Stable Isotope Reveals Tap Water Source Under Different Water Supply Modes in the Eastern Margin of the Qinghai–Tibet Plateau

water

Article Stable Isotope Reveals Tap Water Source under Diﬀerent Water Supply Modes in the Eastern Margin of the Qinghai–Tibet Plateau

Mingxia Du, Mingjun Zhang *, Shengjie Wang , Hongfei Meng, Cunwei Che and Rong Guo

College of Geography and Environmental Science, Northwest Normal University, Lanzhou 730070, China; [email protected] (M.D.); [email protected] (S.W.); [email protected] (H.M.); [email protected] (C.C.); [email protected] (R.G.) * Correspondence: [email protected]; Tel.: +86-0931-797-1750

Received: 7 October 2019; Accepted: 4 December 2019; Published: 6 December 2019

Abstract: Based on 1260 tap water samples gathered monthly and 136 surface water samples collected seasonally in the eastern margin of the Qinghai–Tibet Plateau, the local tap water line, the basic spatiotemporal characteristics of tap water isotopes, and their indication for water source under different water supply modes were discussed, linking the local tap water supply and water source information. A new tap water isotopes data set based on dense sampling sites was established, which was reliable for the analysis of tap water isotope features, tap water supply management, and tap water sources. The main conclusions are: (1) The local tap water lines in Gannan and Longnan are δ2H = (7.06 0.17) δ18O + (3.24 1.75) (r2 = 0.81, p < 0.01) and δ2H = (5.66 0.09) δ18O + ( 8.12 0.82) ± ± ± − ± (r2 = 0.82, p < 0.01), respectively. (2) The annual mean δ2H and δ18O in tap water show an increasing trend from southwest to northeast. The seasonal differences of δ2H and δ18O in tap water in Gannan and Longnan are small. (3) The correlation of tap water isotopes with those in main source water is high, while that of isotopes in tap water with those in non-water source is low. Under the central water supply mode by local tap water company, tap water isotopes in Gannan where groundwater is the direct water source show weak connection with those in surface water and precipitation, and those in tap water in Longnan with surface water as main source water reveal good connection with isotopes in surface water. Under mixed water supply modes, tap water isotopes indicate that surface water is the main tap water source in Gannan and Longnan with multiple water sources.

Keywords: δ2H and δ18O; tap water; surface water; water source; Qinghai–Tibet Plateau

1. Introduction Tap water isotopes are well proven to be essential indicators [1–10] for studies in many ﬁelds [10–22], including hydrology, ecology, climatology, forensic, and so on. With the development of isotope methods, isotopes have been gradually applied to tap water research [23–32]. At present, studies on isotopes in tap water [33,34] throughout the world are relatively few. Their study areas have mainly involved the United States of America (e.g., the San Francisco Bay Area [31], the Salt Lake Valley of northern Utah [30], western and the whole United States [35–37]), South Africa [30], and China [34,38]. The main research contents include basic features (e.g., spatial and temporal variations, local tap water line) of tap water isotopes, tap water source, the applicability of tap water to relevant ﬁelds, and so on. These studies have enriched the knowledge of tap water isotope features, tap water supply mode and management strategy of local water resources. At present, studies of tap water isotopes in China can be seldom found, except for reports on isotopes in tap water throughout China [34,38]. Features of tap water isotopes and their relationship

Water 2019, 11, 2578; doi:10.3390/w11122578 www.mdpi.com/journal/water Water 2019, 11, 2578 2 of 17 Water 2019, 11, 2578 2 of 17 At present, studies of tap water isotopes in China can be seldom found, except for reports on isotopes in tap water throughout China [34,38]. Features of tap water isotopes and their relationship withwith precipitationprecipitation isotopesisotopes werewere exhibited.exhibited. TheThe waterwater supplysupply systemssystems inin China,China, whichwhich havehave aa vastvast territory,territory, areare complexcomplex [[39].39]. FurtherFurther studiesstudies ofof taptap waterwater isotopesisotopes basedbased onon densedense samplingsampling sitessites areare neededneeded toto finelyfinely exploreexplore thethe isotopicisotopic compositioncomposition ofof taptap waterwater andand itsits indicationindication forfor waterwater source,source, especiallyespecially inin areasareas withwith multiplemultiple waterwater supplysupply modes.modes. InIn southernsouthern GansuGansu Province,Province, easterneastern marginmargin ofof Qinghai–TibetQinghai–Tibet Plateau,Plateau, didifferentfferent waterwater supplysupply modes,modes, includingincluding centralcentral waterwater supplysupply (e.g.,(e.g., suppliedsupplied byby ruralrural drinkingdrinking waterwater safetysafety project,project, suppliedsupplied byby taptap waterwater company)company) andand decentralizeddecentralized waterwater supply,supply, coexistcoexist inin thethe GannanGannan TibetanTibetan AutonomousAutonomous PrefecturePrefecture (Gannan)(Gannan) andand Longnan.Longnan. In the existing studies in ChinaChina [[34,38,40],34,38,40], onlyonly fourfour taptap waterwater samplingsampling points were involved involved in in Gannan Gannan and and Long Longnan,nan, which which cannot cannot accurately accurately reflect reflect the thelocal local tap tapwater water isotope isotope landscape, landscape, seasonal seasonal differences, differences, and the and significance the significance of water of watersource sourceindication. indication. In this Instudy, this study,isotopes isotopes in 1396 in water 1396 watersamples samples (tap water (tap water samples: samples: 1260; 1260;surface surface water water samples: samples: 136) were 136) wereanalyzed, analyzed, and andthe thelocal local tap tapwater water line, line, the the basic basic spatiotemporal spatiotemporal characteristics characteristics of of isotopes, andand indicationindication forfor waterwater sourcesource underunder didifferentfferent waterwater supplysupply modesmodes werewere discussed,discussed, combiningcombining thethe fieldfield surveysurvey datadata aboutabout waterwater supplysupply andand sources.sources. ThisThis researchresearch isis beneficialbeneficial forfor understandingunderstanding taptap waterwater isotopes,isotopes, waterwater supply,supply, and and water water source source determination. determination. 2. Study Area 2. Study Area Areas analyzed in this study were located in southern Gansu, including the Gannan Tibetan Areas analyzed in this study were located in southern Gansu, including the Gannan Tibetan Autonomous Prefecture (Gannan) and Longnan (Figure1). Gannan is one of the ten Tibetan Autonomous Prefecture (Gannan) and Longnan (Figure 1). Gannan is one of the ten Tibetan Autonomous Prefectures in China and located between 100 46 –104 44 E and 33 06 –36 10 N, Autonomous Prefectures in China and located between 100°46◦′–104°440 ◦′ E 0and 33°06′–36°10◦ 0 ′ N,◦ with0 with an area of 4.02 104 km2. The elevation in Gannan is between 1100 and 4900 m, mainly above an area of 4.02 × 104 ×km2. The elevation in Gannan is between 1100 and 4900 m, mainly above 3000 m. 3000 m. The temperature difference between day and night is large. There are more than 120 rivers or The temperature difference between day and night is large. There are more than 120 rivers or streams streams flowing in Gannan, such as the Yellow River, Tao River, Da Xia River, Pai-lung River, and so on. flowing in Gannan, such as the Yellow River, Tao River, Da Xia River, Pai-lung River, and so on. Longnan is situated between 104 01 –106 35 E and 32 35 –34 32 N, with an area of 2.79 104 km2. Longnan is situated between 104°01◦ 0′–106°35◦ 0′ E and 32°35◦ 0′–34°32◦ 0′ N, with an area of 2.79× × 104 km2. Longnan is the only region in Gansu which belongs to the Yangtze River system and has a subtropical Longnan is the only region in Gansu which belongs to the Yangtze River system and has a subtropical climate. High mountains, river valley, hills, and basins are interlaced in Longnan. climate. High mountains, river valley, hills, and basins are interlaced in Longnan.

