Theme 2-3-Stream-aquifer interflow estimation - evaluation of common algorithms

Abstract

A credible estimation of insitu stream-aquifer interflow is a prerequisite for modeling of contaminant transport in streams and groundwater aquifers. The interflow is commonly estimated by monitoring the stream stage and the water table elevation in an observation well in the vicinity of the stream. The measured difference between the two levels, distance of the well from the stream, stream-section, aquifer thickness and the hydraulic conductivity are employed to estimate the interflow. The estimation is mostly based upon Dupuit-Forchheimer’s assumptions. It is believed that this solution is exact for fully penetrating streams. For partially penetrating rivers, the additional loss of head due to vertical velocities is accounted for by certain semi-empirical equations. In the present study these algorithms have been evaluated by two-dimensional steady state numerical modeling of the groundwater flow in the vicinity of the stream. The modeling permits simulation of the true stream-aquifer interflow, and the depth-averaged piezometric head representing the water table elevation. The algorithmic stream-aquifer interflow computed from this elevation is compared with the corresponding model-computed value. The comparison reveals that the commonly used algorithms based upon a single water table observation are quite unreliable and uncertain. This indicates the necessity of monitoring the piezometric head distribution in x-z plane by a nest of piezometers, and analyzing the flow net to estimate the interflow.