DELINEATING THE CLAY WINDOW RECHARGE ZONE FOR A RIVER VALLEY AQUIFER - FREDERICTON, NB, CANADA

Like many other places in Atlantic Canada and around the world, the City of Fredericton, New Brunswick, collects most of its water from a river valley aquifer. The semi-confined aquifer consists of glaciofluvial outwash sands including an esker-like ridge up to 40 m thick. It overlies till-covered sandstone and shale bedrock and is in turn buried under a glaciolacustrine clayey silt deposit of variable thickness (0 – 30 m). Since the early 1950s, Fredericton’s water supply has come principally from the Wilmot well field area, where a discontinuity in the clay cap allows a direct hydraulic connection between the aquifer and the bottom of the Saint John River. In order to delineate the recharge zone, riverine seismic and electromagnetic surveys have been carried out. During the summers of 2001 and 2002, a total of 250 line-km of seismic and electromagnetic data were acquired to probe to depths of approximately 50 m. While the seismic method images the clay stratigraphy including its termination against the esker ridge along many profiles, the electromagnetic method provides a more complete plan view of the extent of the clay window recharge zone. The two data sets are highly complimentary and, together with resistivity and borehole data available along the riverbank, should facilitate 3D modelling of the aquifer’s hydraulic connection to the Saint John River. Based on a preliminary map of the river-bottom recharge zone, several piezometers have been hammered down into the aquifer for hydrogeological and hydrogeochemical studies.