Figure 1. Study area in this study. (a) Location of Gannan and Longnan in China. (b) Location ofFigure Gannan 1. Study and Longnanarea in this in Gansustudy. ( Province.a) Location The of Gannan satellite-derived and Longnan land coverin China. base (b map) Location derived of fromGannan Natural and Longnan Earth (http: in //Gansuwww.naturalearthdata.com Province. The satellite-derived). Spatial distributionland cover base of elevation map derived came fromfrom ShuttleNatural RadarEarth (http://www.naturalearthdata.com). Topography Mission 90M Resolution Spat Rawial distribution Elevation of Data elevation (SRTMDEM came from 90M; Shuttle http: //Radarwww.gscloud.cn Topography/sources Mission/?cdataid 90M=302&pdataid Resolution=10 ).Raw Elevation Data (SRTMDEM 90M; http://www.gscloud.cn/sources/?cdataid=302&pdataid=10).

Water 2019,, 11,, 25782578 33 of of 17

3. Data and Method 3. Data and Method 3.1. Collection of Water Samples 3.1. Collection of Water Samples Surface water and tap water samples were gathered by field work of our team in southern Gansu fromSurface May 2017 water to April and tap 2018. water Cold samples tap water were was gathered collected by fieldinto bottles work of after our running team insouthern the tap for Gansu 10 s [41].from Fresh May 2017 surface to April water 2018. was Coldcollec tapted water from wasrivers, collected streams, into or bottles mountain after springs running flowing the tap in for southern 10 s [41]. Gansu.Fresh surface Tap water water samples was collected were gathered from rivers, monthly streams, from or May mountain 2017 to springs April 2018 flowing (except in southern for June Gansu. 2017). SurfaceTap water water samples samples were were gathered collected monthly 5 times from seas Mayonally 2017 between to April 2018May (except2017 and for April June 2017).2018 (in Surface May water2017, July samples 2017, were October collected 2017, 5January times seasonally 2018, and betweenApril 2018). May Sampling 2017 and sites April had 2018 relatively (in May 2017,complete July geographical2017, October coverage 2017, January in southern 2018, and Gansu, April covering 2018). Sampling all the counties sites had and relatively districts complete governed geographical by Gannan andcoverage Longnan in southern (Figure Gansu, 2). As covering Gannan all and the Longnan counties andare districtslocated governedin mountainous by Gannan areas, and tap Longnan water (Figuresamples2). were As Gannan gathered and along Longnan the winding are located roads in mountainous in the mountains, areas, which tap water link samples residential were areas. gathered The totalalong number the winding of tap roads water in and the surface mountains, water which sample links were residential 1260 and areas. 136, The respectively. total number Information of tap water of alland water surface samples water samplesis summed were up 1260 in Table and 136, 1. In respectively. the division Information of four seasons, of all water spring samples includes is summed March, Aprilup in Tableand May,1. In theand divisionsummer of includes four seasons, June, springJuly and includes August, March, and Aprilautumn and includes May, and September, summer Octoberincludes and June, November, July and August, and winter and autumnincludes includes December, September, January October(in the nest and year) November, and February and winter (in theincludes nest year). December, January (in the nest year) and February (in the nest year).

Figure 2. Spatial distribution of of sampling sites sites in in this study. ( a)) All of the tap water (T) and surface water (S) sampling sites in Gannan and Longnan. ( (bb)) Tap Tap water sampling sites supplied centrally by tap water company in Gannan and Longnan (Table(Table S1S1 inin SupplementarySupplementary Materials).Materials).

