Fate and Transport of Nitrate and Microbes in a Simulated Vadose Zone, Capillary Fringe, and Shallow Groundwater Continuum.

Nitrate (NO₃^-) and microbial contaminants are health and ecological threats and their fates in the vadose zone, capillary fringe (CF), and shallow groundwater (SGW) continuum impact their transport to ground and surface waters.  The objectives of this study were to assess the effect of hydrology on NO₃^- fate in the CF and SGW and to evaluate the subsurface transport of indicator microorganisms under various hydrologic conditions. Nitrate fate evaluation was performed in 240-cm-wide, 60-cm-high, and 25-cm-thick flow cells. Two soils with different sand and organic matter contents were packed in separate flow cells.  A solution containing 100 mg L^-1 of both NO₃^- and bromide (Br^-) was passed horizontally through a simulated CF and SGW in the flow cells at various pore-water velocities. Nitrate and Br^- concentrations and redox potential were monitored at various locations in the CF and SGW.  While reduced conditions developed in the SGW, parts of the CF remained aerobic. Nitrate and Br^- concentration in the CF remained comparable suggesting limited denitrification. However, reduced conditions in the SGW were accompanied by up to 100% loss of applied NO₃^-_, while Br^- concentration remained high. Reduction in NO₃-N concentration was likely due to denitrification. To study microbial transport, a suspension of transformed Escherichia coli bacteria that fluoresce under ultraviolet light was applied at various rates to the top and side of a sand-packed 90- × 50- × 3-cm flow cell under various hydrologic conditions. One side of the flow cell was clear glass allowing visual observation of microbial transport under ultraviolet light. Time-lapse photos of microbial transport showed that E. coli were transported horizontally in the CF. The results imply that monitoring of subsurface transport of NO₃^- and microbes should include sampling the CF to prevent possible underestimation of the extent of their transport.