ABSTRACT: Geochemical evolution of induced infiltration in a river-recharged aquifer system: Fredericton, New Brunswick, Canada.

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Abstract

The city of Fredericton, New Brunswick, Canada relies on groundwater from a glacial aquifer in the Saint John River valley. The aquifer is a semi-confined esker discontinuously overlain by clay/silt of glacio-lacustrine and/or marine origin. Recharge to the well field occurs partly from the adjacent river where a discontinuity in the confining layer allows for hydraulic connection with the river. It has been suggested that elevated Mn concentrations in the groundwater supply are related to reductive dissolution of Mn-oxide minerals in the aquifer as a result of the infiltration of dissolved organic carbon from the river. A detailed hydrogeochemical study has been conducted to investigate redox conditions along a flow path from the river bed to a nearby water-supply well. Aqueous geochemical data from multi-level piezometers along the flow path display variations in redox-sensitive solutes (O2, NO3, Mn, Fe, SO4 and HS) in space and time. The redox conditions cycle on a seasonal time scale, likely in response to temperature changes in the infiltrating river water. In the spring and early summer the conditions are relatively oxidizing with elevated concentrations of dissolved O2 and NO3, and low concentrations of Mn and Fe. Toward late summer, and into the fall, the system tends toward more reducing conditions, with concentrations of dissolved O2 and NO3 declining, and concentrations of Mn and Fe increasing. Localized zones of elevated HS concentrations suggest that SO4 reduction occurs, however, the seasonal trend toward reducing conditions is not manifest by a widespread decline in SO4 concentrations as it is for O2 and NO3. The data are generally consistent with trends that are expected based on thermodynamics, with O2 reduction followed by NO3, MnIV, FeIII and SO4 reduction, however, in some locations these respective redox zones are superimposed. The observed overlap of redox zones is likely attributable to a combination of variable reaction kinetics (probably related to mineralogy, and microbiological abundance and activity), and variations in groundwater velocity that may lead to the development of localized redox gradients at a scale that can not be resolved by the sampling. The spatial variations in the data suggest that elevated concentrations of Fe and HS are prevented from breaking through to the production well by redox reactions with Mn-oxide minerals in which FeII and HS are oxidized, and MnIV is reduced. In the future, as Mn-oxide minerals become depleted, it is likely that Fe, followed by HS-, will break through to the well.

Bibliography of Canadian Geomorphology