Determining the groundwater basin and surface watershed boundary of Dalinuoer Lake in the middle of Inner Mongolian Plateau, China and its impacts on the ecological environment

2.1   General setting

Dalinuoer Lake which is the second largest endorheic salt lake in Inner Mongolia lies in the west of Hexigten. It is located at the junction of the major landform of the southern end of the Greater Khingan Mountains, the central part of the Inner Mongolia Plateau, and the northern part of the Otindag Desert. Due to the block of the Greater Khingan Mountains, Yanshan Mountains, Qinghai-Tibet Plateau, Himalayan Mountains and other mountains, water vapor from the Pacific Ocean and the Indian Ocean are difficult to reach and form precipitation. Dalinuoer Lake watershed belongs to the temperate continental monsoon climate zone, with the characteristic of dry climate, strong evaporation and low rainfall. The annual mean temperature ranges from 1°C to 2°C (Xing ZQ et al., 2021), and annual pan evaporation varied from 1521 mm to 2270 mm with an average of 1828 mm (Jiang MY et al., 2020). The annual precipitation ranged from 121 mm to 561 mm with an average of 280 mm, around 70.5% of which occurs in summer from June to August (Zhen ZL et al., 2021). Four inflow rivers, namely the Gonggeer River, the Liangzi River, the Shali River and the Haolai River flow into the lake with a large proportion coming from the Gonggeer River (Fig. 1). The annual surface inflow of the Dalinuoer Lake watershed was 40 ×10⁶ m³ in the last decade (Xing ZQ et al., 2021).

Figure 1.  Groundwater basin and surface watershed boundary as well as the collected water samplings sites from the scientific literature.

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2.2   Geological and hydrogeological setting

Dalinuoer Lake watershed is situated at the intersection of the Yinshan complex fault belt stretching in the EW direction, the Xingmeng tectonic belt stretching in the S-NE direction and the Bayinheshuo fault depression basin, which shows complicated geological structure. It mainly develops three groups faults close to EW, NE and NW directions, which show relatively strong control on river. One of the most important active fault is the Xilamulun River Fault closed to EW direction, which controls the southern boundary of Dalinuoer and Ganggengnuoer Lake, and indirectly forms the boundary between Xilingol Grassland and Otindag Desert (Geng K et al., 1988). It also leads to the elevation differences between the northern and the southern. The Cenozoic fault basin of Dalinuoer Lake stretching in the NE direction is situated at the southeastern margin of Bayinheshuo fault depression basin group developed in late Mesozoic. The Haoluku Anticlinorium Fault developed in the south-central of Dalinuoer Lake watershed, and it controls the distribution of acidic extrusive rocks in Jurassic and intrusive rocks in late Yanshanian. The Haoluku Anticlinorium Fault divides the Dalinuoer Lake watershed into two small basins, Hankela and Yuanshuitou, in which the Quaternary strata with a thickness of more than 200 m accumulated (Fig. 2).

Figure 2.  Hydrogeological sketch map of the Dalinuoer Lake and Xilamulun Riverhead.

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The impermeable basement rocks of the Dalinuoer Lake watershed mainly consist of Mesozoic volcanic rocks. The Neogene sporadically and Quaternary strata widely distributed overlying the basement rock. The Quaternary sedimentary thickness is large and its genetic types are complex, on which successively deposited glacial facies, glacial water-lake facies, alluvial facies, slope diluvium, lake facies, lacustrine facies, eruptive basalt and aeolian facies. The glacial water-lake facies and aeolian sands are mainly distribute in the southern of Dalinuoer Lake, and the lithology is mainly silt, fine sand and gravel. There developed the tertiary basalt platforms in the northwestern, and the east lakeshore is covered by the Quaternary sediments (Fig. 2, Fig. 3). The permian sedimentary rock, shale, quartz-schist, intermediate-acid volcanic rock and a small amount of Jurassic granite are cropped out in the middle and upper reaches of the Gonggeer River, and the Quaternary alluvial-diluvial layer developed in the gullies (Inner Mongolia Autonomous Region 101 Geological Team, 1979).

Figure 3.  I-I' hydrogeological profile map of the Dalinuoer Lake watershed.

