Overview of Huixian Karst Wetland
5.1.1 Geographical location and transportation
Guilin Huixian Karst Wetland is located in Huixian Town and Sitang Township, Lingui County, Guilin City, extending to Wenquan and Huangchatang in the north. It extends to Mutong, Maojia, and Zhatangdi in the south, to Jiutoushan and Mojia in the west, and to Fengjia in the east. The geographical coordinates are: 110°09′50″～110°14′30″ east longitude, 25°05′20″～25°06′45″ north latitude, with a total area of about 35.2km2. The transportation in the area is developed, mainly Guiwu Expressway and Liangyong Secondary Backbone Highway, which are connected to townships and villages into a network. At present, there are fourth-class highways in ***, such as Mutong, Siyi, Xinmin, Wenquan, Zhuyuan, and Dawan, which can directly reach most areas of the wetland, and the transportation is very convenient (Figure 5.1)
Figure 5.1 Traffic location map of Huixian Karst Wetland
5.1.2 Meteorology and Hydrology
5.1.2.1 Meteorology
The climate in the area where Guilin Huixian Karst Wetland is located is warm and humid, belonging to It has a mid-subtropical monsoon climate with no severe cold in winter and no severe heat in summer. The climate change characteristics are shown in Figure 5.2. The average annual rainfall in the study area is 1835.8mm, the maximum annual rainfall is 2452.7mm, and the minimum annual rainfall is 1313.3mm. The rainy season is from March to August, and the rainfall accounts for 80% of the whole year. Among them, April to August is the period when heavy rains occur frequently, and the rainfall accounts for about 50% of the whole year. From August to September, the number of heavy rains decreases, and high temperature and drought weather often occur. In October, the weather is sunny and rainless, with crisp autumn air and pleasant climate.
The multi-year average evaporation in the study area is 1569.7mm. The maximum evaporation month is July, reaching 199mm, accounting for 12.67% of the total annual evaporation. The average annual temperature is 19.5℃, and the coldest month is 1 In July, the average temperature is 8.6°C. The hottest month is July, with an average monthly temperature of 28.9°C. The extreme highest and lowest temperatures are 38.8°C and -3.3°C respectively.
Figure 5.2 Huixian Karst Wetland Climate Change Characteristics
5.1.2.2 Hydrology
The main rivers in the area include Mutong River and Xiangsidai Ancient Canal, respectively. It is located in the middle and south of the wetland, running east-west (Figure 5.3). An overview of the main rivers in the wetland is as follows:
(1) Mutong River
Also known as Shenlong River, its source is Mutong Lake, flowing through the north of Fenghuang Mountain and the south of Jiutou Mountain to the west Inject into the Xiangsi River. The total length of Mutong River is about 4.38km, and the total rainwater collection area is about 23.14km2. This watershed is the main water storage area of the wetland, and the Mutong River is the main discharge zone of the wetland surface water.
(2) Xiangsidai Ancient Canal
Also known as the Ancient Guiliu Canal, it was excavated in the first year of Changshou in the Tang Dynasty (AD 692). The ancient canal is located in the central and southern part of the wetland, with a total length of 14.54km. The ancient canal is divided into east and west sections with Fenshui Pond as the dividing point. The eastern section passes through Dumen Ridge and flows into the Liangfeng River at Jiangjiaba near Liangfeng; the western section passes through the southern part of Mutong Lake and flows into the Huixian River near Mojia. Although the excavation of the ancient canal connected shipping between the Lijiang River and the Liujiang River, it also destroyed the water circulation system inside the wetland. The ancient canal runs through the entire wetland in the east-west direction, and also passes through the wetland's main water storage area - Mutong Lake and the Fenshuitang area, causing the wetland water to drain into the ancient canal, thus accelerating the degradation of the wetland.
