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Title: Critical role of preferential flow in field-scale pathogen transport and retention

Author
item Bradford, Scott
item FEIKE, LEIJ - California State University
item SCHIJVEN, JACK - Utrecht University
item TORKZABAN, SAEED - Commonwealth Scientific And Industrial Research Organisation (CSIRO)

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/24/2017
Publication Date: 4/13/2017
Citation: Bradford, S.A., Feike, L.J., Schijven, J., Torkzaban, S. 2017. Critical role of preferential flow in field-scale pathogen transport and retention. Vadose Zone Journal. 16(4):1-13. doi: 10.2136/vzj2016.12.0127.

Interpretive Summary: Contamination of drinking water and fresh produce by disease causing microorganisms poses a risk to human health, and surveys of groundwater frequently detect low concentrations of pathogens. A mathematical model was modified to simulate the influence of field-scale variations in water velocity on pathogen transport and fate. Results demonstrate that pathogen migration in soil is very sensitive to velocity variations, and that high velocities regions (e.g., preferential flow pathways) will control the ultimate transport potential of pathogens in soils and groundwater. However, these results also depended on the amount of pathogen retention and water exchanged with lower velocity regions. This information will be of interest to scientists, engineers, consultants, government regulators, and health officials that are concerned about the microbial water quality.

Technical Abstract: A stream tube model was applied to simulate pathogen transport and fate in the subsurface at the field scale. Local-scale transport within each stream tube was described deterministically using analytic solutions for pathogen transport and fate in a single or dual permeability porous medium. Important pathogen transport and fate processes that were accounted for in an individual stream tube included: advection, dispersion, reversible and irreversible retention, and decay in the liquid and solid phases. The velocity in a stream tube was related to a median grain size using the Kozeny-Carman equation, and filtration theory was used to predict the dependence of retention on physicochemical factors. The field-scale velocity distribution was described using a unimodal or bimodal lognormal probability density function (PDF). The bimodal lognormal PDF was used in conjunction with the dual permeability model to account for exchange between slow and fast velocity domains. The mean and variance of the field-scale concentrations were calculated from local-scale stream tube information. The setback distance to achieve a selected risk of infection was determined from the modeled concentrations and a simplified risk assessment approach. Simulation results demonstrate that field-scale pathogen transport and setback distance were very sensitive to velocity distribution characteristics. Early breakthrough, higher peak concentrations, and larger setback distances were associated with high velocity domains that had little retention, whereas the opposite trends were associated with low velocity regions. The relative importance of high velocity regions increased under physicochemical conditions that enhanced retention, although the setback distance was also smaller.