Characteristics of Water Environment in Regions of Zhaling and Eling Lakes near Waterhead of Yellow River

The Zhaling and Eling lakes, which play an important role in adjustment and storage of water for the upstream area of the Yellow River, are two of the largest natural freshwater lakes in the Yellow River drainage area. The water environment of the two lakes and their adjacent regions is quite unique. Since the lakes are located on a frozen earth belt of the Qingzang plateau, a study of characteristics and evolutionary trend of this area is helpful to better understand the environment of the Yellow River waterhead region (Li, 1995).

NATURAL ENVIRONMENT OF THE TWO LAKES

Climate

The two lakes lie in a semiarid region of the Qingzang plateau, where the altitude is more than 4 200 m above the sea level. The air is rare, and the radiation of the sun is strong. The annual average temperature is about -4 ℃. Temperature in the daytime ranges from 12 to 16 ℃. Annual average precipitation in the area is 303.42 mm and occurs mostly during May and September (accounting for 85.48 % of the total precipitation). The annually average evaporation is 1 372 mm. The frost-proof period is extremely short. The area is characterized by the typical plateau mainland climate (Zhao, 1995).

Physiognomy

Physiognomies of the Zhaling and Eling lake regions are classified into four types, namely mountainous terrain, alluvial-pluvial plain, lacustrine plain and semifixed dune.

The mountainous terrain occurs in the Buqing mountain to the north, and the Bayankela Mountains to the south, of the two lakes. The elevation of the mountainous terrain ranges from 4 500 to 5 800 m. Glacier-denuded terrain topography such as horn peak, fin ridge, amphitheater, fluted valley and "U" valley have been developed. The melting and freezing function is strong, and there are many stone mountains, valleys and stone rivers (Yu, 1995).

The alluvial-pluvial plains are mainly located in the valley of the Yellow River and its branches such as Duoqu River, Lenaqu River and Zoumaqu River, and the mountainous frontier zones to the south and north of the two lakes, where elevation ranges from 4 300 to 4 600 m. In the Yellow River valley, only Ⅰ-level terrace has been developed, whileⅠ-level, Ⅱ-level terraces can be seen in the valleys of the branches.

Covered by swamps, the area is a main pasture of the Yellow River waterhead region. Frozen-swollen dunes and heatmelted ponds can also be seen. Around the two lakes there is a lacustrine plain, with elevation between 4 280 and 4 300 m, 3 terraces have been developed in the plain. TheⅠ-level terrace, which consists of lacustrine silt-fine sand and gravel with horizontal bedding, is about 1-2 m higher than the lake surface, and about 80-100 m wide. TheⅡ-level terrace, which is composed of upper clayey sand and bottom grit width a forward escarpment of 1-1.5 m, is 3-4 m higher than lake surface and 200-250 m in width. Ⅲ-level terrace, which is 13-15 m higher than the lake surface, generally forms barrancas or stretches into the lakes and forms peninsulas and islands. In addition, there are Ⅳ-level andⅤ-level terraces scattering to the east of the Eling lake. Based on the analysis of sporopollens and paleoclimate, the 3 terraces around the two lakes are thought to be formed at Holocene (Fig. 2).

Figure  2.  Sketch of lacustrine terraces to north of Zhaling lake.

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Semifixed dunes lie in the lake gulf billabong to the southeast shore of the two lakes, where altitude is 4 300-4 400 m. The dunes are about 1-5 m high, and the terrain is mainly made up of sand ridges and dunes. Among the dunes, there are aeolian billabongs, swamps and barrier-lagoon lakes with various sizes (Fig. 3).

Figure  3.  Profile of sandbank lagoon to southeast of Eling lake.

