To better understand the origin of water in the Badain
Jaran Desert, China, water samples were collected from lakes, a spring and
local unconfined aquifer for analyses of radiocarbon (
Arid regions comprise nearly one-third of the Earth's land area. In these regions, surface water and groundwater are scarce, due to the low precipitation and strong evaporation. The scarcity of freshwater poses serious challenges to agriculture and economic development. Hence, it is critical to identify the origins of surface water and groundwater in these regions for proper management and protection of the limited water resources. Although water isotopes have been widely used to identify water sources (e.g., Rademacher et al., 2002; Gates et al., 2008a; Morrissey et al., 2010; Chen et al., 2012), the isotopic composition of groundwater and surface water in arid environments can deviate significantly from that of local precipitation due to evaporation, which complicates the interpretation of water stable isotope data and can lead to equivocal inferences about recharge origin (e.g., Gates et al., 2008a, b; Chen et al., 2012). Previous studies have shown that evaporation results in characteristic water isotopic patterns controlled by local climatic conditions, which could provide useful insight into the mechanisms of recharge (e.g., Clark and Fritz, 1997; Geyh et al., 1998; Oursingbé and Tang, 2010; Tweed et al., 2011; Wood, 2011; Murad and Mirghni, 2012). Despite the progress in the application of isotopes and other geochemical tracers in hydrological studies, the origin and residence time of groundwater in many arid and semi-arid areas remain open questions in water resource research.
The Badain Jaran Desert (BJD) is a hyperarid area located in the western Inner Mongolia, China. The landscape in the BJD is dominated by numerous large sand dunes, interspersed with more than 100 lakes and springs. Most of the lakes are permanent surface water bodies, with no obvious drying-up trends. The presence of these apparently permanent lakes in this hyperarid environment has attracted many researchers to the region to investigate the origin of the lake water (Geyh and Gu, 1992; Geyh et al., 1998; Chen et al., 2004; Ma and Edmunds, 2006; Gates et al., 2008a; Yang et al., 2010; Zhang et al., 2011; Zhao et al., 2012; Wu et al., 2014). Because of the harsh environment and the lack of basic meteorological, climatic and geologic data in the desert, previous studies relied primarily on the isotope analyses of water samples collected from limited localities and times to draw inferences about the source and/or flow path of water in the area. It is generally agreed that groundwater is the main water source to the lakes and the groundwater inputs into the lakes balance the water loss due to intense evaporation as continuous groundwater discharge has been spotted around and under the lakes (Chen et al., 2012; Dong et al., 2013). However, the origin of groundwater in the desert remains a hotly debated issue (see review by Dong et al., 2013). Earlier studies suggest that groundwater in the BJD originated directly from precipitation in the area and the numerous large and highly permeable sand dunes serve as an effective storage for groundwater discharging into the lakes (Hoffmann, 1996; Jäkel, 2002; Wang, 1990). Several recent studies, however, suggest that the groundwater in the BJD originated in neighboring basins or adjacent mountains (Dong et al., 2004; Gates et al., 2008a, b; Ma and Edmunds, 2006; Ma et al., 2007). Other studies suggest that the lake water and groundwater were recharged by local precipitation during the early Holocene when the climate was much wetter than today (Yang and Williams, 2003; Yang, 2006; Yang et al., 2010). It has also been hypothesized that the water in the area may originate in high mountains in the Qilian Mountains and be transported over long distances through deep and large fractures (fault system) underground to the BJD (Chen et al., 2004; Ding and Wang, 2007).
In this study, we present new stable and radioactive isotope data from water samples collected from the local aquifers and lakes as well as from evaporation experiments. The new data are used, in conjunction with data published in the literature, to test the various hypotheses regarding the origin of groundwater and to develop a new conceptual model for groundwater recharge–discharge in the dune–lake systems of the BJD.
Maps showing location of the Badain Jaran Desert
The Badain Jaran Desert is located on the northwestern Alxa Plateau in China,
stretching from
In the southern part of the desert, vegetation coverage is less than 10 %
and consists of a few grasses and shrubs with long roots horizontally
distributed in the sand to maximize the extraction of soil water. Interlayer
structures beneath the dune surface consisting of aeolian sand, flood
deposits and lacustrine deposits in the lowlands between the megadunes were
found from sediment cores drilled at several locations (Gao et al., 1995).
