General situation of Mashan karst underground river basin

7. 1. 1. 1 geographical location

Mashan karst underground river basin is located in mashan county, Guangxi Zhuang Autonomous Region (Figure 7. 1). The geographical location of the basin is about10810 ′ ~10825′, and the north latitude is 23 38 ′ ~ 23 50 ′. Mashan karst underground river system is 50.5km long and its basins meet. Mashan karst underground river system originates from the northern end of Damingshan anticline, and consists of the trunk and two larger tributaries, Jiajuanling and Guzhai. The water quantity of Mashan karst underground river system is mainly discharged into Hongshui River through the big end located in the middle section of Hongshui River, and Bailongtan Hydropower Station of Hongshui River is located at about 1km upstream of the sewage outlet.

Fig. 7. Schematic diagram of1Mashan karst underground river basin.

7. 1. 1.2 landform

Carbonate rocks in Mashan karst underground river basin are widely distributed, with pure and heavy texture and strong karst development, forming a typical karst landscape. There are many karst individual forms in the basin, such as karst cracks, karst ditches, karst caves, karst pipelines, karst wells, tiankeng, karst pools and skylights. Mashan karst underground river basin is located in the transition zone between the western mountain area of Guangxi and the central basin of Guangxi, and the landform types are mainly peak-cluster depressions and peak-cluster valleys. Peak-cluster depression is a karst landform widely distributed in the basin. The individual form of peak cluster is several peaks with the same base. Depression is a closed low-lying land distributed in the middle of peaks and clusters. The bottom of the depression is generally pot bottom or flat basin shape, and karst individual forms such as dissolved wells, dissolved cracks, tiankeng or skylight are often developed. Atmospheric precipitation enters the depression, and karst groundwater is directly replenished through these channels. Peak cluster Gu Duo is a semi-closed negative terrain with entrances and exits, long and narrow, with steep peak forests on both sides and thin soil cover; The bottom of the valley is flat, and karst underground rivers often develop along the valley, with skylights, funnels and dissolved wells.

Mashan karst underground river basin is influenced by structure and stratum lithology. The topography of the whole basin gradually declines from southeast to northwest, and inclines to Hongshui River, forming the valley topography of Hongshui River Basin.

7. 1.2 Meteorology and Hydrology

7. 1.2. 1 meteorological conditions

Mashan karst underground river system is located near the Tropic of Cancer and belongs to subtropical monsoon climate, with southerly winds in summer and northerly winds in winter, which is obviously influenced by monsoon climate. Adequate sunshine, warm and humid all year round, high temperature and high humidity all year round. The annual average temperature is about 265,438+0℃, with high temperature from May to 65,438+00℃, and the highest temperature can reach 39℃. The weather is cold from February to February, and the monthly average temperature is between 65,438+00 and 65,438+03℃. Affected by the subtropical monsoon climate, the annual rainfall in the same period of rain and heat is 1245.8 ~ 2060.3 mm, and the rainy season is from April to September, accounting for about 80.4% of the total annual rainfall. The rainfall for four consecutive months (April-July) accounts for about 67% of the annual rainfall, and the rainfall is mainly concentrated in summer. The dry season is from June 10 to March of the following year, and the dry season rainfall only accounts for about 19.6% of the total annual rainfall (Figure 7.2).

Figure 7.2 Average monthly rainfall in Mashan area

7. 1.2.2 Hydrological conditions

Mashan karst underground river system is located in Hongshui River basin, which belongs to the main stream of Pearl River system, and all the water of Mashan karst underground river system is discharged into Hongshui River. According to the data of Qianjiang Station of Hongshui River, the upstream catchment area of this station is 128 165km2, with the highest water level elevation of 85.44m and the lowest water level elevation of 57.47m, and the water depth in dry season is generally 35 ~ 45m. Guangxi Waterway Engineering Bureau measured the river near Qianjiang River, and the maximum depth of the riverbed reached -30m above sea level, and the maximum depth in dry season could reach 80 ~ 90m. The annual average flow is 2.1×103m3/s, the runoff modulus is16.5l/(s km) 2, and the flow is (2.22×102 ~/kloc-0). With the construction of Ertan Hydropower Station on Hongshuihe River, the outlet of Mashan karst underground river system is submerged by Hongshuihe River, so it is impossible to obtain hydrological data of Mashan karst underground river system.

