The turning factor in the estimation of stream-aquifer seepage.

A combined analytical-numerical approach is presented to characterize properly the exchange flow between a stream and a hydraulically connected aquifer. It eliminates the need to use a three-dimensional fine grid under and in the vicinity of the river cross section in order to obtain accurate results. Basically the approach matches an analytical solution in a vertical two-dimensional (2D) plane with the numerical description of the aquifer behavior in a 2D horizontal plane. The approach is compared with a finite-difference formulation such as used in MODFLOW.