E. coli transport through soil columns: implications for bioretention cell removal efficiency.

Bioretention cells are a developing technology utilizing natural processes to facilitate treatment of surface water during runoff events. In bioretention cells, runoff collects in a shallow depression and percolates through the soil media, eliminating a variety of pollutants ranging from sediment to pathogenic bacteria. A series of column studies was used to evaluate the transport of bacteria through conventional and engineered cell media. The objective was to determine breakthrough concentration curves for the indicator organism Escherichia coli moving through simulated bioretention cell media (coarse sand, loamy sand, and loamy sand with coarse sand plugs), all tested under saturated conditions. Solutions with known concentrations of E. coli were introduced as a continuous source to the lower boundary of the column, and the effluent was analyzed for E. coli. The coarse sand was tested at 7.7 and 2.0 mL/min, where the final effluent concentrations in the higher and lower flow rate experiments were approximately 100% and 80% of the influent concentration, respectively. Final effluent concentrations in experiments with loamy sand at flow rates of 2.0 mL/min were approximately 12% of the influent concentration. However, when sand plugs were added to the loamy sand to improve infiltration capacity, filial effluent concentrations increased from 12% without plugs to approximately 65% of the influent concentration with plugs at the same flow rate. Mechanisms of removal were investigated by fitting concentration data to one-dimensional advection/dispersion equations with sorption and lumped physical straining and adhesion, and demonstrated the importance of both flow velocity and soil particle size distribution on E. coli transport. Design strategies to maximize infiltration in bioretention cells call be counterproductive to bacteria removal.