First, the ground collapses.
(1) allocation
According to the formation conditions, ground collapse can be divided into three types: karst ground collapse, mining ground collapse and Quaternary ground collapse, among which karst ground collapse is the most harmful. The karst ground collapse disasters in Hebei Plain mainly occur in Tangshan City, the water source of Liujiang Basin in Qinhuangdao City, Handan City and the western karst areas of Xingtai City, where the buried karst groundwater and overlying Quaternary pore water are strongly exploited. There are more than 1 10 karst collapses in Tangshan city, of which about 28 are caused by draining water from mineral deposits and exploiting groundwater. Since 1980s, karst collapse has been further developed due to the massive exploitation of karst water and overlying Quaternary pore water, with an influence range of 20km2. During the investigation of Liujiang Basin 1983 ~ 1985 water source in Qinhuangdao City, there were 9 karst collapses. After 1988 water source was put into operation, there were 286 karst collapse pits and 2 ground fissures, which caused damage to two middle school houses in six villages, resulting in direct economic loss of 1.5 million yuan (Yao, 1994). According to the present situation of karst ground collapse in Hebei plain, it can be concluded that the shallow buried karst development zone distributed in Hebei plain is the environmental geological effect of karst ground collapse.
(B) Analysis of influencing factors
The exploitation of groundwater, the discharge of sedimentary water and earthquakes will all induce karst collapse. According to preliminary statistics, there are more than 80 karst collapses caused by earthquakes in Tangshan City, and about 5 due to groundwater exploitation (Duan et al., 1993). Therefore, it can be considered that karst ground collapse is mainly determined by natural factors such as geological structure and earthquake, and human exploitation of groundwater is only one of the inducing factors, with relatively little influence. However, karst ground subsidence caused by groundwater exploitation often occurs in some important areas, such as water sources and urban areas, which will cause great harm. Therefore, in the process of groundwater exploitation, the monitoring of land subsidence should still be strengthened.
Second, land subsidence.
(A) distribution and cause analysis
Since the 1970s, groundwater in Hebei plain has been seriously over-exploited, especially in the deep-water funnel area from the central and eastern plain to the coastal plain, where a large area of land subsidence has occurred, and the subsidence speed and scale have been increasing. The area where the cumulative settlement exceeds 100mm is 36,000 km2, accounting for about 50% of the plain area. The area with subsidence greater than 600 mm is 5,000 square kilometers, accounting for 8%, mainly distributed in Cangzhou-Jizhouheng groundwater compound funnel area. The sinking speed is accelerating day by day. For example, in Cangzhou City, 1970 ~ 1986 occurred 9mm settlement, with an average settlement rate of 45.9mm/a, and 1986 ~ 1990 was 96.8mm/a .. Hengshui City1970 ~/.
The Quaternary loose sediments in eastern China are thick and the muddy soft soil layer is shallow, which belongs to the sensitive area of environmental geological effects of land subsidence. The lithology and structural characteristics of strata are the internal factors of land subsidence. However, in the large-scale exploitation of deep confined water in recent 20 years, the sharp drop of water level constitutes an extremely sufficient external condition for land subsidence.
(2) Analysis of influencing factors and trends
The land subsidence in Hebei plain is the result of regional tectonic subsidence, Quaternary geological conditions, seismic activity and other natural factors, as well as human activities such as groundwater overexploitation and oil and gas exploitation. In different regions and different time periods, the effect is different. For example, the ground heave in the process of ground subsidence caused by over-exploitation of groundwater (table 16-6 and figure 16- 1) shows that in the subsidence area induced by groundwater exploitation, natural factors still dominate in a certain period of time.
From 1968 to 1972, with the formation of the groundwater level drop funnel in Handan city, the settlement speed is accelerated and the scope is expanded. The settlement speed in general areas is 8 ~ 3 1.5 mm/a, which exceeds the settlement background value. The rate of land subsidence around the funnel is 4.25mm/year, which is obviously controlled by the decline of groundwater level. 1972 ~ 1976, bombs were loaded on the ground in Handan city1~ 21mm (figure 16- 1), which may be related to the Tangshan earthquake./kloc
Figure 16- 1 Handan Land Subsidence Profile
Figure 16-2 Relationship Curve between Land Subsidence and Groundwater Level in Handan City
The land subsidence in Cangzhou City was discovered from 1970, and the groundwater exploitation began from 1965, and the exploitation amount increased year by year. Therefore, land subsidence and groundwater exploitation did not happen at the same time, and it was lagging behind in the early (1960s). In 1970s, the amount of subsidence was roughly proportional to the amount of exploitation, and with the increase of exploitation, the ground continued to sink. After 1980s, although the exploitation amount was basically fixed, the water level in the center of the funnel declined and rose, but the ground continued to decline (Table 16-6).
Table 16-6 Ground Deformation Data
(According to Ren Rong 199 1)
To sum up, for the land subsidence in Hebei Plain, from the observation of land subsidence to the present situation, it can be generally considered that human activities, especially the over-exploitation of groundwater, are the leading factors affecting land subsidence. But we should pay attention to: ① the main control of natural factors in a certain period; (2) the lag of land subsidence relative to the decline of groundwater level. That is to say, even if the groundwater level no longer drops, the land subsidence will continue for a period of time, and the length of time is mainly controlled by internal factors such as lithology and rock thickness.
Third, seawater intrusion.
