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Title: Release of E.coli D21g with transients in water content

Author
item WANG, YUSONG - University Of California
item Bradford, Scott
item SIMUNEK, JIRI - University Of California

Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2014
Publication Date: 7/21/2014
Citation: Wang, Y., Bradford, S.A., Simunek, J. 2014. Release of E.coli D21g with transients in water content. Journal of Environmental Science and Technology. 48:9349-9357.

Interpretive Summary: Most microbial transport studies, including those for pathogens, have been conducted under uniform water saturation conditions, and results typically suggest limited mobility of microorganisms in soil. Data presented in this manuscript demonstrates that microbial transport will be highly sensitive to changes in water saturation that commonly occur in unsaturated soils in the vadose zone. In particular, drainage and infiltration of water in soil may mobilize large numbers of microorganisms. The amount of bacteria release strongly depended on the number and location of retained cells, the solution chemistry, and the water saturation dynamics. Changes in water saturation are therefore expected to produce increased risks of microorganism transport through the vadose zone to contaminate groundwater resources. This information will be of use to scientists, engineers, regulators and health professionals in assessing the risks of microbial contamination to water resources.

Technical Abstract: Transients in water content are well known to mobilize microorganisms that are retained in the vadose zone. However, there is no consensus on the relative importance of drainage and imbibition events on microorganism release. To overcome this limitation, we have systematically studied the release of Escherichia coli D21g during cycles of drainage and imbibition under various solution chemistry and initial conditions. Results from these column studies revealed the influence of imbibition and drainage on D21g release. In particular, imbibition efficiently released cells from the air-water interface (AWI) that were initially retained under steady-state unsaturated conditions by expansion of the water films and destruction of the AWI. Conversely, significant release and transport of cells during drainage only occurred below a critical water saturation (water film thickness). In this case, cells that were initially retained on the solid-water interface (SWI) were likely incorporated into the AWI and then transported with the AWI during drainage. The efficiency of cell release from the SWI during drainage was much less than for the AWI during imbibition. Cycles of drainage and imbibition removed cells from the SWI and the AWI, respectively. However, the peak concentration and amount of cells that were released increased with the number of retained cells and the amount of drainage and imbibition, and decreased with the number of drainage and imbibition cycles. Release of cells during drainage and imbibition was found to be more pronounced in the presence of a weak secondary minimum when the ionic strength (IS) was 5 mM NaCl. Increases in the solution IS decreased the influence of water transients on release, especially during drainage. Complete recovery of the retained cells could be achieved using both IS reduction and cycles of drainage and imbibition, even when the cells were retained under favorable attachment conditions. In general, cell release was more pronounced with transients in water content than transients in IS when the IS=5 mM.