DEFINING THE DISTRIBUTION, SOURCE, FATE AND TRANSPORT OF NITRATE IN GROUNDWATER BENEATH AN AGRICULTURALLY INTENSIVE REGION USING HIGH-RESOLUTION PROFILING METHODS

The hydrogeology, stable isotope distribution, and chemical distribution of Cl- and NO3--N within the Battersea Drainage Basin in southern Alberta were investigated. The Battersea Drainage Basin is characterized by widespread spreading of livestock manure on irrigated farmland and a high density of feedlots, creating concern about groundwater quality in the region. Past research has used conventional piezometers to study the source, distribution and fate of nitrate in the shallow groundwater. The key component of this research involved using the new technique of high-resolution profiling to determine the distribution, source, fate, and transport of nitrate in the shallow geological groundwater environment. High-resolution profiles of δ2H indicated groundwater throughout the glaciofluvial deposits and between 5.4 and 13.7 m below ground in glaciolacustrine deposits contained values > -150.0‰ and tritiated waters (> 0.08 TU). This suggested that this water recharged within the past 60 years. At depth 5.4 to 13.5 m BG, lower δ2H values did not coincide with detectable tritium, indicating the groundwater was much older and not vulnerable to agricultural contamination. High-resolution profiles of Cl- and NO3--N (solid core and squeezed pore water data) showed concentrations of these ions up to 411 and 219 mg L-1, respectively, in the glaciofluvial deposits. Concentrations of Cl- and NO3--N decreased to less than 50 mg L-1 (Cl-) and the Drinking Water Standard for NO3--N (10 mg L-1) in the underlying glaciolacustrine and glacial till deposits at most sites. Comparison to the high-resolution δ2H profiles suggested the high nitrate concentration in the glaciofluvial sediments is agricultural in origin. High concentrations for Cl- and NO3--N (up to 257 and 209 mg L-1, respectively) observed in glacial till and glaciolacustrine deposits below 6.0 m BG at two sites (LB5a and LB6) did not coincide with modern water, indicating the source of high nitrate is geologic in origin. The NO3--N to Cl- ratios suggested denitrification was not appreciable in the glaciofluvial deposits. However, denitrification may be a cause of decreased nitrate in the underlying fine textured deposits at certain sites. Interpretation of the high resolution profiles also suggested that the major conduit for nitrate migration is in near-surface glaciofluvial sediments via advection. The distributions of δ2H, Cl- and NO3--N with depth suggest that transport in the underlying glaciolacustrine and glacial till deposits is diffusion dominated, thus acting as a sink and removing nitrate from the permeable zone. However, the presence of fracturing in the oxidized zone of glacial tills and glaciolacustrine deposits suggests that transport may occur via advection through fracturing and diffusion in the material matrix. High-resolution profiling of δ2H, Cl- and NO3--N proved to be valuable in obtaining information regarding the distribution, source, and transport of groundwater and nitrate in the shallow groundwater environment at a level of detail that cannot be readily obtained through use of conventional piezometers