Submitted to: Vadose Zone Journal
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/3/2007
Publication Date: 5/1/2008
Citation: Gargiulo, G., Bradford, S.A., Simunek, J., Ustohal, P., Vereecken, H., Klumpp, E. 2008. Bacteria transport and deposition under unsaturated flow conditions: the role of water content and bacteria surface hydrophobicity. Vadose Zone Journal. Vol 7(2):406-419 Interpretive Summary: Experiments were conducted to investigate the transport behavior of two bacteria strains through sand at several different water contents. These strains are similar in shape and size, and differ primarily in their surface chemistry and tendency to form aggregates (stick together). The moisture content was observed to play an important role in bacteria mobility. Lower concentrations of bacteria were transported through the sand with decreasing water content. This effect was found to be enhanced for the bacteria that formed aggregates. The experimental data were analyzed using a mathematical model that accounts for retention of bacteria in sand by chemical interactions and in small soil pores. Retention of bacteria in small pores was found to play an important role in bacteria removal at lower water contents and for the bacteria that formed aggregates.
Technical Abstract: Packed column experiments were conducted to investigate the transport and deposition behavior of representative hydrophobic (Rhodococcus rhodochrous) and hydrophilic (Deinococcus radiodurans) bacteria strains at several different water contents. These strains are similar in shape (coccoid) and size, and differ primarily in their surface hydrophobicity and tendency to form aggregates. The breakthrough curve and the final deposition profile were measured in each transport experiment. The moisture content was observed to play an important role in bacteria mobility. The recovered amount of bacteria in the effluent tended to diminish with decreasing water content, and the percentage of cells retained in the sand was inversely related to the water content. This effect was found to be enhanced for the more hydrophobic strain, Rhodococcus rhodochrous, presumably due to cell aggregation. The experimental data were analyzed using a mathematical model that accounts for time-dependent attachment and straining processes. Fitted model parameters were found to depend on bacteria surface characteristics, and the deposition profiles showed considerable deviation from the filtration (attachment) theory fit. In particular, most of the cells were found to be deposited close to the column inlet, and hyper-exponential deposition profiles were observed to decrease monotonically with depth. Consideration of depth-dependent straining in the model formulation significantly improved the description of the data. The straining coefficient tended to increase with decreasing water content, especially for the hydrophobic bacteria that formed aggregates. Straining is believed to increase in magnitude with decreasing water content because water flows through smaller pore spaces. The fitted attachment coefficient of the hydrophilic bacteria tended to increase with decreasing water content, presumably due to attachment to the air-water interface that increases with decreasing water content.