Water 2019, 11, 2578 4 of 17

Table 1. Information of all collected water samples in Gannan and Longnan.

Number of Sum of Sampling Number of Sum of Water Type Area Sampling Sampling Frequency Samples (n) Samples Sites (N) Sites Gannan 43 monthly 433 Tap water 123 1260 Longnan 80 monthly 827 Surface Gannan 17 seasonally 82 28 136 water Longnan 11 seasonally 54

3.2. Experimental Analysis δ2H and δ18O in water samples were analyzed by a liquid water isotope analyzer (DLT-100, developed by the Los Gatos Research company of the United States) [42] at the Northwest Normal University. In the test, three standard samples (standard No. 3: δ2H: 96.4 0.5%, δ18O: 13.10 − ± − 0.15%; standard No. 4: δ2H: 51.0 0.5%, δ18O: 7.69 0.15%; standard No. 5: δ2H: 9.5 ± − ± − ± − ± 0.5%, δ18O: 2.80 0.15%; provided by the LGR company) and six gathered water samples were − ± considered as one group. Every sample was tested for six injections. Data of the first two needles were discarded because of the isotope memory effect, and values of the last four needles were calculated as final results [43,44]. The measurement uncertainties for δ18O and δ2H were no more than 0.2% and 0.6%. Results tested were relative to the Vienna Standard Mean Ocean Water (VSMOW). Rsample Rstandard δsample = − 1000% (1) Rstandard ×

In Equation (1), Rsample presents the ratio of 2H/1H(18O/16O) in water samples. Rstandard shows the ratio of 2H/1H(18O/16O) in the VSMOW.

3.3. Other Data Precipitation isotope data used in this paper were detected from a global data product, the Regionalized Cluster-based Water Isotope Prediction (RCWIP, Grabiszy´nska,Poland) model version 1.00 [45–49], providing estimated δ2H and δ18O in precipitation (annual and monthly values). The reliability of this precipitation isotope database in China has been veriﬁed to be good [38].

3.4. Tap Water Isoscape Simulation and Error Test Methods Based on the existing studies about the isoscapes in diﬀerent water bodies (e.g., tap water and precipitation) [31,45,50–52], seven regression models combining spatial (including latitude (L, ◦), longitude (O, ◦), and elevation (E, m)) and meteorological factors (e.g., average temperature and precipitation) (Table2) were selected and compared to develop the isoscape of tap water in southern Gansu. The Shuttle Radar Topography Mission 90M Resolution Raw Elevation Data (SRTMDEM 90M) were applied in isoscape models. Meteorological parameters, including average temperature (T; ◦C), precipitation (P; mm), wind speed (S; m/s), water pressure (V; kPa), and solar radiation (R; kJ/(m2 day)), · with 30 s spatial resolution involved in these models were selected from WorldClim-Global Climate Data (version 2) [53]. Simulation results for all of the seven regression models were evaluated based 2 on adjusted determination coeﬃcient (radj ), mean absolute error (MAE), mean bias error (MBE), and root mean square error (RMSE) (Table3). Models 6 and 7 were selected for the simulation of δ2H and δ18O, respectively, as the optimal model. The simulated contour combined regression estimation and interpolated residuals with the Kriging method [36]. Water 2019, 11, 2578 5 of 17

Table 2. Regression models for the estimation of tap water isoscape in Gannan and Longnan.

Code Regression Model Regression Method Reference 1 δ = aL2 + bL + cE + d Multiple regression [36,50,52] 2 δ = aL + bO + cE + d Multiple regression [34] 3 δ = aT + bP + c Multiple regression [34] 4 δ = aT + bP + cR + dS+ eV + f Multiple regression [38] 5 L, O, E, L2, O2, E2 Stepwise regression [51] 6 T, P, R, S, V, T2, P2, R2, S2, V2 Stepwise regression [51,54] 7 L, O, E, T, P, R, S, V, L2, O2, E2, T2, P2, R2, S2, V2 Stepwise regression [38]

Note: L, latitude (in degree); O, longitude (in degree); E, elevation (in m); T, temperature (in ◦C); P, precipitation (in mm); R, solar radiation (in kJ/(m2 day)); S, wind speed (in m/s); V, water vapor pressure (in kPa). ·

Table 3. Equations of regression models for δ2H and δ18O in tap water in Gannan and Longnan.

