 5–8- mdominant thick. Although soil with thelow compositionpermeability ofcreates the alluvial semi-confined aquifer isconditions generally locally uniform, in some the presence strata, ofwhere clay-dominant the lower soillayer with of the low aquifer permeability yields less creates water semi-confined compared with conditions the overlying locally layer in [32,34] some.strata, where the lower layer of the aquifer yields less water compared with the overlying layer [32,34]. 2.2. Water Sample Collection and Analysis 2.2. Water Sample Collection and Analysis To gain a scientific understanding of the interaction between the Tuul River and groundwater in theTo gainaquifer a scientific and to clarify understanding the spatio-temporal of the interaction variation betweenin NO3 concentrations the Tuul River in and water, groundwater we used inseasonal the aquifer monitoring and to clarify of water the resources spatio-temporal with hydrochemical variation in NOand3 isotopicconcentrations analyses inand water, interpreted we used seasonalthe spatio monitoring-temporal ofdata water using resources statistical with approaches. hydrochemical and isotopic analyses and interpreted the spatio-temporalWe collected 135 data water using samples statistical from approaches. 40 sites to determine the dissolved ion composition and stableWe isotopic collected (i.e. 135, O water and samplesH) ratios from of water. 40 sites The to samples determine were the collected dissolved from ion compositionbedrock wells and in stablethe isotopicGer districts (i.e., O ( andn = 58), H) ratiosshallow of water.wells in The the samples alluvial wereaquifer collected (n = 44), from the bedrockriver (n = wells 24), and in the springsGer districts (n = (n9).= 58), The shallow groundwater wells in samples the alluvial were aquifer taken ( nfrom= 44), drinking the river water (n = 24), wells and and springs shallow (n = 9).wells The tapped groundwater into samplesthe alluvial were aquifer taken from for the drinking central waterpublic wells water and supply shallow in UB. wells Shallow tapped wells into were the alluvial<20 m deep, aquifer and for thedeep central bedrock public wells water were supply >20 m in deep. UB. Five Shallow sampling wells campaigns were <20 mwere deep, conducted and deep during bedrock August wells 2010, were February 2011, October 2011, April 2012, and July 2017 to assess seasonal changes. In addition, two >20 m deep. Five sampling campaigns were conducted during August 2010, February 2011, October 2011, snow and two rain samples were collected from September to October 2010 and in July 2017, April 2012, and July 2017 to assess seasonal changes. In addition, two snow and two rain samples were respectively. Additional precipitation data for UB from 1990 to 2001 were retrieved from the collected from September to October 2010 and in July 2017, respectively. Additional precipitation data for International Atomic Energy Agency database [35]. UB from 1990 to 2001 were retrieved from the International Atomic Energy Agency database [35]. Water samples were filtered through 0.45-μm membrane filters, sealed in 30-mL polyethylene Water samples were filtered through 0.45-µm membrane filters, sealed in 30-mL polyethylene bottles, stored in an ice box, and transported to the Korean Basic Science Institute (KBSI; Daejeon, bottles, stored in an ice box, and transported to the Korean Basic Science Institute (KBSI; Daejeon, Korea)

Water 2018, 10, 750 5 of 18 and Yonsei University (Seoul, Korea). They were analyzed for major ions and trace elements, including Na, K, Ca, Mg, Si, Fe, Mn, Zn, Ni, Cd, Co, Cr, Cu, Pb, As, Hg, HCO3, Cl, SO4, and NO3, using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES, JY Ultima 2; Horiba, Tokyo, Japan) at the KBSI and the Ion Chromatography (IC, model 883 Basic IC plus; Metrohm AG, Herisau, Switzerland) of Yonsei University. Levels of all trace elements were below the detection limits; therefore, they were not considered in the subsequent analyses. Isotopic signatures (δ18O and δ2H) were analyzed using a stable isotope ratio mass spectrometer (IsoPrime; GV Instruments Ltd., Manchester, UK) at the KBSI. All δ18O and δ2H data were reported using the conventional δ notation, according to the Vienna standard mean ocean water (V-SMOW) standard, Equation (1) as follows:

Rsample − RSMOW δsample = × 1000 (%0) (1) RSMOW

18 16 where, Rsample is the isotopic ratio of O/ O or D/H of a water sample, and RSMOW is the isotopic ratio of 18O/16O or D/H of the V-SMOW standard. The analytical reproducibility was ±0.1‰ and ±0.5‰ for δ18O and δ2H, respectively. Using the SPSS software (ver. 16.0; SPSS Inc., Chicago, IL, USA), a multivariate technique was applied to delineate the relationship of the spatial variation in water chemistry to NO3 contamination in the water. Hierarchical cluster analysis (HCA) was applied to identify groups obtaining similarity of water chemistry. Water chemistry of major ions and field parameters was used in the HCA by Wards linkage methods [36,37]. Controlling factors of the hydrogeochemistry and NO3 concentration of the water samples were inferred from the results of a principal component analysis (PCA). PCA has been used widely to analyze and obtain meaningful information about groundwater quality by extracting groups of correlated elements from the initial data set. These groups are included into principle factors, which describe processes occurring in the investigated environment. Identification of the factors allows genetic interpretation of the environment. First factor obtained explains the biggest part of variance. The following factors explain repeatedly smaller parts of variance. In addition, the factor loading shows how the factor characterizes the variables (ion concentration). High factor loadings (close to −1 or 1) indicate strong relationship (positive or negative) between the variable and the factor describing this variable. The factor loadings matrix was rotated to the orthogonal simple structure according to the varimax rotation technique. The result of this operation are high factor loadings obtained for the variable correlated in the factor and low factor loadings obtained for remaining variable [36,38,39]. For the HCA and the PCA analyses, data from the total of 95 groundwater and 20 river water samples obtained from August 2010 to April 2012 were used.

