Effects of wetland restoration on drinking water quality: model-based evaluation with radon-222 and chloride data

The experimental site is located in the alluvial plain of the Wiese River in northwestern Switzerland. The following goals were defined: (a) to create an environment of partially flooded, wetland-type vegetation in ponds used as groundwater recharge fields, (b) to use river water that undergoes considerable fluctuations in discharge and microbiological water quality for artificial recharge, and (c) to maintain high quality drinking water at the adjacent production wells. Monitoring wells are used to provide the following data: distributions of hydraulic heads, surface water and pore water concentrations of chloride and radon gas. Groundwater residence times are determined with the help of measured radon isotope activities ( 222 Rn) for three different layers of the aquifer. A three-dimensional (3-D) numerical flow and solute transport model is automatically calibrated against transient hydraulic head and groundwater chloride concentration data. Both the residence times from the 222 Rn method and the 3-D model predictions are in good agreement for the middle aquifer layer with the highest permeability. The presented modelling efforts provide a valuable evaluation tool for a larger-scale flood plain restoration.