2 2 Isotope Code Equations r radj Sig. N MBE MAE RMSE 1 δ = 0.122L2 0.012E 182.913 0.582 0.575 0.000 123 0.14 4.13 5.13 − − 2 δ = 5.61L + 5.317O 0.004E 803.075 0.665 0.657 0.000 123 –0.66 3.67 4.65 − − 3 δ = 1.053T + 0.048P 101.836 0.436 0.426 0.000 123 –0.01 4.83 5.96 − δ2H (%) 4 δ = 4.505T 0.058P + 0.00006449R 10.554S + 99.162V 64.208 0.66 0.645 0.000 123 0.08 3.67 4.63 − − − − − 5 δ = 0.082L2 + 0.025O2 0.0000009622E2 423.155 0.672 0.664 0.000 123 4.20 4.84 6.19 − − 6 δ = 0.053P + 74.125S + 77.378V 0.098T2 16.107S2 166.133 0.709 0.696 0.000 123 0.02 3.26 4.28 − − − − 7 δ = 0.001R + 65.462V + 0.236L2 0.096T2 209.471 0.730 0.721 0.000 123 10.53 10.53 11.31 − − − 1 δ = 0.021L2 0.002E 30.661 0.541 0.533 0.000 123 0.38 0.71 0.86 − − − 2 δ = 0.889L + 1.057O 0.00003615E 150.125 0.718 0.711 0.000 123 0.01 0.45 0.57 − − − 3 δ = 0.114T + 0.009P 16.187 0.408 0.399 0.000 123 0.15 0.68 0.84 − − δ18O (%) 4 δ = 0.545T 0.006P + 0.00003996R 0.41S + 13.941V 18.462 0.698 0.685 0.000 123 0.20 0.48 0.63 − − − − 5 δ = 0.013L2 + 0.005O2 80.602 0.720 0.715 0.000 123 1.47 1.48 1.58 − − 6 δ = 0.001R + 10.531V 0.02T2 + 0.000000003406R2 + 61.186 0.722 0.713 0.000 123 5.33 5.33 5.36 − − 7 δ = 0.000135R + 13.746S + 8.259V + 0.034L2 3.086S2 47.914 0.793 0.784 0.000 123 0.27 0.41 0.56 − − − Note: N, number of the tap water sampling sites; L, latitude (in degree); O, longitude (in degree); E, elevation (in m); 2 T, temperature (in ◦C); P, precipitation (in mm); R, solar radiation (in kJ/(m day)); S, wind speed (in m/s); V, water vapor pressure (in kPa); MBE, mean bias error; MAE, mean absolute error; RMSE,· root mean square error.

4. Result

4.1. Basic Characteristics of Tap Water Isotopes

4.1.1. Local Tap Water Line Figure3 shows the local tap water line (LTWL), local surface water line (LSWL), and local meteoric water line (LMWL) in Gannan and Longnan. Global meteoric water line (GMWL), δ2H = 8 δ18O + 10 [55], is also presented in Figure3. The LTWLs in Gannan and Longnan are δ2H = (7.06 0.17) δ18O ± + (3.24 1.75) (r2 = 0.81, p < 0.01) and δ2H = (5.66 0.09) δ18O + ( 8.12 0.82) (r2 = 0.82, p < 0.01), ± ± − ± respectively. Lower slopes can be seen in the LTWLs in Gannan and Longnan compared to the GMWL proposed by Craig [55] and Gourcy et al. (δ2H = (8.14 0.02) δ18O + (10.9 0.2), r = 0.98) [56] (in the ± ± Student’s t-test, p > 0.05) [10]. The slopes of the LTWLs in Gannan and Longnan are also lower than the Chinese tap water line δ2H = (7.57 0.04) δ18O + (5.07 0.38) (r2 = 0.93, p < 0.01) (in the Student’s ± ± t-test, p > 0.05) [10], newly calculated combining two exciting data sets of Chinese tap water isotopes (base on monthly tap water isotope values) [34,38]. This may be as a result of diﬀerences in climate and evaporation ratios in water source areas [57]. The slopes of LTWLs, LSWLs, and LMWLs are all higher in Gannan than those in Longnan. Water 2019, 11, 2578 6 of 17 Water 2019, 11, 2578 6 of 17

Figure 3. Relationships between δ2H and δ18O in tap water, surface water, and precipitation in Gannan Figure 3. Relationships between δ2H and δ18O in tap water, surface water, and precipitation in Gannan and Longnan; (a,b) Gannan, (c,d) Longnan. (GMWL: Global meteoric water line; LMWL: Local meteoric and Longnan; (a,b) Gannan, (c,d) Longnan. (GMWL: Global meteoric water line; LMWL: Local water line; LTWL: Local tap water line; LSWL: Local surface water line; n: Number of water samples or meteoric water line; LTWL: Local tap water line; LSWL: Local surface water line; n: Number of water grid sites). samples or grid sites). 4.1.2. Spatial Pattern 4.1.2. Spatial Pattern The simulated tap water isoscape in Gannan and Longnan is exhibited in Figure4 and Figure S1 (in SupplementaryThe simulated Materials).tap water isoscape From southwest in Gannan to northeast,and Longnan the annualis exhibited mean inδ2 FiguresH and δ 184 Oand show S1 (in an 2 18 Supplementaryincreasing trend Materials). on the whole. From In southw southwestest to Gannan, northeast, isotopes the annual are the mean lowest δ H within and δ Gannan,O show with an increasingδ2H values trend lower on than the whole.85.0% Inand southwestδ18O lower Gannan, than 11.0%isotopes. In are the the north lowest and within east parts Gannan, of Gannan, with 2 − 18 − δannualH values mean lowerδ2H than and δ−1885.0‰O in tap and water δ O arelower the than highest −11.0‰. within In Gannan, the north with and some eastδ parts2H values of Gannan, higher 2 18 2 annualthan 75.0%mean δandH andδ18 Oδ higherO in tap than water10.0% are the. Inhighest most within parts of Gannan, Gannan, with the some annual δ H mean values values higher of − 18 − 2 thanδ2H range−75.0‰ from and δ85.0O tohigher65.0% than and −10.0‰. those In of mostδ18O areparts lower of Gannan, than 11.0% the annual. Isotopes mean in values Longnan of δ areH − − 18 − rangehigher from than − those85.0 to in − Gannan65.0‰ and in general. those of Inδ mostO are parts lower of than Longnan, −11.0‰. the Isotopes annual meanin Longnan values are of δ higher2H are 2 thanhigher those than in Gannan65.0% andin general. those of Inδ most18O are parts higher of Longnan, than 10.0% the annual. The mean annual values mean of values δ H are of higherδ2H in − 18 − 2 thantap water −65.0‰ in theand east those part of ofδ LongnanO are higher are thethan highest −10.0‰. (δ2 HThe higher annual than mean55.0% values), andof δ thoseH in tap of δ 18waterO in 2 − 18 insoutheast the east Longnanpart of Longnan are the highestare the highest (δ18O higher (δ H higher than than8.0% −).55.0‰), The simulated and those residuals of δ O in for southeastδ2H and 18 − 2 18 Longnanδ18O are inare Figure the highest4c,d. In (δ mostO higher parts of than Gannan −8.0‰). and The Longnan, simulated residuals residuals for δfor2H δ rangeH and between δ O are 2.0in 2 − Figureand 2.0% 4c,d., andIn most those parts for δof18 OGannan are from and 0.6Longnan, to 0.6% residuals. A jackknife for δ H procedure range between was applied −2.0 and to report2.0‰, 18 − andthe isoscapethose for uncertaintiesδ O are from (Figure −0.6 to5 ).0.6‰. Each A sampling jackknife siteprocedure supplied wa centrallys applied by to tap report water the company isoscape uncertainties(Figure2, Table (Figure S1 in Supplementary5). Each sampling Materials) site supplied in Gannan centrally and by Longnan tap water was company removed (Figure separately 2, Table to S1simulate in Supplementary the isotope landscape. Materials) Correction in Gannan between and Lo estimatedngnan was and removed observation separately annual to mean simulate values the of 2 isotopeδ2H and landscape.δ18O is showed Correction in Figure between5. The estimate equationsd and of observationδ2H and δ18 annualO are close mean to values y = x. of The δ H values and 18 2 18 2 δofOr2 isfor showed the equations in Figure of 5.δ2 HThe and equationsδ18O are of 0.85 δ H and and 0.93, δ O respectively. are close to y The = x. simulated The values water of r isotopefor the 2 18 equationslandscape of map δ H has and a δ goodO are accuracy. 0.85 and 0.93, respectively. The simulated water isotope landscape map has a good accuracy.