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CHARACTERISTICS OF GROUNDWATER ENVIRONMENT

Except the structural melted area of rivers and lakes, 90 % of the Zhaling and Eling lakes area is covered with frozen earth whose lower limit of elevation is about 4 030-4 070 m. With the increasing elevation, the thickness of the frozen earth increases from 8 to 43 m. The depth of seasonal melting ranges from 0.44 to 1.80 m. The characteristics of groundwater of the region are very unique. The main types of the groundwater are as follows: water in fractured basement in the frozen region, water in unconsolidated rock in the frozen region, infrapermafrost water, water in unconsolidated rock in the melted region and water in the structural melted zone. Each type of the water discharges into the Zhaling and Eling lakes.

Water in Fractured Basement Rock in Frozen Region

This type of water is mainly distributed in the northern of the region Buqing mountain and in the southern Bayankela Mountains. The elevation of these regions is high, and it abounds with the precipitation. Fractures formed from weathering make the rocks in the regions permeable. The thickness of the aquifer, which is controlled by the seasonal melted depth, ranges from 0.5 to 1.0 m. After the groundwater in the fractured rocks melts between May and October each year, and is recharged by melted snow-water and precipitation, it flows down the slopes. Some groundwater runs out of the aquifer, as springs and forms streams. These supply plenty of water for grasslands around the mountains and sub-mountains. The TDS of the water is between 0.17 and 0.43 g/L, and HCO₃-Ca type water is commonly found.

Water in Unconsolidated Sediments of Frozen Region

This type of water is widely distributed in the basins among mountains, ravines and piedmonts. The aquifer is composed of lacustrine sand and gravel, drift sheet, drift-pluvial sand and gravel, alluvial sand and gravel of Quaternary age. The thickness of the aquifer controlled by the seasonal melted depth is smaller than 1.5 m, its bottom is made up of permafrost and its water table is about 0.50-2.00 m below the land surface. In summer, the melted water of frozen region begins to flow, often comes out at the river valley and bottomland around the lakes, and recharges the surface water or phreatic water of melted region. Spring discharge is small, and charges seasonally. The spring discharge in June and July is larger than in other months. After November, all the springs are frozen. The water quality is good, and the groundwater is of HCO₃-Ca or HCO₃·Cl-Na·Ca types. The TDS is between 0.19 and 0.47 g/L. This type of water has the closest connection with the vegetation system of the land surface. It moistens and breeds many pastures around the two lakes.

Infrapermafrost Water

This type of groundwater is artesian, and is distributed in the basins among the mountains and the region of the two lakes. Drilling data reveal that the aquifer is composed of sand and gravel of Eopleistocene and Epipleistocene, with thickness of 12 to 40 m. The yielding of groundwater in the aquifer is different from place to place. At the center of the basin and the region with bad recharge condition, the yield to a single well is about 8.08 m³/d; at the nearby of active fault belt, the yielding of water to a single well is up to 765.5 m³/d. Water in the aquifer is of good quality and of the type of HCO₃·Cl-Na·Ca. The water has the TDS between 0.52 and 1.00 g/L.

Water in Unconsolidated Rock in Melted Region

This type of water is often found along two sides of perennial rivers, such as the Yellow River, Duoqu River and Lenaqu River, at the bottom of the Zhaling and Eling lakes, and in the regions around the lakes. The melted regions around the lakes are 100-200 m in width. The widths of the ones along the rivers are usually 10-50 m, depending on the discharge of the rivers. The widest part of the master Yellow River is 700-1 800 m in width. The groundwater in consolidated sediments in the melted region includes phreatic water and artesian one.

Phreatic Water in Melted Region

This type of water is main distributed at the edges of the Zhaling and Eling lakes, and in the valleys of the Yellow River, Lenaqu River, Zoumaqu River and Duoqu River. The aquifers are composed of alluvial, lacustrine, and alluvial-pluvial grit and fine silt of Holocene. The thickness of the aquifers ranges from 10 to 30 m. The depth of the water table is usually 1-4 m. The phreatic water of these regions has a close hydraulic connection with the surface water, and is recharged by the melted superpermafrost water. Water in the aquifer is abundant. The yielding of water to well is 1 000-5 000 m³/d. The groundwater circulates rapidly. Its TDS is smaller than 0.5 g/L, and HCO₃-Ca·Mg or HCO₃-Ca type of water is encountered.