In addition to the main rivers mentioned above, there are also many water bodies of different sizes including ponds, lakes, and swamps (Figure 5.3). There are 15 major water bodies, which are: Mutong Lake, Longshan Lake, Shenlong Pond, Dulong Pond, Fenshuitang, Laodou Marsh, Maojia Fish Pond Marsh, Shuidong Marsh, Fengjia Water Field Marsh, Huangtang Marsh, Longdong Marsh, Anlong Marsh, Wenquan Water Field Marsh, Shenlong Bridge Marsh and Jiutou Mountain grassland, with a total area of about 6.6km2.
Figure 5.3 Huixian karst wetland water system and water body distribution map
5.1.3 Topography and landforms
5.1.3.1 Topography
Huixian The main body of the karst wetland is located at the axis of the Maojia syncline, with the northern edge of the Mamian-Huangcun anticline at the southern edge, the southern part at the northern sloping end of the Jiaqiaoling anticline, and the central Shiziyan area as the watershed between the Liujiang River system and the Lijiang River system. The terrain is generally higher in the north, followed by the south, and the center is the lowest, with ground elevations ranging from 147.0 to 544.3m.
5.1.3.2 Landform
According to the topographic characteristics, genetic type, surface composition materials, underlying bedrock ancient topography and modern landform evolution process of Huixian karst wetland, it can be classified into The landform types are divided into three types: peak cluster valley, solitary peak plain, and residual hill plain (Figure 5.4).
The main characteristics of each landform type are as follows:
(1) Fengcong Valley
Mainly distributed in Dachangshan-Wenquan-Huangchatang in the northern part of the wetland This area covers an area of about 3.30km2, and the ground elevation is between 151.0 and 544.3m. Dachang Mountain is the highest point in the area, with an elevation of 544.3m. This landform type is controlled by geological structures and belongs to the transition zone between the hub uplift zone of the Huangcun-Mamian anticline and the Maojia syncline of the secondary structure of the Guilin arc-shaped structural belt. The lithology of the strata in this area is mainly limestone, with karst development.
(2) Gufeng Plain
Mainly distributed in the east and west of the wetland, with an area of about 3.70km2 and a ground elevation between 146.2 and 157.8m.
The eastern part of the area is centered on Lion Rock, with peak elevations ranging from 176.8 to 292.0m; the western part is centered on Jiutou Mountain and Fenghuang Mountain, with peak elevations ranging from 172.6 to 407.4m. The stratigraphic lithology in this area is mainly limestone, dolomitic limestone and argillaceous limestone, and the karst development is relatively weak. Among them, a large-scale undercurrent develops in the Lion Rock area in the east.
Figure 5.4 Huixian Karst Wetland Landform Map
(3) Residual Hill Plain
It runs through Fenghuang Mountain-Dulong-Mudong-Doumen area, covering an area of It is about 28.2km2, and the ground elevation is between 149.2 and 155.5m. The main body of the Huixian Karst Wetland is located in it. The area is overlain by a Quaternary red-yellow clay layer with a thickness of 1 to 5 meters, and is underlain by the Carboniferous Yanguan Formation muddy and carbon-argillaceous limestone, which was formed during the Yanshan tectonic movement. The karst water storage structural basin forms the natural relatively water-isolating floor of Huixian karst wetland.
5.1.4 Stratum lithology and geological structure
5.1.4.1 Stratum lithology
The exposed strata, area and distribution range of Huixian Karst Wetland are shown in the table 5.1; Figure 5.5.