Water 2019, 11, 2578 7 of 17 Water 2019, 11, 2578 7 of 17

(c)

(d)

Figure 4. SpatialSpatial variations variations of of annual annual mean mean values values of of δ2δH2H ((a ((),a ‰)), % and) and δ18δO18 ((Ob), (( b‰)), % and) and residuals residuals for δfor2H δ((2Hc), ((‰)c), %and) and δ18Oδ18 ((Od), (( d‰)), % in) intap tap water water in in Gannan Gannan and and Longnan. Longnan. Residuals Residuals equal equal to to the observation values minus the estimated values.

Water 2019, 11, 2578 8 of 17 Water 2019, 11, 2578 8 of 17

Water 2019, 11, 2578 8 of 17

FigureFigure 5. 5. Correction between estimated estimated and and obse observationrvation annual annual mean mean values values of ofδ2Hδ2 ((Ha), (( ‰)a), %and) andδ18O

δ((18bO), ((‰)b), %in )tap in tapwater water at the at the sampling sampling sites sites supplied supplied centrally centrally by by tap tap water water company company (Table S1S1 inin 2 18 SupplementarySupplementaryFigure 5. Correction Materials) Materials) between in in Gannan Gannan estimated and an andd Longnan. Longnan. observation The The annual dotted dotted mean line line isvalues is y y= =x. ofx. δ H ((a), ‰) and δ O ((b), ‰) in tap water at the sampling sites supplied centrally by tap water company (Table S1 in 4.1.3.4.1.3. TemporalTemporalSupplementary Variation Variation Materials) in Gannan and Longnan. The dotted line is y = x. 2 18 4.1.3.DiDifferencesﬀerences Temporal of of Variation isotope isotope between between two two adjacent adjacent seasons seasons (∆ (δΔδH2H and and∆ Δδδ18O,O, % ‰)) atat everyevery samplingsampling site site 2 18 were calculated (Figure6). On the whole, ∆δ H2 and ∆δ 18O between two adjacent seasons in Gannan were calculated (Figure 6). On the whole, Δδ H and Δδ O 2between 18two adjacent seasons in Gannan Differences of isotope between two adjacent2 seasons (Δδ H and Δδ O, ‰) at every sampling site and Longnan are small. The values of ∆δ2 H between two adjacent seasons mainly range from 3.0 and Longnan are small. The values of Δδ H between2 two18 adjacent seasons mainly range from −3.0 to were calculated (Figure 6). On the whole, Δδ H and Δδ O between two adjacent18 seasons in Gannan− to 2.0% (red and purple sampling sites in Figure6a,c,e,g). The values of ∆δ 18O between spring and 2.0‰and (red Longnan and arepurple small. sampling The values sites of Δinδ2 HFigure between 6a,c,e,g). two adjacent The values seasons of mainly Δδ O range between from spring−3.0 to and winterwinter2.0‰ mainly mainly (red and vary vary purple from from 0sampling to0 to 0.5% 0.5‰ sites(purple (purple in Figure and and dark 6a,c,e,g).dark purple purple The sampling valuessampling of sites Δ sitesδ18 inO Figureinbetween Figure6h), spring 6h), and and thoseand those of 18 ∆ofδ ΔwinterOδ18 betweenO between mainly other vary other twofrom two adjacent 0 adjacentto 0.5‰ seasons (purple seasons are and are mainly dark mainly purple from from sampling0.5 −0.5 to to 0.2% sites0.2‰ in(red (red Figure and and 6h), purple purple and samplingthose sampling − 2 18 sitessitesof in inΔδ Figure 18FigureO between6 b,d,f).6b,d,f). other Table Table two4 adjacentshows4 shows the seasons the seasonal seasonal are mainly and and annual from annual −0.5 mean tomean 0.2‰δ δ H(red2H and and andδ δpurple18OO inin sampling GannanGannan and and 2 Longnan.Longnan.sites in The FigureThe highest highest 6b,d,f). seasonal seasonal Table mean4 showsmeanδ H theδ in2H seasonal Gannan in Gannan and annual Longnanand Longnan mean both δ2 appearHboth and appearδ in18O summer. in Gannanin summer. The and annual The 2 18 meanannualLongnan.δ Hmean and The δδ2H highestO and in Gannanδ18 seasonalO in Gannan are mean both are lowerδ2H both in thanGannan lower those than and in thoseLongnan Longnan. in Longnan. both appear in summer. The annual mean δ2H and δ18O in Gannan are both lower than those in Longnan.