Extensive calcareous cementation and cemented tubes of dead plant roots were
found in the slopes of the dunes and there are tufa deposits at some of
these lakes (Yang, 2000). Outcrops of Jurassic, Cretaceous and Tertiary
sandstones can be seen along the fringe of the desert (Ma, 2002).
According to Ma et al. (2007) and Wang et al. (2014), groundwater mainly
exists in the continuous unconfined aquifer that consists of Quaternary
sandy sediments in the center of the BJD. This aquifer varies in thickness
from
The southeastern region of the Badain Jaran Desert is near the present-day
northern boundary of the East Asian summer monsoon regime. Summer monsoons
provide the primary source of precipitation, which accounts for
Water samples for this study were collected in August 2012 and July 2013
mainly from the southeastern part of the desert where lakes are
concentrated. Samples were collected from various lakes, including the Sumu
Jaran Lake (
Each groundwater sample for
Schematic diagram showing the cross-section profile between the
Sumu Jaran Lake and the Sumu Barun Jaran Lake as well as the water sampling
points
There was a precipitation event during the week of our field exploration in the Sumu Jaran Lake area in August 2012. It rained at night and continued into the early morning. A rain water sample was collected from a puddle on the ground. This sample, combined with previously published precipitation isotope data (Gates et al., 2008a) and data from Zhangye – the nearest IAEA-GNIP (International Atomic Energy Agency Global Network of Isotopes in Precipitation) station located approximately 170 km to the southwest of the study area at the altitude of 1400 m, (IAEA/WMO, 1986–2003), was used to approximately represent the isotopic composition of the local precipitation in the BJD.
To better understand the effects of strong evaporation on the isotopic characteristics of water in the desert, we conducted surface water evaporation experiments and soil water infiltration–evaporation experiments on the shore of the Sumu Jaran Lake (Fig. 2b) in July 2013. The observation site and experimental methods are schematically presented in Fig. 2b and c.
Evaporation from open water was monitored in two experiments using local
groundwater (Pan-1) and lake water (Pan-2), respectively (Fig. 2c). The two
plastic pans, which have the same cylinder shape with a diameter of 21 cm
and height of 10 cm, were used in the evaporation experiments. They were
filled with water to an initial depth of 6 cm and then put on a flat ground
surface to allow water to evaporate. With continuous evaporation, the water
levels in the pans gradually decreased. We measured the water depths every
day at 07:00 and 19:00 LT. Isotope samples from both pans were collected at 19:00 LT every day after the measurements of the water level. These experiments
continued for 6 days. The air temperature and humidity at the height of 2 m
above the ground surface were also measured every 2 h in daytime. A
total of six water isotope samples were obtained from each of the experiments.
During the period of the experiments, the air temperature varied between
17 and 42
Evaporation of soil water was observed in three experiments, ES-1, ES-2 and
ES-3 (Fig. 2c). Three pits were excavated to different depths below a flat
ground surface to install the evaporation–infiltration systems. Each of the
evaporation–infiltration systems was composed of a sandbox with a funnel and
a bottle attached to the bottom. The sandbox is 0.5 m
The water samples were collected for various chemical analyses. Among them,
21 were prepared for stable hydrogen and oxygen isotope analyses, including
12 lake water samples, 1 spring sample, 1 precipitation sample and 7
groundwater samples. The depths of water tables and the conductivities of
water bodies in the desert were measured using a TLC (temperature, level,
conductivity) meter (Model 107 TLC, Solinst). Seven groundwater samples were
analyzed for radiocarbon contents of dissolved inorganic carbonate (DIC) and
four samples were analyzed for tritium (
The radiocarbon contents of DIC samples were analyzed using an accelerator
mass spectrometer. The
The
The relationship between
The
Results from the pan evaporation experiments.
