ABSTRACT: Windows under the Water: Geophysics in the study of a river valley aquifer.

CGRG Bibliography of Canadian Geomorphology
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The glaciofluvial sand and gravel aquifer underlying the City of Fredericton, New Brunswick in the Saint John River valley is one of many river valley aquifers that serve municipalities in Atlantic Canada and world-wide. It is currently the focus of a multi-disciplinary study of water quality and supply issues in alluvial aquifers that exchange water with rivers above and fractured bedrock below. Geophysicists are contributing by searching for evidence of bedrock faulting or structure that could represent significant pathways for infiltration, and by searching for areas where the aquifer may be in direct hydraulic connection with the river bed. Delineation of the river-bottom recharge zone for Fredericton’s well field is a project priority as knowledge of that infiltration pathway is required in order to site piezometers for hydrogeological and hydrogeochemical studies. Infiltration of river water into the Fredericton aquifer occurs where elevated portions of the sand and gravel unit, widely believed to be parts of a buried esker, protrude through an overlying clay/silt aquitard to the floor of the Saint John River. Riverine seismic and riverbank resistivity surveys are proving to be highly effective and complimentary in imaging these so-called clay windows. Over 30 line-km of high resolution, single-channel seismic reflection and side-scan sonar data were collected on the Saint John River using real-time kinematic GPS for navigation in late May, 2001. Equipment was deployed from UNB’s 40-foot research vessel Mary-O in water as shallow as 1.5 metres. Data quality seemed to be dependent on river bottom type but in most areas the ‘Seistec’ seismic profiling system with it’s boomer source and special line-in-cone hydrophone receiver generated excellent broadband records showing reflection events with dominant frequencies of approximately 2.5 kHz at depths as great as approximately 50 m. The edges of inferred recharge zones are identified as boundaries where reflectors present within the clay/silt layer suddenly terminate and are draped on the sides of an interpreted esker. Shallow electromagnetic (EM31) and resistivity imaging data collected along the shoreline have helped to constrain interpretation of the seismic data and have yielded a detailed cross-section through the recharge zone opposite the Fredericton well field. Areas of high resistivity, where sand extends to the surface, are bounded by areas of much lower resistivity where clay is present. There is evidence of isolated clay pods within the recharge zone - presumably remnants of clay that filled depressions in the esker as it was buried. The presence of these pods has important implications for design of the planned piezometric monitoring array, and riverine resistivity or electromagnetic surveys are planned for the summer of 2002 to determine their extent and under the river.

Bibliography of Canadian Geomorphology