Using periodic analytic elements to analyze aquifer tests near surface waters

Abstract

In riverbank filtration (RBF), the objective is to indirectly pump water from a river into wells near the river, by inducing recharge through the river sediments. Design models must quantify the rate of induced infiltration from surface waters, which is often controlled by the hydraulic resistance between the surface water body and the aquifer. Traditional tools for aquifer-test analysis near surface streams have limited utility for directly estimating the resistance of a streambed or riverbed. The authors have developed software tools for aquifer test analysis using an analytic element model for periodically varying flow called Wigaem. The Wigaem code can explicitly represent the plan-view geometry of surface streams and the aquifer perimeter, including tributary streams when necessary. All boundary conditions in the model may vary periodically, including water levels in surface waters and pumping rates of wells in the simulation. A constant-discharge pumping test may be simulated by extending the pumping regime in time such that it becomes periodic and the total discharge during the period of simulation equals zero. Problems caused by the Gibbs phenomenon are reduced through application of a moving average filter, which makes Wigaem suitable for very general aquifer test problems. By calibrating the model to match observed water-level data, the model is used to estimate the transmissivity, storativity, and the entry resistance of surface waters, which are crucial for the design of RBF systems. This approach is applied to a site in central Illinois, United States.