Groundwater responses to recharge and flood in riparian zones of layered aquifers: An analytical model

A riparian zone is an important element in a river-aquifer system, controlling water exchange and other chemical and biological processes between a river and an aquifer. Complex groundwater flow patterns may occur due to aquifer heterogeneity within a riparian zone. The purpose of this study is to investigate the impacts of layered heterogeneity on water exchange in the riparian zone using a mathematical model for groundwater flow in a two-layer aquifer that is recharged by precipitation and floods. A semi-analytical solution is derived for the hydraulic head, lateral discharge, and fluxes between the layers. Results demonstrate that the hydraulic conductivity difference between the two layers enhances lateral flow in the higher permeable layer and, more importantly, generates vertical flow between the two layers. The vertical flow induced by the recharge event is downward while it could be upward or downward induced by the flood event, which is determined by the contrast in permeabilities of the two layers. Using an equivalent hydraulic conductivity approach underestimates discharge of the two-layer aquifer due to recharge or flood. The analytical solution closely matched the observed hydraulic heads in riparian zone well of White Clay Creek and provided reasonable estimates of aquifer parameters. The present solution provides a valuable basis for further study of chemical and biological processes in riparian zone.