3. Results and Discussion Various water sources, including shallow groundwater wells tapped into the unconsolidated aquifer along the Tuul River, relatively deep groundwater wells tapped into bedrock aquifers, and hand-dug wells, have been developed to meet the increasing demand in UB. We characterized the chemical compositions of these water sources.

3.1. Hydrochemistry of Water Resources Table1 presents basic statistics for the hydrochemical characteristics of the groundwater and surface water. Most hydrochemical parameters varied substantially among the surface water and groundwater samples. For example, in surface water, electrical conductivity (EC) ranged from 43 to 841 µS/cm and increased from the upstream to downstream areas of the city due to the accumulating influences of human activities and effluent discharge from the central wastewater treatment plant (R-7 in Figure1). In groundwater, the EC values ranged from 65 to 2035 µS/cm. Water 2018, 10, 750 6 of 18

Table 1. Descriptive statistics for the hydrochemical parameters of groundwater and surface water.

Ground Water (N = 95) River Water (N = 20) Parameter Min. Max. Median Std. Dev Min. Max. Median Std. Dev EC (µS/cm) 65.0 2035.0 426.0 424.1 43.0 841.0 96.0 182.6 pH 6.3 8.9 7.2 0.5 6.5 8.9 7.8 0.6 Temp. (◦C) 1.2 14.9 5.9 2.5 1.9 21.4 8.0 6.0 HCO3 (mg/L) 24.4 473.5 154.7 111.2 14.6 250.5 38.6 59.1 SO4 (mg/L) BDL 104.6 22.3 30.2 4.0 65.0 6.6 14.0 Cl (mg/L) 0.6 206.1 7.2 35.3 0.4 62.4 1.8 14.9 NO3 (mg/L) BDL 305.4 12.0 65.7 0.1 6.5 1.3 1.9 SiO2 (mg/L) 0.2 18.6 10.4 4.4 BDL 9.6 7.0 3.1 Ca (mg/L) 7.0 197.1 48.9 53.2 6.8 50.9 11.5 12.8 Mg (mg/L) 1.4 73.6 9.9 11.8 1.0 7.9 1.6 2.0 Na (mg/L) 1.4 72.0 12.0 15.2 1.6 62.2 2.8 13.6 K (mg/L) BDL 6.3 0.9 1.2 BDL 14.0 0.7 3.0 BDL: below the detection limit.

Groundwater had significantly higher concentrations of most dissolved constituents, indicative of water–rock interactions along the groundwater flow path. Moreover, NO3 levels were significantly higher in groundwater (median: 12 mg/L) than in surface water (median: 1.3 mg/L), suggesting that N sources affected water quality via groundwater flow. The dominant factors controlling the groundwater hydrochemistry were extracted based on a principal component analysis of the major ion concentrations (i.e., HCO3, SO4, Cl, NO3, Ca, Mg, Na, K and SiO2; Table2).

Table 2. Results of the principal component analysis for groundwater parameters after varimax rotation.

Parameter PC 1 PC 2

HCO3 0.30 0.70 SO4 0.57 0.64 Cl 0.75 0.55 NO3 0.76 0.56 SiO2 0.04 0.87 Ca 0.62 0.72 Mg 0.68 0.63 Na 0.72 0.42 K 0.84 −0.03 % variance explained by eigenvalues 67.8 10.0 % variance explained by squared sum of loadings 40.4 37.4 Principal component loadings > 0.7: bold with underline; Principal Component (PC).

Principal Component (PC) 1 and 2 explained approximately 77.8% of the total variation in the hydrochemical data. PC 1, explaining about 68% of the variance, included the influences of K, NO3, Cl, and Na. Of these parameters, NO3 and Cl could be explained by groundwater contamination in urban areas without proper sewage systems. The highest factor loading of K might have originated from detergent in wastewater, which is a source of Cl and K [40]. PC 2 accounted for another 10% of the total variance, and predominantly included the influences of SiO2, Ca, and HCO3. These ions reflected natural water–rock interactions, characterized by weathering of silicate and carbonate minerals in the local aquifer system. Consequently, the groundwater chemistry and quality in the study area appeared to be controlled by anthropogenic pollution overlapping natural water–rock interactions. To delineate the spatial distribution of water quality based on the potential for NO3 pollution, we employed a hierarchical cluster analysis (HCA) [36,37] to identify groups with similar major ion compositions and field parameters. Figure3 presents a tree diagram produced from the HCA, which divides the samples into three groups. Most shallow groundwater samples from the alluvial aquifer were clustered in Group 1 (Sh-1–Sh-12, D-15, and D-21). Group 2 included spring water Water 2018, 10, x FOR PEER REVIEW 7 of 18 Water 2018, 10, 750 7 of 18 divides the samples into three groups. Most shallow groundwater samples from the alluvial aquifer were clustered in Group 1 (Sh-1–Sh-12, D-15, and D-21). Group 2 included spring water (Sp-1) and (Sp-1)bedrock and bedrock groundwate groundwaterr (D-1, -2, -3, (D-1, -4, -8, -2, -9, -3,-14, -4, and -8, -19). -9, Finally, -14, and Gro -19).up 3 Finally,accounted Group for the 3 remaining accounted for the remainingbedrock groundwater bedrock groundwater and spring samples and spring (D-6, - samples7, -10, -11, (D-6, -16, - -7,17, -10,-18, - -11,20, and -16, Sp -17,-1). -18, -20, and Sp-1).

