Transport of Escherichia coli through a Thick Vadose Zone

Livestock manure applications on fields can be a source of contamination in water resources, including groundwater. Although fecal indicators like Escherichia coli have often been detected in tile drainage systems, few studies have monitored groundwater at depth after manure treatments, especially at sites with a deep, heterogeneous vadose zone. Our hypothesis was that microbial transport through a thick vadose zone would be limited or nonexistent due to attenuation processes, subsurface thickness, and heterogeneity. This study tested this hypothesis by monitoring E. coli concentrations beneath a 12-m-thick vadose zone of coarse, heterogeneous glacial sediments after surface application of liquid swine manure. Escherichia coli was detected on all 23 sample dates over the 5-mo period (4 Apr. 2012-13 Aug. 2012), with particularly elevated concentrations 1 wk after application and lasting for 5 wk. Variable low-level concentrations before and after the elevated period suggest remobilization and delayed transport of microorganisms to the water table without additional loadings within the flow field. These findings suggest preferential flow pathways allowing deep infiltration of manure bacteria as well as a continued source of bacteria, with variable retention and travel times, over several months. Preferential flow pathways at this site include soil macropores, depression focused infiltration, and pathways related to subsurface heterogeneity and/or fracture flow through finer-grained diamict beds. Further research is needed to confirm the relative contribution of sources, constrain travel times, and define specific transport pathways.