(Anhui Geological Survey Institute, Hefei 23000 1)
This paper discusses the hydrogeological conditions, the history and present situation of groundwater exploitation, the allowable groundwater exploitation and over-exploitation status, the causes, present situation and characteristics of land subsidence, and the harm caused by land subsidence.
Keywords: aquifer; Groundwater dynamics; Groundwater exploitation; Allowable exploitation amount; land subsidence
Fuyang City, Anhui Province is located in the west of Huaibei Plain, with a flat and open terrain, with an altitude of 26 ~ 33m. The terrain is generally high in the northwest and low in the southeast, with an average ground slope of about 1/8000. Geomorphological genetic types are alluvial plain and alluvial-denudation plain.
1 regional geological environment conditions
1. 1 layer
The regional stratum belongs to the North China Stratigraphic Zone, Huaihe River, stratigraphic regionalization and Fuyang Stratigraphic Zone. There are Huoqiu Group and Wuhe Group in Upper Paleozoic, Cambrian, Ordovician, Carboniferous and Permian in Paleozoic, Cretaceous in Mesozoic and Paleogene, Paleogene and Quaternary strata in Cenozoic.
This area is mainly covered by Quaternary and Tertiary loose layers, with a thickness of about 800 ~ 900 m, of which the thickness of Quaternary is about130 ~150 m..
1.2 Hydrogeological characteristics
Groundwater in Fuyang city is a single loose rock pore water. According to its burial conditions, hydraulic characteristics and its relationship with atmospheric precipitation and surface water, it can be divided into shallow groundwater and deep groundwater from top to bottom. Shallow groundwater occurs in Holocene and Late Pleistocene strata, with a depth of 50m, which is closely related to atmospheric precipitation and surface water, and can be called the first aquifer group (shallow layer) according to the up-down relationship. Deep groundwater exists in the stratum below 50m, which is not closely related to atmospheric precipitation and surface water. According to the hydrogeological structure and the current exploitation situation, the deep groundwater is divided into two aquifer groups, namely the second aquifer group (middle and deep, with a buried depth of 50-150-500m) and the third aquifer group (deep, with a buried depth of 1 50-500m (Figure1)).
1.2. 1 first aquifer group
It is mainly composed of Pleistocene series and Holocene series, widely distributed in the whole region, with a buried depth of 50 m, which is closely related to atmospheric precipitation and surface water. The buried depth of the water-bearing sand layer is 4.00 ~ 17.6 m, and the buried depth of the floor is 7.5~48.54m m. The lithology is mainly grayish yellow and brownish yellow silty sand with loose structure and good sorting. The thickness of sand layer is controlled by ancient river channel, and the maximum thickness of sand layer in ancient river channel can reach 16m. Single well water inflow 147.74 ~ 2578.63m3/d, water level buried depth 1.05 ~ 4.97m ... The hydrochemical type is calcium-magnesium type, and the salinity is less than1g/L.
The groundwater in this layer is mainly replenished by atmospheric precipitation. From northwest to southeast, the water level varies from 1 ~ 2m. Evaporation is the main way of excretion.
1.2.2 second aquifer group
It is mainly composed of the middle and lower Pleistocene series of Quaternary, with the buried depth of water-bearing sand layer of 49.68 ~ 100.85 m and the buried depth of floor118.00 ~147.00 m. The lithology is mainly grayish yellow, brownish yellow, blue-gray fine sand, silty fine sand and medium. Its structure is loose and its sequencing is good. Generally, there are 4 ~ 1 1 layer, the cumulative thickness is18.20 ~ 38.1/m, and the water inflow of a single well is 761.00 ~ 2556.97m3/d. hydrochemistry.
Figure 1 Schematic Profile of Fuyang Aquifer Model (according to Anhui First Hydrogeological Engineering Geological Team 1990)
The cohesive soil layer distributed in this aquifer group has weak water content, poor water permeability and strong compressibility, which is the main compression layer that causes land subsidence in Fuyang City.
1.2.3 third aquifer group
It consists of the upper strata of the Late Tertiary. Buried depth150 ~ 500m. The buried depth of water-bearing sand layer is 147.50 ~ 175.70 m, and the buried depth of floor is about 500m. The lithology is mainly bluish gray, grayish white, grayish yellow medium sand, medium fine sand, fine sand and silty sand, with loose structure and general sorting. * * 5 ~ 9 layers are developed, and the cumulative thickness of sand layer is 28.32 ~ 100 meters. The water inflow of a single well is1514.49 ~ 3,570.00 m3/d, the hydrochemical type is HCO-3Na, and the salinity is about1g/L. ..