Figure 3. Dendrogram of the groundwater sample groups based on their hydrochemical properties. Figure 3. Dendrogram of the groundwater sample groups based on their hydrochemical properties. Based on Gibbs’ [41] plots (Figure 4), water–rock interactions appeared to be the major mechanismBased on Gibbs’ controlling [41] plots the hydrochemistry (Figure4), water–rock of water interactionsin UB for shallow appeared alluvial to beaquifer the major and bedrock mechanism controllingaquifer groundwater, the hydrochemistry as well as river of water water. inAll UBresources for shallow had similar alluvial Na/(Na aquifer + Ca) ratios; and however, bedrock they aquifer groundwater,were clearly as differentiated well as river based water. on Allthe resourcesTDS concentrations, had similar which Na/(Na were higher + Ca) for ratios; resources however, with longer they were clearlyresidence differentiated times. Group based 1 groundwater on the TDS and concentrations, river water samples which were were characteri higherz fored by resources similar ranges with of longer residenceTDS, Na times./(Na + Group Ca), and 1 groundwaterCl/(Cl + HCO3), and implying river vig waterorous samples interchange. were TDS characterized and Cl/(Cl + HCO by similar3) values ranges for groundwater samples showed increasing trends from Group 1 to Groups 2 and 3. of TDS, Na/(Na + Ca), and Cl/(Cl + HCO3), implying vigorous interchange. TDS and Cl/(Cl + HCO3) values for groundwater samples showed increasing trends from Group 1 to Groups 2 and 3.

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Figure 4. Variation in the Na/(Na + Ca) and Cl/(Cl + HCO3) ratios (w/w) as a function of the total Figure 4. Variation in the Na/(Na + Ca) and Cl/(Cl + HCO3) ratios (w/w) as a function of the total dissolveddissolved solids solids (TDS) (TDS) in thein the water water resources resources(adapted (adapted from [4 [411]).]).

The hydrochemicalThe hydrochemical compositions compositions of of thethe waterwater samples plotted plotted on on the the modified modified Piper Piper diagram diagram indicated gradual changes in water chemistry and quality from Ca-HCO3–type to Ca-[HCO3 + Cl + indicated gradual changes in water chemistry and quality from Ca-HCO3–type to Ca-[HCO3 + Cl NO3]–type water (Figure 5). Surface waters were predominantly of the [Ca + Na]-HCO3 type, similar + NO3]–type water (Figure5). Surface waters were predominantly of the [Ca + Na]-HCO 3 type, to Group 1 groundwater samples from the riverbank area. Group 1 groundwater samples were similar to Group 1 groundwater samples from the riverbank area. Group 1 groundwater samples typically of the Ca-HCO3 type, whereas Group 2 samples were categorized more broadly as being of were typically of the Ca-HCO type, whereas Group 2 samples were categorized more broadly as the [Ca + Na]-[HCO3 + Cl] type.3 Group 3 samples were typically of the Ca-[Cl + NO3] type, probably beingreflecting of the [Ca the + influence Na]-[HCO of the3 + anthropogenic Cl] type. Group input 3 samplesof Cl and wereNO3. typically of the Ca-[Cl + NO3] type, probablyFigure reflecting 6 shows the influencethe distribution of the of anthropogenic the groundwater input hydrochemical of Cl and characteristic NO3. groups. Group Figure1 wells,6 distributed shows the along distribution the Tuul of River, the groundwater were mostly shallow hydrochemical wells tapped characteristic into the alluvial groups. aquifer. Group 1 wells,Group distributed 2 wells alongare distributed the Tuul along River, the were east- mostlywestern shallowside of the wells study tapped area between into the Group alluvial 1 and aquifer. GroupGroup 2 wells 3. Finally, are distributed Group 3 wells, along on the the east-westernnorth-central side of of UB, the were study bedrock area between wells and Group springs 1 and Groupcharacterized 3. Finally, by Group water 3–rock wells, interactions on the north-central in the bedrock side aquifer. of UB, In Group were bedrock3, the impact wells of andthe Ger springs districts on the slope is probably exemplified, which lack proper sewage disposal systems. characterized by water–rock interactions in the bedrock aquifer. In Group 3, the impact of the Ger districts on the slope is probably exemplified, which lack proper sewage disposal systems.

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Water 2018, 10, x FOR PEER REVIEW 9 of 18

Figure 5. Piper diagrams of the chemical compositions of groundwater and surface water in UB. FigureFigure 5. Pip 5. erPip diagramser diagrams of of the the chemical chemical compositionscompositions of of groundwater groundwater and and surface surface water water in UB. in UB.

FigureFigure 6. Spatial 6. Spatial distribution distribution of the groundwater of the groundwater well groups well with groups different with hydrochemical different hydrochemical characteristics characteristics in the study area. in the study area. FigureBased 6. Spatial on the factor distribution analysis of (Table the groundwater 2), the dominant well water groups–rock with interactions different were hydrochemical related to Basedsilicatecharacteristics on and the carbonate factor in the analysisstudy weathering area. (Table and2), the dissolution dominant–precipitation water–rock interactionsprocesses [42,43]. were The related Group to silicate 1 and carbonategroundwater weathering samples were and plotted dissolution–precipitation below the HCO3 versus processes SiO2 10:1 [slope42,43 (Figure]. The Group7a), indicating 1 groundwater that samples Based were on plotted the factor below analysis the HCO (Table3 versus 2), the SiO dominant2 10:1 slope water (Figure–rock 7 interactionsa), indicating were that related silicate to silicate and carbonate weathering and dissolution–precipitation processes [42,43]. The Group 1 groundwater samples were plotted below the HCO3 versus SiO2 10:1 slope (Figure 7a), indicating that