Figure 5.5 Geological map of Huixian Karst Wetland
Table 5.1 Chronology of exposed strata in Huixian Karst Wetland
5.1.4.2 Geological structure
< p>According to the research results of the Institute of Karst Geology of the Chinese Academy of Geological Sciences in the 1980s, the Huixian karst wetland distribution area is located at the intersection of the Nanling zonal structural belt, the east Hunan-east Guangxi meridional structure, and the east wing of the Guangxi mountain-shaped structure. Based on the relationship between structural traces and genetic assemblage, Huixian karst wetland spans three structural belts: Guilin arc structural belt, east-west structural belt and northwest linear structural belt (Figure 5.6).(1) Guilin Arc Structural Belt
The Guilin Arc Structural Belt belongs to the first-level structural belt of Huixian Karst Wetland. Its structural traces are the deformation of north-south fold structures, consisting of It is composed of middle and upper Devonian and lower Carboniferous carbonate rock formations. The Fengcong Valley area on the northern boundary of Huixian Karst Wetland is located at the southern end of the Mamian anticline, a secondary structural unit in the middle of the Guilin arc-shaped structural belt.
Figure 5.6 Schematic structural diagram of Huixian karst wetland region
(2) East-west structural belt
The east-west structural belt is a third-level structural unit with hairy Home syncline. The main body of Huixian karst wetland is located in this structural zone. This syncline structure is a nearly east-west-trending third-level structural basin formed by the regional north-south geostress during the Yanshan tectonic movement at the end of the Late Triassic. It is composed of Lower Carboniferous carbonate rocks. This structural basin is Huixian Karst wetlands provide unique water storage conditions.
(3) Northwest linear tectonic zone
The northwest linear tectonic zone consists of a series of fault zones trending 290° to 330°. It was generated during the Indosinian tectonic movement. It has experienced multiple stages of tectonic movements in Yanshan and Himalayas, and is a neotectonic activity zone with multiple activities. The southern boundary of Huixian Wetland is located at the northern sloping end of the Jiaqiaoling anticline of this structural zone.
5.1.5 Hydrogeological conditions
5.1.5.1 Division of water-bearing rock groups and water richness
Huixian Karst Wetland can be roughly divided into three water-bearing rocks The groups are: single-layer loose rock water-bearing rock group (I), continuous pure carbonate medium-thick layer water-bearing rock group (II1), and interlayer impure carbonate water-bearing rock group (II2). The rock division, distribution and water richness of each water-bearing rock group are shown in Table 5.2 and Figure 5.7(a).
Figure 5.7 Hydrogeology and profile sketch of Huixian Karst Wetland
(a) Hydrogeology sketch of Huixian Karst Wetland; (b) A-A' profile sketch; (c) B —B' Profile Sketch
Table 5.2 Lithology and distribution characteristics of the water-bearing rock formation in Huixian Karst Wetland
5.1.5.2 Groundwater recharge, diameter and drainage conditions
The recharge, runoff and discharge of groundwater in Huixian Karst Wetland are mainly controlled by factors such as meteorology, hydrology, topography, strata lithology and geological structure. The recharge, runoff and discharge patterns are shown in Figure 5.7 (b) and (c).
(1) Groundwater recharge
The main recharge source of groundwater in Huixian Karst Wetland is atmospheric precipitation. There are three main forms of recharge: one is direct infiltration recharge from atmospheric precipitation, the second is infiltration recharge from swamp water bodies, and the third is lateral recharge from external water. Among them, the first two are the main recharge forms of groundwater.
The supply of groundwater in the loose layer is mainly the direct infiltration supply of atmospheric precipitation and the infiltration supply of swamp water. In addition, the middle part of the wetland also receives vertical recharge from the underlying rock strata during the wet season. Because this area is located at the axis of the Maojia syncline, groundwater collects from the south and north sides, and will be recharged upward under the pressure of the loose layer. of groundwater.
The recharge of karst groundwater is mainly direct infiltration recharge from atmospheric precipitation and lateral recharge from external sources. In special drought years, it may also receive vertical recharge from the overlying loose layer. The direct infiltration and recharge of atmospheric precipitation mainly occurs in karst exposed areas. The mountains in this area are affected by dissolution and weathering, and vertical karst fissures and karst pipes develop. After receiving atmospheric rainfall, rainwater flows along vertical karst fissures and karst pipes under the action of gravity. The infiltration of pipelines recharges groundwater; the lateral recharge of external water mainly refers to the lateral recharge of groundwater around the southern and northern boundaries of the wetland.
