TOWARDS AN INTEGRATED GEOPHYSICAL, HYDROLOGICAL, GEOCHEMICAL AND MICROBIOLOGICAL CHARACTERIZATION OF RIVER-AQUIFER INTERACTIONS: THE EXAMPLE OF FREDERICTON, NEW BRUNSWICK, CANADA

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 glaciolacustrine clay/silt. It has long been suspected that recharge to the well field occurs from the adjacent Saint John River where a discontinuity in the confining layer may allow for hydraulic connection with the river. It has also 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 DOC from the river.

Riverine seismic and electromagnetic surveys (EM31 and EM34) were carried out to delineate stratigraphic controls on recharge from the river. While the seismic method imaged the extent of the clay stratigraphy, thickness and its termination against the esker ridge, the electromagnetic method provided a more complete plan view of the extent of the discontinuity in the clay/silt unit. These results guided more direct investigations of the river recharge zone.

Groundwater temperatures in the recharge area are being used as an environmental tracer to assess travel time from the river to the well field. Time-series measurements of groundwater temperature versus depth have been collected at two locations in the recharge area. Water temperature displays variations of at least 12 degrees C to depths as great as 17 m below the riverbed. The data indicate high infiltration velocities, which will be further constrained using numerical modeling.

Aqueous geochemical data from piezometers along the flow path to the well field suggest that the groundwater system evolves to increasingly reducing conditions, with a decrease in DOC coinciding with a decrease in NO3, and increases in the concentrations of alkalinity, Mn, Fe and sulfide. The data are consistent with trends that are expected from microbially-mediated NO3 reduction, followed by reductive dissolution of Mn and Fe (oxy)hydroxides. The influence of micro-organisms on the redox zonation is supported by enumerations of bacterial populations in the groundwater by plate-count and MPN methods, as well as identification of species using DNA sequencing. The influence of this redox evolution on trace-element mobility in the system is under investigation.