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Confined Two-Aquifer SystemNeuman and Witherspoon (1969) derived a solution for flow to a pumping well in a confined two-aquifer system that accounts for leakage through a storative confining unit and drawdown in the unpumped aquifer. The Neuman-Witherspoon solution differs from the solution by Hantush and Jacob (1955) which assumes the unpumped aquifer acts as a constant-head boundary and the aquitard releases no water from storage. Neuman and Witherspoon provided examples to illustrate errors introduced by the limiting assumptions of Hantush-Jacob solution on the estimation of transmissivity (T) for the pumped aquifer. Figure 1 shows a new example using a synthetic data set prepared with the following properties for a confined two-aquifer system:
This synthetic data prepared using the Neuman-Witherspoon solution include the effects of drawdown in the unpumped aquifer (i.e., a constant-head boundary condition is not assumed) and storage in the aquitard. The match of the Neuman-Witherspoon (1969) solution in Figure 1 shows the "true" response of the pumped aquifer at the observation well.
Figure 2 shows the Hantush-Jacob (1955) solution matched to the synthetic data set for the confined two-aquifer system. Although the fit of this solution to the data appears satisfactory, the Hantush-Jacob solution overstates the "true" transmissivity of the aquifer by 24%. Other examples presented by Neuman and Witherspoon showed potentially larger errors in the estimates of T when using the Hantush-Jacob solution.
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