(I) Causes of the Continuing Decline in Regional Groundwater Levels
Many regions of the world where groundwater is extracted have experienced a large and significant decline in groundwater levels. Analyzing from the groundwater equilibrium, the main reason is: on the whole aquifer or certain sections of the aquifer, due to the groundwater exploitation for a long time exceeds the recharge (i.e., over-exploitation), and gradually depletes the storage capacity, and does not get recovered in a certain period of time as a result of the results. In determining whether a sustained regional decline in groundwater levels will occur in a given area, special attention should be paid to distinguishing it from the phenomenon of declining water levels caused by cyclical changes in precipitation recharge. The continuous decline of regional groundwater level caused by over-exploitation in a certain area is shown in Fig. 11-14. The reasons for the continuous decline of regional groundwater level can be summarized in four aspects:
(1) Incomplete understanding of the regional hydrogeological conditions, especially the formation conditions of the groundwater resources, and the calculated permissible exploitation amount is on the large side, which results in the exploitation amount being larger than the recharge amount for a long period of time, and causes the continuous decline of the regional groundwater level. The groundwater level continues to fall. This phenomenon of continuous decline in water level is generally most evident in the change in the annual minimum water level 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 unreasonable mining, mainly refers to the mining lot, mining level and mining time on the "three concentration" mining, as well as mining in the anarchic state. Sometimes, although the recharge of the entire aquifer is basically balanced with the amount of extraction, the extraction intensity is too high due to the over-concentration of the extraction wells in certain localized areas or in certain aquifer levels (or at a certain depth), which will result in the continuous and substantial decline of the water level in the localized areas or in certain aquifers. For example, the Shanghai Municipal District **** 5 aquifers, groundwater storage is quite rich, but 87% of the amount of mining is concentrated in the Ⅱ, Ⅲ two aquifers, of which 84% of the wells and 80% to 90% of the amount of mining, but also in the two aquifers concentrated in the east of Shanghai Yangshupu, Hongkou and the west of Shanghai, Putuo, Changning, Jing'an a few industrial zones, and the water wells are mostly for the summer (May to September) cooling and cooling. Therefore, in the above sections of these two aquifers, the interference between wells is intensified, the water output is reduced, and the regional groundwater level drops considerably, forming the center of the regional groundwater level drop funnel, and they are also the sections that produce the most serious ground subsidence.
(3) Reduction of groundwater level recharge due to changes in anthropogenic or natural factors, causing regional groundwater level decline: ① Due to anthropogenic or natural reasons, the surface water flow, which is the main source of groundwater recharge, is reduced or cut off, or the riverbed is silted up, resulting in a reduction in the recharge of surface water to groundwater. For example, in the Wuwei pre-mountain plain area of the Northwest Hexi Corridor, for more than a decade, due to the construction of water conservancy projects in the upstream mountainous areas, the river water recharge to the groundwater of the alluvial fan has been greatly reduced, resulting in the alluvial fan leading edge of the area of the water table has dropped by 3-10m, so that the flow of the spring in the overflow zone has been reduced by 30% to 90%; ② due to the destruction of forests and excessive reclamation and other reasons, resulting in regional climate change, precipitation, and reduce the ground infiltration conditions. Ground infiltration conditions become worse, so that the recharge is less than the amount of mining, resulting in a decline in the regional water table; ③ in the same hydrogeological unit of the water source, due to the deep dredging of mineral deposits or other underground engineering, or due to the interception of new wells in the upstream of the water source or the peripheral areas of the water wells to increase the depth of mining and other man-made reasons, can also be caused by a substantial decline in the groundwater level of some water sources; ④ increase in the depth of the groundwater, urban construction and the increasing area of concrete, asphalt and other water-insulating pavement, so that the amount of atmospheric precipitation infiltration recharge is reduced.
(4) Due to the development of economic construction, population growth and improvement of living standards, the demand for water has increased significantly. Some areas have exceeded the permissible groundwater extraction and the formation of regional groundwater level decline, but still want to expand the extraction, so that it is more to promote the rate of decline in the groundwater level, leading to a series of environmental and geological problems.
(2) The harm of the 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 a lot of environmental geological problems, and the main harms are in the following aspects:
(1) Due to the decline of the regional groundwater level, the amount of water produced by water extraction projects decreases, and the pumping equipment must be replaced in order to extract water, which makes the cost of pumping water decrease. To withdraw water, so that the cost of pumping is increasing, and in serious cases, even make the wells scrapped. Many large water sources and well irrigation areas, there is this problem. For example, Shandong Zihe alluvial 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. Well scrapping, playing deep wells and replacement of water pumps, economic losses amounted to more than 5,000 million yuan, the entire irrigation district more than 140 million yuan per year in electricity costs. Because of the water level has dropped significantly, China's annual scrapping of water wells amounted to tens of thousands of eyes.
