Iron and manganese reduction driven by organic matter and mixing of fresh and saline groundwater in the Fraser River Delta aquifer, Vancouver, {Canada

We present results of field investigations of the biogeochemistry of an aquifer a few km from the ocean adjacent to the Fraser River in Vancouver, Canada. At the site, a wedge of relatively dense saline ocean water enters the aquifer in the hyporheic zone at the river bottom, migrates away from the river along the base of the aquifer to a maximum distance of approximately 500m inland, where it overturns and mixes with fresh groundwater. The mixed saline - fresh water then flows back under a regional freshwater gradient and eventually discharges to the river at the top of the saline wedge. Pore waters show iron concentrations peak at over 300 ppm (5.4 mM) and manganese at 7 ppm (0.13 mM) at the interface between terrestrial recharge and top of the overturned saline groundwater. The reducible concentrations on the sediment are approximately 5000 ppm (solid/solid) iron and 70 ppm manganese. At present flow rates and fluxes of organic matter, between 300 and 1500 pore volumes are required to flush iron completely from the aquifer. Since the presence of organic matter, the dominant process is reductive dissolution of iron and manganese oxide minerals via organic matter oxidation, although acid-volatile sulfide and methane measurements show that both sulfate reduction and methanogenesis are also occurring. Dissolved organic matter concentrations range between 5 and 30 ppm. Excitation - emission fluorescence spectroscopy is used to help identify the distinct sources of dissolved organic matter, which include terrestrial from fresh recharge, detrital from sediments and from inflowing ocean water. Kinetic reactive-transport modeling that includes primary mineral redox reactions and secondary mineral precipitation was used to: i) interpret the role of mixing of fresh and saline water, ii) to constrain reduction rate parameters and metabolic activity levels from field data, including oxidation rate of organic matter by iron and manganese oxides, probably accompanied with sulfate reduction and methanogenesis. iii) to understand how other secondary minerals further control aqueous ferrous iron and manganese concentration through mineral precipitation/dissolution processes.