Research papers
An isotope study of the Shule River Basin, Northwest China: Sources and groundwater residence time, sulfate sources and climate change

https://doi.org/10.1016/j.jhydrol.2022.128043Get rights and content

Highlights

  • The Indian summer monsoon invaded into the upper Shule River Basin in August 2018, interrupting the usual pattern of westerly moisture flow.

  • Surface water and groundwater originate from both recent precipitation and ancient precipitation archived in ice.

  • Groundwater with pre-evaporation δ18O < −14‰ may represent incursions of monsoonal moisture prior to 12 ka in the Hexi Corridor.

  • δ34SSO4 and δ18OSO4 in both surface water and groundwater fail to distinguish agricultural sulfate pollution from natural sulfate sources.

Abstract

Isotopes (δ18O and δD, δ34SSO4 and δ18OSO4, tritium and 14C) were employed to reveal moisture sources in precipitation and sources of surface water and groundwater, as well as groundwater residence times and sulfate sources in the Shule River Basin (SRB). Groundwater originates in the Qilian Mountains as high-altitude precipitation and meltwater from ice archives. The local meteoric water line (LMWL) is δD = 7.8δ18O + 18.1. Precipitation from westerly circulation has a characteristic annual cycle of δ18O and δD, high (δ18O > −5‰) in summer and low (δ18O < −10‰) at other times. This pattern was interrupted by an incursion of the Indian summer monsoon in August 2018, resulting in abnormally low δ18O and δD values. Surface water in the upper SRB yields an evaporation trend of slope near 5, with an origin near δ18O = −10‰ on the LMWL. Other catchments of similar altitude in the Qilian Mountains have evaporation trends with different origin points, indicating different input fractions of meltwater from ancient ice for each catchment. Groundwater δ18O and δD data plot along mixing trends, different in each sub-basin, between three water types: (1) recent Shule River runoff; (2) water like that archived in the Dunde ice sheet, representing precipitation over the last 12 ka; and (3) evaporated water that cannot be explained as precipitation from the last 12 ka. Type (3) water originated as water with δ18O values between −14 and −20‰ on the LMWL, and may represent incursion of monsoonal circulation prior to 12 ka. Tritium and 14C data identify post-bomb recharge, but 14C is of limited use in dating older groundwater mixtures. Sulfate isotopes (δ34SSO4 and δ18OSO4) in dissolved sulfate from groundwater and surface water indicate mixing of sulfur derived from evaporite and sulfide, but do not identify sulfate pollution from fertilizer. Future climate change may lead to water shortage as ancient ice is consumed by melting.

Introduction

Shortage of water resources and discrepancies between supply and demand are increasingly prominent in the inland area of northwest China, restricting possibilities for economic and social development in the region. Over recent decades, population has increased rapidly, leading to a sharp increase in water demand in an area with limited resources (Guo et al., 2015a, Guo et al., 2015b). Groundwater is a vital source of irrigation and drinking water in arid and semiarid regions, and water withdrawals commonly exceed natural rates of renewal (Edmunds, 2003), causing large declines in groundwater levels (Qiu, 2010); this is the case for China. In addition, salinization of groundwater occurs in many areas as a result of river regulation, land-use change, irrigation and groundwater exploitation (Huang and Pang, 2012). Given the rapid economic development and increasing population in the Shule River Basin (SRB), an improved understanding of the regional water cycle, including moisture sources and groundwater recharge mechanisms, is essential for regional planning.

