The role of deuterium excess in determining the water salinisation mechanism: A case study of the arid Tarim River Basin, NW China
Graphical abstract
Highlights
► We present a stable isotopic method for determining the water salinisation mechanism. ► We show the relation between deuterium excess and the residual fraction during evaporation. ► The mineral dissolution and/or transpiration contributes most salinity in groundwaters in the Tarim River Basin.
Introduction
As the total area of arid and semiarid regions accounts for 1/3 of the continental area globally, water salinisation is a major environmental issue in water resources, partly due to river regulation, land-use changes, irrigation and groundwater exploitation (Williams, 1999, Gaye, 2001). An understanding of the chemical evolution and salinisation mechanism of groundwater can provide insight into the interaction between water and the environment and can contribute to rational water resource management (Adams et al., 2001, Edmunds, 2009), especially in (semi)arid regions with a fragile ecosystem and intense anthropogenic interference (Bennetts et al., 2006). In the Nile delta, for example, an evaporation experiment (Simpson et al., 1987) showed that the increase in groundwater salinity is mainly caused by plant transpiration (unusable salts rejected by plants), and changes in the irrigation pattern to reduce water loss did not reduce the salinity. Land-use and hydrological regime changes have resulted in the mobilisation of large amounts of stored salts in the Murray River Basin, Australia (Allison et al., 1990, Simpson and Herczeg, 1991).
Groundwater chemistry is also important for ecosystem restoration because water quality can control vegetation growth and soil characteristics if the salinity is high enough. The negative impacts of high-salinity water on vegetation growth include preventing vegetation from absorbing moisture and decreasing soil fertility (He et al., 2006, Manchanda and Garg, 2008). Plant features, including the richness, diversity and composition of species are significantly related to salinity, and deteriorate with increasing salinity (Lymbery et al., 2003). Water quality issues can also emerge, threatening and further diminishing the amount of renewable water resources in (semi)arid areas (Edmunds, 2003).
In arid areas, mineral dissolution, evaporation and transpiration are the main salinisation mechanisms for water bodies in inland plain areas (Hao et al., 2000). However, quantifying the contribution of each of these factors to the total salinity is difficult, and no simple and reliable methods are currently available (Hao et al., 2000). Because evaporation enriches water bodies with heavy isotopes, some studies have used the relationship between δ18O and δ2H, and δ18O (or δ2H) and salinity to determine the sources of salinity (Gaye, 2001) and the evapoconcentration effect (Simpson et al., 1987, Hao et al., 2000, Phillips et al., 2003). For example, when a water body loses 1% of its mass relative to the initial water, the δ2H in the remaining water can increase by 0.65‰, according to an evaporation experiment in Egypt conducted by Simpson et al. (1987). In the study of two seasonal lakes in the northwestern Sahara, Fontes and Gonfiantini (1967) concluded that when the water bodies lost 1% of their mass relative to initial water, the δ2H in the remaining water increased by 0.78‰. This value was 0.62‰ in the Murray River, Australia (Simpson and Herczeg, 1991). Thus, the contribution of evapoconcentration to salinity can be determined by the mass loss ratio.
However, the aforementioned method has some limitations, especially when the isotopic compositions of the initial water vary. A fixed isotopic composition at the basin scale is not common because precipitation with different isotopic compositions (monthly variations and spatial differences) may recharge rivers or groundwaters. For example, when water samples (river or groundwater) A2 and C2 with the same value of δ18O are obtained, as shown in Fig. 1, it is difficult to identify the water loss and salinisation mechanism by the relationship between δ18O and salinity unless their initial water (precipitation or source water, A1 and C1) is known. The water samples A2 and B2 show different values of δ18O, however, the mass loss ratios are same.
In addition to evaporation, mineral dissolution and/or transpiration are important factors controlling the salinity of water bodies in arid areas. Because deuterium excess (Dansgaard, 1964) decreases during evaporation and is unrelated to the isotopic composition of the initial water, it can be used to assess the effect of evaporation. The remaining factors, i.e. mineral dissolution and/or transpiration, cannot be distinguished by using stable isotopes. This is important for water resource and salt management, at least in arid areas where surface reservoirs are commonly used and a large amount of the water loss from a reservoir is caused by open-water evaporation. This scenario is especially true in Xinjiang and the Hexi Corridor, NW China. To the present authors’ knowledge, this study is the first to use deuterium excess to quantify the salinisation mechanism. The extremely arid Tarim River Basin (TRB) is used as an example to demonstrate the main factors controlling groundwater salinity and how deuterium excess is useful in determining the salinisation mechanism. The data are taken from the authors’ previous papers (Pang et al., 2010, Huang and Pang, 2010), which mainly focused on diminished groundwater recharge and circulation relative to degradation of riparian vegetation in the Middle Tarim River (MTR) as well as the response of groundwater to water diversion and the scope of modern recharge in the Lower Tarim River (LTR).
Section snippets
Methodology
Globally, the stable isotopic composition of precipitation can be described by the following equation:
When precipitation undergoes evaporation on recharging rivers, lakes and groundwater through soil water infiltration, the slope of the evaporation line will be less than 8, commonly 4–6 for open water and even 2–3 for soil evaporation (Barnes and Allison, 1988). However, there is no isotopic fractionation during salt drainage, plant transpiration (Zimmermann et al., 1967, Foerstel,
Study area and data source
The TRB is located in the south of Xinjiang, NW China (Fig. 4) and is a typical inland basin for examining the evolution of salinity and deuterium excess from source streams, rivers, lake/water reservoirs and groundwater. The Middle and Lower Tarim River is dominated by a typical continental temperate arid climate. According to the Tikanlik meteorological station (TG in Fig. 4) data, the average precipitation is approximately 40 mm/a, the potential evaporation is 2590 mm/a, and the annual average
Conclusions
The use of deuterium excess does not require the knowledge of initial stable isotopic composition of water, and is a physically based, effective and reliable approach to determining the contribution of evapoconcentration to salinity. In the TRB, mineral dissolution contributes most of the salinity (67–77%) in Boston Lake, the Kongque River and the Tarim River. For groundwater, the increase in salinity is not accompanied by a significant evaporative isotopic signal. Mineral dissolution and/or
Acknowledgements
This research is supported by the National Science Foundation of China (Grants 41202183, 40872162, 40672171) and the China Postdoctoral Science Foundation Funded Project (Grant 20110490581). Special thanks are due to Prof. Klaus Froehlich for helpful discussions. The authors wish to express their appreciation to Prof. W. Mike Edmunds, Dr. Victor Heilweil and an anonymous reviewer, whose detailed comments were very helpful in improving the clarity and focus of the manuscript.
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