Water 2018, 10, 750 10 of 18

Water 2018, 10, x FOR PEER REVIEW 10 of 18 weathering was the dominant process. As these wells were tapped into the upper layer of alluvium distributedsilicate along weathering the Tuul was River, the dominant silicate weathering process. As couldthese wells occur were in the tapped coarse into to the ﬁne upper sands layer and of gravels derivedalluvium from shale, distributed quartz along sandstone, the Tuul River, and granite silicate weathering [24]. By contrast, could occur groundwater in the coarse andto fine even sands surface water samplesand gravels from derived Groups from 2shale, and quartz 3 were sandstone, plotted aboveand granite the 10:1[24]. By slope, contrast, indicating groundwater that carbonateand even surface water samples from Groups 2 and 3 were plotted above the 10:1 slope, indicating that weathering was the dominant process. We used the HCO3 versus Ca concentration plot of the samples to assesscarbonate the dissolution weathering and was precipitation the dominant ofprocess. carbonate We used minerals the HCO (Figure3 versus7b). Ca Groupconcentration 3 wells plot exhibited of the samples to assess the dissolution and precipitation of carbonate minerals (Figure 7b). Group 3 stronger CaCO3 dissolution than CaCO3 precipitation, whereas Group 2 wells showed the opposite wells exhibited stronger CaCO3 dissolution than CaCO3 precipitation, whereas Group 2 wells showed trend. Thesethe opposite results trend. imply These that results more imply freshwater that more recharge freshwater occurred recharge inoccurred Group in 3 Group wells 3 than wells in than Group 2 wells, suggestingin Group 2 wells, that theysuggesting are exposed that they to are the exposed risk of to pollution the risk of frompollution near-surface from near-surface sources. sources.

FigureFigure 7. Assessment 7. Assessment of the of dominantthe dominant water–rock water–rock interactionsinteractions of of the the water water samples samples plotted plotted based basedon on (a) HCO3 vs. SiO2 and (b) HCO3 vs. Ca2+2+ concentrations. (a) HCO3 vs. SiO2 and (b) HCO3 vs. Ca concentrations. In summary, water in UB could be differentiated into three groups based on its hydrochemical Incharacteristics. summary, water Group in 1 UB samples could were be differentiatedlocated along the into Tuul three River, groups Group based2 samples on were its hydrochemical from the characteristics.north-eastern Group area of 1 UB, samples and Group were 3 locatedsamples were along from the the Tuul north River,-western Group area. 2Group samples 1 included were from the north-easternthe alluvial area aquifer of UB, along and the Group river and 3 samples showed were silicate from weathering the north-western–dominant water area.–rock Group 1 includedinteractions. the alluvial By aquifer contrast, along samples the river from and Groups showed 2 and silicate 3 reflected weathering–dominant predominantly carbonate water–rock weathering. However, Group 3 was under the influence of CaCO3 dissolution conditions, indicative interactions. By contrast, samples from Groups 2 and 3 reﬂected predominantly carbonate weathering. of more freshwater recharge, making it more vulnerable than Group 2 to surface pollution sources. However, Group 3 was under the inﬂuence of CaCO3 dissolution conditions, indicative of more freshwater3.2. Isotopic recharge, Signatures making of Water it more Resources vulnerable than Group 2 to surface pollution sources. The relationships among various water sources (i.e., precipitation, surface water, and 3.2. Isotopic Signatures of Water Resources groundwater) were inferred based on isotopic signatures (δ18O and δ2H). Precipitation isotopic data Thewere relationships obtained from among 1990 to various 2001 [35]. water δ18O sourcesranged from (i.e., −30.4 precipitation,‰ to −1.6‰ surface (median: water, −11.5‰), and groundwater)and δ2H were inferredranged from based −236.5 on isotopic‰ to −16.6‰ signatures (median: (δ 18 −81.4‰;O and Figureδ2H). Precipitation8). From these isotopicdata, the data local weremeteoric obtained water line (LMWL) was defined as Equation (2). from 1990 to 2001 [35]. δ18O ranged from −30.4‰ to −1.6‰ (median: −11.5‰), and δ2H ranged from −236.5‰ to −16.6‰ (median: −81.4‰;δ Figure2H = 7.88 ).δ18 FromO + 2.7 these (r2 = 0.98) data, the local meteoric water line(2) (LMWL) was definedThe as Equation slope of the (2). LMWL was 7.8, which was close to the equilibrium value of 8.0 [44]. The intercept of the LMWL was 2.7, muchδ2H =lower 7.8 δthan18O the + 2.7global (r2 meteoric= 0.98) water line value of 10, indicative (2) of lower deuterium excess values in the warm season due to the evaporation effect of summer The slopeprecipitation of the LMWL [45–48]. was The 7.8, long which-term was precipitation close to data the equilibrium showed seasonal value differences of 8.0 [44 in]. The isotopic intercept of the LMWLvalues, with was higher 2.7, much values lower in summer than the than global in winter meteoric (Figure water 8a). Such line valueseasonal of variation 10, indicative has been of lower deuteriumreported excess as valuesan effect in of the temperatur warm seasone, as dueprecipitation to the evaporation under warmer effect temperatures of summer condenses precipitation with[45 –48]. The long-termheavier isotopesprecipitation due to data evaporation showed seasonal during rainfall, differences and in consequently isotopic values, has higher with higher isotopic values in summercompositions than in than winter does (Figureprecipitation8a). under Such colder seasonal conditions variation [49,50]. has All been median reported isotopic assignatures an effect of of the groundwater and surface water samples were lower than the weighted median precipitation, temperature, as precipitation under warmer temperatures condenses with heavier isotopes due to evaporation during rainfall, and consequently has higher isotopic compositions than does precipitation under colder conditions [49,50]. All median isotopic signatures of the groundwater and surface water samples were lower than the weighted median precipitation, indicating that groundwater and surface Water 2018, 10, 750 11 of 18

Water 2018, 10, x FOR PEER REVIEW 11 of 18 water, whichindicating provide that groundwater the main and water surface supply water, for which the provide city, are the likely main water recharged supply fromfor the present-day city, meteoricare water. likely recharged from present-day meteoric water.

