Modeling the release of E. coli D21g with transients in water content

Abstract

Transients in water content are well known to mobilize colloids that are retained in the vadose zone. However, there is no consensus on the proper model formulation to simulate colloid release during drainage and imbibition. We present a model that relates colloid release to changes in the air-water interfacial area (A aw ) with transients in water content. Colloid release from the solid-water interface (SWI) is modeled in two steps. First, a fraction of the colloids on the SWI partitions to the mobile aqueous phase and airwater interface (AWI) when the A aw increases during drainage. Second, colloids that are retained on the AWI or at the air-water-solid triple line are released during imbibition as the AWI is destroyed. The developed model was used to describe the release of Escherichia coli D21g during cycles of drainage and imbibition under various saturation conditions. Simulations provided a reasonable description of experimental D21g release results. Only two model parameters were optimized to the D21g release data: (i) the cell fraction that was released from the SWI (f r ) and (ii) the cell fraction that partitioned from the SWI to the AWI (f awi ). Numerical simulations indicated that cell release was proportional to f r and the initial amount of retention on the SWI and AWI. Drainage to a lower water content enhanced cell release, especially during subsequent imbibition, because more bacteria on the SWI were partitioned to the AWI and/or aqueous phase. Imbibition to a larger water content produced greater colloid release because of higher flow rates, and more destruction of the AWI (smaller A aw ). Variation in the value of f awi was found to have a pronounced influence on the amount of cell release in both drainage and imbibition due to changes in the partitioning of cells from the SWI to the aqueous phase and the AWI.

Role of air‐water interfaces in colloid transport in porous media: A review

“…While conceptual and theoretical understanding of capillary forces and interfacial energetics is well‐advanced, the incorporation of these concepts and processes into flow and transport models is hampered by the need of knowledge of the local configuration of air‐water interfaces in porous media, which is a prerequisite to calculate interaction energies and capillary forces. Continuum models on colloid transport under variably saturated flow conditions need to explicitly take into consideration these effects on colloid mobilization and transport, and promising advances in this regard have been made [ Zhang et al ., ; Bradford et al ., ]. Such types of models need to be further developed, tested, and examined against theoretical and experimental evidence.…”

Recent Developments and Applications of the HYDRUS Computer Software Packages

“…Particle‐facilitated solute transport (often referred to also as colloid‐facilitated transport when solutes are transported sorbed to colloids) is often observed for many strongly sorbing contaminants such as heavy metals and radionuclides. This computational module has been used by Pang and Šimůnek (2006) among others. Colloid Transport with Changing Water Contents: This module can simulate particle transport, similarly as the standard HYDRUS‐1D computational modules, while additionally considering the effects of changes in the water content on colloid and bacteria transport and attachment and detachment to or from solid–water and air–water interfaces (e.g., Bradford et al, 2015). For example, when the air–water interface disappears during imbibition, particles residing on this interface are released into the liquid phase.…”

“…This module can simulate particle transport, similarly as the standard HYDRUS-1D computational modules, while additionally considering the effects of changes in the water content on colloid and bacteria transport and attachment and detachment to or from solid-water and air-water interfaces (e.g., Bradford et al, 2015). For example, when the air-water interface disappears during imbibition, particles residing on this interface are released into the liquid phase.…”

“…Torkzaban et al (2010) and Bradford et al (2012) additionally evaluated the effects of dynamic changes in the solution ionic strength on the transport and release of colloids and microorganisms in soils. Bradford et al (2015) further considered the effects of changing water contents on E. coli D21g transport and attachment and detachment to or from solid-water and air-water interfaces. We emphasize that at present a specialized nonstandard HYDRUS-1D module must be used to consider the effects of changes in solution chemistry and water contents on the transport and release of colloids (see Nonstandard Modules section)…”

Colloid release and clogging in porous media: Effects of solution ionic strength and flow velocity

“…first-order kinetic rate unless the total releasable colloid mass is also determined and adjusted in the model (Roy and Dzombak 1996;2015b). It should be mentioned that some attempts have been made to resolve the discrepancies between DLVO predictions and observed release behaviour by considering non-DLVO forces (e.g., Lewis acid-base interactions and Born repulsion) and the influence of nano-scale physical and/or chemical heterogeneities on colloid and grain surfaces (Duffadar and Davis, 2007;Shen et al, 2012).…”