Bacteria-mediated Reduction and Precipitation of Fe(OH)3 and FeS in the Subsurface of a Coastal Aquifer: A Numerical Investigation

A mathematical model is developed to simulate the reduction processes and precipitation of Fe(OH)3 and FeS in an unconfined coastal aquifer. The available measured data of the selected aquifer shows that the saltwater region of the coastal aquifer is highly reduced. Mn2+, Fe2+ and HS− have been formed in the reduced saltwater region of the aquifer as a result of the bacteria-mediated reduction of MnO2, Fe(OH)3 and SO $$_{4}^{\ \ 2-}$$ respectively. Formed Fe2+ is oxidised again when it reaches the oxygen available mixing zone. Moreover, the formed Fe2+ and HS− precipitate as FeS in the reduced saltwater region. The objective of the present study is to simulate the above-mentioned reduction and oxidation processes by coupling solute transport, bacteria-mediated reduction and precipitation together. Bacterial growth is assumed to follow double Monod kinetic equation. Four bacterial groups (X1, X2, X3 and X4) are described. Bacteria group X1 uses oxygen under aerobic conditions and NO $$_{3}^{\ \ -}$$ under anaerobic conditions as electron acceptor. Anaerobic bacterial groups X2, X3 and X4 use respectively MnO2, Fe(OH)3 and SO $$_{4}^{\ \ 2-}$$ as electron acceptors. Organic carbon which behaves as the electron donor is considered as the most important factor for the bacteria-mediated reduction processes. Numerical results are presented for ten years of calculation and are discussed highlighting the importance of more detailed studies on the biogeochemical aspects of coastal aquifers and their numerical simulations.