Many areas in the world where groundwater is extracted have experienced a large and substantial continuous decline in groundwater levels. The essence of this is that in the entire aquifer or certain sections of the aquifer, the amount of groundwater extracted exceeds the amount of recharge for a long period of time, gradually depleting the permanent storage, and in a certain period of recharge cannot be recovered. When judging whether a regional groundwater level decline will occur in a certain area, special attention should be paid to the difference between it and the water level decline phenomenon caused by cyclical changes in precipitation recharge. For example, in the Shijiazhuang area, the amount of groundwater extraction exceeds its recharge, causing a sustained decline in the regional groundwater level that cannot be recovered. In the Baiquan area of Xingtai, although the groundwater level has been declining for many years due to cyclical changes in pumping, the water level can be recovered over a longer period of time, so it is not a problem of continuous decline in the regional groundwater level. The reasons for the continuous decline of regional groundwater level can be attributed to four aspects:
(1) The incomplete understanding of the regional hydrogeological conditions, especially the formation conditions of groundwater resources, the calculated allowable extraction is large, thus leading to the extraction of long-term greater than the amount of recharge, resulting in the continuous decline of regional groundwater level. This continuous decline in water level is generally most evident in the change in the lowest water level of the calendar year at the center of the regional water level decline funnel.
(2) The sustained and substantial decline of groundwater level caused by unreasonable exploitation. The so-called irrational exploitation is mainly caused by the "three concentrations" on the exploitation lot, exploitation level and time, as well as the anarchic state on the exploitation management. Sometimes, although the recharge of the entire aquifer is basically balanced with the volume of exploitation, the over-concentration and over-intensity of exploitation of certain localized areas or aquifer levels (or at a certain depth) will also result in a continuous and substantial decline in the water level of a localized area or aquifer. For example, there are five aquifers in the Shanghai Municipal District***, which are quite rich in groundwater storage. However, 87% of the mining volume is concentrated in Ⅱ, Ⅰ two aquifers; among them, 84% of the wells and 80% to 90% of the mining volume, but also concentrated in the two aquifers in Yangshupu, Hongkou, east of Shanghai and west of Shanghai, Putuo, Changning, Jing'an several industrial areas. As a result, a strong regional groundwater level decline is formed in the above sections of these two aquifers, forming the center of the regional groundwater level decline funnel. This is also the section that produces the most serious ground subsidence. In the periphery of the centralized mining area or Ⅳ, Ⅴ aquifer, groundwater level decline is not significant. Some water sources, due to excessive concentration of mining time caused by the groundwater level in a period of substantial decline, although not necessarily sustainable, but it will also affect the normal operation of pumping equipment, and bring other hazards. For example, agricultural irrigation wells, if its density is large, and in the dry years of the dry season centralized mining, it can cause the wells interfere with each other, and produce a substantial drop in water level, so that the amount of water out of the reduction, and even appear "hanging pump" phenomenon. Shanghai Municipal District, some for the summer (May to September) cooling and cooling with the wells, due to the extraction of a particularly large, resulting in the period of groundwater level dropped significantly, the interference between the wells intensified, the amount of water out of the reduction; at the same time, but also to increase the rate of ground subsidence.
(3) Decrease in groundwater recharge due to changes in man-made or natural factors, causing a decline in regional groundwater levels.
Due to man-made or natural reasons, the surface water flow, which is the main source of groundwater recharge, is reduced or even cut off, or the riverbed is silted up, resulting in a reduction in the amount of surface water recharge to groundwater. For example, in the Wuwei pre-mountain plains area, over the past two decades, due to the construction of water conservancy projects in the upstream mountainous areas, the amount of river water recharging the groundwater of the alluvial fan has been greatly reduced, resulting in the groundwater level in the area in front of the alluvial fan dropping by 3 to 10 m, and the flow of spring water in the overflow zone has been reduced by 30% to 70%. In some water sources near rivers, the groundwater recharge is reduced due to the decrease of river flow or the increase of days of broken flow, or due to the siltation of the riverbed and the deterioration of permeability, etc., and the resulting decline of the groundwater level is even more obvious.
First of all, due to the destruction of forest vegetation and other reasons, resulting in regional climate change, precipitation is reduced, the ground infiltration conditions deteriorate, so that the amount of recharge is less than the amount of extraction, causing the regional groundwater level to fall.
Secondly, within the same hydrogeological unit of the water source, due to the deep dredging of mineral deposits or other underground works, or due to the interception of new well clusters in the upper reaches of the water source, or the peripheral areas of wells to increase the depth of mining and other man-made reasons, can also cause a substantial decline in the groundwater level of certain water sources.