7. 1.3 geological background

7. 1.3. 1 formation lithology

The strata in Mashan karst underground river basin are mainly Devonian and Carboniferous (Table 7.1; Figure 7.3).

Table 7. 1 Overview of Strata Distribution in Mashan Karst Underground River Basin] 1]

7. 1.3.2 regional geological structure

Mashan karst underground river basin is located in Du 'an-Mashan structural subregion and northwest of Damingshan structural subregion, which is a NW-trending compression zone, mainly composed of dense folds and regional large faults (Table 7.2; Figure 7.3).

Table 7.2 Introduction to Main Geological Structures of Mashan Karst Underground River Basin [1]

Study on the particularity of groundwater and environment in karst area

1- underground river; 2- boundary of underground river basin; 3- Stratigraphic boundary; 4- Normal fault; 5- Reverse failure; 6- rivers; 7- Underground river skylight; 8- the seat of the county government; 9- Township; 10-Lower Permian Maokou Formation; 1 1- Lower Permian Qixia Formation; 12- Upper Carboniferous; 13- Middle Carboniferous; 14- Lower Carboniferous Datang Formation; 15- Lower Carboniferous Yan Guan Formation; 16-Upper Devonian; 17- Upper member of Middle Devonian Dongling Formation; 18- Middle Devonian Dongling Formation

7. 1.4 Karst hydrogeological conditions and chemical characteristics of karst water

According to formation lithology, lithologic combination characteristics, karst development degree and water-bearing characteristics, carbonate rocks in Mashan karst underground river basin can be divided into two aquifer groups, namely thick layered limestone and dolomitic limestone aquifer group, which are mainly composed of D3, C3, P 1m and other rock groups. The medium-thick limestone aquifer group is mainly composed of D2, C2, T2, C 1y and P1q. See Table 7.3 for the division and hydrogeological characteristics of each aquifer group.

Table 7.3 Division of Karst Aquifer Groups in Mashan Karst Underground River Basin [1]

sequential

During the circulation and migration of karst groundwater, the chemical composition and ion concentration of water change with the change of action conditions through the interaction with surrounding rock media. Under certain geographical, geological, hydrogeological and geochemical conditions, specific hydrochemical characteristics are formed. Its formation is mainly leaching and mixing, and its strength is related to the properties of solution (surrounding rock medium), solvent viscosity and migration speed. The main factors affecting the hydrochemical characteristics of karst groundwater are: atmospheric precipitation, surface water, topography, soil, vegetation and rock medium. All of them can directly or indirectly affect the formation of groundwater chemical composition and cause the change of hydrochemical composition.

Mashan karst underground river basin is an open geochemical system, with warm and humid climate and mutual transformation of surface water and groundwater. Through the investigation of karst groundwater exposure and occurrence conditions, hydrochemical sampling and testing, combined with the analysis of hydrochemical genetic conditions, the hydrochemical characteristics of Mashan karst underground river basin are shown in Table 7.4.

Table 7.4 Hydrochemical Characteristics of Mashan Karst Underground River Basin [1]

Fig. 7.4 Distribution of Main Cations at Water Quality Observation Points in Mashan Karst Underground River Basin

Fig. 7.5 Distribution of Main Anions at Water Quality Observation Points in Mashan Karst Underground River Basin

According to the composition characteristics of karst groundwater, most of the karst groundwater in Mashan usually belongs to HCO 3-CA type water. Generally speaking, on the one hand, the formation of hydrogeochemical characteristics of Mashan karst underground river system is related to the formation conditions of karst water in peak-cluster depressions and peak-cluster valleys; On the other hand, it also reflects the material basis and environmental conditions of Mashan karst underground river basin.

