Colloid Transport Behavior in Agricultural Soils: Microbes and Microspheres
University of Calgary
Pathogens originating from agricultural fields have the potential to adversely impact groundwater quality. In Canada, this was clearly brought to light by the Walkerton groundwater tragedy where E. coli contamination of shallow groundwater resulted in the deaths of seven residents. Yet increasing field evidence suggests that pathogen detections occur infrequently in both time and space even in vulnerable aquifers. There is clearly a need for field based studies to better understand the processes controlling biocolloid transport in the subsurface. Here we present a series of field studies that were conducted to achieve the following: (1) determine the suitability of using microspheres as surrogates for E. coli; (2) evaluate colloid transport and attenuation during infiltration through undisturbed agricultural field soils; and (3) investigate the importance of macropores on preferential transport of colloids. Small-scale infiltration experiments were conducted at three separate field sites in southern Ontario. A tension infiltrometer was used to infiltrate a solution of Brilliant Blue dye and surface applied colloids (different sized microspheres and E. coli RS2g) under partially saturated conditions. Upon completion of infiltration, excavations were completed to examine the dye-stained flow patterns, map soil and macropore features, and collect soil samples for enumeration of microspheres and E. coli. All soils contained significant macroporosity, predominantly in the form of vertical worm burrows. Colloid properties had a significant influence on transport, but results showed that microspheres with similar size and surface properties to E. coli RS2g exhibited similar transport behavior. Despite differences in soil type, infiltration rates and flow patterns, colloid transport behavior was similar across all sites. Colloids were readily attenuated near surface with >2 log reduction in the top 5 to 10 cm of the soil profile. At greater depths colloid concentrations remained relatively consistent and were closely related to the intensity of dye staining. Flow along macropores was the most important factor controlling deep vertical migration of both dye and colloids. These field tests highlight the need for suitable methods to monitor and describe field-scale pathogen migration in order to better assess the vulnerability of shallow groundwater resources.