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Dalinuoer Lake is a tectonic dammed lake which was formed by basalt weir in northwestern on the basis of the tectonic dammed lake formed during the subsidence of a faulted basin. The groundwater and spring flow into the lake from the south, with a relatively high discharge. The groundwater types of Dalinuoer Lake watershed can be divided into five types: Pore water in unconsolidated rocks, basalt cave fissure-pore water in unconsolidated rocks, basalt cave fissure water-basalt volcanic cone group, magmatic fissure water and pore-fissure water in clastic rocks. The main aquifers of pore water in unconsolidated rocks are glacial deposits and diluvia layer, which distribute in the southern high plain and the eastern valley terraces with an area of 3309 km². The main aquifers of basalt cave fissure-pore water in unconsolidated rocks are the first basalt platform or Quaternary cover, which mainly distribute in the northwestern of Dalinuoer Lake with an area of 739 km². The magmatic fissure water and pore-fissure water in clastic rocks are poor in water-abundance, with an area of 498 km² and 63 km² respectively (Fig. 2).

4.1   Identification of the surface watershed

The point of the surface watershed is to identify the surface watershed ridge, water catchment line and range. In Fig. 1, connecting the highest watershed ridge of the northern, southern, western and eastern of the Dalinuoer Lake based on the DEM model, a encirclement Dalinuoer Lake watershed including regional A and B with an area of 6139 km² was delineated clearly. The eastern watershed ridge is the remnant of the Greater Khingan Mountains lies in Hexigten, which forms the natural boundary of the Gonggeer River system flowing into Dalinuoer Lake. The western and northern of Dalinuoer Lake is the basalt platform formed by multiple Cenozoic volcanic eruptions or covered by Quaternary sand, which is widely distributed with volcanic cones. In the northwestern of the watershed, the river system is non-developed. The western watershed ridge is the natural boundary of the Huri Chaganor River system. The northern watershed ridge is the natural boundary of the Xilingol River system. Southeastern of the watershed ridge is the natural boundary of the Xilamulun Riverhead watershed located in Hexitenqi. The developmental rivers in the watershed eventually flow into the Dalinuoer Lake.

In this study, the drainage area of the Dalinuoer Lake watershed is 1943 km² larger than the area proposed by the water resources department (Xing ZQ et al., 2021). It is mainly differentiated by the regional B which lies in the southern of the Haoluku Anticlinorium Fault. Yang XP et al. (2015) mentioned that a relict channel flowing northward from the Otindag Desert to Dalinuoer Lake existed in regional B before ca. 4.2 ka. Owing to the northern hemisphere climate shifts to extraordinary drought as a whole, the important geomorphic and hydrological events appeared in the Otindag Desert. In addition, the westward retrogressive erosion into the interior of the Otindag Desert which is caused by the Xilamulun Riverhead is being exacerbated. As a result, the groundwater is sapped, and the already scarce surface water flows into the Xilamulun River through the groundwater. It leads to the rapid decline of groundwater level and the lake vanished and degradation, as well as the river cut off. The authors of this study believe that once the high intensity rainfall events occur under climate change condition, the relict channel would likely to reflow. So the surface watershed of Dalinuoer Lake should contain regional A and regional B.

4.2   Identification of the groundwater basin divide

Based on the comprehensive analysis of terrain landform, geological structure and hydrogeological condition, the water resistance ability of the folds, faults and stratigraphic boundaries are usually identified to determine the groundwater basin divide. The relative water-resisting stratigraphic boundary is usually the metamorphic rock, igneous rock and fine grain clastic rock with a certain thickness. According to the occurrence, fracture development and filling condition, folds and fracture which show various degree of water-resisting ability become the impervious boundary.

4.2.1   Evidence from the geological structure, hydrogeology and groundwater flow system

Fig. 2 showed that the northern, eastern and northwestern of the groundwater basin divide of Dalinuoer Lake are consistent with the surface watershed ridge. It is speculated that the southwestern boundary is the movable groundwater divide. The Haoluku Anticlinorium Fault in SW-NE direction which developed in the southern of the surface watershed of Dalinuoer Lake controlled the distribution of acidic extrusive rocks in the Jurassic and intrusive rocks in the late Yanshanian. Because of its obvious water-resisting character, it becomes the intermittent outcropped groundwater basin divide. The Haoluku Anticlinorium Fault divides the surface watershed of Dalinuoer Lake into the northern regional A named the groundwater system of Dalinuoer Lake and southern regional B named the groundwater system of the Xilamulun Riverhead (Fig. 2, Fig. 3). The area of regional A and B is 4838 km² and 1301 km² respectively. Fig. 4 showed that the groundwater level elevation was not consistent in both sides of the water-blocking anticlinorium fault. The groundwater level elevation of the upstream of surface watershed named regional B is lower than the downstream named regional A, and the isoline of groundwater level elevation is perpendicular to the basin divide. It is well confirms the Haoluku Anticlinorium Fault acts as the groundwater basin divide with water-resisting character. The regional A and B are two independent groundwater system, and the groundwater discharges in the direction of Dalinuoer Lake and Xilamulun River separately.