In the natural state, deep groundwater is mainly replenished by lateral runoff. In the mining state, the main sources of groundwater recharge are regional lateral recharge, vertical overflow recharge and water released by cohesive soil compaction. Due to the large-scale exploitation of deep groundwater in Fuyang city, the groundwater level has been declining.
1.3 engineering geology and compressive layer
According to the previous research results, the main physical and mechanical properties of cohesive soil in this area, such as natural void ratio and compressibility, decrease with the increase of depth from top to bottom, and the compressibility of soil gradually decreases. The permeability of soil directly controls the consolidation rate of soil. In the natural state, the permeability of soil becomes worse from top to bottom. According to the measurement results of pore water pressure, the hydraulic gradient vector direction of each cohesive soil points to the water-bearing sand layer in the mining state.
According to geological age, lithologic combination and physical and mechanical properties, 250m shallow soil is divided into four engineering geological layers. From top to bottom:
1.3. 1 The first engineering geological stratum: engineering geological stratum of cohesive soil (plastic) and sandy soil (g1);
1.3.2 second engineering geological stratum group: engineering geological stratum group of cohesive soil (hard plastic) and sandy soil (G2);
1.3.3 third engineering geological stratum: engineering geological stratum of sandy soil and cohesive soil (hard plastic) (G3);
1.3.4 fourth engineering geological stratum group: micro-diagenetic cohesive soil (hard) and sandy soil engineering geological stratum group (G4).
The shallow soil at 156m can be further divided into 13 engineering geological layers, including 6 compressed layers, 6 water-bearing sand layers and 1 hard soil layer (see Figure 2).
Fig. 2 Comprehensive histogram of land subsidence
1.4 Dynamic characteristics of groundwater
1.4. 1 shallow groundwater dynamics
The shallow groundwater in this area is only mined sporadically, which is mainly used for agricultural irrigation in suburbs and domestic water in urban fringe, and the dynamic influence of mining water level is not obvious. Precipitation is the main factor affecting the dynamics of shallow groundwater in this area, which is characterized by typical infiltration and evaporation.
The buried depth of shallow groundwater is generally 1 ~ 3m, and the water level has obvious periodic changes for many years, and the water level fluctuation is generally 1 ~ 2m. Annual change 1 April, the water level changes little, but it is buried deeply (generally 2 ~ 3m, and it can reach 4m locally), and1month has the lowest water level. The water level gradually rises from May to August, and reaches the peak in August and September, with the buried depth of about 1m, and gradually falls back from 10 to 12.
The water level changes of Heying River and Quanhe River in the area have no obvious influence on the groundwater level on both banks. Only in the reach above Heying Gate, the water level will be raised by closing the gate from May to 65438+February, and Heying River will replenish the groundwater in the dike in flood season. The shallow groundwater level on both sides of the river increased slightly.
1.4.2 groundwater dynamics in the middle and deep layers
In the early 1960s, the confined water level of reclaimed water (the second aquifer group) in urban areas was 0. 19m higher than the surface, showing a natural dynamic type. After the sixties and seventies, the water level decreased year by year. According to the data analysis, the annual decline of water level is 1 ~ 3m. After centralized mining in the urban area, the maximum buried depth of the groundwater level in the second aquifer group has reached 72.3m, and the water level has a downward trend, with a fluctuation range of1~ 2 m. Each level has a peak period and a low period every year. Generally, the water level is low from July to August, that is, the water level rises from 65438+February to February of the following year. The water level drops at a speed of 0.9 ~ 1.7m, and the annual drop of water level is small, generally less than 0. 1m from the center of the funnel. The maximum buried depth of the water level of the third aquifer group also reached 72.14m; ; The dynamic change characteristics of water level are basically consistent with those of middle and deep groundwater, but the annual water level decline is more than 2.5m, and the marginal decline is also about1m.
According to the observation data of regional flow field, the water level range of the second aquifer group below 20m is about 452km2, and the water level range of the third aquifer group above 20m is about 434km2, which expands outward at the speed of 30 ~ 50km2/a. In the mining center, the water level of the 400m deep well has reached 60 ~ 64m, and the current expansion speed of the funnel is relatively slow, about 5 ~10km2. ..
2 groundwater development status and existing problems
2. 1 Present situation of groundwater development and utilization
The development and utilization of groundwater in Fuyang city is mainly divided into three aspects: first, water supply companies concentrate on water supply; Second, enterprises and institutions exploit their own water sources; The third is to take shallow groundwater in rural areas.