Artesian Water in Melted Region

The distributing regions of this type of water are almost coincident with the ones of the phreatic water. Aquifer is made up of lacustrine sandy clay and drift argillaceous sand-pebblestone of Pleistocene to Eopleistocene. The aquifer is 10-30 m wide, and is recharged by the water below from the frozen layer. Along the northern shore of Eling lake, the water quantity is abundant. The aquifer commonly yields 1 000-5 000 m³/d of the water to wells. The TDS is 0.5-1.0 g/L, and the water is of HCO₃·Cl-Na·Ca type. Between the two lakes, the total thickness of the aquifer is up to 30.70 m, and yielding to a well is 916 m³/d. In the central part of the two lakes, the path of ground water flow is long, and the water quality is relatively bad. The TDS is up to 15.8 g/L, and the type of water belongs to Cl-Na·Mg type.

Water Structural in Melted Region

This type of water is distributed along the active faults, and never freezes in whole year. Through the belts of faults, groundwater discharges in the form of artesian springs appearing in lines. The quality of water is good, and of HCO₃-Ca·Mg or HCO₃·Cl-Na·Ca type. Its TDS is 0.3-0.6 g/L. Generally, the structural melted region across different types of aquifers including the infrapermafrost water, the superpermafrost water, and become perennial regional transporting channels of groundwater.

CHARACTERISTICS OF SURFACE WATER ENVIRONMENT

Characteristics of Water Environment of Yellow River

The region of the two lakes belongs to the Yellow River water system. The Yellow River originates from Yueguzonglie, winding through Xingxiuhai marsh plain from west to east, enters the Zhaling lake, flows out at the southeast corner of the lake, crosses the wide valley between two lakes, inputs into Eling lake, and discharges at the northeast corner. The total collection area of the Yellow River before the exit of Eling lake is 18 428 km². In this region, the Yellow River valley is not obvious; the river bed is wide and shallow, and the streams disperse. According to the data of Hydrological Station of Maduo Huangheyan, the annual average runoff is 20.35 m³/s; the annual throughflow is 6.4×108 m³; the annual throughflow modulus is 0.972 L/s; the annually averaged silt concentration of ages is 0.12 kg/m³; the maximum runoff of the past years is 152 m³/s (Otc.1, 1981), and the minimum runoff is zero. Since 1950s, the Yellow River once stopped flowing 3 times (in 1960, 1980 and 1997 respectively), and the duration of each break shows an increasing trend. Water in the Maduo section of the Yellow River is of HCO₃·Cl-Ca·Na·Mg type, and has TDS of 0.4 g/L and the pH of 8.36.

There are many branches that enter the Yellow River, the Zhaling lake and the Eling lake. The main branches include Maqu, Zaqu, Lenaqu and Duoqu Rivers. The nature of rivers differs from place to place. Rivers in the south originating from the Bayankela Mountains are perennial rivers. Those in the north are seasonal rivers. Water quality of northern rivers is good. The water is of HCO₃-Ca and HCO₃-Ca·Mg type, and has TDS of 0.35-0.46 g/L, and pH of 8.36-8.60.

Water Environment of Zhaling and Eling Lakes

The surface area of the Zhaling lake is 526.1 km², and the elevation of the water surface is 4 293.2 m. The gradient of the water surface is 0.1 %. The maximum depth is 13.1 m, and the average depth is 8.9 m. The capacity of this lake is 46.7×108 m³. The water of the Zhaling lake is of HCO₃·Cl-Na·Mg·Ca type, and TDS of 0.504 g/L (Table 1).The Eling lake has a surface area of 610.7 km², and the elevation of the water surface is 4 268.7 m. The gradient of the water surface is 0.1 %. The maximum depth is 34.7 m, and the average depth is 17.7 m. The capacity of lake is 107.6×10⁸ m³. Its annual average outflow is 4.86×10⁸ m³. The water of this lake is of HCO₃·Cl-Na·Mg·Ca type, and has TDS of 0.376 g/L (Table 1).