The sampling location and data and EC,
The
The radiocarbon activities in groundwater samples, reported as a percentage
of modern carbon (pMC) range from 35.81 to 72.71, and the
Radiocarbon and
It is well known that evaporation preferentially removes the light isotopes
from the liquid phase, resulting in a progressive isotopic enrichment in the
remaining water following an evaporation line in a
The evaporation experiments were conducted in the summer and may not
represent evaporation conditions in other seasons. However, the similarity
between the evaporation line determined through our evaporation experiments
(Fig. 3a) and those derived from measurements of natural water samples (Fig. 4) implies that seasonal variations in meteorological conditions do not
significantly alter the evaporative
The
Water samples collected from various lakes in the BJD are significantly
enriched in the heavy isotopes
Previous studies have also determined the evaporation line in the BJD (Chen et
al., 2004, 2012; Wu et al., 2014). The evaporation line reported by Wu et
al. (2014; i.e.,
The intersection of the evaporation line and the GMWL in the
Although a number of studies have used the natural abundances of stable
isotopes to infer the origin of water in the BJD (Chen et al., 2004; Ding
and Wang, 2007; Gates et al., 2008a, b; Ma and Edmunds,
2006), the effects of evaporation on the isotopic ratios and the
The plot of
As shown in Fig. 5a, the isotope data from the BJD and the Qilian Mountain
do not support the hypothesis regarding the Qilian Mountain being the
recharge area for groundwater in the BJD. The evaporation line EL
Figure 5a also shows that the shallow groundwater samples from the Yabulai
Mountain area fall above the evaporation lines in the BJD, but follow a
trend line (EL Yabulai:
The conceptual model of the
Based on the hydrogeological conditions and the water isotope data, we
propose a new conceptual model for the origination of water in the BJD,
which is schematically shown in Fig. 6. The sources of groundwater and
lake water in the BJD are mainly derived from meteoric precipitation in the
desert and adjacent mountains. These mountains include the Zongnai Mountain
in the east, the Yabulai Mountain in the southeast and the Beida Mountain in
the south, providing lateral groundwater flow toward the lakes in the BJD,
especially the fresh and brackish lakes near the margin of the desert.
Groundwater feeding most of the lakes, particularly the ones located farther
away from the mountains in the BJD, however, receives at least partial
recharge from infiltration water originated from local precipitation on the
highly permeable sand dunes in the area. Similar results were reported from
arid central Australia (Tweed et al., 2011). Based on the stable isotope
(
Only a few other studies also reported
Comparison of
Assuming that water in the lakes originated from the local precipitation
in the desert, the water balance of the lakes can be analyzed using the
following mass-balance equation:
The radiocarbon ages of DIC in groundwater are shown in Table 3, which range
between 2 and 9 ka. This does not indicate very old water since
groundwater ages in deserts are generally higher than 10 ka (Bentley et al.,
1986; Kronfel et al., 1993; Sultan et al., 1997; Edmunds et al., 2006;
Hagedorn, 2015). These ages do not represent the residence time of water due
to the input of
The flow model of groundwater near the Sumu Jaran Lake. The
HCO
The DIC of groundwater is likely composed of two sources: soil CO
For example, the calculated
Furthermore, these groundwater samples contained detectable amounts of
tritium, indicating at least some component of modern recharge (Fig. 8;
Table 3). This is because tritium is a radioactive isotope with a short
half-life of 12.43 years and its concentration in a sample should fall below
detection limit after 7–10 half-lives (
Isotopic analyses of water samples from natural water bodies (including
groundwater and lake water) as well as from evaporation experiments provide
new insights into several key aspects of the water cycle in the Badain Jaran
Desert.
The lakes in the area all had significantly higher The negative The groundwater in the unconfined aquifer in the desert was derived
primarily from modern meteoric precipitation in the region and possible
recharge areas include the eastern and southeastern areas of the BJD and the
adjacent mountains to the east and south of the desert. The radiocarbon ages of DIC in groundwater in the BJD do not represent the
residence time of water in the unconfined aquifer and are too old due to the
addition of old DIC from the dissolution of ancient carbonates in the
aquifer. Although the residence time of groundwater remains to be
determined, the variation patterns in
This study provides direct evidence showing that evaporation is an important
factor influencing the isotope composition of infiltration water and needs
to be considered when using the deuterium and oxygen isotopes to infer water
resources of semi-arid and arid regions. This study also demonstrated that
the characteristic water isotopic patterns resulting from evaporation could
be utilized to help resolve ambiguities in the interpretation of water
isotope data in terms of recharge sources, especially, in the arid regions,
such as the central Australia and the deserts of United Arab Emirates.
The data in this study are available from the publications and the websites that are listed in the reference section.
XSW designed and carried out the field experiments. BXH studied the water circle system in the study area. XW and YW analyzed the data and interpreted the results. XW prepared the paper with contributions from all coauthors.
Author Bill X. Hu is a member of the editorial board of the journal.
This research was sponsored by National Science Foundation of China (no. 91125024). Xiujie Wu received financial support from the China Scholarship Council (CSC). Stable isotope analysis was performed at the National High Magnetic Field Laboratory, which is supported by US National Science Foundation Cooperative (agreement no. DMR-1157490) and the State of Florida.Edited by: Christine Stumpp Reviewed by: two anonymous referees