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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
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UC Riverside Electronic Theses and Dissertations (2013)

An understanding of the processes that influence the transport and fate of microorganisms in porous media is needed to protect water resources from pathogenic microorganisms and contaminants associated with the colloidal phase, and to optimize bioremediation strategies. As the physical and chemical factors influencing the transport of microorganisms have been well understood in homogeneous systems, accurate description of microorganism transport in field scale is hampered by the existence of preferential flow.

The objective of this research was to better understand and quantify physical and chemical factors that influence microorganism transport and fate in soils that exhibit preferential flow. E. coli D21g and coliphage phiX174 were selected in this study as the representative bacterial pathogen and the surrogate for human viruses, respectively. The transport of microorganisms was examined in well defined and controlled soil columns containing artificial macropores (sand lenses) of different length and configuration under different solution ionic strength (IS).

Transport experiments demonstrated that retention of E. coli D21g and φX174 increased with IS in both homogeneous and heterogeneous systems. The importance of preferential flow on microbe transport was found to be enhanced at higher IS, even though the overall transport decreased. Deposition profiles revealed significant cell retention at the interface of the coarse sand lens and fine sand matrix as a result of mass transfer. The length and configuration of the artificial macropore proved to have a great impact on the transport of E.coli D21g, especially under high ionic strength conditions. At low ionic strength, more extensive transport in the preferential path and earlier arrival time were observed for E.coli D21g compared to bromide as a result of size exclusion. Cell release from the preferential flow system with a reduction of solution IS exhibited multi-pulse breakthrough behavior that was strongly dependent on the initial amount of cell retention, especially at the lens-matrix interface, and the lens configuration. Simulations in 2D models were capable to describe the transport and deposition, and the release process during transients in chemistry in preferential flow systems. Dual-permeability models were also successfully applied to simulate microorganism transport in preferential flow systems with different configuration and length of preferential path, and correlations found between the parameters of dual-permeability model and preferential path characters could promote the upcaling from local to field scale for microorganism transport in preferential flow systems.

    Cover page: Physical and Chemical Factors Influencing the Transport and Fate of Microorganisms in Soils With Preferential Flow
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2014)
    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. © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA.
      Cover page: Physicochemical Factors Influencing the Preferential Transport of Escherichia coli in Soils
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2014)
      Dual-permeability models are increasingly used to quantify the transport of solutes and microorganisms in soils with preferential flow. An ability to accurately determine the model parameters and their variation with preferential pathway characteristics is crucial for predicting the transport of microorganisms in the field. The dual-permeability model with optimized parameters was able to accurately describe the transport of E. coli D21g in columns with artificial macropores of different configurations and lengths at two ionic strength levels (1 and 20mM NaCl). Correlations between the model parameters and the structural geometry of the preferential flow path were subsequently investigated. Decreasing the macropore length produced a decrease in the apparent saturated hydraulic conductivity of the macropore domain and an increase in the mass transfer between the macropore and matrix domains. The mass transfer coefficient was also found to be dependent on the configuration of the preferential flow pathway. A linear superposition approach was used to estimate field-scale preferential transport behavior for hypothetical fields with different amounts and configurations of macropores. Upscaling procedures were numerically investigated to predict this field-scale transport behavior from column-scale parameters. The upscaling method provided a satisfactory prediction of the field results under the tested scenarios. This information will be useful in assessing the risks of microbial transport due to preferential flow.
        Cover page: Estimation and upscaling of dual-permeability model parameters for the transport of E. coli D21g in soils with preferential flow
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2014)
        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.
          Cover page: Release of E. coli D21g with Transients in Water Content
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2014)
          An understanding of microbial transport and survival in the subsurface is needed for public health, environmental applications, and industrial processes. Much research has therefore been directed to quantify mechanisms influencing microbial fate, and the results demonstrate a complex coupling among many physical, chemical, and biological factors. Mathematical models can be used to help understand and predict the complexities of microbial transport and survival in the subsurface under given assumptions and conditions. This review highlights existing model formulations that can be used for this purpose. In particular, we discuss models based on the advection-dispersion equation, with terms for kinetic retention to solid-water and/or air-water interfaces; blocking and ripening; release that is dependent on the resident time, diffusion, and transients in solution chemistry, water velocity, and water saturation; and microbial decay (first-order and Weibull) and growth (logistic and Monod) that is dependent on temperature, nutrient concentration, and/or microbial concentration. We highlight a two-region model to account for microbe migration in the vicinity of a solid phase and use it to simulate the coupled transport and survival of species under a variety of environmentally relevant scenarios. This review identifies challenges and limitations of models to describe and predict microbial transport and survival. In particular, many model parameters have to be optimized to simulate a diversity of observed transport, retention, and survival behavior at the laboratory scale. Improved theory and models are needed to predict the fate of microorganisms in natural subsurface systems that are highly dynamic and heterogeneous.
            Cover page: Modeling Microorganism Transport and Survival in the Subsurface
            • Article
            • Peer Reviewed
            UC Riverside Previously Published Works (2015)
            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 (Aaw) 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 air-water interface (AWI) when the Aaw 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 (fr) and (ii) the cell fraction that partitioned from the SWI to the AWI (fawi). Numerical simulations indicated that cell release was proportional to fr 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 Aaw). Variation in the value of fawi 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.
              Cover page: Modeling the release of E. coli D21g with transients in water content
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