FigureFigure 6. Di6. ﬀDifferenceserences for forδ2 δH2H and andδ 18δ18OO (that(that is, ∆Δδ2HH and and Δ∆δδ1818O,O, ‰) % in) intap tap water water in two in two adjacent adjacent seasons at each sampling site in Gannan and Longnan; (a,b) summer minus spring, (c,d) autumn seasonsFigure at6. eachDifferences sampling for site δ2 inH Gannanand δ18O and (that Longnan; is, Δδ2H (a ,band) summer Δδ18O, minus‰) in spring, tap water (c,d )in autumn two adjacent minus minus summer, (e,f) winter minus autumn, (g,h) spring minus winter. summer,seasons (ate, feach) winter sampling minus site autumn, in Gannan (g,h) spring and Longnan; minus winter. (a,b) summer minus spring, (c,d) autumn

minus summer, (e,f) winter minus autumn, (g,h) spring minus winter.

Water 2019, 11, 2578 9 of 17

Table 4. Seasonal and annual mean δ2H and δ18O in tap water at all sampling sites in Gannan and Longnan.

Isotope Area Spring Summer Autumn Winter Annual Gannan 70.3 70.0 71.0 71.2 70.6 δ2H (%) − − − − − Longnan 58.4 57.4 59.2 59.5 58.6 − − − − − Gannan 10.3 10.4 10.6 10.6 10.5 δ18O (%) − − − − − Longnan 8.8 8.8 9.0 9.1 8.9 − − − − −

4.2. Comparison of Isotopes in Tap Water under Diﬀerent Water Supply Modes with Those in Precipitation and Surface Water

4.2.1. Under the Mode of Central Water Supply by Local Tap Water Company To detect the connection of tap water isotopes under different water supply modes with other water bodies, tap water samples under the mode of centralized water supply by local tap water companies in Gannan and Longnan (Figure2b, Table S1 in Supplementary Materials) were selected and analyzed. On the whole, the correlation of isotopes between tap water supplied centrally by local water companies and precipitation is weak in Gannan and Longnan (Table5). The correlation coefficients at most sampling sites are less than 0.5. In Figure7, the correlation coe fficients in the fitting equations for δ2H and δ18O between tap water and surface water in Gannan (r < 0.5) are smaller than those in Longnan (r > 0.5). Relatively speaking, weak connection of isotopes can be seen between tap water and surface water in Gannan, while better connection presents in Longnan.

Table 5. Correlation coeﬃcients (r) of monthly δ2H and δ18O between tap water and precipitation under the mode of central water supply by local tap water companies in Gannan and Longnan.

p < 0.05 p < 0.05 Area Sampling Sites r of δ2H r of δ18O (Yes/No) (Yes/No) Hezuo 0.31 N 0.49 N Xiahe Xian 0.45 N 0.59 N Lintan Xian 0.85 Y 0.92 Y Jone Xian 0.70 Y 0.75 Y Gannan Luqu Xian 0.19 N 0.24 N − Maqu Xian 0.59 N 0.36 N Tewo Xian 0.49 N 0.16 N Zhugqu Xian 0.16 N 0.25 N − Wudu 0.12 N 0.53 N Dangchang 0.55 N 0.47 N Xian Wen Xian 0.73 Y 0.47 N − − Longnan Kang Xian 0.24 N 0.51 N Cheng Xian 0.59 N 0.54 N Hui Xian 0.26 N 0.12 N Liangdang 0.46 N 0.54 N Xian Xihe Xian 0.35 N 0.32 N Li Xian 0.48 N 0.31 N Water 2019, 11, 2578 10 of 17 Water 2019, 11, 2578 10 of 17