The SRB is an inland river basin located at the western end of the Hexi Corridor in Northwest China (Fig. 1). The Shule River rises in the Qilian Mountains and flows through the Yumen-Tashi sub-basin to the lower Gobi Desert, where it disappears (Ma et al., 2005). Groundwater and river water are important sources for oases in the middle and lower regions of the SRB. The water is used mainly for farmland irrigation near Changma (in the Yumen-Tashi sub-basin), Huahai (in the Huahai sub-basin) and Shuangta (in the Guazhou sub-basin) (Guo et al., 2015a, Guo et al., 2015b, Zhao, 2017). By 2020, the population had increased to 5.4 × 105, as a result of the migration of a large agricultural workforce to the region. Concurrently, the area of irrigated farmland has grown to 1.3 × 103 km2 (Water Resources Bulletin of Gansu Province, 2020). Over-pumping of groundwater for irrigation has caused the hydraulic heads in aquifers to decreased by 0.15 m/a from 2000 to 2010 (Wang et al., 2016). Large-scale agriculture and other human activities have also led to severe sulfate pollution and deterioration of drinking water quality through acidification and salinization (WHO 1998). Examples of high SO42- concentrations in river water (average, 199 mg/L) and shallow groundwater (average, 383 mg/L) are documented in the Heihe River Basin (HRB) adjacent to the study area (Li et al., 2013a, Li et al., 2013b). Moreover, the disparity between high evaporation rates and low precipitation has caused water resource shortage and degradation. Therefore, understanding the sources and quality of groundwater and surface water in this region is of great significance to the sustainable development and effective management of water resources in the basin.

In hydrological studies, stable isotopes (δ18O and δD) have been used to constrain atmospheric moisture sources and the sources and interactions of surface water and groundwater (e.g., Fontes, 1980, Gat, 1996, Clark and Fritz, 1997). Tritium (3H, T) has a half-life of 12.43 years (Unterweger, 1980); and is useful for differentiating recharge prior to and since the atmospheric testing of nuclear weapons (Eastoe et al., 2011). The annually-averaged concentration of tritium in precipitation at mid-latitudes over most of the world was lower than 10 TU before 1952 (Clark and Fritz, 1997); in the region including the study area, natural tritium concentrations appear to be near 10 TU (Zhao et al., 2018). An anthropogenic pulse of tritium unrelated to atmospheric testing of nuclear weapons is recorded in 1987–2002 rainwater from Zhangye in the HRB (Zhao et al., 2018) and is likely to have affected the SRB also. For older groundwater, radiocarbon in dissolved inorganic carbon is useful in constraining residence time (Geyh, 2000, Eastoe et al., 2010). Values of δ34SSO4 and δ18OSO4 can be used to determine the sources of sulfate in surface water and groundwater and the effects of fractionating processes such as redox reactions (Seal et al., 2000). Sulfate sources can be identified more precisely if δ18OSO4 values are used in conjunction with δ34SSO4, for example where oxidation of sulfide is a source of sulfate, or where δ34SSO4 values in source materials overlap.

Previous studies focused on separate parts of the SRB. For example, Wang et al. (2015) focused on the upper (UR) and middle (MR) reaches; He et al., 2015, Guo et al., 2017 on the MR, Guo et al. (2015) on the MR and lower reaches (LR), and Ma et al., 2013, Wang et al., 2016 on the LR only. None of them attempted to interpret the isotope data at the basin scale, or in the context of an extensive dataset for O and H isotopes in dated ice of the region, spanning the last 12 ka (Thompson, 2000, Takeuchi et al., 2009, Li et al., 2015).

The Qilian Mountains, which function as a natural water tower for the Hexi Corridor of China, are essentially the only local source of water (Sun et al., 2016, Wang et al., 2018). In the high-altitude area of the Qilian Mountains where water resources originate (Li et al., 2013a, Li et al., 2013b) and in the lower parts of the SRB, there has been little systematic study of the origins of surface water and ages of groundwater. A new dataset consisting of multiple isotope parameters (δ18O, δD, 3H, 14C and sulfate isotopes) is presented (Table S1, S2, S3 and S4) in our study. Our data represent 163 precipitation samples and 13 snowpack samples collected in the UR, and 153 surface water samples and 80 groundwater samples from throughout the SRB. We also use previously published data for δ18O and δD as follows: To add to the dataset for the local meteoric water line (LMWL) in the headwaters of the SRB, we use data for precipitation in the UR (Wu et al., 2016). To relate ice archives to regional hydrology, we cite isotope data to for the Dunde ice sheet (Thompson, 2000, Takeuchi et al., 2009), and for ice meltwater of Laohugou (Wu et al., 2016) and Shiyi (Li et al., 2015) glaciers. We include previous datasets for surface water and groundwater in the MR and LR (He, 2013, Li et al., 2015, Ma et al., 2013, Wang et al., 2015a, Wang et al., 2015b, Wang et al., 2016) to provide a full regional picture of the contributions of meltwater to aquifers of the SRB. In addition, previously published δ34SSO4 and δ18OSO4 of surface water and groundwater from the neighboring HRB (Li et al., 2013a, Li et al., 2013b) are compared with similar data for the SRB.