Table  1.  HYDROCHEMICAL DATA OF ZHALING AND ELING LAKES

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Water Environment of Lakes around Two Lakes

There are a number of small lakes star-studded around the Zhaling and Eling lakes. Their types are complex, such as freshwater lakes, salt-water lakes, brine water lakes and playas. Their evolutionary processes reflect the vicissitude of the water environment of the two lakes.

Water environment of four sister lakes including Ayonggamacuo lake

The four sister lakes, Ayonggamacuo, Ayonggongmacuo, Ayongwamacuo and Longrecuo lakes, which line along the Yellow River in west-east direction in Maduonan. They originate from the glacier valley, and their total area is 109.65 km². They all belong to seasonal releasing lakes that have sources and discharge the Yellow River, except that the Long-recuo lake belongs to obturated lake that has a source. It can be seen from Table 2, that the water types of the four lakes are HCO₃·Cl-Na·Mg·Ca type and were coincident with other releasing lakes originally. After they became seasonal releasing lakes, the pH value of the water increased and were developed into alkalescent lakes at first, then the types of water changed into Cl·HCO₃-Na·Mg type. When they become obturated lakes, the type of the water is Cl·SO₄-Na·Mg. The water environment condition of the four lakes represents the water environmental characteristics, indicating that the lakes have transformed from releasing ones into seasonal releasing lakes and obturated lakes.

Table  2.  HYDROCHEMICAL CHARACTERISTICS OF FOUR SISTER LAKES

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Water Environment of Chacuo, Chamucuo and Nancuo Lakes

The ripicolous regions around the Zhaling and Eling lakes surrounded by many small lagoons left from atrophied lakes. They were separated from their mother lakes by the sandbank, and became relative cut-lake basins. They are mainly replenished from precipitation, water above the frozen layer, phreatic water of melted region and structural melted region water. Under the condition that the evaporation is 4.25 times as much as the precipitation, the evaporation and inspissation are strong. An evolutionary model of the lakes has been established, indicating that freshwater lakes were first transformed into salt water lakes, then into salt lakes and finally into playas being. The quantity of replenishment and the water quality of replenishing sources determine the evolutionary process and the hydrochemical characteristics. The evolutionary process of these lakes is similar to that of the other many small lakes (Fig. 1).

Figure  1.  Map showing hydrochemical figure of water in frozen layer and melted zone water. 1. HCO₃-Ca type water; 2. HCO₃-Ca·Mg type water; 3. HCO₃·Cl-Na type water; 4. HCO₃-Na·Ca type water, 5. Cl·HCO₃-Na type water; 6.Cl·HCO₃-Na·Ca type water; 7. salt water lake; 8. active fault; 9. fresh water lake.

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Chamucuo lake lies in the southeastern shore of the Zhaling lake. The surface area of the Chamucuo lake is 4.44 km²; the altitude is 4 290 m, and the collection area is 12 km². The Heihebei mountain to Bayanheqian active fault crosses through the lake. The water is transparent, lucid and bitter-salt; the water temperature is 15-18 ℃ (higher than those of the other lakes); the TDS is 84.2 g/L, and the water type is of Cl-Na·Mg type (Table 3).

Table  3.  HYDROCHEMICAL DATA OF CHAMUCUO AND CHACUO LAKES

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Chacuo lake lies in the western shore of the Eling lake. The surface area of Chacuo lake is 1.04 km²; the altitude is 4 298 m, and the collection area is 10.5 km². The lake has water in summer and is dry in winter. There are salt granule depositions on the lakeside. The TDS of the lake water is 310.97 g/L, and the water belongs to Cl-Mg·Na type. The hydrochemical characteristics are listed in Table 3.

