Aqueous- and solid-phase biogeochemistry of a calcareous aquifer system downgradient from a municipal solid waste landfill (Winterthur, Switzerland)

This study addresses the biogeochemical changes that take place in a calcareous aquifer system under and downgradient from a municipal solid waste landfill. Aqueous-phase chemical analysis of the redox-sensitive species indicates the presence of aerobic respiration, denitrification/NO3- reduction, and Fe(III), Mn(III/IV), and SO4 reduction processes under the landfill. Because available and released organic matter is limited, reduction processes downgradient from the landfill do not go far beyond aerobic respiration, denitrification, and Mn(III/IV) reduction. Assuming steady-state conditions, STEADYQL computer program has been used to model the biogeochemical processes by taking into account the kinetics of the redox reactions, calcite precipitation and dilution. Dilution has the most significant influence on the concentrations of the dissolved organic and inorganic carbon. Dissolved Mn(II) concentrations in the entire anaerobic zone are controlled by the solubility of rhodocrocite [MnCO3(S)]. At selected locations under the landfill where SO4 reduction takes place, dissolved Fe(II) concentrations are regulated by the solubility of amorphous FeS. Chemical extraction of the aquifer solid phase indicates that the oxidation capacity of this aquifer system is largely controlled by iron(III) (hydr)oxides. Due to Fe(III)-reducing processes, the concentration of iron(III) (hydr)oxides has diminished under the landfill. Downgradient along the leachate plume, where no Fe(III)-reducing processes are currently observed, decreased concentrations of solid Fe(III) phases and the presence of iron(II) sulfide phases suggest that, in the past, Fe(III)- and SO4-reducing processes have indeed taken place. This is presumably due to the presence of higher concentrations of available organic matter in the past.