The major goals of the study were: (1) to investigate the modern moisture sources in the headwaters of SRB; (2) to relate values of δ18O and δD in surface water to those in the regional ice archives; (3) to improve the understanding of the origins of surface water and groundwater in the SRB, along with residence times of the latter; (4) to investigate the role of climate change, in particular changes in sources of moisture over time, in generating the large ranges of δ18O and δD values in SRB groundwater and surface water; (5) to constrain the sources of dissolved sulfate in the SRB. Our results will provide an improved basis for planning to prevent further degradation of the regional environment in the SRB.

Section snippets

Study area

The SRB is located in the western part of the Hexi Corridor and stretches from 93°10′ to 99°00′E and from 38°00′ to 42°48′N. The basin covers approximately 4.13 × 104 km2 (Fig. 1). The Shule River rises in areas of high precipitation and meltwater availability in the Qilian Mountains. The river enters the Hexi Corridor plain from the south, then flows west as a perennial stream across a piedmont alluvial plain that forms part of the Gobi Desert. It terminates about 48 km northwest of Dunhuang

Precipitation

Precipitation δ18O and δD at Suli in the UR ranged from −20.8‰ to + 3.3‰ and from −146.9‰ to + 39.5‰, with arithmetic means −7.9‰ and −45.1‰, respectively. The LMWL for the UR of the SRB, based on this study and published data of Wu et al. (2016), was δD = 7.8δ18O + 18.1 (Fig. 3; R2 = 0.95, n = 285). For 2018, when precipitation amounts were available, an amount-weighted LMWL (Hughes and Crawford, 2012) was calculated: δD = 8.0δ18O + 19.1 (n = 136). Similar LMWLs were obtained from this study

Usefulness of tritium and 14C data

In Fig. 8, groundwater can be classified into three groups. Group 1 (green area) has high uncorrected 14C activity (>80 pMC), and represents groundwater of short residence time (mainly post-bomb recharge, with some mixing with older water). Group 2 (blue area) has low tritium (<10 TU) and 14C activity (<80 pMC) with shallow and deep groundwater in the SRB, indicating recharge mainly from pre-bomb precipitation. Group 3 (yellow area) is an intermediate group with uncorrected 14C values of 15–80

Conclusions

  • 1.

    The similar LMWLs for the UR of the SRB (δD = 7.8δ18O + 18.1) and the UR of the HRB, indicate that similar moisture sources and condensation processes operate throughout the Qilian Mountains. However, in August 2018, strong monsoon activity advected moisture to the SRB headwaters, generating precipitation with δ18O and δD values lower than those observed in years with westerly moisture sources.

  • 2.

    Values of δ18O and δD indicate an evaporation trend of slope near 5 in UR surface water of the Shule

CRediT authorship contribution statement

Cong Xie: Investigation, Methodology, Writing – original draft. Liangju Zhao: Supervision, Project administration. Christopher J. Eastoe: Writing – review & editing. Ninglian Wang: Writing – review & editing. Xiying Dong: Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 41771028, 41730751 and 42130516), the National Key Research and Development Program of China (2017YFC0404302), and the Opening Foundation of State Key Laboratory of Continental Dynamics, Northwest University (19LCD04). We thank the anonymous reviewers for their help in improving the manuscript.

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