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Title: Physicochemical factors influencing the preferential transport of Escherichia coli in soils

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

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/18/2013
Publication Date: 1/13/2014
Citation: Wang, Y., Bradford, S.A., Simunek, J. 2014. Physicochemical factors influencing the preferential transport of Escherichia coli in soils. Vadose Zone Journal. DOI: 10.2136/vzj2013.07.0120.

Interpretive Summary: The root zone serves as an important barrier to protect groundwater from disease causing microorganisms that pose a risk to human health. In this research we investigate the influence of three factors that are known to enhance the transport potential of microbes in the root zone: (i) preferential flow; (ii) changes in solution chemistry; and (iii) size exclusion. Experimental and modeling results demonstrate that the length and configuration of preferential flow paths had a strong influence on the transport of a representative bacteria species (E. coli D21g), especially at high solution ionic strengths (IS). Size exclusion was found to enhance the transport of cells in comparison with bromide. Decreasing the ionic strength with pure water resulted in the release of most of the retained cells. Our model formulation was able to accurately simulate most of the observed transport, retention, and release behavior in systems with preferential flow. This information will be of interest to scientists and engineers concerned with predicting the fate of microbial pathogens in agricultural soils.

Technical Abstract: Laboratory and numerical studies were conducted to investigate the transport and release of Escherichia coli D21g in preferential flow systems with artificial macropores under different ionic strength (IS) conditions. Macropores were created by embedding coarse sand lenses in a fine sand matrix and altering the length, continuity, and vertical position of the lens. The length of an artificial macropore proved to have a great impact on the preferential transport of E. coli D21g, especially under high-IS conditions. A discontinuous macropore (interrupted by fine sand) was found to have less preferential transport of E. coli D21g than a continuous macropore of the same length that was open to either the top or bottom boundary. At low IS, more extensive transport in the preferential path and earlier arrival time were observed for E. coli D21g than Br- as a result of size exclusion. Two release pulses (one from the preferential path and the other from the matrix) were observed following a reduction of the solution IS for flow systems with macropores that were open to either the top or bottom boundary, whereas three pulses (two from the preferential path and another from the matrix) were observed for systems with discontinuous macropores. Numerical simulations of E. coli D21g under both constant and transient solution chemistry conditions had very high agreement with the experiment data, except for their capability to predict some subtle differences in transport between the various lens configurations.