Groundwater recharge and evolution in the Dunhuang Basin, northwestern China
Highlights
► The dominant geochemical processes of the aquifers of the groundwater in the Dunhuang Basin were determined. ► Multiple environmental tracers (chemicals and isotopes) were used. ► We identify the recharge sources, recharge environment and the residence time of the groundwater. ► The confined groundwater was recharged during the last glaciations under a cold climate and unrenewable.
Introduction
Water demand for agricultural, household and environmental uses is rapidly rising due to continuously increasing population especially in the developing world, and more and more areas are expected to experience imbalance of supply and demand for water in the near future (Vairavamoorthy et al., 2008). Furthermore, climate change, population growth, and economic development will likely affect the future availability of water resources in arid and semi-arid regions. One of the main issues of these areas that is affected by climate change is the quality and quantity of the water resources that are supplied to the growing population. Increasing population density, economic activity, and unsustainable water management practices have led to over-exploitation of many of the more easily accessible freshwater resources around the globe (Vörösmarty et al., 2010). Many of the world’s largest rivers such as the Nile, Colorado, Yellow river, Indus and Ganges no longer reach the sea for part of the year (Conway and Hulme, 1993, Shiklomanov, 1997, Archer, 2003, Christensen et al., 2004, Liu and Jun, 2004, Conway, 2005). In sub-humid to arid areas, the total global groundwater depletion has increased from 126 km3 a−1 in 1960 to 283 km3 a−1 in 2000 due to increased groundwater abstraction, especially in the world’s major agricultural regions, including NW India, north China, and the central USA (Wada et al., 2010). These regions are water-stressed and the anticipated climatic trends for the near future further threaten this precarious situation (Gleick et al., 2006, Bates et al., 2008). Alcamo et al. (2000) found that the areas affected by severe water stress will expand and intensify, growing globally from 36.4 to 38.6 million km2 between 1995 and 2025. Some cities in the drier parts of the world are likely to have exhausted their groundwater reserves in little more than a decade. Water scarcity has brought into focus the urgent need for planned action to manage water resources effectively as it is widely acknowledged that water is a major limiting factor in the socio-economic development of a world with a rapidly expanding population. Sustainable management of water resources to meet human and ecosystem needs will require accurate estimates of groundwater recharge, as the surface water resources are generally scarce and highly unstable in semiarid and arid regions, with the result that groundwater is the primary source of water in these regions (Kinzelbach et al., 2003, Scanlon et al., 2006).
Situated deep in the hinterland of Eurasia, China’s northwestern inland zone has a very arid climate. The landscape in these regions is fragile due to the low and irregular precipitation, high temperatures and evaporation, and notable drought periods (Ma et al., 2005). In such arid and semi-arid environments, groundwater is not only an important source of public water supply, but is also important to the regional ecology. Since the 1950s, human activities, and particularly the exploitation of land and water resources associated with dramatic population growth, have led to great changes in the water regime and have created serious environmental consequences, including declines in the regional groundwater levels, desertification, and drying of rivers and lakes in the lower reaches of river basins (Feng et al., 2005, Zhang, 2005, Chen et al., 2010, Li, 2010, Huang and Pang, 2010). For example, the groundwater level has dropped widely by as much as 35 m in the Minqin Basin since the 1960s (Ma et al., 2005). In the lower Tarim River, runoff has ceased to flow into the 350 km long river since the construction of the Daxihaizi Water Reservoir in 1972, causing severe damage to the riparian forest dominated by Populus euphratica (Song et al., 2000). The so-called ‘‘Green Corridor” and the highway along the Lower Tarim River (Kuala to Ruoqiang, part of National Highway G218) are endangered by degrading riparian vegetation and desertification (Huang and Pang, 2010). Desertification in the Shule River Basin is also a very severe problem. The sand dune along the Shule River has evolved from immobilization to mobile dunes because of the degeneration of the vegetation cover (Ding et al., 2001). If this situation continues, further deterioration of the environment and ecosystems of this vast area is unavoidable (Zhu et al., 2008). Careful studies of the characteristics of the groundwater and its evolution under natural water circulation processes help to provide scientific guidelines for sustainable exploitation of the region’s water resources and prevention of further degradation of the regional environment.
