Redox chemistry of iron and manganese minerals in river-recharged aquifers: A model interpretation of a column experiment

Abstract The transport of Fe and Mn observed in a laboratory column filled with sections of riverbed sediments and fine grained aquifer materials is modeled for steady-state conditions. The simulated processes include oxidation of organic matter by O 2 and NO - 3 in the riverbed sediment section, reductive dissolution of Fe and Mn (hydr) oxides, and precipitation of dissolved Fe and Mn, mainly as amorphous FeS. Local equilibrium for aqueous species is assumed. Kinetic reactions are used in the calculations for the oxidation of organic matter and for dissolution and precipitation of minerals. The observed concentration profiles of H + , NO - 3 , SO 2- 4 , and dissolved Fe and Mn in the laboratory column are reasonably explained by the model. The most sensitive model parameters are the rate constant for decomposition of organic matter and those for the dissolution of Fe and Mn (hydr) oxides. The results of a sensitivity analysis suggest that the decomposition rate of organic matter is responsible for the observed seasonal variations in concentrations of dissolved Fe and Mn in river-recharged aquifers, such as at Glattfelden in Switzerland. The observed reactions are mediated by bacteria.