General situation of karst wetland in Huixian county

5. 1. 1 Geographical location and traffic

Guilin Huixian Karst Wetland is located in Huixian Town and Sitang Township, Lingui County, Guilin City. There are hot springs and yellow tea ponds in the north, mudong, Maojia and Zhatang in the south, Jiutou Mountain and Mojia in the west and Feng Jia in the east. Geographical coordinates are: east longitude11009' 50 "~1kloc-0/4' 30", north latitude 25 05' 20 "~ 25 06' 45", with a total area of about 35.2km2. At present, * * has four highways, namely Mudong, Siyi, Xinmin, Wenquan, Zhuyuan and Dawan, which can directly reach the wetlands in most areas, and the traffic is very convenient (Figure 5. 1).

Fig. 5. 1 Huixian karst wetland traffic location map

5. 1.2 Meteorology and Hydrology

5. 1.2. 1 meteorology

The climate of Guilin Huixian Karst Wetland is warm and humid, belonging to subtropical monsoon climate, with no severe cold in winter and no heat in summer. See Figure 5.2 for climate change characteristics. The average annual rainfall in the study area is 1835.8mm, the maximum annual rainfall is 2452.7mm, the minimum annual rainfall is 13 13.3 mm, and the rainy season is from March to August, accounting for 80% of the annual rainfall. Among them, April to August is a period of frequent rainstorms, and the rainfall accounts for about 50% of the whole year. From August to September, the number of heavy rains decreased, and high temperature and dry weather often appeared. 5438+ 10 June, the weather is sunny and rainy, the autumn is crisp and the climate is pleasant.

The average annual evaporation in the study area is 1569.7mm, and the maximum evaporation is in July, reaching 199mm, accounting for 12.67% of the total annual evaporation. The annual average temperature 19.5℃, the maximum Leng Yue 1, the average temperature of 8.6℃, the hottest month of July, the monthly average temperature of 28.9℃, the extreme maximum temperature of 38.8℃ and the extreme minimum temperature of -3.3℃.

Figure 5.2 Climate Change Characteristics of Huixian Karst Wetland

5. 1.2.2 Hydrology

The main rivers in the area are Mudong River and the ancient canal in Acacia Belt, which are located in the south-central part of the wetland and near the east-west direction (Figure 5.3). The main rivers in the wetland are summarized as follows:

(1) Mudong River

Also known as Shenlong River, its source is Mudong Lake, which flows through the northern part of Fenghuang Mountain and the southern part of Jiutou Mountain and flows westward into Xiang Si. The total length of Mudong River is about 4.38km, and the total rainwater collection area is about 23. 14km2. This basin is the main water storage area of wetland, and Mudong River is the main drainage area of wetland surface water.

(2) Acacia belt ancient canal

Also known as Gugui Liuqu, it was dug in the first year of longevity in Tang Dynasty (AD 692). The ancient canal is located in the south-central part of the wetland, with a total length of 14.54km. The ancient canal is divided into two parts, the east and the west, with the reservoir as the dividing point, and the east flows into the Liangfeng River through the Jiangjiaba near the cool wind at Dumenling. The western section flows into Huixian River near Mojia through the south of Mudong Lake. Although the excavation of the ancient canal communicated the shipping between Lijiang River and Liujiang River, it also destroyed the water circulation system inside the wetland. The ancient canal runs through the whole wetland from east to west, and also passes through the main water storage areas of the wetland-Mudong Lake and Fenshui Reservoir, which leads to the discharge of wetland water to the ancient canal, thus accelerating the degradation of the wetland.

In addition to the development of the above major rivers, there are many water bodies of different sizes, including ponds, lakes and swamps (Figure 5.3). There are 15 main water bodies, namely Mudong Lake, Longshan Lake, Shenlongtang, Dulongtang, Fenshui Pond, Laodan Marsh, Maojia Fish Pond Marsh, Shuidong Marsh, Fengjia Shuitian Marsh, Tanghuang Marsh and Longdong Marsh.

Fig. 5.3 Distribution Map of Water System and Water Body of Huixian Karst Wetland

5. 1.3 Terrain

Terrain 5. 1.3

The main body of Huixian karst wetland is located in the axis of Maojia syncline, with the southern margin of Mamian-Huangcun anticline in the north, the northern oblique end of Jiaqiaoling anticline in the south, and the lion mountain area in the middle as the watershed between Liujiang River system and Lijiang River system. The terrain is generally high in the north and low in the south, and the ground elevation is147.0 ~ 544.3 m.

5. 1.3.2 Terrain

According to the topographical features, genetic types, surface composition, underlying bedrock paleotopography and modern landform evolution process, Huixian karst wetland can be divided into three types: peak cluster valley, butte plain and residual hill plain (Figure 5.4).