2 18 FigureFigure 7. 7.Relationship Relationship ofofδ δ2HH andandδ δ18O (%(‰)) in tap water under thethe modemode ofof centralizedcentralized waterwater supplysupply byby locallocal taptap water water companies companies withwith thosethose inin surface surface water water in in Gannan Gannan ( a(a,b,b)) and and Longnan Longnan ( c(,cd,d).). Solid Solid circlescircles and and error error bars bars show show the the arithmetic arithmetic average average and and standard standard deviation, deviation, respectively. respectively. 4.2.2. Under Mixed Tap Water Supply Modes 4.2.2. Under Mixed Tap Water Supply Modes The relationship between isotopes in tap water under mixed water supply modes (based on all tap waterThe relationship samples) and between those in isotopes other two in tap water wate bodiesr under in mixed Longnan water and supply Gannan modes was (based analyzed on inall tap water samples) and those in other two water bodies in Longnan and Gannan was analyzed in detail in terms of temporal variation, numerical difference, and correlation. In general, the temporal detail in terms of temporal variation, numerical difference, and correlation. In general, the temporal features of isotopes between in tap water under mixed water supply modes and precipitation show features of isotopes between in tap water under mixed water supply modes and precipitation show great differences (Figure8). The monthly δ2H in Gannan and Longnan change little throughout the great differences (Figure 8). The monthly δ2H in Gannan and Longnan change little throughout the year, and so are the δ18O values. Isotopes in precipitation in Gannan are higher in summer, with lower year, and so are the δ18O values. Isotopes in precipitation in Gannan are higher in summer, with lower values in winter. In Longnan, the seasonal mean δ2H and δ18O in precipitation are the highest in spring values in winter. In Longnan, the seasonal mean δ2H and δ18O in precipitation are the highest in (δ2H: 53.7%, δ18O: 7.8%) and the lowest (δ2H: 82.0%, δ18O: 11.6%) in winter. The isotopic spring− (δ2H: −53.7‰,− δ18O: −7.8‰) and the lowest (δ−2H: −82.0‰, δ18O:− −11.6‰) in winter. The isotopic seasonal differences of surface water are small, so are tap water (Figure9). The lowest seasonal values seasonal differences of surface water are small, so are tap water (Figure 9). The lowest seasonal values δ2H( 74.7%) and δ18O( 11.0%) in surface water in Gannan present in autumn. The highest seasonal δ2H (−−74.7‰) and δ18O (−11.0‰) in surface water in Gannan present in autumn. The highest seasonal values δ2H( 57.1%) and δ18O( 8.8%) in surface water in Longnan appear in spring. values δ2H (−−57.1‰) and δ18O (−−8.8‰) in surface water in Longnan appear in spring.

WaterWater2019 2019, 11, 11, 2578, 2578 11 ofof 1717 Water 2019, 11, 2578 11 of 17

Figure 8. Comparison of monthly δ2H and δ18O (‰) between tap water and precipitation in Gannan Figure 8. Comparison of monthly 2δ2H and δ1818O (‰) between tap water and precipitation in Gannan Figure(a,b) and 8. Comparison Longnan (c,d of). monthly δ H and δ O (%) between tap water and precipitation in Gannan (a(,ab,b)) and and Longnan Longnan ( c(,cd,d).).

FigureFigure 9.9. Comparison of of seasonal seasonal and and annual annual δ2δH2 Hand and δ18Oδ18 (‰)O (% between) between tap water tap water and surface and surface water Figure 9. Comparison of seasonal and annual δ2H and δ18O (‰) between tap water and surface water waterin Gannan in Gannan (a,b) and (a,b Longnan) and Longnan (c,d). The (c,d top). The and topbottom and of bottom the box of exhibit the box the exhibit 75th and the 25th 75th percentiles, and 25th in Gannan (a,b) and Longnan (c,d). The top and bottom of the box exhibit the 75th and 25th percentiles, percentiles,the line in the box line signs in the the box 50th signs percentile the 50th (med percentileian), whiskers (median), mark whiskers the 10th mark and the 90th 10th percentiles; and 90th the line in the box signs the 50th percentile (median), whiskers mark the 10th and 90th percentiles; percentiles;points below points and belowabove andthe whiskers above the indicate whiskers lowe indicater than lowerthe 10th than and the higher 10th andthan higher 90th percentiles, than 90th points below and above the whiskers indicate lower than the 10th and higher than 90th percentiles, percentiles,respectively. respectively. respectively.

Water 2019, 11, 2578 12 of 17

Numerical differences of tap water isotopes with precipitation are much larger than those with surface water (Figure 10). Differences of monthly mean δ2H in tap water with that in precipitation in Gannan and Longnan mainly range from 0 to 30.0‰ (Figure 10a), and those for δ18O mainly range from −2.0 to 4.0‰ (Figure 10b). The differences of monthly mean δ2H and δ18O between tap water and precipitation in Gannan are larger than those in Longnan (Figure 10a,b). The differences of seasonal mean δ2H in tap water with surface water in Gannan and Longnan mainly range from −1.0 Waterto 3.0‰2019, 11(Figure, 2578 10c), and those for δ18O mainly range from −0.1 to 0.4‰ (Figure 10d). Differences12 of 17 of seasonal mean δ2H between surface water and tap water in Gannan are larger than those in Longnan (FigureNumerical 10c,d). differences of tap water isotopes with precipitation are much larger than those with 2 18 surfaceThe water relationships (Figure 10 of). Diδ Hfferences and δ ofO monthlyin tap water mean withδ2H those in tap in water the other with two that water in precipitation bodies are inpresented Gannan in and Figures Longnan 11 and mainly 12, respectively. range from Overall, 0 to 30.0% tap water(Figure isotopes 10a), andshow those better for correlationδ18O mainly with 2 18 rangesurface from water2.0 than to 4.0% precipitation. (Figure 10 Asb). δ H The and di ffδerencesO values of in monthly surface mean waterδ increase,2H and δ those18O between in tap water tap − 2 18 watershow and an upward precipitation trend in (Figure Gannan 12). are In larger contrast, than with those the in Longnanincreasing (Figure of δ H 10 anda,b). δ TheO in di precipitation,fferences of seasonalthose in mean tap waterδ2H indo tap not water show witha uniform surface changing water in trend Gannan (Figure and Longnan11). For tap mainly water range and precipitation, from 1.0 to 2 18 − 3.0%the correlation(Figure 10 coefficientsc), and those in fortheδ fitting18O mainly equations range for from δ H and0.1 δ toO 0.4% are all(Figure less than 10d). 0.5 Diin ffGannanerences and of − 2 18 seasonalLongnan mean (Figureδ2H 11). between To tap surface water waterand surface and tap water, water those in Gannan for δ H are and larger δ O than are thoseall larger in Longnan than 0.7 (Figure(Figure 10 12).c,d).

