Form Princple And Controlling-adjusting Research About Deep Groundwater Depression Cone In Dezhou

It is widespread concerned that over-exploitation of Quaternary deep confined water (referred to "deep water", the same below) in Haihe Plain has already affected development of regional economy and society. Aimed at continuous drawdown of water table for over-exploitation and other geo-environmental problems, like land subsidence, this thesis selected distribution area of deep groundwater depression cone in Dezhou City as focus area. And it took variation of deep groundwater table and corresponding clay layers in the three main exploitation layers of 300～500 m, 500～800 m and 800～940 m as study targets. There were two key scientific points which should be cleared fist. One was lithology and structure of the formation where land subsidence happened due to drawdown of deep groundwater table and threshold value of buried depth. The other was recharge source of deep confined water and mechanism of variation of regeneration ability under large-scale mining condition. In this thesis, information was mainly achieved from deep groundwater dynamic monitoring, water chemistry composition and isotopic samples analysis. Using groundwater dynamic methods and isotopic technology, the thesis made following research. First, the thesis studied physic structure of aquifers in Dezhou deep groundwater depression cone area, including lithology characteristics, variation of thickness, correlation relationship and spatial distribution rules. Second, it studied formation and evolution process of deep groundwater depression cone and the affection of recharge, exploitation quantity, exploitation depth, characteristics and structure of the aquifers. Third, it studied impact mechanism and pattern of human activities on recharge of deep groundwater. Through these researches, the thesis achieved main knowledge and advances as follows.1. Dezhou deep groundwater system was typical and specific. Its exploitation formation included six aquifers and seven clay layers so that the structure was very complex. Depth o f exploitation was from 300 m to 900 m, with the maximum amounting to 940 m, and it was the deepest layer for exploiting in Haihe Plain. At present, water table of central deep groundwater depression cone dropped to about -120 m, water buried depth reached 140 m. In 2005, depression cone area was 11068.8km~2 in Dezhou. And it connected with Cangzhou deep groundwater depression cone, Wen'an-Dacheng depression cone and Tianjin depression cone, which became a regional compositive depression cone. In study area, geo-environmental problems emerged, like land subsidence and interface of salt groundwater downward and southward.2. Exploitation resources of Dezhou deep groundwater was composed of four parts, such as lateral runoff recharge, crossflow recharge, elastic releasing water of aquifers and compression releasing water of clay layers. Lateral runoff recharge took 20.6% of the total recharge and it came from regional runoff of external deep groundwater systems, reflecting natural regeneration ability of deep groundwater. Isotopic data showed that Dezhou deep groundwater was from precipitation in the last glacial period. It had the characteristics of leaching, and affected by evaporation and invasion of salt water in the process of water cycle. Latter, it was interfered by seawater invasion for many times. HCO3-Na type of water was prior than any other type, with high fluorine, high iodine, high sodium, low calcium, low hardness and alkalescent, low concentration of hydrogen and oxygen steady isotope and low tritium content as well.3. Results of correlation analysis showed that artificial over-exploitation was the main factor that resulted in formation and evolution of deep groundwater depression cone. The relationship between exploitation and water table of central depression cone was non-linear, but was characteristics of cubic polynomial curve instead.4. Strata of filling deep groundwater in Dezhou were well consolidated. Although being a long period of time after consolidation, releasing water would happen when pressure and additional stress exceeding pre-consolation stress. Land subsidence and drawdown of water table of deep groundwater had a good correlation relationship. Based on regression calculation of land subsidence and deep groundwater table, threshold value of deep groundwater table was -51.12 m, allowable drop of water table was 70.24m.5. When exploitation regulated reasonably, water table of central deep groundwater depression cone was recoverable. Under the condition of reducing exploitation for 60×10~4～100×10~4m~3 one year, water table of central depression cone began to rise slowly, and it would disappear in the year about 2020. But bearing capacity of deep groundwater resources was uncertain and various, for it closely related to condition of exploitation, mining horizon and the structure of formation.The main results and innovation were as follows.①According to buried depth of formation in research area, lithology, types of causes, deposition time, physical and mechanical properties of soil, water power features and hydrological characteristics, strata above 800 m were first dived into 8 aquifers, 9 compression layers (aquitard) and one layer of surface soil, water-rock dynamic research system was also built. At the same time, formations between the third aquifer and the fifth aquifer as well as aquitard were generalized into III bearing formation group. Aquifers and aquitard between the sixth aquifer and eighth aquifer were generalized into IV bearing formation group. The III and IV bearing formation groups were the main exploitation layers in Dezhou.②Storage resources of deep groundwater in Dezhou were mainly from precipitation recharge in the last glacial period. Exploitation resources mainly included lateral runoff recharge and crossflow recharge, which were from regional runoff systems, with 17.5% of elastic releasing water and 61.9% of compression releasing water.③The thesis revealed formation and evolution of Dezhou deep groundwater depression cone, and identified and determined dominant factors inducing formation and evolution of depression cone, using geo-statistical methods. The results laid a solid foundation to predict and make reasonable regulation countermeasures.④The thesis created the model revealing relationship between variation of Dezhou deep groundwater depression cone and artificial exploitation. And it predicted water table of central depression cone and mean water table under different exploitation condition. The thesis also analyzed regulation characteristics of deep groundwater. Besides, the thesis established the correlation relationship between land subsidence and water table of deep groundwater for the first time, and it calculated the threshold water table and allowable drawdown of water table. At last, in order to better prevent geo-environment disasters, the thesis put forward regulation projects for controlling deep groundwater depression cone in Dezhou.