In recent years, the Chinese government and scientists have carried out many studies to assess the characteristics and utilization of the groundwater resources in northwestern China’s arid regions. Most of the research has taken place in the eastern and middle parts of the Hexi Corridor (i.e., the Zhangye, Ejina, and Minqin Basins), with important results that provide guidance for regional water management. For example, the deep groundwater of the Minqin Basin was recharged under cool and wet conditions during the late Pleistocene to Holocene periods based on an analysis of isotopic, noble gas, and chemical indicators (Ma et al., 2005, Edmunds et al., 2006, Zhu et al., 2008). Some authors have studied the hydro-geochemical evolution and residence time of water along the groundwater flow path in the Zhangye and Ejina Basins (Feng et al., 2005, Chen et al., 2006). The exchanges between groundwater and surface water in the Zhangye Basin have also been examined, using either a 3D groundwater flow simulation model or 222Ra analyses (Wu et al., 2004, Hu et al., 2007). As yet, similar studies have not been performed in the western part of the Hexi Corridor, Dunhuang Basin is a type locality of this area, and Dunhuang was an important city on the ancient Silk Road, and now is a renowned tourist city famous for the Mogao Caves, Crescent Lake and Mingsha Shan (literally, Echoing-Sand Mountain in the Kumtag Desert). However, increasing attention has been paid to Dunhuang recently because its famed Crescent Lake has been rapidly shrinking into the desert sand due to groundwater depletion (Yardley, 2005, Jiao, 2010), and because of the inter-basin water allocation programme in the Shule River catchment including Dunhuang Basin. Crescent Lake has dropped more than 7.5 m in the past three decades, while the groundwater table elsewhere in the basin has fallen by as much as 10 m, and the central government has indicated a national priority to rehabilitate this important and historic area. However, characteristics of groundwater beneath Dunhuang Basin remain poorly understood (Piao et al., 2003). It is certain that increasing surface water in the basin will increase the groundwater recharge.
The main objective of this study is to present the results from a wide selection of geochemical and isotopic indicators revealing the main characteristics of the groundwater in the Dunhuang Basin. The specific goals included: (1) using stable environmental isotopes (18O, 2H and 14C) to determine the evolution and age of the groundwater under natural conditions; (2) using the chemistry of major ions to determine the dominant geochemical processes that take place along the groundwater flow paths. The results of this study will not only improve understanding of the groundwater system in the whole Hexi Corridor, but will also provide essential information and a theoretical basis for the design of effective water resources management in the Dunhuang Basin.
Section snippets
General setting
Dunhuang, located at the western end of the Hexi Corridor in northwestern China’s Gansu Province, was an important town on the ancient Silk Road (Fig. 1). The basin lies between the Mingsha Shan (Echoing-Sand Mountain in the Kumtag Desert) and Sanwei Mountains in the SE and the Mazong Mountains in the north, and spreads into the Gobi desert in the west (Fig. 2). The basin has a drainage area of 6290.8 km2, but the actual area covered by the present study only included the oasis area, which
Materials and methods
To obtain sufficient data to cover the study area, field work was carried out during most of a year, from July 2010 to June 2011. A total of 25 representative samples were obtained, including four surface water samples from the Danghe Reservoir and the main channel of the Danghe River (covering the region from the upper to lower reaches), and 19 groundwater samples from water supply and irrigation boreholes as well as from springs within the Quaternary aquifer across the study area. To examine
Groundwater geochemistry evolution along the flow path
The principal characteristics of the surface water and groundwater and changes along the transect from the southwestern part of the basin (near Nanhu) to the northeastern part of the basin (near Xihu) are shown in Table 1 and Fig. 4. This transect represents the overall groundwater flow pattern within the Dunhuang Basin, and in Fig. 3, all of the groundwater sample locations are projected onto the corresponding position on this line. In the analysis, we divided the groundwater data into three
Conclusions
Isotopic data and hydrogeochemical tracers were used to understand the recharge and geochemical evolution of groundwater in the Quaternary aquifers beneath the Dunhuang Basin. The Quaternary aquifers are all closely connected with streams originating in the Qilian Mountains. The mean values of δ18O and δ2H in the unconfined groundwater tended to be enriched along the overall groundwater flow path from the SW to the NE, increasing from −11.0‰ to −7.5‰ for δ18O and from −76.3‰ to −61.5‰ for δ2H,
Acknowledgements
This research was supported by the National Natural Science Foundation of China (No. 41271039), the Fundamental Research Funds for the Central Universities, and the Keygrant Project of the Chinese Ministry of Education (No. 310005). This work also forms part of the 111 project (No. B06026) and the wider UK–China research collaboration. We thank Mrs. Jingfang Wang and Dr. Fred Leaney of the CSIRO Land and Water Laboratory for their assistance with our field work and laboratory analysis. We also
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