Figure 4.  Isoline map of groundwater level elevation.

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It is noted that an EW fault extending westward through Jingfeng, Ganggenore, Dalainore, Duolunnore and Baiyinkulunnore existed in the southern of Dalinuoer Lake. The fault which formed by the compression in the NS direction is called the EW orientation major concealed fracture of the southern of Dalinuoer Lake in our research. It is speculated that it could be extended to the east and connected with the Xilamulun River Fault, and has the insidious step water blocking effect. This fault could further divide the groundwater system of Dalinuoer Lake (regional A) into the northern and southern as two secondary groundwater systems. The aquifer thickness of the northern secondary groundwater system is small, the groundwater discharges into the river directly. While the aquifer thickness of the southern secondary groundwater system is large, so the northward flowing groundwater is blocked on the concealed fault boundary and overflows upward, eventually forms springs or discharges into the lake. The characteristics of groundwater flow field on both sides of this fault are also inconsistent (Fig. 4).

In addition, the eastern, western and southern divide of the Xilamulun Riverhead groudwater basin are consistent with the surface watershed ridge. But because the groundwater of regional B recharges the groundwater of regional C, the southwestern boundary of the groundwater system of Xilamulun Riverhead crosses the surface watershed boundary, and it extends westward to the Haoluku Anticlinorium Fault.

4.2.2   Evidence from the hydrogen and oxygen stable isotopes

The hydrogen and oxygen stable isotopes analytical data of the water samplings collected from the scientific literature are listed in Table 1. As shown in Table 1, the stable isotope compositions of the river samplings collected in the southern of regional A ranging from −84.04‰ to −81.94‰ for δD and −12.64‰ to −13.01‰ for δ¹⁸O, respectively. The arithmetic mean values of δD and δ¹⁸O are −83.32‰ and −12.64‰, respectively. The stable isotope compositions of the spring samplings collected in the southern of regional A ranging from −84.3‰ to −82.21‰ for δD and −13.33‰ to −12.81‰ for δ¹⁸O, respectively. The arithmetic mean values of δD and δ¹⁸O are −83.26‰ and −13.07‰, respectively. The stable isotope compositions of the groundwater samplings collected in the southern of regional A ranging from −82.64‰ to −85.61‰ for δD and −13.11‰ to −13.53‰ for δ¹⁸O, respectively. The arithmetic mean values of δD and δ¹⁸O are −84.25‰ and −13.32‰, respectively. The rivers in the southern of regional A are all centrally recharged by springs, in which the hydrogen and oxygen isotopes are basically the same with its surrounding groundwater samplings. In Fig. 5, the river and groundwater samplings in the southern of regional A centrally located in the domain a2, which further demonstrated that the surface water and groundwater in the southern of regional A compose the integrated fluvial aquifer system.

Table 1.  The stable isotope compositions of all the water samplings collected from the scientific literature.

ID	δ2H/ ‰	δ18O/ ‰	Region	Samplings	Source
DLW-1	−83.73	−13.28	A	Groundwater	Zhen ZL et al. 2014
DLW-2	−82.64	−13.11	A	Groundwater
DLW-3	−85.61	−13.44	A	Groundwater
DLW-4	−84.75	−13.3	A	Groundwater
DLW-5	−57.53	−8.36	A	Groundwater
DLW-7	−84.9	−13.53	A	Groundwater
DLW-8	−83.87	−13.23	A	Groundwater
DLS-1	−84.3	−13.33	A	Spring
DLS-2	−82.21	−12.81	A	Spring
A7	−36.44	−3.46	A	Lake
B4	−37.78	−3.37	A	Lake
B6	−37.23	−3.43	A	Lake
C5	−37.05	−3.3	A	Lake
D4	−36.96	−3.31	A	Lake
D6	−37.46	−3.27	A	Lake
E1	−36.76	−3.34	A	Lake
E2	−37.12	−3.41	A	Lake
E5	−36.33	−3.46	A	Lake
DLR-1	−101.83	−15.74	A	River
DLR-2	−84.04	−13.01	A	River
DLR-3	−81.94	−12.81	A	River
DLR-4	−83.32	−12.64	A	River
G1	−66.7	−8.9	B	Groundwater	Zhu BQ et al., 2019
G2	−64.8	−9.34	B	Groundwater
G3	−63.4	−8.64	B	Groundwater
G4	−66.1	−9.62	B	Groundwater
G5	−65.5	−9.8	B	Groundwater
G6	−68.9	−10.5	B	Groundwater
G7	−73.1	−10.7	B	Groundwater
G8	−73.7	−11	B	Groundwater
G9	−72.5	−11	B	Groundwater
G10	−74.4	−11.1	B	Groundwater
G11	−75.9	−11.3	B	Groundwater
L6	−52.9	−6.15	B	Lake
s2	−72.6	−10.5	C	Spring
R4	−85.2	−11.8	C	River
R5	−75	−10.1	C	River
s1	−70.8	−10.3	D	Spring
L1	−53.1	−6.55	D	Lake
L2	−50.7	−6.32	D	Lake
L3	−42.9	−4.29	D	Lake
L4	−34.2	0.381	D	Lake
L5	−45.1	−4.99	D	Lake
R1	−66.2	−10.1	D	River
R2	−65	−9.55	D	River
R3	−73.8	−11.1	D	River

