River bank filtration: modelling of the changes in water chemistry with emphasis on nitrogen species

Bank-filtrated water is an important component of the drinking water production in many countries. The changes in the water chemistry during the transfer from the river to the aquifer have important implications for the quality of the produced water. In this paper, we first describe certain features of the evolution of the water chemistry during bank-filtration in the case of an experimental site, part of a large well field (Seine river, France). Here, bank-filtration leads to highly reducing conditions in the aquifer. A conceptual and numerical macroscopic model of this evolution, focusing on nitrogen compounds, is then presented. The model is designed to simulate organic matter mineralization and redox reactions catalyzed by bacteria in the river bed sediments where water infiltrates. Growth and decay of bacteria are explicitly accounted for and a numerical solution is found with an operator splitting technique. The model is able to reproduce column experiments by von Gunten and Zobrist (1993) designed to simulate infiltration of organically polluted river water into an aquifer. A model application to the characteristics of the experimental site is also presented. Results of a sensitivity analysis highlight the importance of. (1) the flow rate of water infiltrating river bed sediments; and (2) the organic carbon content of these sediments, for the evolution of the water quality during transfer from the river to the aquifer.