2. 1. 1 centralized mining by water supply company.
At present, Fuyang Water Supply Company covers an area of 25km2 in the urban area, mainly in the west of Heying River (the old city), and its daily production is about 5 1 ~ 60000 m3.
2. 1.2 Self-provided wells of enterprises and institutions
There are 263 self-provided wells in enterprises and institutions in Fuyang City, which are self-contained, with a daily output of 62,700 m3, accounting for 55% of the city's total water supply. The total water supply is industrial water 1697 19000 m3, domestic water 156 15600 m3 and public works of 7.874 million m3.
2. 1.3 The daily exploitation of shallow groundwater in suburbs is 5.74 million m3, and the average annual water intake is about 1.57 million m3.
Groundwater exploitation in Fuyang city consists of centralized exploitation by water supply companies and decentralized exploitation by self-provided wells of various units. Fuyang waterworks was established in 1976, and water supply company was established in 1998. Two water distribution plants, Hexi and Hedong, exploit the second and third floors of groundwater as water supply sources. Before the factory was built, urban residents drank untreated river water and shallow groundwater. With the development of economy and the increase of population, the exploitation capacity and degree of groundwater have also increased rapidly. From a few wells in the 1950s to dozens in the 1970s, from 100 wells in the late 1980s to 275 wells in the middle and deep layers of the city. In March, 2004, Fuyang municipal government counted 263 middle and deep wells. Up to now, after more than 20 years' development, the water supply company has built 82 water wells and 3 secondary pressure stations, with the designed water supply capacity of 94,000 m3/d, annual water supply reaching 1.86 1.5 million m3/d, and the average daily water supply is 51.00 million m3/d, accounting for Fuyang City. At present, the groundwater in Fuyang city has formed a pattern of centralized exploitation by water supply companies and decentralized exploitation by self-provided wells of various units. In 2002, the production reached 5.7 million m3. See table 1 for groundwater exploitation in different years.
Table 1 Annual Exploitation Statistics of Middle and Deep Groundwater in Fuyang
2.2 Fuyang City Groundwater Exploitation History (Deep)
From 1950s to the end of 1970s, the middle groundwater was fully exploited, and the maximum water intake was 654.38+04.97 million m3/a, which was less than the exploitable resources of the middle groundwater.
In the mid-1980s, middle-level groundwater was mainly mined, and deep groundwater was laid, with an exploitation amount of about 20 million m3/a, which tends to be balanced with the exploitable amount of middle-level groundwater.
In the late 1980s, due to the lack of planning and unreasonable layout of water intakes, two high-intensity catchment areas were formed in the west and east of Heying River. At the same time, the deep groundwater level began to decline continuously, and the central groundwater formed a funnel area of water level decline, which induced environmental geological problems such as land subsidence. The number of deep wells and the exploitation of deep groundwater in cities have increased sharply, and the annual exploitation of groundwater has approached and exceeded 30 million cubic meters/year; In 1990s, the exploitation of middle and deep groundwater exceeded 40 million m3/a, which was greater than the exploitable resources of middle and deep groundwater. At the same time, the exploitation of middle groundwater is gradually changing to the exploitation of deep groundwater, which leads to the continuous decline of middle groundwater level and the intensification of land subsidence.
Due to mining, the deep groundwater level continues to decline (Figure 3), and the rate of water level decline is generally 1 ~ 2m/a, forming a large-scale groundwater level decline funnel. At present, the maximum buried depth of the central water level in the urban area has reached 89.0m, and the falling funnel range of the deep groundwater with a depth above 20m is greater than 550km2 (Figure 4), and the falling funnel range with a depth above 10m is greater than 1200km2. The water level in the descending funnel continues to drop. At present, the scope of the groundwater level drop funnel has been extended to the periphery of the city, and it is connected with the surrounding drop funnels such as Lixin and Taihe, forming a regional groundwater level drop funnel.
Fig. 3 Variation curve of deep groundwater depth in hole FC 810
With the continuous exploitation of deep groundwater, the amount of deep groundwater resources will continue to decrease, which will inevitably affect the urban construction and development of Fuyang City and the lives of urban residents. The difficulty and cost of groundwater exploitation will also increase accordingly.