Figure 8. δ18O and δ2H values of water in the study area: (a) precipitation from 1990 to 2001 (data Figure 8. δ18O and δ2H values of water in the study area: (a) precipitation from 1990 to 2001 obtained from [35]) and (b) water sample groups of this study from 2010 to 2017. WMP indicates the (data obtained from [35]) and (b) water sample groups of this study from 2010 to 2017. WMP indicates weighted median precipitation of UB, and the Global Meteoric Water line (GMWL) and Local the weightedMeteoric median Water line precipitation (LMWL) are presented of UB, and as solid the and Global dotted Meteoric lines, respectively. Waterline GW and (GMWL) RW denote and Local Meteoricgroundwater Water line and (LMWL) river water are presented, respectively. as solid and dotted lines, respectively. GW and RW denote groundwater and river water, respectively. The δ18O and δ2H values of the groundwater and surface water samples ranged widely from −16.9‰ to −11.1‰ and −129.3‰ to −87.2‰, respectively (Table 3). The δ18O and δ2H values in river 18 2 Thewaterδ O ranged and δfromH values−16.9‰ ofto −12.5‰ the groundwater and −127.0‰ and to −96.0‰, surface respectively water samples (Figure ranged8b). The Group widely from 18 2 −16.9‰1 groundwaterto −11.1‰ samplesand −129.3 from ‰the toalluvial−87.2 aquifer‰, respectively close to the Tuul (Table Rive3r). had The δ18Oδ andO δ and2H valuesδ H valuesof in river water−16.3‰ ranged to −13.2‰ from and− −129.316.9‰‰ to −100.0‰,−12.5‰ respectively.and −127.0 Meanwhile,‰ to − 96.0the δ‰18O, and respectively δ2H values of (Figure the 8b). The GroupGroup 1 groundwater 2 samples ranged samples from −14.4 from‰ to the −11.5‰ alluvial and aquifer −110.2‰ close to −87.7‰, to the respectively. Tuul River Finally, had δ18 thoseO and δ2H of Group 3 groundwater samples ranged from −13.7‰ to −11.1‰ and −108.4‰ to −87.2‰, values of −16.3‰ to −13.2‰ and −129.3‰ to −100.0‰, respectively. Meanwhile, the δ18O and δ2H respectively. Group 1 shallow groundwater samples were relatively depleted in isotopic ratios − − − − values ofcompared the Group with 2 the samples bedrock ranged groundwater from in14.4 Groups‰ to 2 and11.5 3, but‰ andtheir values110.2 were‰ to similar87.7 to‰ those, respectively. of Finally,surface those of water Group. These 3 groundwater results suggestsamples that bedrock ranged groundwater from −13.7 experienced‰ to − 11.1 a relatively‰ and longer−108.4 ‰ to −87.2‰residence, respectively. period under Group the 1influence shallow of groundwaterevaporation along samples the flow werepaths from relatively the land depleted surface to inthe isotopic ratios comparedlocal groundwater with the discharge bedrock points groundwater [51,52]. in Groups 2 and 3, but their values were similar to those of surface water. These results suggest that bedrock groundwater experienced a relatively longer Table 3. δ2H and δ18O values of the water resources (unit: ‰). residence period under the influence of evaporation along the flow paths from the land surface to δ18O δ2H the local groundwaterWater Resource discharge points [51,52]. Min. Max. Median Std. Dev. Min. Max. Median Std. Dev. Precipitation −30.5 −1.6 −11.5 7.5 −236.5 −16.6 −81.4 59 2 18 River waterTable 3. δ H−16.9 and δ−12.5O values−15.1 of the water0.9 resources−127 − (unit:96 ‰−113.9). 6.8 Group 1 −16.3 −13.2 −15.3 0.7 −129.3 −100 −113.5 6.5 Groundwater Group 2 −14.4 −11.5 δ18−O12.5 0.9 −110.2 −87.7 −δ97.32H 6.3 Water ResourceGroup 3 −13.7 −11.1 −12.4 0.7 −108.4 −87.2 −96.7 5.3 Min. Max. Median Std. Dev. Min. Max. Median Std. Dev. − − − − − − PrecipitationAs the groundwater discharged30.5 1.6 into the11.5 river through 7.5 the shallow236.5 alluvial16.6 aquifer,81.4 these 59 two River water −16.9 −12.5 −15.1 0.9 −127 −96 −113.9 6.8 sources mixed dynamically with river water, which has lower isotopic signatures, via pumping effects. River waterGroup can 1 become−16.3 isotopically−13.2 depleted−15.3 due 0.7 to high-−inte129.3nsity rainfall−100 [51],−113.5 recharge6.5 from − − − − − − Groundwaterupstream highlandGroup areas 2 due14.4 to altitude11.5 effects,12.5 or snowmelt 0.9 water110.2 inflow [49].87.7 Considering97.3 that 6.3 UB Group 3 −13.7 −11.1 −12.4 0.7 −108.4 −87.2 −96.7 5.3 is surrounded by high mountains and has a dry climate, the low isotopic values in the Tuul River could be the result of snowmelt input in upstream areas. Group 2 groundwater was not clearly As the groundwater discharged into the river through the shallow alluvial aquifer, these two sources mixed dynamically with river water, which has lower isotopic signatures, via pumping effects. River water can become isotopically depleted due to high-intensity rainfall [51], recharge from upstream highland areas due to altitude effects, or snowmelt water inflow [49]. Considering that UB is surrounded by high mountains and has a dry climate, the low isotopic values in the Tuul River could be the result of snowmelt input in upstream areas. Group 2 groundwater was not clearly distinguished Water 2018, 10, 750 12 of 18 from that of Group 3, probably because the wells of these groups were located in geographically similar areas on slopes, resulting in similar groundwater flow paths. In summary, the stable isotopic signatures of the water sources in UB exhibited various fractionation effects, including seasonal variation in long-term changes and evaporation via altitude effects. The isotopic values of groundwater and surface water indicate that they are of meteoric origin. Due to pumping-induced recharge, the shallow groundwater from the alluvial aquifer along the Tuul River appeared to mix dynamically with river water originating from the upstream area fed by snowmelt water. The bedrock groundwater samples from wells of Groups 2 and 3 showed normal hydrological cycles of precipitation transferring to infiltration, evaporation, recharge, and discharge into the Tuul River.