7. 1.5 Structure and Function Analysis of Mashan Karst Underground River System

7. 1.5. 1 development and distribution of Mashan karst underground river system

Mashan karst underground river system consists of two main tributaries and trunk lines (Figure 7.6), with a total length of 50.5km. The outlet of Mashan underground river system is located on the floodplain at the downstream of Bailongtan Hydropower Station 1km, and water flows out only in flood season, and the rest of the time is cut off (before the construction of Hongshuihe multi-stage hydropower station). The exit is a double-layer channel structure. The lower outlet is located below the water level of Hongshui River in dry season, so it cannot be observed, and the upper outlet is only exposed in dry season. With the construction of Hongshuihe multi-stage hydropower station, the double outlet of Mashan karst underground river system is submerged by Hongshuihe, so it is impossible to obtain the monitoring flow data directly. It is necessary to calculate the flow of the discharge port through the data of other observation stations. In addition, there is a flood discharge channel on the west side of the upstream trunk of Mashan karst underground river, which flows westward into Mashan Valley. The sewage outlet is located in the upper reaches of the river basin, and its scale is small. Only a small amount of water is discharged during the flood season, and finally it is injected into the Hongshui River through the Girl River. Because the displacement is very small, the displacement is ignored in the later research for the convenience of calculation.

Figure 7.6 Hydrogeological Map of Mashan Karst Underground River Basin

D2d2-Middle Devonian Dongling Formation; D2d 3—— Upper member of Dongling Formation of Middle Devonian; D3— Upper Devonian; C1y-lower carboniferous Yan Guan formation; C 1d—— Lower Carboniferous Datang Formation; C2- Middle Carboniferous; C3- Upper Carboniferous

(1) Jiajuanling tributary

Jiajuanling tributary originates from Carboniferous Yan Guan Formation (C 1y), and its lithology is mainly thin and medium-thick limestone, with karst development, large karst cave scale and deep water table. This tributary flows through the carboniferous Datang Formation (C 1d), and its lithology is similar to that of the carboniferous Yan Guan Formation (C 1y), mainly limestone, and the karst is well developed. Upper Devonian (D3) strata are widely distributed in the catchment area of this tributary, and its lithologic characteristics are thick layered dolomite and dolomitic limestone in the lower part and thick layered limestone in the upper part. In the process of development and distribution, this tributary is affected by the poor karst development of dolomite and dolomite limestone, and its distribution is hindered by its westward development. Finally, the tributary flows into the lower reaches of Mashan karst underground river in Wang He.

Jiajuanling tributary is located in the mountainous area with high karst peaks and clusters, and there are few valleys and small scale along the way. The tributaries are developed along Carboniferous strata, with good karst development conditions and few water outlets, and only a small skylight is exposed in the upper reaches. In dry season, the buried depth of water level is between 30-60m, and the hydraulic gradient is about 4. The form of water flow is mainly undercurrent, and there is no alternation between open and undercurrent.

Jiajuanling tributary is located in Fengcongwa area, with less dew point and deep water level. There is only one water quantity observation station-Guben underground river outlet flow observation station within the catchment range of this tributary.

(2) Tributary of Guzhai

The tributary of Guzhai originates from Donggangling Formation (D2d2) of Middle Devonian in the north of Daming Mountain, and its lithology is mainly medium-thick layered limestone and dolomitic limestone. The Devonian Donggangling Formation on the east and west sides of Daming Mountain is characterized by limestone deposits decreasing southward, mainly shale and siliceous shale mixed with siliceous rocks. The tributaries of Guzhai developed to the south first and then to the west. The main reason is that the lithology of Devonian strata here is mainly thick layered dolomite and dolomitic limestone, and the karst development is weak, which limits the distribution of tributaries of Guzhai, and some water overflows the surface, changing from underground undercurrent to open current. When the tributary of Guzhai flows through the Carboniferous Yan Guan Formation (C 1y) and Carboniferous Datang Formation (C 1d), because the lithology of this layer is mainly thin and medium-thick layered limestone, karst develops and water flows underground, forming an undercurrent. Finally, the tributary of Guzhai meets the trunk of Mashan karst underground river at Longhe.

There is a NW-SE reverse fault in the southeast corner of the tributary catchment area of Guzhai, along which a narrow valley of Treaty 10km is formed. The terrain in the valley is relatively flat and covered with a thin layer of soil. Guzhai tributary is located in the middle and upper reaches of Mashan karst underground river system, with only shallow rivers, mainly peaks and valleys, and many semi-closed valleys of different sizes along the way. The water intake point and cave are small in scale, the buried depth of water level is less than 10m, and the hydraulic gradient is about 10.