2.3 Allowable exploitation of groundwater
According to 1989 ~ 19 1 Investigation and Evaluation Report on Water Environment Geology in Fuyang City, the annual maximum allowable exploitation amount of middle and deep groundwater in Fuyang is 24,072,520 m3, and the daily exploitation amount is 66,000 m3. Considering that the overlying weak permeable layer in Fuyang city is constantly compacted and consolidated with the land subsidence, the upper permeability becomes worse. According to the model calculation, the allowable annual exploitation amount of the middle and deep groundwater below 50m is 22.5 million m3/a, and the allowable daily exploitation amount is 66.5438+600,000 m3/a. ..
Fig. 4 Isogram of buried depth of deep groundwater level (65438+February 30, 2000)
2.4 Existing problems
2.4. 1 Centralized over-exploitation of middle and deep groundwater
At present, the water supply for residents' life, industrial enterprises and public utilities in Fuyang City is basically a single centralized exploitation of the middle and deep groundwater in the urban area, and the water supply has formed a pattern of single decentralized over-exploitation of the deep groundwater. There is a lack of unified management in the development and utilization of groundwater resources in urban areas. There are 27 deep wells per square kilometer in the area with high well point density, and the dense Lianhua Road deep well group reaches 25 wells per square kilometer, far exceeding the limit of 2 deep wells per square kilometer. According to the survey, the exploitation amount of Fuyang City was 57 million m3 in 2002, 50 million m3 in 2004, and the over-exploitation amount was about 30 million m3/a. According to statistics, the actual daily groundwater exploitation amount in Fuyang City exceeded 6,543.8+200,000 m3 (all middle and deep groundwater), and the over-exploitation coefficient was 654.38+0.74, which was a serious over-exploitation.
2.4.2 The water level continues to decline, and the descending funnel expands.
Due to blind over-exploitation of groundwater, Fuyang City has formed a large area of water level drop funnel. Since the mid-1980s, a large number of deep groundwater has been exploited, and the groundwater level has dropped rapidly. Long-term over-concentrated exploitation of middle and deep groundwater has caused a large-scale and substantial year-on-year decline in the middle and deep groundwater level in urban areas. At present, the underground water level drop funnel above 1.200 km2 has been formed, the confined water level in the middle layer of concentrated mining area has been reduced to 50-70m, and the static water level in the center of the funnel is 64mm. Compared with 65,438+0,968 before large-scale mining, the water level in the centralized mining area decreased by 40-67m, with an average decrease of 65,438+0,968+0,985. 1985 ~ 1999, with an average annual decrease of1.2 ~1.92m. After 1999, the water level rose slightly, but fluctuated greatly. It leads to the scrapping of deep wells and a great waste of energy and mechanical equipment investment. At present, the scope of the landing funnel is still expanding. Especially, the descending funnel of deep groundwater expands rapidly at a speed of 30 ~ 50km2/a (Figure 5).
Fig. 5 Isobaric map of groundwater in Fuyang City
3 the present situation of land subsidence and its harm
3. 1 land subsidence status
The lateral runoff of deep groundwater is very weak in natural state. The groundwater level drops sharply, resulting in a huge head difference, which makes cohesive soil compress and release water, which not only brings crisis to water supply, but also causes and produces land subsidence. The characteristics of land subsidence in Fuyang city are gradually formed and developed with the increase of deep groundwater exploitation, which is a dynamic process. Before the 1960s, the ground was not deformed due to the small amount of groundwater exploitation. In 1970s, the land subsidence was less than 100 mm; ; In 1980s, with the rapid increase of deep groundwater exploitation, the scope of land subsidence also expanded by more than seven times. During the period of 0980 1990 10, the range of land subsidence has exceeded Fuyang city. According to the leveling data, the urban land subsidence is a shallow oval funnel with the major axis NW-SE, about 25km, and the minor axis NE-SW, about 2 1.2km, with the maximum subsidence. From 1980 to 1990, the cumulative maximum settlement is 8 17.6mm, the settlement rate is 73.39mm/a, the cumulative settlement is 1990 ~ 1995, and the settlement rate is 25.48mm/a The maximum settlement of the center reached 14 18mm, and by 2002, the settlement of the center was 150 1.82mm (Table 2).
Table 2 List of settlement changes of some ground settlement monitoring points
3.2 Hazards caused by land subsidence
According to the investigation, the surface deformation in Fuyang is mainly manifested as linear engineering deformation and deep-buried engineering deformation, that is, bridge deformation and damage in settlement area and deep water well deformation. No other signs of deformation such as ground fissures were found.