The main features of each landform type are as follows:

(1) peak cluster valley

It is mainly distributed in Dachangshan-Wenquan-Huangchatang area in the north of the wetland, with an area of about 3.30km2, and the ground elevation is between151.0 ~ 544.3m, of which Dachangshan is the highest point in this area, with an elevation of 544.3 m. This landform type is controlled by geological structure and belongs to Huangcun-Mamian anticline hub uplift zone and Guilin arc. The lithology of strata in this area is mainly limestone with karst development.

(2) Gu Feng Plain

It is mainly distributed in the east and west of the wetland, covering an area of about 3.70km2, and the ground elevation is between146.2 ~157.8m. In the east of this area, Lion Mountain is the core, and the peak elevation is between176.8 ~ 292.0m.: In the west, Jiutoushan-Fenghuang Mountain is the core, with the highest peak elevation of172.6 ~ 407.4m. The stratum lithology in this area is mainly limestone, dolomite limestone and argillaceous limestone, and the karst development is relatively weak. Among them, there is a large-scale undercurrent in the eastern lion mountain area.

Figure 5.4 Geomorphology Map of Huixian Karst Wetland

(3) Canopy Plain

It runs through Fenghuang Mountain-Dulong-Mudong-Doumen area, covering an area of about 28.2km2, and the ground elevation is between149.2 ~155.5m. The main body of Huixian karst wetland is located in it. This area covers the Quaternary red-yellow clay layer with a thickness of 1 ~ 5m, and the underlying Carboniferous Yan Guan Formation argillaceous and carbonaceous argillaceous limestone, which is a karst water storage structural basin formed during the Yanshan tectonic movement and constitutes the natural relative water-resisting layer of Huixian karst wetland.

5. 1.4 Formation lithology and geological structure

5. 1.4. 1 formation lithology

See Table 5. 1: Figure 5.5 for the exposed stratum, area and distribution range of Huixian karst wetland.

Figure 5.5 Geological Map of Huixian Karst Wetland

Table 5. 1 Annual Representative of Exposed Strata of Huixian Karst Wetland

5. 1.4.2 Geological structure

According to the research results of Institute of Karst Geology, Chinese Academy of Geological Sciences in 1980s, Huixian karst wetland is located at the intersection of Nanling belt structure, eastern Hunan-eastern Guangxi meridional structure and the east wing of Guangxi gable structure. Based on the relationship between structural characteristics and genetic combination, Huixian karst wetland spans three structural zones: Guilin arc structural zone, east-west structural zone and northwest linear structural zone (Figure 5.6).

(1) Guilin arc structural belt

Guilin arc structural belt belongs to the first-class structural belt of Huixian karst wetland, and its structural feature is north-south folding deformation, which is composed of middle-upper Devonian and lower Carboniferous carbonate strata. The peaks and valleys in the northern boundary of Huixian karst wetland are located at the southern end of the Ma-mian anticline, a secondary structural unit in the middle of Guilin arc structural belt.

Figure 5.6 Schematic Diagram of Regional Structure of Huixian Karst Wetland

(2) East-west structural belt

The east-west structural belt is a three-level structural unit with Maojia syncline. The main body of Huixian karst wetland is located in this structural belt. The syncline structure is a scene of Yanshan tectonic movement at the end of late Triassic. It is a near-east-west tertiary tectonic basin formed by regional N-S geostress, and is composed of carbonate rocks of Lower Carboniferous. This tectonic basin provides unique water storage conditions for Huixian karst wetland.

(3) NW-trending linear structural belt

The NW-trending linear tectonic belt is composed of a series of fault zones with strike of 290 ~ 330, which originated from Indosinian tectonic movement, and then experienced Yanshanian and Himalayan multi-stage tectonic movements. It is a new tectonic active zone with frequent activities. The southern boundary of Huixian wetland is located at the northern inclined end of Qiaojialing anticline in this structural belt.

5. 1.5 Hydrogeological conditions

5. 1.5. 1 division of water-bearing rock groups and water abundance

Huixian karst wetland can be roughly divided into three water-bearing rock formations, namely: single-layer loose rock water-bearing rock formation (Ⅰ), continuous pure carbonate medium-thick water-bearing rock formation (Ⅱ1) and interlayer impure carbonate water-bearing rock formation (Ⅱ 2). See Table 5.2 and Figure 5.7(a) for the rock division, distribution and water abundance of each water-bearing rock group.

Fig. 5.7 Hydrogeology and Profile of Huixian Karst Wetland

(1) Hydrogeological sketch of Huixian karst wetland; (b) Sketch of section A-A'; (c) Schematic diagram of section B-B'

Table 5.2 Lithology and Distribution Characteristics of Water-bearing Rock 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 meteorological and hydrological factors, topography, stratum lithology and geological structure, and their recharge, runoff and discharge modes 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 the direct infiltration recharge of atmospheric precipitation, the other is the infiltration recharge of swamp water, and the third is the lateral recharge of exogenous water. Among them, the first two are the main forms of groundwater recharge.

