REDUCTIVE DISSOLUTION OF MN OXIDES IN RIVER-RECHARGED AQUIFERS: LABORATORY-SCALE INVESTIGATIONS
A one-dimensional flow path from the river to the aquifer was simulated using sand-filled columns. One column was inoculated with bacteria and a second column was treated with ethanol in an attempt to sterilize the system. Both columns received the same influent solution, formulated with a major-ion composition similar to the Saint John River and acetate as a source of DOC. The inorganic geochemistry of the column effluent was monitored for 200 days.
The results of the experiment demonstrate that the two principal controls on Mn concentrations in the column effluent are cation exchange reactions and microbially-mediated reductive dissolution of Mn oxides. There is an initial period during which Mn concentrations in the effluent are relatively high in response to the establishment of exchange equilibrium. This is followed by near steady-state conditions with a relatively low but constant Mn flux resulting from reductive dissolution. A one-dimensional reactive transport model was used to simulate the processes that occurred in the columns and the results supported the conceptual model that was developed on the basis of the experimental data.