The Study On Deep Groundwater Resources And Subsidence Caused By Withdrawal With Method Of Numerical Simulation In Tianjin District

Deep fresh groundwater is the significant water supply resources, which plays a vital role in promoting the development of economy of Tianjin District. Unfortunately, because of shortage of water resources on the whole, a fact that extensive exploitation of groundwater has not been effectively controlled results in, to some extent, unbalance of groundwater. So groundwater level continues to decline, and therefore a lot of environmental geological problems appear, such as land subsidence. Accordingly, a precise evaluation of groundwater resources at area of concern is indispensable for scientific management of groundwater resources.In accordance to the project of“Digitization of Hydrogeological Conditions of Deep Groundwater and Simulation and Prediction of Withdrawal of Tianjin District”, which is implemented by Tianjin Hydrology & Water Resources Reconnaissance Management Center, a hydrogeological conceptual model of Tianjin plain terrain was constructed by using software GMS with data from 2006 to 2008. Coupled with the application of MAPGIS, ARCVIEW, a large number of data about sources and sinks are stored by ACCESS in a specific format as required by GMS, and then the numerical model of groundwater is build up through VBA programming. In addition, parameters needed in SUB package, which are including equivalent thickness, factor, vertical hydraulic conductivity, elastic skeletal specific storage, inelastic skeletal specific storage, are obtained by statistical analysis of 275 borehole’s data. Simultaneously, lowest previous head of various aquifers of study area are dealt with so as to construct numerical model of land subsidence with MODFLOW2005. Trial and error method is used by identification of hydrogeological parameters, data inversion of volume of withdrawal and water released by interbeds, fitting flow field, subsidence filed, hydrograph and subsidence observed data, all of which shows that the numerical model is valid, and results indicate that:(1) The main sources of deep groundwater system are leakage recharge, lateral inflow, water released by interbeds, and the average annual value of those three sources from 2006 to 2008 are 3.01×10~8m~3/a, 2.39×10~8m~3/a, 2.91×10~8m~3/a, which account for 35%, 28% and 34% respectively, of total recharge of deep groundwater system. Meanwhile, the major sink of deep groundwater system is withdrawal, which takes up almost 92% of total discharge of deep groundwater system. Moreover, the volumes of withdrawal of groundwater system from 2006 to 2008 are 9.03×10~8m~3/a, 9.13×10~8m~3/a, 8.45×10~8m~3/a and volumes of water released by interbeds from 2006 to 2008 are 3.18×10~8m~3/a, 3.38×10~8m~3/a, 3.06×10~8m~3/a.(2) Parameters related to land subsidence are adjusted continually through method of trial and error in order to properly identify numerical model. The results calculated through model reflect the reality of land subsidence filed in general and the data of representative subsidence observation is matching calculated value successively. Eventually, volumes of water released by interbeds from 2006 to 2008 are 3.13×10~8m~3/a, 2.86×10~8m~3/a, 2.75×10~8m~3/a, which is verified by valid subsidence numerical model.