Bacteria and Colloids In the Vadose Zone: Filter or Freeway?.

Bacterial transport through the vadose zone to groundwater poses potentially serious risks to human health.  Water flow and solute transport in natural field soils are complex and difficult to describe due to processes such as preferential flow.  The complexity of colloid and bacterial transport is further exacerbated by colloid-specific transport behavior (such as filtration and attachment to air-water interfaces) and biological processes (such as bacterial survival).  Here, we present findings from studies that were designed to evaluate biocolloid transport in the subsurface at several different sites across Canada.  These studies included both artificial and natural colloid tracers under a range of different infiltration scenarios.  Despite distinct differences in soil type, infiltration rates, and flow regimes, the resulting colloid transport and attenuation were relatively consistent across all sites.  There were also key differences between sites, particularly related the role of macropores in preferential transport of bacteria to depth.  The results point to the need for improved methods to monitor and describe field-scale microbial transport in order to better assess the vulnerability of shallow groundwater resources.