3.3. Nitrate Contamination of Water Resources

NO3 in water resources can originate from various sources, including atmospheric deposition, chemical and organic fertilizers, industrial waste effluent, and improperly maintained septic systems [53]. Natural NO3 levels in groundwater are generally very low (<10 mg/L NO3), but these levels can increase in association with anthropogenic inputs. In urban areas, NO3 concentrations are often above 50 mg/L, reflecting N loading from high-density housing without sewage sanitation systems [54]. As such, NO3 is the most common water pollutant, and causes significant health issues associated with water resource use. The World Health Organization has established a drinking-water NO3 guideline level of 50 mg/L based on health concerns [55]. However, NO3 concentrations of 10−20 mg/L have been linked to health effects; Ward et al. [56] reported that long-term exposure to such NO3 levels from municipal water supply systems has possible links to bladder and ovarian cancers and non-Hodgkin’s lymphoma in the United States [56,57]. In the water samples from UB, NO3 concentrations ranged from below the detection limit to 305.4 mg/L (Table4). River water samples showed no indication of NO 3 pollution (median: 1.1 mg/L) and the 48 shallow groundwater samples from the alluvial aquifer along the Tuul River in Group 1 also appeared to have background levels with maximum of 5.18 mg/L. Several exceptions were found in shallow dug well Sh-5 and municipal monitoring wells Sh-9 and D-21, showing potential pollution risks indicated by higher ranges from BDL to 27.8 mg/L. Groundwater from Group 2 wells had NO3 concentrations of 8.0–47.4 mg/L (median: 15.8 mg/L), implying that these wells were influenced by NO3 contamination sources, although the NO3 levels were within the drinking-water guideline. By contrast, Group 3 groundwater samples, with NO3 concentrations of 64.0–305.4 mg/L (median: 101.3 mg/L), were highly contaminated. Wells in Groups 2 and 3 comprised 25% and 32% of the sampled wells, respectively, indicating that about 60% of the groundwater in the study area could be under the influence of NO3 contamination sources.

Table 4. Distribution of NO3 concentrations in various water resources in UB (unit: mg/L).

Water Resource Number of Samples Min. Max. Median Std. Dev. Group 1 48 BDL 27.8 1.7 6 Groundwater Group 2 28 8 47.4 15.8 11 Group 3 35 64 305.4 101.3 57.8 River water 24 BDL 6.5 1.1 1.8 BDL: below the detection limit.

Groundwater NO3 sources were delineated based on the hydrochemistry of the water samples. Figure9a presents the Na versus Cl concentrations of the water samples, where a slope of 1:1 indicates that Na and Cl originate from meteoric sources [53]. Most surface water samples and Groups 1 and 2 groundwater samples were plotted below the 1:1 slope line, indicative of excess Na ions from the geological matrix. Silicate mineral weathering could account for the additional Na in the surface water and Group 1 groundwater (Figure7a) [ 58]. Meanwhile, for Group 2 groundwater, dominated by carbonate weathering, cation exchange processes could account for the additional Na from the geologic WaterWater 20182018,, 1010,, x 750 FOR PEER REVIEW 1313 of of 18 18 Water 2018, 10, x FOR PEER REVIEW 13 of 18 geologic matrix via Ca substitution (Figure 7b). By contrast, the excess Cl concentrations in Group 3 geologicgroundwatermatrix via matrix Ca appeared substitution via Ca substitution to (Figureoriginate7b). (Figurefrom By contrast, the 7b). same By the sourcescontrast, excess as Cl the NO concentrations excess3 (Figure Cl concentrations 9b). in Group 3 groundwater in Group 3 groundwaterappeared to originateappeared from to originate the same from sources the same as NO sources3 (Figure as9 NOb). 3 (Figure 9b).