(2) Due to the regional water table decline, can cause ground subsidence, ground cracks and ground subsidence and other serious environmental and geological problems. Ground subsidence is a serious problem faced by many plains 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, 8m; Shanghai, 2.37m (1921-1965). At least 36 other large and medium-sized cities in China, such as Tianjin, Xi'an, Taiyuan, Suzhou and Taipei, have successively experienced problems of ground settlement or cracking and subsidence. Cities such as Tokyo, Bangkok, London, and Venice are in danger of having part of their downtown areas flooded by seawater due to ground subsidence; cities such as Bangkok and Shanghai are in danger of having urban sewage and rainwater frequently accumulate in their downtown areas and not being able to be drained in a timely manner due to ground subsidence. Located in the United States of America, Arizona, between the city of Pinar and Miley Copa well irrigation area, in 1948 to 1967 years, the groundwater level lowered 70 to 100m, the amount of ground subsidence up to 1.2m (up to 2.5m). The uneven ground subsidence and accompanying ground cracks damaged the entire irrigation system, roads, railroads, and aqueducts in the area.
There are many causes of ground settlement. At present, it is internationally recognized that, due to the large amount of groundwater extraction, the groundwater level has dropped significantly, prompting the discharge of pore water in the upper easily compressible clay layer, causing the consolidation and compression of the soil layer, resulting in ground subsidence. On the other hand, according to the principle of effective stress, in any plane of the aquifer, the total pressure of the overlying rock layer by the water-bearing rock solid particles skeleton effective stress and pore water pressure *** with the same bear, and is in a state of equilibrium, due to the water level drops significantly, so that the pore water pressure decreases, the skeleton of the effective stress increases, which caused the ground layer compression and ground subsidence. The above is the most common cause of ground settlement. From Shanghai's ground subsidence and groundwater level and the amount of exploitation of the observation data over the years, can be seen that the amount of ground subsidence and groundwater exploitation and water level depth changes in consistency (Figure 11-15).
Figure 11-15 Relationship between Groundwater Level, Extraction Volume and Ground Subsidence Rate in Shanghai
(3) In the coastal areas, the substantial decline in regional groundwater level destroys the natural equilibrium conditions of salty and fresh water, causes seawater intrusion, and deteriorates the quality of water in the extracted aquifers. In some areas, and even make the fresh water completely into salt water, lose the value of development and utilization, and sometimes also make the good land into saline. China's Dalian, Laizhou, Qinhuangdao, Ningbo, Yantai and other coastal cities are such problems. Bohai Bay area of seawater intrusion has reached more than 1,480km2.
(4) Due to the decline in the regional water table, so that some of the famous karst springs dry up, destroying the spring landscape as a feature of tourism resources. The famous Jinan Baotu in Shandong and other four major springs, Huixian Hundred Springs in Henan, Hebei Xingtai Hundred Springs, Shanxi Taiyuan Jin word springs, Beijing Northwest Yuquanshan Springs, etc., since the 1970s, due to the substantial decline in regional water table, resulting in the flow of springs and gushing potential to greatly reduce the flow of springs, and even a long time of dryness and discontinuation of the flow of springs, so that the ornamental value of the springs tourism greatly reduce the attractions of tourism resources eclipsed and the spring mouth of the diversion project Abandoned.
(5) Due to the substantial decline in groundwater level, the ecological environment deterioration. Arid and semi-arid areas have fewer water resources, for a long time in order to solve the problem of water supply for industrial and agricultural life, often in front of the mountains along the construction of reservoirs, storage of river water, the plains area vigorously develop groundwater. Due to the neglect of ecological water demand, resulting in increasingly serious degradation of vegetation, ecological environment deterioration. For example, more than 90 small and medium-sized reservoirs have been built in the upstream mountainous areas of the Heihe River basin in China's Hexi Corridor, drastically reducing the amount of groundwater recharge in the middle and lower reaches. Under these circumstances, the middle and lower reaches of the river have increased groundwater exploitation to address the demand for water for economic development and domestic use. This pattern of exploitation eventually led to the drying up and disappearance of many rivers. Under the effect of evaporation and concentration, the salinity of the remaining surface water and shallow groundwater increased. The West Juyan Sea was 88 g/L in 1960, and reached 100-200 g/L before drying up; the East Juyan Sea was once a freshwater lake, but after the 1970s, it was continuously salinized, with a mineralization level of 10 g/L. The quality of the shallow groundwater, in addition to freshwater along the two sides of the river, has been increasing, and in the lower reaches it can be as much as 3 g/L. The serious dislocation of water and salinity has degraded or even disappeared the riverine forest and scrub meadows that had been developed along the river. The serious water and salt imbalance makes the original development of riparian forest and scrub grassland along the river degraded or even disappeared. The original in the middle reaches of the region widely distributed swamp and aquatic vegetation with the spring water reduction and decline, replaced by dry and saline meadows.
Additionally, the groundwater level drops significantly, but also can make the land sand. For example, in Minqin County, Gansu Province, due to the over-exploitation of groundwater in the Shiyang River Basin, the water level has dropped significantly, and 724hm2 of forest land has been sandy. Groundwater level decline, but also can cause the underground air oxygen deficiency disaster, if in the construction of underground projects encountered the above oxygen deficiency air, they will be by some channel suddenly through the site, to the construction workers bring serious consequences.