Modeling the 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 proper model formulation to simulate microorganism release during drainage and imbibition events.   We present a model that relates microorganism release to changes in the air-water interfacial area with transients in water saturation, and then use this model to describe the release of Escherichia coli D21g during cycles of drainage and imbibition under various solution chemistry and initial conditions.  Cell release is modeled in two steps.  First, a fraction of the retained cells on the solid-water interface (SWI) may partition to the air-water interface (AWI) during drainage, and below a critical water film thickness may be transported on the AWI by advection.  Second, cells that are retained on the AWI or at the air-water-solid triple point are efficiently released from the AWI during imbibition as water films expand and the AWI is destroyed.  Model simulations provide a promising description of experimental release results.