(1) Distribution and reasons
Seawater intrusion generally refers to the phenomenon that over-exploitation of groundwater makes the groundwater level lower than the sea level, which makes seawater replenish groundwater along the aquifer, resulting in salinization of water quality. The main areas of seawater intrusion in Hebei Province are Zaoyuan Belt in Funing County, western Qinhuangdao, Songkashan-Xiaobaozhai-Binhai Forest Farm triangle, with an invasion area of about 50km2 and an invasion rate of 16 ~ 22m/a (Yao, 1994). In 1960s, seawater intrusion in Zaoyuan water source area was very small, limited to both sides of the coastline. At that time, the pumping capacity was 6.5438+0.25 million m3/a.. After the 1960s, the production increased year by year, and 199 1 year reached 8.97 million m3/a, which greatly reduced the groundwater level and formed a water level drop funnel centered on Zaoyuan. A large number of pumping changes the natural flow field of groundwater, resulting in a large amount of seawater being replenished from three sides along aquifers and rivers in the south of the water source area, which leads to the gradual deterioration of water quality in the water source area.
After the Qing-Qin water transfer project199/kloc-0 was put into operation in July, the water supply output of Zaoyuan water source began to decrease, but the seawater intrusion area and Cl- content were still not much different from those of 1986, and the Cl- content of1255 mg was sampled and analyzed in March, 1992. According to this, it can be considered that seawater and fresh groundwater keep a dynamic balance at the boundary, groundwater often moves to the sea side, while groundwater decreases and the boundary moves to the land. In the natural state, there is a moving range, and human exploitation of groundwater can break this dynamic balance. There are 4.63km2 seawater intrusion areas in Songka River area and coastal forest farm south of Songka Mountain, which are non-exploitation areas of groundwater. /observation well kloc-0/9 1990, about 2.5km away from the sea surface, with TDS of10 ~16g/l; The Cl- content is 4007.8mg/L, while the TDS of observation well No.55, which is about 0.5km away from the sea, is 5. 1g/L, and the Cl- content is 2 136.4mg/L, which is contrary to shallow seawater intrusion. It is considered by geophysical prospecting and electrical sounding that it belongs to deep seawater intrusion, and seawater invades along the structural fracture zone, and its equilibrium surface is only affected.
(2) Analysis of influencing factors and trends
According to the above-mentioned seawater intrusion, the reasons can be summarized as follows: ① the loose aquifer and structural fracture zone connected with seawater in the offshore area constitute the channel for seawater to invade inland groundwater; (2) Overexploitation of groundwater: When the groundwater descending funnel extends to the coast or the edge of the river ditch, seawater will flow backwards; (3) Water conservancy projects: Because water conservancy projects reduce the inflow of river water and sediment, seawater can flow upstream along the river at high tide, and at the same time, water conservancy projects reduce the recharge of river water to groundwater in the estuary area; (4) Greenhouse effect: Rising sea level and decreasing groundwater recharge are the external environments that cause seawater intrusion. Seawater intrusion also occurs in the sensitive area of seawater intrusion environmental effect and is controlled by its natural conditions. In the alluvial-diluvial fans in the lower reaches of Tanghe River, Yanghe River and Daihe River, there is an obvious hydraulic connection between groundwater, surface water and seawater because the underground aquiclude is mostly distributed in a strip shape and does not form a complete aquiclude. This hydrogeological condition makes it a sensitive area for the environmental effect of seawater intrusion. Groundwater exploitation in Zaoyuan, Qinhuangdao is the main inducing factor of local seawater intrusion. At present, the scale of invasion is mainly related to the intensity of groundwater exploitation. In Songkashan area, it is mainly caused by natural conditions and influenced by natural factors such as tides and greenhouse effect.
Although over-exploitation of groundwater is the main cause of seawater intrusion, there are many uncertain factors in its occurrence and development. Here, it is regarded as a grey system, and the scale development trend of seawater intrusion in Qinhuangdao city is predicted by using grey system theory. The forecast results (table 16-7) show that if the water supply of the water diversion project from Qinghai to Qin is normal, the seawater intrusion in Qinhuangdao area will decrease year by year under the current mining intensity.
Table 16-7 Seawater Invasion Scale
At present, there is no seawater intrusion in other areas of Hebei Plain, such as Cangzhou, but it is not excluded that the quantitative change will become qualitative change under the influence of human activities, especially a certain degree of quantitative change will become a sensitive area for seawater intrusion under the influence of natural disasters such as earthquakes. Therefore, it is still necessary to control groundwater exploitation and strengthen monitoring to avoid the harm of seawater intrusion.
Four. abstract
Because predecessors have done a lot of work on the above-mentioned geological disasters in Hebei Plain, this book briefly quotes the research results of predecessors and puts forward the concept of geological disaster sensitive area under the influence of human activities. The large-scale exploitation of groundwater has produced different types of disasters in different areas of Hebei Plain, which shows that the same human activities will have different performances in different sensitive areas of environmental effects, and also shows that natural geology and geographical conditions are the main controlling factors of geological disasters in Hebei Plain, while human activities are only inducing factors. In a certain period (from the date of geological disaster observation to the present), human activities are important influencing factors, but it does not rule out that in a smaller period, natural factors will still be the main influencing factors in some areas, such as ground uplift in land subsidence caused by groundwater exploitation. Therefore, it is necessary to analyze the specific environmental impact from different time periods and different regions.
Discussion: ① Land subsidence, land subsidence and seawater intrusion are direct disasters caused by human exploitation of groundwater. Although predecessors have done a lot of work, further work is needed to reduce the losses caused by disasters; (2) The groundwater drop funnel caused by groundwater exploitation, as well as land subsidence and land subsidence have caused changes in the terrestrial water circulation system, which should be paid attention to.