Figure 9. Relationships of (a) Na vs. Cl and (b) NO3 vs. Cl in various water resources. FigureFigure 9. 9. RelationshipsRelationships of of ( (aa)) Na Na vs vs.. Cl Cl and and ( (bb)) NO NO33 vs.vs. Cl Cl in in various various water water resources. resources. During the rapid development of UB over the last decade, population growth has surpassed that of theDuringDuring city’s theinfrastructure, the rapid rapid development development and Ger of ofdistricts UB UB over over have the the lastexpanded last decade, decade, to population population surround growththe growth city has hascenter. surpassed surpassed Given that thatthe oflackof the the of city’s city’sproper infrastructure, infrastructure andand (e.g.Ger Ger, pipelinedistricts districts water have have- expandeddistribution expanded to tosystems surround surround and the thesewage city city center. disposalcenter. Given Given systems), the lackthe lackgroundwaterof proper of proper infrastructure wells,infrastructure such (e.g., as (e.g. those pipeline, pipeline in Groups water-distribution water 2 - anddistribution 3, have systems becomesystems andtheand mainsewage sources disposal disposal of watersystems), systems), for groundwaterresidentsgroundwater through wells, wells, water such such kiosks. as as those those At inthe in Groups Groupssame time, 2 2 and and the 3, 3,groundwater have have become become de the theveloped main main along sources sources the of ofslope water water areas for for residentshasresidents become through through threatened water by kiosks.kiosks. contamination AtAt thethe samesame sources, time, time, the includingthe groundwater groundwater unmanaged developed developed pit latrines along along the and the slope septicslope areas areastanks has has[55].become become Thethreatened correlation threatened by between contaminationby contamination high NO sources,3 concentrations sources, including including and unmanaged unmanagedheavier isotopic pit latrinespit latrines values and andin septic groundwater septic tanks tanks [55]. [55]. The correlation between high NO3 concentrations and heavier isotopic values in groundwater (FigureThe correlation 10) indicates between that high thes NOe sources3 concentrations could become and heavier contaminated isotopic valuesin the inGer groundwater districts. Ultimately, (Figure 10 ) (Figurecontaminatedindicates 10) that indicates thesegroundwater sources that thes couldfrome sources the become upslope could contaminated becomerecharge contaminated area in the flowsGer districts.downward in the Ger Ultimately, toward districts. the contaminated Ultimately, discharge contaminatedarea,groundwater the Tuul fromgroundwater River, the slowly upslope from and recharge thesteadily upslope area expanding flows recharge downward the area region flows toward impacted downward the discharge by towardcontamination area, the the discharge Tuul to River, the area,riverbankslowly the and Tuul alluvial steadily River, aquifer, expanding slowly threatening and the steadily region the impactedshallowexpanding groundwater, by the contamination region whichimpacted to is the the by riverbank major contamination water alluvial supply aquifer,to th fore riverbankthethreatening city center. alluvial the shallow aquifer, groundwater, threatening whichthe shallow is the majorgroundwater, water supply which for is thethe citymajor center. water supply for the city center.

1818 FigureFigure 10. 10. IdentificationIdentification of of source source areas areas of of groundwater groundwater contamination contamination based on a plot of NO33 vs. δ O.O. Figure 10. Identification of source areas of groundwater contamination based on a plot of NO3 vs. δ18O. Figure 11 presents the results of the seasonal sampling campaigns. In Group 1, the NO3 Figure 11 presents the results of the seasonal sampling campaigns. In Group 1, the NO3 concentrationFigure 11 was presents stable, thewith results the lowest of the median seasonal value sampling of 1.7 mg/L. campaigns. This value In likely Group represents 1, the NO the3 concentration was stable, with the lowest median value of 1.7 mg/L. This value likely represents concentrationnatural NO3 concentration was stable, with in alluvial the lowest aquifer median groundwater, value of 1.7 which mg/L. actively This value interacts likely and represents mixes with the the natural NO3 concentration in alluvial aquifer groundwater, which actively interacts and mixes with naturalthe Tuul NO River.3 concentration By contrast, in the alluvial median aquifer NO3 levelsgroundwater, in Group which 2 wells actively ranged interacts from 15.9 and to mixes 27.7 mg/L, with the Tuul River. By contrast, the median NO3 levels in Group 2 wells ranged from 15.9 to 27.7 mg/L, thewith Tuul no significant River. By contrast,temporal the variation. median The NO most3 levels contaminated in Group 2 groundwater,wells ranged from the15.9 Group to 27.7 3 mg/L,wells, with no signiﬁcant temporal variation. The most contaminated groundwater, from the Group 3 withshowed no significant relatively temporal wide variation variation. among The most the samplingcontaminated perio groundwater,ds. The highest from median the Group value 3 wells, was wells, showed relatively wide variation among the sampling periods. The highest median value was showedobserved relatively during late wide summer variation in October among 2011, the and sampling the lowest perio valueds. The was highest observed median during value winter was in observed during late summer in October 2011, and the lowest value was observed during winter in observedFebruary during2011. In late river summer water samples, in October highest 2011, nitrateand the levels lowest were value also was recorded observed in October during 2011winter with in February 2011. In river water samples, highest nitrate levels were also recorded in October 2011 with Februaryhigh rainfalls, 2011. but In river low iwatern July samples, 2017 without highest much nitrate rainfall. levels This were implies also recorded that nitrate in October levels in 2011 the withriver high rainfalls, but low in July 2017 without much rainfall. This implies that nitrate levels in the river

Water 2018, 10, 750 14 of 18 high rainfalls, but low in July 2017 without much rainfall. This implies that nitrate levels in the river Water 2018, 10, x FOR PEER REVIEW 14 of 18 water could be affected by introduction of NO3 sources through direct surface runoff from the areas of Groupwater 3 wells could that be affected are exposed by introduction to the same of NO sources3 sources bythrough recharge direct processes.surface runoff In from summary, the areas the wide variationof inGroup NO 3 wellsconcentrations that are exposed during to the each same campaignsources by recharge suggest processes that the. In degree summary, of the contamination wide varies signiﬁcantlyvariation in NO on3 aconcentrations spatial scale, during probably each campaign due to local suggest sources that the of degree contamination. of contamination In addition, seasonalvaries changes significantly of nitrate on backgrounda spatial scale, levelsprobably of thedue Riverto local water sources and of contamination. the Group 3 seem In addition, to be affected seasonal changes of nitrate background levels of the River water and the Group 3 seem to be affected by rainfall, more seasonal data should be obtained to identify any signiﬁcant trends. by rainfall, more seasonal data should be obtained to identify any significant trends.

