Research On Relationship Between Deep Groundwater Exploitation And Land Subsidence In Dezhou

Dezhou City has long been exceedingly withdrawing deep groundwater, which has given rise to several regional descending funnels of deep groundwater and caused land subsidence. At present, this has become a serious environmental geological problem hindering the sustainable development of economy and society in Dezhou and demanding prompt solution. This study is made ever more significant by this practical dilemma. The author is to systematically analyze the development of deep groundwater exploitation and land subsidence and their relationship, to set up a correlative model between land subsidence and groundwater level, and ultimately to seek to predict and prevent land subsidence in Dezhou district.To better understand abovementioned geological problem, this thesis systematically analyzes the dynamic change of groundwater by applying Visual MODFLOW model and simulating spacial and temporal development of the hydrodynamic field and descending funnels of the deep groundwater. It further proposes a groundwater exploitation program which may check the expansion of the funnels.The study evidences that primary cause of the funnel is the excessive exploitation of deep groundwater. As exploitation proceeds, the funnel extends both horizontally and vertically. The water level elevation at funnel center is -108.00m and that at edge is -50.00m. That water level contour extends outward at 387.50m/a. Water level at funnel center is also descending at 3.55m/a. With a contemporary exploitation of 2047×104m3/a, it may be reasonably predicted that the depression depth may go even further. When t is 5 years, depression depth of the controlled peepholes will be 5.60～23.44m and depression speed 1.12～4.68m/a. When t is 10 years, depression depth will be 15.06～33.66m, depression speed 1.50～3.40m/a, and average water level elevation at funnel center -118.90m. Predications based on various exploitation designs all prove that the groundwater level will fall continuously if the current exploitation of 2047×104m3/a drags on. In 2012, the buried depth will reach -145.72m. With exploitation reduced to 1950×104m3/a, the buried depth of groundwater will reach -134.70m in 2012. If the exploitation is further reduced 1750×104m3/a, groundwater level will stop falling and regain rising tendency in 2011. And in 2012, the buried depth of groundwater in funnel center will rise to -120.40m.The study on groundwater exploitation and land subsidence in Dezhou also shows that correlation coefficient between the two reaches as high as 0.9992, which gives further evidence to the proposal that excessive exploitation constitutes primary cause of land subsidence in this district. This paves the way to analyzing the land subsidence mechanism in Dezhou.Based on abovementioned systematic analysis, the researcher has divided Dezhou into several settlement districts according to their settlement conditions, hydrogeological and engineering geological features. Each district is provided with an independent land subsidence/groundwater level correlative model. Analysis of model data, especially the accumulated settlement and groundwater buried depth, gives prominence to representative observation points in each district which may be used as a base for simulation. The simulation results show that the correlative model in each district complies with actual land subsidence conditions in Dezhou and may be used to predict further settlement in exploitation districts. All models predict that the settlement at all major reference points will continue growing along with excessive groundwater exploitation and falling groundwater level. As is predicted, the settlement center will fix at No.1 National Textile Company in 2016 with a groundwater level elevation of -128.47m and an accumulated land subsidence of 1332.16mm. Across the whole region, areas with a settlement of more than 1200mm will cover 45km2. Areas with a settlement of more than 1000mm will cover 126km2. Areas with a settlement of more than 800mm will cover 275km2.Land subsidence is caused by the joint compression of several aquifers and aquicludes which have been proven extremely difficult to quantify. The S curvilinear land subsidence model, however, successfully avoids this problem and gives a direct insight into the land subsidence mechanism. The model at each settlement district may be readily modified by updated data of land subsidence and groundwater. The coefficientsa , k in the model are in constant change along with the development of settlement. In a word, the self-adjusting feature of this model ensures a more reliable prediction on settlement tendencty.Researches have shown that the study area is now at gradual-speedy- qualitative change stage, an optimal period for measures to be taken to control land subsidence. To effectively prevent land subsidence from deteriorating, the city planners shall take proactive action such as scientific planning, adjusting deep groundwater exploitation, deep groundwater recharging, supplying water in varied qualities and meticulously monitoring land subsidence.