The governing equation of solute transport in the well skin produces multiple parameter estimates because of the equifinality of modeling radially convergent tracer tests. A new transport equation for the skin of an extraction well (i.e., a new transient Robin boundary condition) is proposed. A new analytical model was developed to test a fully penetrating extraction well. The analytical solution of the model was obtained using the Laplace transform and finite Fourier cosine transform. A finite element solution was acquired for the test in a partially penetrating extraction well. Results suggest the skin governing equation produces the estimates of the skin width w and formation vertical dispersivity α_z are arbitrary values chosen from the ranges of 0.5 m≤w≤1 m and 0.08 m≤α_z ≤0.1 m. These ranges exclude the default values. In contrast, the new Robin boundary condition accurately reflects the skin effect when the Peclet number, defined as the ratio of w to the longitudinal dispersivity of the skin, is less than 1. The present solution relying on this boundary condition predicts the single optimal estimates of w and α_z . The estimates (w=0.31 m, α_z =0.17 m) approach their default values. The present solution applies to field tests.