Among them, the lateral recharge at the northern boundary is mainly karst groundwater supply, and the southern boundary is pore water and karst groundwater supply.
(2) Groundwater runoff
Controlled by topography and structure, the flow directions of loose layer groundwater and karst groundwater in Huixian Karst Wetland are roughly the same, generally from south to north. The runoff is fan-shaped to the west, middle and east of the wetland, see Figure 5.7(a). In the Phoenix Mountain and Lion Rock areas, affected by reverse faults and topography, karst groundwater flows to the west and east respectively. In the Fenghuang Mountain-Wenquan Mountain-Fengjia area, some karst groundwater runoff will also be blocked due to the influence of changes in stratum lithology in the northeast direction.
(3) Groundwater discharge
The discharge areas of groundwater in Huixian Karst Wetland are relatively scattered, mainly distributed in Mutong River, ancient canal, Huangmao-Doumen and Dachangshan- Wenquan Mountain-Dulong-Fengjia area. The main forms of excretion include undercurrents, springs and undercurrents.
The groundwater in the loose layer is basically discharged into the Mutong River and ancient canals in the form of undercurrents. Non-karst springs are found in the canals near the lower reaches of the Mutong River and upstream Jiutou Mountain.
Karst groundwater has many forms of discharge. In the wet season, karst groundwater is discharged in the form of undercurrents, springs and undercurrents; in the dry season, its discharge is mainly in the form of undercurrents. During wet periods or when concentrated heavy rainfall occurs, karst groundwater can gather in a short period of time, and the flow rate increases sharply. The runoff reaches the Dachangshan-Wenquanshan-Dulong-Fengjia area. Since the formation lithology is composed of pure carbonic acid The salt rock turns into interlayered impure carbonate rock, and the karst groundwater runoff is blocked, and most of the runoff will be discharged to the surface in the form of karst springs. The Huangmao-Doumen area is located in the anticlinal structural belt, and karst springs are also developed; during the dry season or when there is no rainfall for a long period of time, the amount of karst groundwater becomes smaller, the flow rate slows down, and the springs basically stop flowing. At this time, the discharge of karst groundwater is mainly undercurrent.
An undercurrent develops in the Lion Rock area. After the area receives atmospheric rainfall, part of the rainfall infiltrates into the undercurrent along the karst fissures, and is finally discharged from the undercurrent outlet. But most of the water in this undercurrent comes from the swamp water body in the south of Huangchatang.
5.1.5.3 Chemical Type of Groundwater
The hydrochemical characteristics of groundwater in Huixian Karst Wetland are shown in Table 5.3. It can be seen from the table that the chemical type of karst groundwater is mainly HCO3-Ca type, with a pH value of 7.18 to 7.76 and a total hardness of less than 150 mg/s. Among them, the total hardness, salinity and pH value of the interlayer impure carbonate rock area are higher than those of the pure carbonate rock area. For example, the total hardness and salinity of groundwater in Fengjiaming Well are 269.97mg/L and 415.65mg/L respectively, while the total hardness and salinity of groundwater in Dulongbei Spring Point are 149.40mg/L and 222.82mg/L respectively; loose layer groundwater The chemical types are mainly HCO3·Cl-Ca·K type and HCO3-Ca·K type, and the total hardness and mineralization are also significantly increased. This chemical type indicates that the loose layer groundwater in these areas is closely connected with the karst groundwater. Generally, factors such as the development of karst pipes will be taken into consideration when selecting the location of private wells. For example, the Qixing Well, according to villagers, there are karst pipes at the bottom of the well, and the pore water is closely connected with karst groundwater. Therefore, the chemical type of groundwater in the loose layer obtained in this study cannot fully represent the chemical type of groundwater in the entire region.
Table 5.3 Chemical characteristics of groundwater in Huixian Karst Wetland