Bacterial filtration rates in repacked and weathered soil columns

Knowledge of the transport and deposition behavior for pathogenic Escherichia coli is needed to assess contamination and protect water resources. We evaluated the effects of repacked (R) and physically and biologically weathered (PBW) columns of sandy loam and clay loam soils on the transport and retention of E. coli and bromide under saturated and unsaturated flow conditions. PBW soil columns were weathered outdoors by exposure to wetting, drying, freezing and thawing cycles during the winter of 2008 and spring and summer of 2009. These physically weathered soil columns were subjected to additional 6 months of biological weathering by 20 earthworms placed onto each soil columns. Nalidixic acid-resistant E. coli (E. coli NAR) and bromide (Br) were employed as pulse tracers and their temporal and spatial variabilities were monitored in the leachate samples. The flow regimes (inlet suction 0 for saturated and 5 hPa for unsaturated) were controlled using a tension infiltrometer. Cumulative percentage and maximum concentrations (Cmax) of Br and E. coli NAR and filtration coefficients (λf) of E. coli NAR were assessed. In the repacked soil columns, average bacterial concentration and Cmax in leachate were lower, likely due to enhanced pollutant/soil particle interactions at low water fluxes. The largest λf occurred in the repacked clay loam soil. Accelerated water velocities through preferential flow pathways were likely responsible for the smaller filtration coefficients in the PBW columns. Unsaturated flow conditions resulted in larger λf values for E. coli NAR and smaller Cmax values for both tracers.

► Bacteria and bromide filtration rates in repaired soil columns were studied.
► Pore networks in weathered columns led to accelerated bacteria and solute movement.
► Unsaturated condition showed higher filtration compared to saturated condition.
► Greatest filtration coefficient was observed for the repacked clay loam soil.
► Breakthrough sequence justified the preferential flow in the weathered columns.