GROUNDWATER GEOCHEMISTRY IN THE FREDERICTON AQUIFER, FREDERICTON, NEW BRUNSWICK: PRELIMINARY RESULTS

The Fredericton aquifer is comprised of glaciofluvial sediments within the Saint John River Valley, and is known from previous studies to receive recharge from the Saint John River. A field-scale investigation is underway to determine the effects of infiltrating river-water on the aqueous geochemistry within the aquifer. The investigation focuses on an area of the riverbed where downward-directed hydraulic gradients indicate that river water infiltrates to the aquifer. Three nests of four piezometers, ranging in depth from 4 to 25 meters below the riverbed, were installed to assess geochemical changes along an infiltration flow-path. Sampling was conducted for major cations, anions, sulphide and dissolved organic carbon (DOC).

There is a general decrease in DOC with depth that correlates with an increase in alkalinity. This is consistent with the occurrence of microbially-mediated redox reactions, where river-derived DOC is an electron donor, and O2, NO3, Mn(IV), Fe(III), and SO4 are the corresponding sequential terminal electron accepters.

Variability in observed concentrations versus depth suggests that there are stratigraphic controls on the geochemical characteristics of the groundwater. This is most apparent from the data for Cl and the redox-sensitive species NO3, SO4, Fe(II), and Mn(II). The observed variations in the concentrations of these species versus depth may be a result of differences in the redox conditions, mineralogy, groundwater flow path and velocity that correlate to stratigraphy. For example, the variations in dissolved Fe(II) and Mn(II) versus depth may be related to differences in the local abundance of Fe and Mn oxides, or alternatively, to variations in hydraulic conductivity (groundwater velocity or residence time) that are controlled by stratigraphy. Future research plans include the collection of drill cores from the aquifer in order to address these issues.