FigureFigure 10. 10.Di Differencesﬀerences (% (‰)) ofofδ 2δH2H andandδ 18δ18OO inin taptap waterwater withwith precipitation precipitation and and surface surface water water in in GannanGannan and and Longnan. Longnan. ( a(,ab,b)) tap tap water water minus minus precipitation, precipitation, (c (,cd,)d tap) tap water water minus minus surface surface water. water.

The relationships of δ2H and δ18O in tap water with those in the other two water bodies are presented in Figures 11 and 12, respectively. Overall, tap water isotopes show better correlation with surface water than precipitation. As δ2H and δ18O values in surface water increase, those in tap water show an upward trend (Figure 12). In contrast, with the increasing of δ2H and δ18O in precipitation, those in tap water do not show a uniform changing trend (Figure 11). For tap water and precipitation, the correlation coeﬃcients in the ﬁtting equations for δ2H and δ18O are all less than 0.5 in Gannan and Longnan (Figure 11). To tap water and surface water, those for δ2H and δ18O are all larger than 0.7 (Figure 12).

Water 2019, 11, 2578 13 of 17 Water 2019, 11, 2578 13 of 17 Water 2019, 11, 2578 13 of 17

Figure 11. Relationship of δ2H and δ18O (‰) between tap water and precipitation in Gannan (a,b ) and Longnan (c,d). Solid circles2 and error18 bars show the arithmetic average and standard deviation, FigureFigure 11.11. RelationshipRelationship of ofδ Hδ2 andH and δ Oδ 18(‰)O (%between) between tap water tap and water precipitation and precipitation in Gannan in ( Gannana,b) and respectively. (aLongnan,b) and Longnan(c,d). Solid (c ,dcircles). Solid and circles error andbars errorshow bars the showarithmetic the arithmetic average and average standard and deviation, standard deviation,respectively. respectively.

FigureFigure 12. 12.Relationship Relationship ofofδ δ2H2H andandδ δ1818OO (%(‰)) betweenbetween taptap waterwater andand surfacesurface waterwater inin ﬁvefive seasonal seasonal samplingFiguresampling 12. timestimes Relationship in in Gannan Gannan of ( δa(2a,Hb,b) ) and and δ Longnan 18LongnanO (‰) (between c(c,d,d).). SolidSolid tap circlescircles water and andand error errorsurface bars bars water show show in the the five arithmetic arithmetic seasonal averagesamplingaverage and and times standard standard in Gannan deviation, deviation, (a,b) respectively. andrespectively. Longnan (c,d). Solid circles and error bars show the arithmetic average and standard deviation, respectively.

Water 2019, 11, 2578 14 of 17

5. Discussion on Water Source Information of Tap Water Isotopes Under the mode of central water supply by local tap water companies, for tap water isotopes, weak connection can be seen in Gannan with isotopes in precipitation and surface water, while tap water isotopes in Longnan present better correlation with surface water isotopes. According to water supply information from tap water companies and our field investigation data about water source, groundwater and surface water are the main tap water sources in Gannan and Longnan, respectively, under the this mode of water supply. Correlation of tap water isotopes with isotopic composition of main water source is high, while that with isotopic composition of non-water source is low. Under mixed tap water supply modes, in Gannan and Longnan, tap water isotopes both show better correlation with isotopic composition of surface water, including seasonal variations, numerical differences, and correlation. On the basis of our field investigation data, tap water sampling sites with surface water as water source account for larger proportion, both in Gannan and Longnan, compared with other water sources (precipitation and groundwater). It can be found that isotopes in tap water can well indicate the main water source, in the case of multiple water sources.

6. Conclusions The isotopic composition in tap water shows great significance for the understanding of local natural and humanistic environment background, water supply management, and regional water resources. In this study, δ2H and δ18O in 1260 tap water and 136 surface water samples were analyzed to exhibit the spatiotemporal characteristics of tap water isotopes and their water source signals under different water supply modes. Main results are: The LTWLs in Gannan and Longnan are δ2H = (7.06 0.17) δ18O + (3.24 1.75) (r2 = 0.81, p < 0.01) and δ2H = (5.66 0.09) δ18O + ( 8.12 0.82) ± ± ± − ± (r2 = 0.82, p < 0.01), respectively. Isotopes in tap water show an increasing trend from southwest to northeast, and their seasonal differences are small. Correlation of tap water isotopes with isotopic composition of main water source is high, while that with isotopic composition of non-water source is low. Under the central water supply mode by local tap water companies, tap water isotopes in Gannan where groundwater is the direct water source show weak connection with isotopes in surface water and precipitation, and for tap water isotopes in Longnan where surface water is the main water source, good connection present with surface water isotopes. Under mixed water supply modes, tap water isotopes indicate that surface water is the main source of tap water in Gannan and Longnan. Isotopic composition of tap water can well indicate the main water source, in the case of multiple water sources.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4441/11/12/2578/s1: Table S1. Information of tap water sampling sites serviced by tap water companies among all the tap water sampling sites in Gannan and Longnan; Figure S1. Spatial variations of annual mean values of d-excess (%) in tap water in Gannan and Longnan. The map of d-excess was calculated according to d-excess = δ2H 8 δ18O, based − on the maps of δ2H and δ18O. Author Contributions: Sample collection, H.M. and C.C.; experimental analysis and writing—original draft preparation, M.D.; writing—review and editing, S.W. and R.G.; funding acquisition, M.Z. Funding: This research was supported by the National Natural Science Foundation of China (No. 41701028), and the Scientific Research Program of Higher Education Institutions of Gansu Province (No. 2018C-02). Acknowledgments: The authors are very thankful for the team partners’ help during the processes of sample collection and experiment analysis. Conflicts of Interest: The authors declare no conflict of interest.

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