Fuyang Heying Sluice is located in the center of the ground subsidence funnel in Fuyang City. No.4, No.6 and No.8 piers are cracked, and the crack width is 6 ~ 10 cm. After the reinforcement and transformation of 1998, a new tensile crack with a width of 0.3 ~ 0.8 cm was produced. The investigation found that surface deformation phenomena such as well tube uplift and ground cracking of well platform occurred in deep wells due to the exploitation of groundwater. Fuyang Clothing Factory (now a water supply company), located in the center of land subsidence, has a deep well, the well platform is obviously raised compared with the surrounding ground, and the ground of the well platform is cracked and the cracks are radial. Well pipe lifting has affected the normal use of the well, and the water pipeline must be bent before it can be used.
Deep water wells in textile mills, power supply bureaus, cigarette factories and other units are all near the centers of ground subsidence, well tube uplift and ground cracking of well platforms.
Although land subsidence is a slowly changing geological disaster, after years of development, it has directly or indirectly caused certain harm to the urban construction and economic development of Fuyang City, mainly as follows:
3.2. 1 Destroy water conservancy facilities and lower flood control standards.
Fuyang land subsidence center is located at the intersection of Heying River and Quanhe River in the city center. The control gate of Heying River and Fuyang River with a total length of 1.80 meters is also the main traffic artery of the city. Due to uneven settlement, the bottom plate of the gate cracked in many places, the pier was dislocated and the hinge seat was tilted. As a result, the opening and closing of the gate is ineffective, and the cracks in the door body widen year by year. 1995 to 1997, gate foundation reinforcement and gate renewal. From March 5438+0, 2006 to June 2002, the bridge was renewed. After the repair, new cracks appeared in Heying sluice, which seriously threatened the operation safety of the sluice. In 2003, the maximum flood discharge capacity of sluice decreased from 3500m3/s when 1959 was built to 2500m3/s, and there was a downward trend year by year. At present, the motorway on the gate has been closed and can only be used by pedestrians.
3.2.2 Reduce flood control standards and aggravate flood disasters.
The heights of Heying and Quanhe dikes in Fuyang city, which are located in the subsidence area, all decrease with the ground subsidence, and can not meet the original design flood control standard of once every 20 years. Once a major flood disaster occurs, it will endanger personal and property safety. The ground elevation of other subsidence areas is also lower than the river flood level 1 ~ 2m, and the land subsidence intensifies its flood disaster.
3.2.3 Destruction of municipal and water supply and drainage facilities.
Ground subsidence caused some deep groundwater exploitation wells to be tilted and dislocated, the well pipes to be relatively lifted, and the well platform to be cracked and deformed.
3.2.4 Destruction of roads, bridges and buildings
According to the investigation, cracks and expansion joints appear on the bridge deck of long-span buildings in Fuyang, such as Fuyu Bridge, Heying Bridge and Quanhe Highway Bridge. Some road sections cause dislocation and cracking of drainage pipes, and the sewage that could have been discharged smoothly is concentrated in low-lying places, causing urban waterlogging.
Fig. 6 Distribution map of land subsidence in Fuyang City Unit: mm
3.2.5 Destruction of Urban Surveying Control Network
The second leveling line around Fuyang, with Fuyang as the center, laid by the State Seismological Bureau, was disturbed by the ground subsidence, which affected the earthquake monitoring. There is a phenomenon of falling slope in the subsidence area, which seriously interferes with the prediction of warning water level in flood season.
4 conclusion
Due to the development of urban construction, in recent years, Fuyang concentrated over-exploitation of groundwater, which led to the decline of groundwater level, the formation of a large area of falling funnel, and the formation of land subsidence disasters. Its geological disasters are characterized by slow development, great harm and great difficulty in management. Therefore, in the future, it is necessary to establish a land subsidence network and a prevention and control system, rationally develop and utilize groundwater resources, and promote the coordinated development and unity of economic development, natural resources development and utilization, and geological ecological environment protection.
Study and solve the development and change law of land subsidence in Fuyang city, put forward prevention and control plans, and establish a reasonable groundwater exploitation model to serve the sustainable development of social economy and the harmonious coexistence of nature.
Overexploitation of groundwater and characteristics of land subsidence in Fuyang City, Anhui Province
Yang zedong
(Anhui Geological Survey Institute, Hefei 2300 1)
Abstract: On the basis of studying the hydrogeological conditions, the history and present situation of groundwater overexploitation and the allowable exploitation amount of groundwater in Fuyang City, this paper analyzes the present situation and characteristics of land subsidence in Fuyang City, the causes of land subsidence and its harm.
Keywords: aquifer; Groundwater trend; Groundwater exploitation; Permitted mining quantity; Surface collapse