In addition, due to the exploitation of groundwater to increase the depth of groundwater in the region, the thickness of the air-packed belt increased, so that the infiltration of atmospheric precipitation to reduce the amount of recharge, but also to promote the regional groundwater level continued to decline. This effect is particularly pronounced in certain water sources that are recharged by precipitation infiltration when the depth of decline in the mined water table is greater.
Finally, with the intensification of human modification of the morphological nature of the ground, such as urban buildings and water-blocking pavement covering an increasing area, the former running water channel or catchment depression leveling, the improvement of the regional drainage system, etc., will affect the infiltration of precipitation to groundwater recharge.
(4) As a result of the development of economic construction, population growth and the improvement of living standards, the demand for water has increased significantly. People know that has exceeded the permissible amount of groundwater extraction and has formed a regional groundwater level continues to decline, but also to expand the amount of extraction, which is more to promote the rate of decline in the regional groundwater level, leading to a series of environmental and geological problems. Obviously, this is not reasonable. However, this situation is very common, and should attract great attention and take preventive measures.
2. Hazards of continuous decline of regional groundwater level
The substantial and continuous decline of regional groundwater level not only brings huge economic losses to the water source, but also generates various environmental geological problems. The main hazards are:
(1) Due to the decline of regional water table, the water yield of the water intake project is constantly reduced, and sometimes it is necessary to replace the pumping equipment in order to draw water, so that the cost of pumping is constantly increasing; in serious cases, even the wells are scrapped. Many large water sources and well irrigation areas, there is this problem. Shandong Zihe alluvial flood fan area, due to a large number of water withdrawals, 15 years the region's water table generally fell 10m, the largest up to 30m or more, so that the original more than 2,000 shallow wells and nearby springs are all depleted. The economic loss of scrapping machine wells, drilling deep machine wells and replacing water pumps amounted to more than 50 million yuan. In addition, the increase in power consumption, so that the cost of watering from the original 0.40 ~ 0.50 yuan per mu increased to more than 1.50 yuan, so that the whole irrigation district more than 200 million yuan per year electricity costs. Dawu water source in the area of groundwater level decline is greater, only in 1989 than in 1988 fell 21.75m, down to -1.0m below the maximum depth of 131.71m.
(2) due to the regional decline in groundwater level, can cause the ground subsidence, ground cracks and ground subsidence and other serious environmental and geological problems. Ground subsidence is a serious problem faced by many plain areas in the world where groundwater is extracted, especially coastal cities. The maximum values of ground subsidence in some areas are as follows: Long Beach, USA: 9.5m; Tokyo: 4.6m; Osaka: 2.88m; Mexico City: 6m; Shanghai: 2.37m. At least 36 large and medium-sized cities in China, such as Tianjin, Xi'an, Taiyuan, Suzhou, and Taipei, have successively experienced problems of ground subsidence or cracks and subsidence, etc. Tokyo, Bangkok, London, Venice, etc., are also facing serious environmental geological problems. Cities such as Tokyo, Bangkok, London and Venice are facing the danger of part of their downtown areas being flooded by seawater due to ground subsidence; cities such as Bangkok and Shanghai are not able to discharge urban sewage and rainwater in a timely manner due to ground subsidence, which often accumulates in the downtown area. Located in the United States of America, Arizona, between the city of Pinar and Miley Copa well irrigation area, as early as 1948 to 1967, the groundwater level fell 70 to 100m, the ground subsidence amounted to 1.2m (up to 2.5m). Uneven ground subsidence and accompanying ground cracks damaged the entire irrigation system, roads, railroads, aqueducts, etc. in the area. Recently, a number of ecologists and engineers have suggested that over-exploitation of groundwater was one of the causes of major earthquakes in certain areas (e.g., September 19, 1985, Mexico City).
There are many reasons for ground subsidence, currently internationally recognized as a result of the large amount of groundwater extraction, the water table has dropped significantly, prompting the upper easily compressed clay layer of pore water discharged, caused by the soil layer of the solidification of the compression, resulting in ground subsidence. According to Shanghai's ground subsidence and groundwater level and the amount of exploitation of all years of observation data can be seen, the amount of ground subsidence and the amount of groundwater exploitation and water level changes in the depth of the drop has consistency.
