Groundwater Risk Assessment And Regulation In The Water Intake Area Of The South-to-North Water Transfer Project In Tianjin City

Tianjin,as one of the water shortage cities in northern China,is highly dependent on groundwater.Along with the development of economy and society in Tianjin,the contradiction between water supply and water demand increases,and the amount of groundwater exploitation also increases,even over mining groundwater,resulting in environmental geological problems such as groundwater pollution,land subsidence and saline water descending,bringing great risk to people's living and working condition.In order to solve these problems,the South-to-North Water Transfer Project was carried out.The arrival of the water of Yangtze River is changing the structure of water supply in the water intake area in Tianjin,which can restrain the widely existing phenomenon of overexploitation of deep groundwater.Therefore,for the water intake area,it is crucial to evaluate groundwater risk values reasonably,so that groundwater of high risk can be regulated specifically.In this paper,the water intake area of the South-to-North Water Transfer Project in Tianjin was selected to be the study region,and the assessment and regulation of groundwater risk were researched.The main conclusions and innovations are summarized as follows:(1)The index systems of risk evaluation for shallow and deep groundwater were constructed,respectively,and methods of Principal Component Analysis and Analytic Hierarchy Process were combined innovatively to achieve a kind of synthetic weight which not only reflected index significance but also reduced index redundancy.So the groundwater risk in the water intake area was evaluated reasonably.(2)Indicator Kriging was employed to analyze the uncertainty of groundwater risk.Accordingly,the most optimistic risk and the most pessimistic risk of shallow and deep groundwater in the water intake area were obtained.The results show that shallow groundwater in Zhongtang Town of Dagang District,deep groundwater in Shengli Street of Dagang District,the west areas of Hangu District and the east of Jinnan District should be the management targets with priority.(3)Based on the results of groundwater risk assessment,the contribution rates of groundwater risk factors were analyzed,and the factors controlling the whole water intake area(general factors)as well as the factors only affecting some regions(local factors)were summarized.Four meteorology situations and three groundwater mining situations were devised for the two general factors,buried depth of shallow groundwater table and exploitation of deep groundwater,and 12 regulation schemes of general groundwater risk were combined accordingly,on the basis of the principles of risk regulation.(4)The numerical model of groundwater,Modflow-2005,was used to regulate general factors of the water intake area.The results show that shallow groundwater can meet the requirements of ecological water utilization and has certain potential for exploitation.For salinity of shallow groundwater,the best plan was selected to fulfill the aim of diluting salinity utmost which was to exploit shallow groundwater of 401million m~3 in normal year.For deep groundwater,the plan with the minimum amount of mining was the best,because it could restore the water table of deep groundwater,control land subsidence and reduce saline water descending better.(5)The local risk factors of fluoride concentration exceeding normal standard in Beichen District,high sewage density in Urban District and low engineering geological stability in Hangu District were regulated.The reasons leading to these problems were analyzed and their corresponding regulation countermeasures were given.The results can provide references for risk management in the water intake area in Tianjin.