The recharge of groundwater in loose layer is mainly the direct infiltration recharge of atmospheric precipitation and the infiltration recharge of swamp water. In addition, the middle part of the wetland is also vertically replenished by the underlying rocks during the wet season, because this area is located at the axis of the Maojia syncline. After the groundwater is collected here from the south and north sides, it will replenish the groundwater in the loose layer under pressure.

The recharge of karst groundwater is mainly the direct infiltration of atmospheric precipitation and the lateral recharge of external water, and it may also accept the vertical recharge of overlying loose layer in special drought years. The direct infiltration and recharge of atmospheric precipitation mainly occurs in the karst bare area, where the mountain is affected by dissolution and weathering, and vertical karst cracks and karst pipes are developed. After receiving atmospheric rainfall, rainwater will penetrate downward along vertical karst cracks and karst pipelines under the action of gravity to replenish groundwater. The lateral recharge of exogenous water mainly refers to the lateral recharge of groundwater around the north-south boundary of wetland. Among them, the lateral recharge of the northern boundary is mainly karst groundwater recharge, and the southern boundary is pore water and karst groundwater recharge.

(2) Groundwater runoff

Controlled by topography and structure, the flow direction of groundwater in the loose layer of Huixian karst wetland is roughly the same as that of karst groundwater, and generally flows in a fan shape from south to north to west, middle and east of the wetland, as shown in Figure 5.7(a). In Fenghuangshan and Shiziyan areas, karst groundwater flows west and east respectively due to the influence of reverse faults and topography. In Fenghuangshan-Wenquanshan-Feng Jia area, due to the influence of lithology changes in the northeast stratum, some karst groundwater runoff will also be blocked.

(3) Groundwater discharge

The groundwater discharge area of Huixian karst wetland is scattered, mainly distributed in Mudong River, Ancient Canal, Huang Mao-Doumen and Dachangshan-Wenquan Mountain-Dulong-Feng Jia. The main forms of discharge are undercurrent, spring point and undercurrent.

Groundwater in loose layer is basically discharged into Mudong River and ancient canal in the form of undercurrent, and there are non-karst springs in the canal near Jiutoushan in the downstream and upstream of Mudong River.

There are many forms of discharge of karst groundwater. In wet season, karst groundwater is discharged by undercurrent, spring and undercurrent; In the dry season, its discharge form is mainly undercurrent. In the rainy season or when there is concentrated heavy rainfall, karst groundwater can be collected in a short time, and the flow rate increases sharply, flowing to Dachangshan-Wenquan Mountain-Dulong-Feng Jia area. Because the formation lithology has changed from pure carbonate rock to interlayer impure carbonate rock, the runoff of karst groundwater will be blocked, and most of the runoff will be discharged to the surface in the form of karst springs. Huang Mao-Doumen area is located in the anticline structural belt, and karst springs are also well developed. In dry season or when it doesn't rain for a long time, the amount of karst groundwater decreases, the flow rate slows down and the spring point is basically cut off. At this time, the discharge of karst groundwater is mainly undercurrent.

There is an undercurrent in Shiziyan area. After receiving atmospheric rainfall, part of the rainfall seeps into the undercurrent along the karst fissure and is finally discharged from the undercurrent outlet. But most of the water in this undercurrent comes from the swamp water south of Huangchatang.

5. 1.5.3 Chemical types of groundwater

The hydrochemical characteristics of groundwater in Huixian karst wetland are shown in Table 5.3. As can be seen from the table, the chemical types of karst groundwater are mainly HCO3-Ca type, with a pH value of 7. 18 ~ 7.76, and the total hardness is lower than 150mg/s/s, among which the total hardness, salinity and pH value of interlayer impure carbonate rocks are higher than those of pure carbonate rocks. For example, the total hardness and salinity of groundwater in Fengjiamin well are 269.97mg/L and 415.65 mg/L, respectively, while the total hardness and salinity of groundwater in Dulong Beiquan point are149.40 mg/L and 222.82 mg/L, respectively. The chemical types of groundwater in loose layer are mainly HCO _ 3 Cl-Cak type and HCO _ 3-Cak type, and the total hardness and salinity are also obviously increased. This chemical type shows that the loose groundwater in these areas is closely related to karst groundwater. The location of general civil wells will consider the factors of karst pipeline development, such as Qixing civil well. According to the villagers, there are karst pipelines at the bottom of the well, and pore water is closely related to karst groundwater. Therefore, the chemical types of groundwater in loose layer obtained in this study cannot fully represent the chemical types of groundwater in the whole region.

Table 5.3 Chemical Characteristics of Groundwater in Huixian Karst Wetland