Simulating Pathogen and Nutrient Transport From the Ground Surface to a Crystalline Fractured Rock Aquifer, Perth, Ontario, Canada

Fractured rock aquifers are particularly vulnerable to contamination in settings where overburden is thin and outcrops are common. A 72 hour, surface to aquifer, tracer experiment was completed in the Precambrian Shield of Eastern Ontario to explore transport rates of pathogens and nutrients in this inherently anisotropic environment. Previous studies at the site suggested a direct connection between the surface to the bedrock aquifer in wells drilled on outcrops. For this experiment, a 1200L dammed area was constructed adjacent to a rock outcrop. Lissamine FF was used to simulated nitrate transport while 0.3 μm and 1.75 μm microspheres were used to simulate E. coli transport. Three sample locations were isolated in two wells that were located 5 m and 10 m from the dammed area. Steady state conditions were created by pumping the deep section in the closer well at the same rate of water influx into the dammed area (7 L/min). The Lissamine samples were analyzed using a Turner Designs Au-10 field fluorometer. Microspheres were enumerated using epifluorescence microscopy and direct counting methods. Tracer experiments indicate that transport times can be very fast, with arrival times between 45 minutes in the shallow section of the closer well to five hours in the further well after tracer application on the rock outcrop.  Microspheres arrive earlier than the conservative flow, but straining is evident.  All results from the tracer experiments indicate that wells drilled on or near rock outcrops are extremely vulnerable to surface contamination from agricultural processes.