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

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.

1. Introduction

Tap water isotopes are well proven to be essential indicators [1,2,3,4,5,6,7,8,9,10] for studies in many fields [10,11,12,13,14,15,16,17,18,19,20,21,22], including hydrology, ecology, climatology, forensic, and so on. With the development of isotope methods, isotopes have been gradually applied to tap water research [23,24,25,26,27,28,29,30,31,32]. 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 [35,36,37]), 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 with precipitation isotopes were exhibited. The water supply systems in China, which have a vast territory, are complex [39]. Further studies of tap water isotopes based on dense sampling sites are needed to finely explore the isotopic composition of tap water and its indication for water source, especially in areas with multiple water supply modes.

In southern Gansu Province, eastern margin of Qinghai–Tibet Plateau, different water supply modes, including central water supply (e.g., supplied by rural drinking water safety project, supplied by tap water company) and decentralized water supply, coexist in the Gannan Tibetan Autonomous Prefecture (Gannan) and Longnan. In the existing studies in China [34,38,40], only four tap water sampling points were involved in Gannan and Longnan, which cannot accurately reflect the local tap water isotope landscape, seasonal differences, and the significance of water source indication. In this study, isotopes in 1396 water samples (tap water samples: 1260; surface water samples: 136) were analyzed, and the local tap water line, the basic spatiotemporal characteristics of isotopes, and indication for water source under different water supply modes were discussed, combining the field survey data about water supply and sources. This research is beneficial for understanding tap water isotopes, water supply, and water source determination.

2. Study Area

Areas analyzed in this study were located in southern Gansu, including the Gannan 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, with an area of 4.02 × 10⁴ km². The elevation in Gannan is between 1100 and 4900 m, mainly above 3000 m. The temperature difference between day and night is large. There are more than 120 rivers or streams 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 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.

3. Data and Method

3.1. Collection of Water Samples

Surface water and tap water samples were gathered by field work of our team in southern Gansu from May 2017 to April 2018. Cold tap water was collected into bottles after running the tap for 10 s [41]. Fresh surface water was collected from rivers, streams, or mountain springs flowing in southern Gansu. Tap water samples were gathered monthly from May 2017 to April 2018 (except for June 2017). Surface water samples were collected 5 times seasonally between May 2017 and April 2018 (in May 2017, July 2017, October 2017, January 2018, and April 2018). Sampling sites had relatively complete geographical coverage in southern Gansu, covering all the counties and districts governed by Gannan and Longnan (Figure 2). As Gannan and Longnan are located in mountainous areas, tap water samples were gathered along the winding roads in the mountains, which link residential areas. The total number of tap water and surface water samples were 1260 and 136, respectively. Information of all water samples is summed up in

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

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

. In the division of four seasons, spring includes March, April and May, and summer includes June, July and August, and autumn includes September, October and November, and winter includes December, January (in the nest year) and February (in the nest year).

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).

$${\delta }_{\mathrm{sample}}=\frac{(R_{\mathrm{sample}}-R_{\mathrm{standard}})}{R_{\mathrm{standard}}}\times 1000‰$$

(1)

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, Grabiszyńska, Poland) model version 1.00 [45,46,47,48,49], 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) [31,45,50,51,52], seven regression models combining spatial (including latitude (L, °), longitude (O, °), and elevation (E, m)) and meteorological factors (e.g., average temperature and precipitation) (

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).

) 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 on adjusted determination coefficient (r_adj²), mean absolute error (MAE), mean bias error (MBE), and root mean square error (RMSE) (

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

Isotope	Code	Equations	r2	radj2	Sig.	N	MBE	MAE	RMSE
δ2H (‰)	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
	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
δ18O (‰)	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
	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); 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); MBE, mean bias error; MAE, mean absolute error; RMSE, root mean square error.

). 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].

4. Result

4.1. Basic Characteristics of Tap Water Isotopes

4.1.1. Local Tap Water Line

Figure 3 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 Figure 3. 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.

