| Nantong area, covering an area of 8544 km2, is located in southeast of Jiangsu province. The demand for groundwater resource increases urgently with the development of economy and increase of population. The exploitation is mainly in the confined aquifer Ⅲ because of low quality of the phreatic aquiferand confined aquifers Ⅰ, Ⅱ. The groundwater levels are declining in local areas as a result of irrational distribution of groundwater exploitation and over-exploitation. It also causes environmental problems such as land subsidence and fresh water salinization, impeding sustainable development of the local economy. Thus, it is necessary to develop a mathematical model to simulate the groundwater flow, land subsidence and water salinization.The thesis systematically collects dataset of groundwater levels, land subsidence and water quality in Nantong and reveals the time and spatial variation characteristics in the past 8 years. The mathematical models are constructed according to the geological and hydrogeological conditions in the study area and the models are identified and verified by the observation data from 2005 to 2013. Based on the emergent water source determined by previous field work and the different planned levels determined by city planning, the models forecast target areas that can be exploited in priority in the case of emergency. The variations of groundwater levels, land subsidence and water quality RE modeled based on the Scenarios of Yellow, Orange and Red Levels and the conclusions are as follows:1. The groundwater levels of confined aquifer Ⅲ are mainly from -20 to -30 m, except Haian and Rugao city with a level about -15m. A cone of depression forms among downtown of Nantong, Qidong, Haimen and Rudong city. The largest difference of groundwater is about 5 m.2. The main compressible layers include the confined aquifer Ⅲ, the aquitard Ⅳ and the confined aquifer Ⅳ. They mainly present a feature of elastic deformation. The value of cumulative land subsidence is around 0-100 mm for Nantong area, except the downtown and city of Haimen, which has a larger value over 100 mm. The average rate is 8 mm/a.3. The over-exploitation causes significantly decrease of groundwater level and unbalance of the interface between fresh water and saline water. The leakage recharge of the saline water in confined aquifer Ⅱ to confined aquifer Ⅲ results in local salinization of the confined aquifer Ⅲ.4. The mathematical models basically forecast the flow trend of groundwater and the cumulative land subsidence values agree with the observed data. Also the location of subsidence (i.e. downtown of Nantong city and south of Haimen city) obtained by modeling agree with the field observation, only differs in terms of the values of land subsidence in different areas. The good correlation between modeled chloride concentration and observed concentration suggests that the model can reflect the solute concentration field. Based on the results, the solute concentration fields of confined aquifers Ⅱ and Ⅳ change little. While the water quality of the confined aquifer Ⅲ show a salinization trend.5. The model also suggests water supply in emergency under different scenarios will not cause serious land subsidence and water salinizaiton. This may provide reference for the emergency water supply strategy of Nantong Area. |
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