The determination of the suitability of ground penetrating radar to map "windows" in the protective clay/silt aquitard of the Fredericton aquifer

Abstract

Ground Penetrating Radar (GPR) is a non-invasive geophysical technique that can be used to determine the subsurface features at a site. This method utilizes changes in material properties (i.e. conductivity, dielectric constant) to provide a detailed image of the underlying geology stratigraphy. The City of Fredericton's main source of potable water is a glaciofluvial sand and gravel aquifer that is overlain by a lacustrine clay/silt unit of variable thickness and a fluvial sand and gravel deposit. The sites chosen for this investigation are located near the Fredericton well field in the Wilmot Park and Smythe Street area of the City's Downtown. Through previous borehole data (Violette, 1990) available for these locations, the confining clay/silt layer is known to exhibit breaks or 'windows' that can be potential sources of recharge or contamination for the aquifer. It would therefore be useful to locate these 'windows' to prevent possible migration of hazardous material into the water supply and to aid in determining the extent of recharge to the aquifer at these locations. The method of GPR profiling with 50 and 100 MHz antennas proved to be an effective means of determining the presence (or lack) of clay/silt due to the high attenuation caused by this highly conductive lacustrine sediment. The clay/silt layer was interpreted as a strong reflector that appears to have a gently slope to the South. This was evidenced by the fact that the reflector varied from - 8 0 to -105 ns in the N-S profiles (9, 11, 12 and 13). The location of the clay window near Smythe Street and in the Southeast corner of Wilmot Park have been positioned using the interpreted radar data.