4.1.2. Spatial Pattern

The simulated tap water isoscape in Gannan and Longnan is exhibited in Figure 4 and Figure S1 (in Supplementary Materials). From southwest to northeast, the annual mean δ²H and δ¹⁸O show an increasing trend on the whole. In southwest Gannan, isotopes are the lowest within Gannan, with δ²H values lower than −85.0‰ and δ¹⁸O lower than −11.0‰. In the north and east parts of Gannan, annual mean δ²H and δ¹⁸O in tap water are the highest within Gannan, with some δ²H values higher than −75.0‰ and δ¹⁸O higher than −10.0‰. In most parts of Gannan, the annual mean values of δ²H range from −85.0 to −65.0‰ and those of δ¹⁸O are lower than −11.0‰. Isotopes in Longnan are higher than those in Gannan in general. In most parts of Longnan, the annual mean values of δ²H are higher than −65.0‰ and those of δ¹⁸O are higher than −10.0‰. The annual mean values of δ²H in tap water in the east part of Longnan are the highest (δ²H higher than −55.0‰), and those of δ¹⁸O in southeast Longnan are the highest (δ¹⁸O higher than −8.0‰). The simulated residuals for δ²H and δ¹⁸O are in Figure 4c,d. In most parts of Gannan and Longnan, residuals for δ²H range between −2.0 and 2.0‰, and those for δ¹⁸O are from −0.6 to 0.6‰. A jackknife procedure was applied to report the isoscape uncertainties (Figure 5). Each sampling site supplied centrally by tap water company (Figure 2, Table S1 in Supplementary Materials) in Gannan and Longnan was removed separately to simulate the isotope landscape. Correction between estimated and observation annual mean values of δ²H and δ¹⁸O is showed in Figure 5. The equations of δ²H and δ¹⁸O are close to y = x. The values of r² for the equations of δ²H and δ¹⁸O are 0.85 and 0.93, respectively. The simulated water isotope landscape map has a good accuracy.

4.1.3. Temporal Variation

Differences of isotope between two adjacent seasons (Δδ²H and Δδ¹⁸O, ‰) at every sampling site were calculated (Figure 6). On the whole, Δδ²H and Δδ¹⁸O between two adjacent seasons in Gannan and Longnan are small. The values of Δδ²H between two adjacent seasons mainly range from −3.0 to 2.0‰ (red and purple sampling sites in Figure 6a,c,e,g). The values of Δδ¹⁸O between spring and winter mainly vary from 0 to 0.5‰ (purple and dark purple sampling sites in Figure 6h), and those of Δδ¹⁸O between other two adjacent seasons are mainly from −0.5 to 0.2‰ (red and purple sampling sites in Figure 6b,d,f).

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
δ2H (‰)	Gannan	−70.3	−70.0	−71.0	−71.2	−70.6
	Longnan	−58.4	−57.4	−59.2	−59.5	−58.6
δ18O (‰)	Gannan	−10.3	−10.4	−10.6	−10.6	−10.5
	Longnan	−8.8	−8.8	−9.0	−9.1	−8.9

shows the seasonal and annual mean δ²H and δ¹⁸O in Gannan and Longnan. The highest seasonal mean δ²H in Gannan and Longnan both appear in summer. The annual mean δ²H and δ¹⁸O in Gannan are both lower than those in Longnan.

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 (Figure 2b, 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 (

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.

Area	Sampling Sites	r of δ2H	p < 0.05 (Yes/No)	r of δ18O	p < 0.05 (Yes/No)
Gannan	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
	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
Longnan	Wudu	0.12	N	0.53	N
	Dangchang Xian	0.55	N	0.47	N
	Wen Xian	−0.73	Y	−0.47	N
	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 Xian	0.46	N	0.54	N
	Xihe Xian	0.35	N	0.32	N
	Li Xian	0.48	N	0.31	N

). The correlation coefficients at most sampling sites are less than 0.5. In Figure 7, the correlation coefficients 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.

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 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 features of isotopes between in tap water under mixed water supply modes and precipitation show 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 values in winter. In Longnan, the seasonal mean δ²H and δ¹⁸O in precipitation are the highest in spring (δ²H: −53.7‰, δ¹⁸O: −7.8‰) and the lowest (δ²H: −82.0‰, δ¹⁸O: −11.6‰) in winter. The isotopic 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 values δ²H (−57.1‰) and δ¹⁸O (−8.8‰) in surface water in Longnan appear in spring.

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 to 3.0‰ (Figure 10c), and those for δ¹⁸O mainly range from −0.1 to 0.4‰ (Figure 10d). Differences of seasonal mean δ²H between surface water and tap water in Gannan are larger than those in Longnan (Figure 10c,d).

The relationships of δ²H and δ¹⁸O in tap water with those in the other two water bodies are presented in Figure 11 and Figure 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).

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.