AbstractsEarth & Environmental Science

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 mg/L (5.4 mM) and manganese at 7 mg/L (0.13 mM) at the upper mixing zone - the interface between terrestrial recharge and top of the overturned saline groundwater. The reducible concentrations on the sediment are approximately 784-2,576 mg/kg (14-46 mM/kg) iron and 110-330 mg/kg (2-6mM/kg) manganese. The dominant process is the 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 (DOM) concentrations ranged between 5 and 30 mg/L. Excitation – emission fluorescence spectroscopy is used to help identify the distinct sources of DOM, which include terrestrial from fresh recharge, detrital from sediments and from inflowing ocean water. One-dimensional 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 the 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 concentrations through mineral precipitation/dissolution processes; v) to gain insight into the long-term evolution of the geochemistry at the site.