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

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 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, a fraction of the cells that were initially retained on the solid-water interface (SWI) partitioned into the mobile aqueous phase and the AWI as the receding water film thickness decreased 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.