There are two skylight water level observation stations and Longhe skylight water level observation station, a water quantity observation station-Guzhai underground river outlet flow observation station, and a rainfall observation station-Guzhai rainfall observation station in the tributary catchment area of Guzhai-Luo Long.

(3) The trunk of Mashan karst underground river system.

The trunk of Mashan karst underground river system starts from the skylight of Longhe River at the confluence of tributaries of Guzhai, and flows into Hongshui River from south to north in Dawen. The trunk crosses the Longhe-Ganwan fault, develops in a smaller normal fault zone parallel to the west side of the fault, and finally flows into Hongshui River from south to north. The strata through which the trunk flows are mainly Middle Carboniferous (C2) and Upper Carboniferous (C3), and the lithology is mainly medium-thick layered limestone, dolomite limestone and dolomite. There is a deep channel in the spillway from the skylight of Longhe River to Mashan Valley, but there is no obvious pipeline from the skylight of Longhe River to Wanghe River. According to connectivity test, the velocity of groundwater movement in this section is 94m/h[2].

There is a deep ditch in the downstream section below Wanghe Tianguang, and the dew points along the way are all shallow ditch water points, and there is water in dry season or only in dry season. The characteristics of the development of shallow and deep double-layer pipelines in Mashan karst underground river basin are as follows: the drainage channel of Mashan karst underground river system and the double-layer channel of the main downstream reach near Hongshui River are obviously developed, and only shallow pipelines are generally developed in other reaches.

7. 1.5.2 watershed boundary conditions

The northwest side of Mashan karst underground river basin is Hongshui River, which receives all the water of Mashan karst underground river system, so the northwest side of the basin is river (flow) boundary; The western part of the basin is adjacent to the Guniang River, which flows through the Mashan Valley from south to north and flows into the Hongshui River, so there is a surface watershed on the west side of the basin. The southwest of the basin is adjacent to the clastic rock distribution area, so the southwest of the basin is the water-resisting boundary between karst area and clastic rock area. There is a groundwater watershed between the southern part of the basin and the ancient zero basin; The east of the basin is adjacent to Dalongdong karst underground river system, so there is an underground watershed on the east side of the basin. The northern part of the basin is a relatively water-tight boundary (Figure 7.6).

To sum up, the boundary of Mashan karst underground river basin is relatively complete and the boundary of surface basin is clear; The boundary of the underground basin is long, so the underground basin will not move with the change of season or precipitation. The rest are relatively stable water-resisting boundaries and relatively water-resisting boundaries. The recharge source of water resources in Mashan karst underground river basin basically comes from all kinds of recharge (mainly rainfall) at the upper boundary, and there is no lateral recharge and basically no overflow recharge at the boundary.

7. 1.5.3 Characteristics of recharge, runoff and discharge in the basin

(1) replenishment conditions

The main water supply source of karst underground river system is atmospheric precipitation. Its recharge methods mainly include: after atmospheric precipitation falls to the ground, part of it is intercepted and consumed by topsoil, filled depressions and vegetation; The other part is directly poured into the ground or slowly infiltrated into the ground in the form of slope flow (when rainfall is stronger than infiltration speed) through surface karst channels (karst ditches, karst troughs, tiankeng, underground river skylights, cracks, etc.). ) to supplement karst groundwater. The recharge of karst groundwater is restricted by topography, fault structure, karst development degree, soil vegetation, precipitation and rainfall intensity. According to predecessors' work, the average precipitation infiltration coefficient is 0.443 in areas with strong karst development (C2, C3 and D3) and 0.348 in areas with medium karst development (C 1y, C 1d and D2).

(2) Runoff conditions

Restricted by the types of karst water-bearing media and topographic conditions, karst groundwater has different runoff patterns under the comprehensive influence of various conditions. There are two main runoff modes of karst groundwater in this basin: dispersed seepage and concentrated pipe flow.

1) dispersed seepage. When the rainwater reaches the ground, it not only meets the requirements of plant interception, but also penetrates into the vadose zone to replenish karst groundwater. Its runoff direction moves from high to low according to the terrain. Karst groundwater has no fixed water surface, and the groundwater level changes greatly, but it is basically consistent with the slope of the terrain, the runoff path is short, and the recharge area is basically consistent with the runoff area.