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Figure 5.  Relations of δD and δ¹⁸O in all the water samplings.

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The stable isotope compositions of the groundwater samplings collected in the regional B ranging from −75.9‰ to −63.4‰ for δD and −11.3‰ to −8.64‰ for δ¹⁸O, respectively. The arithmetic mean values of δD and δ¹⁸O are −69.55‰ and −10.17‰, respectively. They are centrally located in the domain b, which are differentiate from the groundwater in the southern of regional A. This further illustrates that the regional A and B belongs to different groundwater systems. At the same time, the hydrogen and oxygen stable isotope compositions of spring samplings collected in the regional C are −72.6‰ and −10.5‰ respectively, which are close to the groundwater samplings collected in the regional B. This declares that the regional B and C belong to the same groundwater system, and together constitute the groundwater system of the Xilamulun Riverhead. It is found that the stable isotope compositions of groundwater samplings are different between the northern and southern of the regional B with small area, which shows northern-southern zone features obviously. The value of the samplings collected in the southern are significantly poorer than in the northern of regional B, which is probably due to the elevation effects. At the same time, the groundwater in the southern does not flow into the northern, but flows towards the Xilamulun River. The TDS of groundwater samplings collected in the southern is higher than that of the northern (Zhu BQ et al., 2019), which further verified this flow pattern.

In Fig. 5, the river sampling collected in the northern Gonggeer River system is independent appeared in the domain a1. The Gonggeer Riverhead originates from the southern remnant end of the Greater Khingan Mountains, so the precipitation-runoff generation area has high elevation, and the rivers flowing into the lake are relatively poor in isotopes.

The southern of regional A, and regional B, C and D all located in the northern of the Otindag Desert, where the precipitation is the only recharge source for the groundwater and the recharge can account for more than 70% of precipitation normally. Therefore, the hydrogen and oxygen stable isotope compositions of the whole samplings collected in that regions all appeared near the GMWL, and had obvious zone features. So the samplings collected from the southern of regional A, and regional B, C and D appeared in the domain a2 and domain b+c+d separately (Fig. 5). The groundwater and spring samplings collected in the regional D appeared in the domain d, the corresponding lake samplings formed by evaporation appeared in the domain d’. For this purpose, the evaporation line in regional D is calculated, and its slope value was 3.62. With the consideration that the groundwater and springs have similarity in geographical and hydrogeological conditions in the regional B, C and D, it is speculated that they have similar evaporation character. The combined evaporation line slope of regional B, C and D were calculated using the groundwater, spring, river and lake samplings collected in the regional B, C and D. Results showed that the combined evaporation line slope value is 4.10. Therefore, the evaporation line slope in this area was roughly 3.62–4.10 (Fig. 5, Fig. 6).

Figure 6.  Relations of δD and δ¹⁸O in the water samplings collected from domain b+c+d.

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In Fig. 5, the samplings collected in the Dalinuoer Lake appeared in the domain a’. In order to explore the Dalinuoer Lake recharge source ration from the northern domain a1 and the southern domain a2, it is assumed that the recharge sources come from the Gonggeer River representing domain a1 and the southern part representing domain a2. The evaporation line of domain a1-domain a’ and domain a2-domain a’ named were plotted with its slope value of 5.11 and 4.78 respectively. The latter slope is closer to the aforementioned slope of the combined evaporation line of regional B, C and D. It is speculated that the original water recharging the Dalinuoer Lake should be the mixed water from the northern part and southern part, and the mixed water domain should appear in the middle of the northern domain a1 and southern domain a2. But the mixed water domain should be appeared more close to the domain a2. It is illustrated that in the current precipitation condition and water resources development and utilization situation, the effect of the surface water and groundwater in the southern of regional A play an even greater role than the northeastern Gonggeer River system in the Dalinuoer Lake recharge. Consequently, it is of great significance to protect water resources conservation area and reduce the development and utilization of groundwater resources in the southern sand of regional A for maintaining the ecology safety of the Dalinuoer Lake.