From Table 3, we can see that the contents of B₂O₃ of Chamucuo and Chacuo lakes are as high as 487.02 mg/L and 123.17 mg/L respectively. This phenomenon is concerned with the replenishment that comes from the deep water that contains abundant B₂O₃ and is obtained through active fault.

Nancuo lake lies in the lake-gulf billabong in the eastern shores of the Eling lake. The surface area is 0.92 km², and the altitude is 4 280 m. The lake is a brine lake with TDS of 444.6 g/L, and 30 % of its surface is covered by crystal salt of 5-20 cm. In the center of the lake, there are islands made up of crystal salt. NaCl accounts for 98.61 %, MgCl₂ for 0.65 %, KCl for 0.44 %, CaSO₄ for 0.03 %, and FeSO₄ for 0.27 % respectively in the crystal salt.

EVOLUTIONARY TREND OF WATER ENVIRONMENT IN ZHALING AND ELING LAKES

Main Control Factor of Water Environmental Evolution

Based on the obtained data, we conclude that the Zhaling and Eling lakes belong to structural fault-trough lakes. During Epipleistocene, they were inland lakes, and the Yellow River was not connected with them. In early Holocene, the Yellow River cracked along its valley, began the headwater erosion, established the contact with the lakes, and made the lakes become releasing lakes. After the lake water discharged out, the water table decreased, and the size of the lakes reduced. The Xingxiuhai region above the Zhaling lake evolved into a wide swamp after the lake water subsided (exhausted out). The neoteric water system layout of the two lakes came into being.

With the ascending of the Qingzang plateau, the climate of the region becomes more and more dry and cool, and the lakes keep on atrophying. The distribution of the 3 terraces around the Zhaling and Eling lakes indicates that the region has ever experienced three times of subsiding. In addition, according to the Maduo Hydrological Station, water table of the Eling lake declined 60 cm between 1952 and 1978, and the annual average declination is 2.4 cm. Since 1980, the water table has declined rapidly. The number of cutoff times of Maduo Section increases. After 1998, the section between the two lakes of the Yellow River, which has never stopped flowing, has cut off three times. The status shows that the atrophy of the lakes still continues. The evolvement situation of the lakes controlled by climate and structure determines the evolutionary trend the water environment in the region of the two lakes. In addition, the effect of the human activities, greenhouse effect and the increasing temperature of Qingzang plateau on the water environment need to be further studied in the future.

Evolutionary Trend of Water Environment of Two Lakes

The equilibrium of the water quantity in the two lakes is maladjustment in dry and cold climate in which evaporation is greater than precipitation. The atrophy of the two lakes will continue. In the long geological years of future, the ripicolous region around the two lakes will continuously generate new lagoons which are separated from native lakes, and the evolutionary process that freshwater lakes transform into salt water lakes, salt lakes and playas will also continue. The star-studded barrier-lagoon lakes around the two lakes will evolve into playas eventually, and become playa belt that distributes around the two lakes discontinuously.

Because of the replenishment and discharge, the runoffs of the two lakes are relatively smooth, and the type of the water still belongs to the type of HCO₃·Cl-Na·Mg·Ca. With the atrophy of the lakes, the pH of the water will probably rise, and the lakes will develop into alkalescent lakes.

CONCLUSIONS

Conditions of the water environment in the region of the Zhaling and Eling lakes, which are located on the zone of frozen earth on the Qingzang plateau, are quite unique. The two lakes are mainly recharged from precipitation, from infrapermafrost water and superpermafrost water, the melted zone water and the structural melted zone water. The equilibrium of the water quantity of the two lakes is maladjustment in dry and cold climate in which evaporation is greater than precipitation. The Zhaling and Eling lakes have been atrophying and left three terraces and star-studded lagoons around the lakes since Holocene. When these lagoons were separated from the original lakes, the lagoons became cut-lake basins, and transformed from fresh water lakes into salt water lakes and salt lakes or salt playas under strong evaporation. This kind of evolutionary process will continue in the future.