2) Centralized pipe flow. It mainly occurs in karst aquifers with strong karst development. Rainwater and surface runoff are directly poured into the ground through surface karst channels, such as tiankeng, funnel and underground river skylight. Groundwater is discharged centrally along karst cracks and pipelines, and its velocity depends on the size of recharge water, the shape of pipelines and the slope of pipeline floor.

When karst groundwater is controlled by water-bearing media such as karst fissures, it always moves along karst fissures, with slow flow rate, stable flow rate and small dynamic change, and the water flow presents a slow linear laminar flow pattern; However, karst groundwater flows in steep slopes and large tunnels in karst caves. The groundwater velocity is quite fast, the flow rate changes dramatically and is extremely unstable. From the instantaneous flood peak to the outlet of karst underground river, there is a rapid runoff discharge, and the water flow presents a nonlinear turbulent flow pattern.

(3) Excretion conditions

Hongshui River flows through the northern part of Mashan karst underground river basin from west to east, which is the lowest discharge datum, and most of the karst groundwater flows into Hongshui River by centralized pipe flow. At the same time, controlled by the structure, the strike of rock strata in the basin is mainly north-south, which also provides favorable geological conditions for the discharge of karst groundwater to Hongshui River.

7. 1.5.4 Karst development law and its controlling factors in the basin

Karst development is restricted by many factors, but the most important factors are lithologic structure characteristics, geological structure and hydrogeological conditions. The karst development in Mashan karst underground river basin has the following characteristics.

(1) Karst development is obviously influenced by lithology.

Except for a small amount of clastic rocks in the southwest corner of the basin, carbonate rocks are most widely distributed in the basin, and karst development is obviously affected by lithology. The exposed strata in Mashan karst underground river basin mainly include middle-upper Devonian and middle-upper Carboniferous. According to the results of rock sampling analysis (Table 7.5), the ratio of CaO/MgO in limestone is much higher than that in dolomite, and the ratio of CaO/MgO in carbonate rock distribution area can reflect the intensity of karst development, so the karst development in limestone distribution area of Mashan karst underground river basin is strong, followed by dolomite distribution area.

Table 7.5 Statistical Results of Petrochemical Composition Analysis in Mashan Karst Underground River Basin [1]

(2) Karst development is influenced by structure.

Mashan karst underground river basin is located in Damingshan structural sub-region, and the fault structure in the northern part of Damingshan anticline is relatively developed, among which the Yang Xu-Mashan compression zone in the northwest direction is relatively large, crossing the Hongshui River from south to north. Affected by this structure, the trunk of Mashan underground river extends from south to north, and the direction is basically consistent with the regional tectonic line.

(3) The development of Mashan karst underground river system is restricted by regional hydrogeological conditions.

Mashan karst underground river system is located in Hongshui River basin, all the water is discharged into Hongshui River, and the groundwater in the basin moves from south to north. Hongshui River, as the lowest drainage datum in the basin, has extremely low water level in dry season and deep river cutting depth, which leads to the increase of hydraulic gradient of karst underground river system and the formation of deep-buried karst underground river. At the same time, the development scale of karst underground rivers and the collection degree of karst groundwater have also increased accordingly.

With the construction of Hongshuihe multi-stage hydropower station, the water level of Hongshuihe River rises, flooding the outlet of Mashan karst underground river system, resulting in the rise of discharge datum level of Mashan karst underground river system, the decline of hydraulic gradient, and the slow movement and collection speed of karst groundwater in the basin.

(4) Karst types are related to karst development.

Mashan karst underground river system is developed in areas dominated by peak-cluster depressions and peak-cluster valleys. The karst development in the peak-cluster depression is strong, the karst caves and sinkholes are densely distributed, and the vertical development of karst is strong. The karst underground river system in Fengcongwa area is deeply buried. The anisotropy of karst development in peak-cluster valley is obvious. Along the long axis of the river valley, the lithology of the distribution area is mostly dolomite, and the vertical depth of karst development is not large. The buried depth of karst underground river system in the peak-cluster canyon area is generally shallow, and the skylights developed along the underground river are mostly dissolved pools and wells. Different types of areas have different degrees of karst development. Generally speaking, the karst development depth of peak-cluster depression is greater than that of peak-cluster valley, while the lateral development degree of peak-cluster valley is greater than that of peak-cluster depression.