Redox zonation at the saline-influenced boundaries of a permeable surficial aquifer: effects of physical forcing on the biogeochemical cycling of iron and manganese

Research investigating geochemical changes accompanying subsurface mixing of fresh and saline water has primarily focused on cation exchange and mineral dissolution/precipitation reactions. In this study, we report on redox species zonation at the boundaries of a freshwater lens confined beneath a small, permeable island surrounded by saline marshes and tidal creeks and located on the estuary side of Sapelo Island, Georgia. The spatial and temporal distribution of the chemical species in the aquifer implies that the freshwater lens resists saline intrusion by maintaining constant advection across the salinity gradient. As a result, the biogeochemical processes in this aquifer seem to have reached a quasi steady-state very close to equilibrium. Redox reactions associated with natural organic matter oxidation may also play an important role at the salinity transition. Surprisingly, aerobic respiration and microbial iron reduction seem to be the main pathways for natural organic matter oxidation. Sulfate reduction is not significant despite the high concentration of sulfate available, and manganese oxides are probably chemically reduced by dissolved sulfide and Fe2+. This study is the first to demonstrate that iron and manganese reduction takes place at the salinity transitions bounding both sides of an island freshwater lens and that microbial iron reduction accounts for most of anaerobic respiration of natural organic matter at these transitions.