Figure 11.FigureBox 11.plot Box showing plot showing the NO the3 (mg/L) NO3 (mg/L) concentration concentration (including (including the standard the standard deviation, deviation, minimum, maximum,minimum, and median maximum, values) and during median sampling values) during campaigns sampling from campaigns 2010 to from 2017. 2010 NO to3 concentrations 2017. NO3 are presentedconcentrations on a power are of presented the exponent on a power scale (0.5);of the (exponenta) Group scale 3, ( b(0.5)) Group; (a) Group 2, (c )3, Group (b) Group 1, and 2, (c ()d ) River Group 1, and (d) River water, respectively. water, respectively. “*”, “o” denotes outliers.

4. Conclusions UB is one of the fastest growing cities worldwide. However, its rapid urbanization has surpassed urban infrastructure development (e.g., sewage disposal systems), posing a signiﬁcant Water 2018, 10, 750 15 of 18 risk of water pollution. Therefore, we evaluated the sustainability of UB from the perspective of water-resource quality issues. Based on ﬁve sampling campaigns and hydrochemical and isotopic analyses of various water samples from the Tuul River, shallow groundwater from the alluvial aquifer, bedrock groundwater, springs, and precipitation, we came to the following conclusions:

1. The hydrochemistry of the water samples from various resources is typically dependent on two major factors: NO3 contamination processes and mineral weathering processes according to water–rock interactions of carbonate and silicate minerals in the geologic matrix. 2. Based on their chemical composition, the groundwater samples could be classified into three groups. Group 1 included shallow groundwater from the alluvial aquifer distributed along the Tuul River, exhibiting silicate weathering–dominant water–rock interactions with near-natural NO3 concentrations. Group 2 samples, distributed mostly on the north-eastern side of the study area, appeared to reflect carbonate hydrochemical reactions of carbonate weathering, as they had intermediate NO3 levels. Group 3 samples were dominated by CaCO3 dissolution, probably due to freshwater recharge, and had significantly elevated NO3 levels exceeding the World Health Organization drinking water guideline of 50 mg/L.

3. The correlation of NO3 with Cl in the groundwater samples and the isotopic signatures implied that NO3 in groundwater resources originated from unsuitable sanitary and sewage disposal practices in the Ger districts, typically located in the upstream area surrounding UB with dense and poor living conditions. 4. The stable isotope signatures delineated the typical groundwater flow in UB. Precipitation recharges into bedrock groundwater in the upslope area, flows downward, and eventually discharges to the local base level of the basin, the Tuul River, through the alluvial aquifer. In the alluvial aquifer along the river, active pumping from multiple wells using riverbank filtration drives dynamic mixing of the groundwater and river water.

5. The Tuul River and shallow groundwater from the alluvial aquifer had natural background NO3 concentrations. Meanwhile, Group 3 groundwater, accounting for 32% of the total groundwater samples, showed NO3 contamination, with minimum and maximum concentrations of 64.0 and 305.6 mg/L, respectively. Finally, Group 2 groundwater samples had a maximum NO3 concentration of 47.4 mg/L and median of 15.8 mg/L, indicating that this groundwater was inﬂuenced by anthropogenic sources.

Figure 12 presents a schematic diagram of the groundwater system and its relation to NO3 contamination of water resources. The hydrochemical characteristics and stable isotope ratios of water samples indicate that groundwater ﬂows from the upslope area to the downslope city center toward the Tuul River through the alluvial aquifer. As the groundwater moves downward, it could carry NO3 from anthropogenic sources, slowly and steadily spreading contamination. Although Group 2 groundwater in the nearby city area has NO3 levels below the guideline of 50 mg/L, its water quality will likely deteriorate in the future and warrants continuous monitoring. As one of the fastest-growing cities worldwide, UB is facing major water supply issues, including water shortages due to over-exploitation driven by increased demand and water-quality degradation (e.g., NO3 contamination) driven by inadequate infrastructure. As identiﬁed in this study, NO3 is the major threat to water quality in UB and appears to be exerting a continuously increasing impact on water resources. In addition, rainy and dry seasonal effects appear on the water quality of river water, probably through the direct runoff from slope areas with potential sources exposed to the land surface. As water resources of adequate quality and quantity must be secured for the sustainable development of UB, appropriate institutional measures should be implemented to protect and preserve the existing water resources with systematic spatio-temporal monitoring. Water 2018, 10, 750 16 of 18 Water 2018, 10, x FOR PEER REVIEW 16 of 18

Figure 12. Schematic diagram of the groundwater flow system in Ulaanbaatar. Figure 12. Schematic diagram of the groundwater ﬂow system in Ulaanbaatar. Author Contributions: N.B. and N.C.W. conceived and designed the research and wrote the paper; N.B. and J.M.L. performed sampling and analysis of the data and proofread a manuscript; and B.N. contributed data Authorcollection Contributions: and discussedN.B. about and N.C.W.the problems. conceived and designed the research and wrote the paper; N.B. and J.M.L. performed sampling and analysis of the data and proofread a manuscript; and B.N. contributed data Funding: This research was funded by [National Research Foundation of Korea] grant number [NRF- collection and discussed about the problems. 2016K1A3A1A08953551]. Funding: This research was funded by [National Research Foundation of Korea] grant number [NRF-2016K1A3A1A08953551].Acknowledgments: We extend our deepest appreciation to Enkhbayar Dandar, Batchimeg Gansukh, and Selenge Ganbold for their support and assistance throughout the study. This paper is a part of Naranchimeg Acknowledgments:Batsaikhan’s Ph.D.We thesis extend at Yonsei our deepest University. appreciation to Enkhbayar Dandar, Batchimeg Gansukh, and Selenge Ganbold for their support and assistance throughout the study. This paper is a part of Naranchimeg Batsaikhan’s Ph.D. thesisConflicts at Yonseiof Interest: University. The authors declare no conflict of interest.

ConﬂictsReferences of Interest: The authors declare no conﬂict of interest.

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