REDUCTIVE DISSOLUTION OF MN OXIDES IN RIVER-RECHARGED AQUIFERS: LABORATORY-SCALE INVESTIGATIONS

River-recharged aquifers are a common source of drinking water throughout the world. The infiltration of river water into adjacent aquifers decreases the concentrations of suspended solids and some harmful micro-organisms. However, the infiltration of dissolved organic carbon from the river may create reducing conditions with potentially detrimental effects on the groundwater supply. Groundwater from production wells installed adjacent to the Saint John River in Fredericton, New Brunswick, Canada contains Mn concentrations in excess of the Canadian Drinking Water Guideline (0.05 mg/L). This study was conducted to test the hypothesis that the elevated Mn concentrations may be explained by microbially-mediated reductive dissolution of Mn-oxide minerals present in the aquifer sediments.

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.