Experimental And Numerical Studies On The Problem Of Water Yield In Single Well

In hydrogeological surveies,the yield of single well which reflects to the output ability of groundwater,is frequently used for classifying the level of groundwaterabundance.It's usually quantitatively represented by the pumping rate of a well with specified raduis and drawdown.The well yield deponds on a lot of factors,including the characteristics of pumping well,the aquifer structure,the recharge conditions and the operation manner of pumping test.Generally,it's obtained by the pumping tests with three drawdown levels for steady state flow.The specific capacity is the yield of single well with a unit drawdown.To interpret the meaning of the specific capacity of a pumping well capturing lateral groundwater flow,Lan(2007)proposed an equation where the specific capacity is equal to the transmissivity(q=T)and shown evidences from hydraulic tests in sand boxes.However,the derivation of this equation was not on the robust basis of groundwater hydraulics.The boundary conditions of the sand-box tests were equivalent to a rectangular confined aquifer with one or two constant head boundaries.It reveals that the specific capacity is proportional to the transmissivity and the coefficient(q/T)is subject to the aquifer shape(D/L)and the radius of the pumping well(rw).In some conditions,q=T could be caused by the special geometric characteristics of the aquifer-well system.The hydraulic tests in sand boxes presented in Lan(2007)just satisfied such special conditions but did not indicate that q=T is generally applicable.New tests in sand boxes without these special geometric characteristics have shown significant different results from the q=T equation.The numerical simulation was performed to solve the question of specific capacity,using the software PMWIN.It shows that the ratio of the specific capacity to transmissivity(q/T)relies on the aquifer shape(D/L),the radius of the pumping well(rw),the aquifer depth which partially penetrating well penetrated and the distance of pumping well(x/L,y/L),except hydraulic gradient.Though the specific capacity is equal to the transmissivity when special conditions is combined,the result would be biased from the q=T equation once we changed one of the factors.Two kinds of strategies were considered to evaluate groundwater allowable withdrawal: the simplified calculation method which directly multiplies the specific capacity and the maximum permissible drawdown to represent the groundwater allowable withdrawal;the modeling method which forecastes the pumping test scheme under maximum permissible drawdown,using numerical simulation procedures.In this paper,a comparative study of the water source of Shimenzhai in Qinhuangdao is carried out.the groundwater level would not be settled lower than limestone cope level as limited conditions in order to prevent karst collapse.The result shows that the groundwater allowable withdrawal derived from simple evaluation method was 101341 m3/d,while the groundwater allowable withdrawal gained from simulation evaluation method was 31300 m3/d.There is a big difference between the two.Therefore,the traditional one is not reliable to evaluate the allowable withdrawal by simply using the water yield in single well.The ratio of the specific capacity to transmissivity(q/T)derived from simulation evaluation method was over 0.2 to 0.71.It is also bias from the condition of q=T.