(3) In coastal areas, the substantial decline in the regional water table destroys the natural equilibrium conditions of salty and fresh water, causing seawater intrusion and deterioration of the water quality of the extracted aquifer. Such as the coastal areas of California, the United States, the groundwater level is still above sea level 7 ~ 23m, ten years, due to excessive groundwater withdrawal, the groundwater level fell to 5.2m below sea level, causing serious seawater intrusion, invasion of coastal 13 aquifers, so that the deterioration of irrigation water quality, most of the good land into saline and alkaline land. In the suburbs of Dalian, before 1964, the amount of groundwater extraction was not large, and basically there was no problem of seawater intrusion. Since then, due to the large amount of over-exploitation of groundwater, by the late 1990s, the groundwater level than in 1964 fell by 10-40m, resulting in serious seawater intrusion, the chlorine ion content of some water sources has been as high as 1,300mg/L.
(4) Due to the decline in the regional groundwater level, so that some of the famous karst springs dry up, destroying the spring landscape as a feature of the tourism resources. The famous Jinan Baotu and other four major springs, Huixian hundred springs in Henan and Taiyuan Jinci springs, etc., since the 1970s, due to a substantial decline in the regional water table, resulting in a substantial reduction in the spring flow and gushing potential, and even a long time of drying up and cutting off the flow, so that the springs of the tourism value of the ornamental value of the great reduction, and make the spring mouth of the diversion project abandoned.
(5) the decline in groundwater level, but also can cause the underground air hypoxia disaster. In some cities, due to intense water extraction, so that the groundwater level is rapidly reduced. The aquifer (section) that is drained is penetrated into the air through channels such as wells or underground works. When the space of the drained aquifer is a strongly reducing environment, the air penetrating into it interacts with the pore wall material of the rock layer and the residual water to turn low-valent iron into high-valent iron, thus consuming a large amount of oxygen in the air; the organic material, soil colloid, and unstable salts also need to consume oxygen, and thus the air penetrating into the space can be severely deprived of oxygen. If the above oxygen-deficient air is encountered during the construction of underground works, they will suddenly penetrate into the construction site from certain passages, which will bring serious consequences to the construction workers. 1971, since 1971, there have been a number of casualties caused by this in the construction of buildings in Tokyo, Japan.
3. Measures to Prevent Continuing Substantial Declines in Regional Groundwater Levels
The best measure is prevention, solving problems before they arise to avoid serious consequences. In the development and design of the water source, according to the permissible extraction of groundwater and the formation and distribution of water resources, the amount of extraction and the layout of the extraction wells to make reasonable arrangements to avoid the extraction of groundwater levels continue to fall substantially and other environmental and geological problems. To this end, it is required to conduct a unified evaluation of regional surface water and groundwater resources on the basis of groundwater basins (systems) or groundwater basins; to formulate a unified water resources scheduling and development plan; to deal with the problems of water supply and drainage in the region in an integrated and balanced manner, and to make predictions and propose preventive and curative measures for possible environmental geological problems, so as to achieve the goal of making full use of the water resources and minimizing the hazards thereof. In practice, however, it is difficult to solve problems before a crisis arises. On the one hand, it is difficult to calculate the amount of underground water resources accurately; on the other hand, it is wrong that people do not know enough about the importance of protecting underground water resources, and in order to meet the needs of production and life, they do not hesitate to over-exploit, and often wait for the problems to become serious before they are dealt with. Generally, the following measures can be taken to prevent and control:
(1) Close certain water sources or reduce the number of extraction wells, compression of the amount of extraction to the source of groundwater recharge permitted by the range. This is a negative approach in areas where there are no conditions for artificial groundwater recharge.
(2) adjust the mining layout. This is used to improve the water level of certain aquifers due to irrational mining caused by a substantial decline in the approach taken. For example, the density of water wells can be reduced, expanding the mining area or mining layer, the thickness of aquifers or multi-layer aquifers, can be implemented in sections or layers of water extraction program. In order to reduce the intensity of exploitation of pressurized aquifers, Shanghai Municipality has stipulated that the newly constructed wells shall be mainly drilled in the aquifers adjacent to the upper and lower ones.
(3) Strengthening groundwater management and establishing a reasonable exploitation system. In order to prevent over-exploitation and centralized exploitation, certain provisions can be made to limit the depth of the water level drop in wells, the amount of exploitation, the time of exploitation and the distance between wells.
(4) Artificial recharge of groundwater to aquifers to increase the total amount of groundwater that can be extracted. This is the most active measure currently being taken by countries around the world to prevent a significant drop in regional groundwater levels and to expand groundwater resources. This aspect will be introduced later.
(5) Establish and improve the groundwater dynamic monitoring network, strengthen the water monitoring and forecasting, as early as possible to find the problem, and take timely preventive and remedial measures.