4.3   The impact of surface watershed and groundwater basin on the ecosystem of Dalinuoer Lake

The westerly winds circulation and the EASM circulation are two extremely important wind systems in the earth (An CB and Chen FH, 2009; Gan TY, 2000). Although the two circulations belong to different subsystems of atmospheric circulation, they are closely related. The two joint influence the regional precipitation and climate environment change in the middle and high latitudes in the northern hemisphere. The previous study showed that the boundary of the EASM and westerly winds lies through the Inner Mongolia Plateau (Chen FH et al., 2010; Zhu BQ et al., 2019, Li WB et al., 2017). The author of this article found that the arithmetic mean values of δD and δ¹⁸O of the groundwater samplings collected in the regional B, C and D which located in the south of the Haoluku Anticlinorium Fault are −69.88‰ and −10.21‰, respectively, and the groundwater samplings centered in the domain b+c+d. The arithmetic mean values of δD and δ¹⁸O of the groundwater samplings collected in the southern of the regional A are −84.00‰ and −13.25‰, respectively, and the groundwater samplings centered in the domain a2. There is no intersection between the domains, and the value of domain a2 is poorer than the domain b+c+d (Fig. 5). This not only denies the possibility of water vapor originating from the Pacific Ocean, but also indicates that the southern of regional A and regional B, C and D are affected by different climatic water vapor sources. As a result, the Haoluku Anticlinorium Fault orientation is presumed to be the boundary of the EASM and westerly winds.

The ecology of the Dalinuoer Lake watershed is extremely sensitive to global climate change. During 52 years, the average precipitation decline rate was −10.5 mm/10a, and the trend rate of temperature increase was +0.35°C/10a, which performs a prominent warming-drying trend (Ma C et al. 2017). The observation of Baiyinaobao weather station showed that the evaporation is 3.6 times greater than rainfall (Zhen ZL et al., 2014). The recharge and discharge relationship between groundwater and surface water of the Dalinuoer Lake watershed is that the groundwater in the southern recharges the river, and groundwater and the spring-feeding river jointly recharge the Dalinuoer Lake. Warm and dry climate causes temperature rising and precipitation declining, which leads to the evaporation increases and runoff decreases. The Dalinuoer Lake water recharge is less than evaporation. In the 20^th century, the human influence was weak. The relationship between man and land relationship showed less tension and the climate controls the ecological evolution of Dalinuoer Lake watershed. There are certain relations between the lake area and precipitation, but the evaporation does not have significant effect on the lake area. Rivers and groundwater have been exploited and utilized by the agriculture, industry and animal husbandry activities since the 21^st century, which lead to soil erosion, land desertification and lake area shrinkage. The human activities have become the main contributor to the regional ecological environment degradation (Ma C et al., 2017).

The TDS, alkalinity, pH and hardness are important indicators for fish survival in the water body. The Dalinuoer Lake area shrinkage leads to the dissolved solids condensed during the evaporation process. As a result, the Dalinuoer Lake evolves into a saltwater lake. Yang FY et al. (2020) found that the average alkalinity and hardness of the water in Dalinuoer Lake were 69.34 mmol/L and 2.61 mmol/L respectively, with an average pH 8.80 in July 2019. From 1975 to 2019, the average alkalinity and hardness increased by 1.01% and 0.38% annually, respectively, while the average annual pH decreased by 0.16%. The high saline-alkaline water of Dalinuoer Lake had no serious negative impact on the original saline-alkali tolerant fish, such as Varsijaro and Dali Lake Plateau Loach now, but it might limit the migration of other ordinary economic freshwater fish species. So far, it was only found that naked carp from Qinghai Lake could better adapt to the high saline and alkaline water environment of Dalinuoer Lake. As the lake area shrinkage and water salinization of Dalinuoer Lake, the suitability of fish breeding habitat is getting worse, and the potential risk of fish species decline is increasing, which will have an important impact on the ecological diversity of Dalinuoer Lake (Yang FY et al., 2020).