Elsevier

Journal of Hydrology

Volume 579, December 2019, 124212
Journal of Hydrology

Research papers
Partitioning groundwater recharge sources in multiple aquifers system within a desert oasis environment: Implications for water resources management in endorheic basins

https://doi.org/10.1016/j.jhydrol.2019.124212Get rights and content

Highlights

  • Recharge sources of three groundwater aquifers have been quantified in Ejina Oasis by using EMMA.

  • Increase in river discharge into the Oasis can restore shallow groundwater levels.

  • Semi-confined groundwater levels continued to decline for over-exploited groundwater in MHR.

  • Lateral groundwater flow from the Qilian Mountain was impossible to recharge confined aquifer.

  • Effective integrated water management policies were suggested.

Abstract

The global expansion of irrigation-based agricultural practices is causing an overexploitation of groundwater resources. A thorough understanding of groundwater recharge processes is necessary for restoring groundwater and improving water resources management, especially in desert oasis aquifers in endorheic basins. In this paper, the Ejina Oasis in the downstream of Heihe River Basin was selected as the representative case for such an endeavor. Based on stable isotopes and end member mixing analysis, we found that the shallow groundwater was modern water, with 13% originating from local precipitation and 87% from river water infiltration. The semi-confined groundwater is a mixture of shallow groundwater (30%), lateral groundwater flowing from the middle Heihe River Basin (MHR; 52%), and confined groundwater (18%). Confined groundwater was old water formed in paleo-climatic humid and cold conditions. Shallow groundwater levels were restored substantially after implementation of the Ecological Water Diversion Project (EWDP) began in 2000 in the Heihe River Basin, but the semi-confined groundwater levels still declined slowly. These different responses to EWDP are attributed to the differences in primary recharge sources of the two aquifers. Because river water infiltration was the main sources of shallow groundwater, increases in river discharge into the Ejina Oasis resulted in increased shallow groundwater levels over a short time scale. Continued over-pumping of groundwater for irrigation in the MHR reduced lateral groundwater flow into the lower Heihe River Basin, which was the primary source of semi-confined groundwater. For successful groundwater restoration and maintenance of groundwater for the future, EWDP implementation must integrate water management of river water and groundwater diversion across the whole basin. Our results can provide useful information on policy decision makers for sustainable use of water resources in the oases of other similar watersheds around the world.

Introduction

In arid/semi-arid climates, desert oases are the basis of human life and centers for economic development (Gonçalvès et al., 2013). Given limited precipitation and surface water resources, groundwater plays a pivotal role in sustaining the health of ecological systems, maintaining human activities and buffering the advance of the desert in these regions (Aeschbach-Hertig and Gleeson, 2012, Lezzaik et al., 2018, Wang et al., 2018). The aquifers of these oases are vulnerable to subtle perturbations in fluxes, which can be exacerbated by global warming and human activities (Liu et al., 2018). In recent decades, water scarcity, rapid population growth and economic development, and overexploitation of water resources, has led to groundwater depletion and is a seriously threat to the sustainable development practices of desert oases environments around the world (Knowling et al., 2015, Hao et al., 2017). New, efficient water resource management strategies that address issues of economic development, ecosystem restoration, and water resource preservation are necessary (Steward and Allen, 2016). The successful implementation of water resource projects requires a clear understanding of site-specific hydrological processes, such as groundwater recharge, which until now have been neglected when implementing management strategies in endorheic basins with multiple aquifers (Zhao et al., 2016, Yu et al., 2017).

In the Heihe River Basin of northwestern China (Fig. 1a), more than 80% of river runoff has been diverted to the oases of the middle reaches between 1960 and 1990, primarily due to expansion of the irrigated agriculture (Xi et al., 2015, Xu et al., 2019), has resulted in decreased river discharge, lowered groundwater levels, dried out lakes, and the degradation of important ecosystems in the Ejina Oasis at the downstream of the basin. This situation not only dramatically increased the number of sandstorms in local region but also induced greater numbers of sandstorms in northern China (Feng et al., 2013). To address these issues, the Ecological Water Diversion Project (EWDP) in the Heihe River Basin was implemented in 2000 to increase the amount of water diverted to the Ejina Oasis to prevent further deterioration of the environment and restore the ecological and hydrological functions of the Oasis (Cheng et al., 2014). The natural desert riparian woods and the nature desert ecosystem are dependent on the groundwater sources, which are the primary available water resource in the Ejina Oasis (Feng et al., 2013). River water from the upper and middle stream reservoirs has been released into the Ejina Oasis two to five times per year after 2000 (Si et al., 2015). After 15 years of integrated water resource management, shallow groundwater levels in the Ejina Oasis aquifer have gradually recovered, but groundwater levels in the semi-confined aquifer continues to decrease since the 1990s, even though there has been approximately 10.47 × 108 m3/yr river water diverted to the Oasis after the implementation of EWDP (Si et al., 2015, Li et al., 2018). Groundwater, as the critical factor in maintaining the ecological balance in the Ejina Oasis, has attracted the attention of eco-hydrological researchers for a long period of time but the focus on this specific region has increased more recently. Previous studies on the groundwater in the Oasis can be summarized into the following three aspects: First, to explore the relationship between the groundwater levels variations and the vegetation changes (Zhu et al., 2009, Wang et al., 2014a, Si et al., 2015, Zhang et al., 2017, Zhou et al., 2018); Second, to focus on assessment interactions of surface water and shallow groundwater (Yao et al., 2015, Xi et al., 2018, Xi et al., 2015); Third, to qualitatively describe the hydrogeological and geochemical characteristics of groundwater (Su et al., 2007, Wen et al., 2008, Wang et al., 2014b, Zhao et al., 2018). Although the existing researches on the Ejina Oasis have provided useful insights into the restoration of shallow groundwater levels, a quantitative estimation of the sources of groundwater recharge to both the shallow and semi-confined aquifers is still necessary, especially as the semi-confined groundwater resources have become a more prominent sources of freshwater in the Ejina Oasis (Xi et al., 2018). Lacking definitive information about groundwater recharge sources, government decision makers have not appropriately managed the health of these aquifers (Xi et al., 2010, Shen et al., 2017, Yu et al., 2018). If this situation continues, it is likely that the groundwater degradation in the semi-confined aquifer will become an intractable problem for the Ejina Oasis.

The significant differences in isotopic compositions among precipitation, surface water and groundwater, making it possible to determine mixing fractions of different water masses involved in groundwater fluxes (Séraphin et al., 2016, Yang et al., 2019). Hence, stable oxygen and hydrogen isotopes (18O and D) of water have been widely used as environmental indicator in the field of groundwater origin and recharge (Prada et al., 2016, Jasechko et al., 2017). The end member mixing analysis (EMMA) approach can be exploited to identify and quantify water sources contributing to the recharge of various aquifers based on the mass balance of tracers (Peng et al., 2016), and these results will provide significant insights for creating sustainable water resources management plans (Guo et al., 2019). In this paper, a comprehensive analysis on the stable isotopic and geochemical methods as well as end member mixing analysis was conducted. We aim to expand the body of knowledge about sources of groundwater recharge in the Ejina Oasis by (1) investigating the isotopic and geochemical signatures of shallow, semi-confined, confined aquifers, (2) quantifying the recharge sources of different aquifers, and (3) exploring the responses of the shallow and semi-confined aquifers to the EWDP implementation to inform integrated water resources management efforts. Our results will provide fundamental information on groundwater recharge sources in multiple aquifers system in the Ejina Oasis, which can be used to optimize the strategies of EWDP on conjunctive management of surface water and groundwater and to explore a balance between recharge and abstraction of groundwater in Heihe River Basin, with a much broader implications for other parts of the world with similar watersheds.

Section snippets

General setting

The Heihe River Basin is the second largest endorheic basin in the arid region of northwestern China. The river originates in the Qilian Mountains and then flows northward through the Zhangye, Jiuquan, Jinta and Dingxin Oasis in the middle reaches and the Ejina Oasis in the lower reaches (Fig. 1a and b). In the Ejina Oasis, the Heihe River forks into eastern and western branches from the Langxin Mountains and then disappears in the East Juyan Lake and West Juyan Lake (dried), respectively (Fig.

Sampling and analysis

River water and groundwater samples were taken during August 2015 and July 2016 to document spatial variation in the isotopic and geochemical signatures in the middle reaches of the Heihe River (MHR) and of the different aquifers in the Ejina Oasis. We obtained 10 groundwater from deep wells (with depths range from 60 to 120 m) and 21 river water samples from the MHR, and 76 groundwater samples from the Ejina Oasis, including shallow (n = 33; well depths less than 30 m), semi-confined

Stable isotope compositions of waters

The stable isotope compositions of precipitation in the Ejina Oasis varied widely with the values ranging from −10.02‰ to 8.04‰ for δ18O and −63.21‰ to 13.79‰ for δD. The arithmetic mean values of δ18O and δD is −3.65‰ and −29.83‰, respectively. The d-excess values changed from −50.50‰ to 16.98‰ with an average of −0.66‰. A synthesis of precipitation stable isotope values from our samples and the published literature (Zhao et al., 2011; GNIP) was used to establish the local meteoric water line

Origin of groundwater in confined aquifer

Given the much depleted isotope values and the low EC and Cl concentrations of the confined groundwater samples, it is unlikely that recharge from the shallow and the semi-confined aquifer to the confined aquifer occurred. Additionally, groundwater pressure of the confined aquifer was higher than other two upper aquifers (Ruan et al., 2015). The confined groundwater samples had relatively homogeneous stable isotope compositions and were significantly more depleted than the weighted average

Conclusions

This research is a systematic attempt to identify recharge sources of shallow, semi-confined and confined aquifers in the Ejina Oasis using water isotopes (δ18O and δD) and geochemistry (EC and Cl). Shallow groundwater is modern water, which originates from 13% of local precipitation and 87% of river water infiltration recharge, while the confined groundwater, likely formed in the paleo-climatic humid and cold conditions that prevailed during the late Pleistocene and the early Holocene, is old

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

We express our gratitude to the Chief Editor, Editor Huaming Guo, and two anonymous reviewers for their constructive and insightful comments and suggestions. Great thanks to Dr Hofmeister K. from the Michigan Technological University have helped us to improve language of this manuscript. This research was funded by the National Key R&D Program of China (Nos. 2017YFC0404306, 2016YFC0400908), the National Natural Science Foundation of China (Nos. 41601029, 41501071), the Key R&D Program of Inner

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