Présentations Colloques

Oral Presentation
Session 7.03: Agricultural and sanitation contaminants and implications for water services and health
Bradford Scott A.
Critical Role of High Permeability Zones on Field-Scale Pathogen Transport and Retention, Infection Risk, and Setback Distance
A stochastic stream tube model was extended to simulate field-scale pathogen transport and fate in the subsurface. 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 physicochemical factors on retention. The field-scale velocity distribution was described using a lognormal probability density function (PDF). Alternatively, a bimodal lognormal PDF was used in conjunction with the dual permeability model to predict the influence of preferential flow and 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 safe setback distance were very sensitivity 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. In contrast, increasing the rate of exchange between low and high velocity regions diminished the relative importance of high velocity regions on pathogen transport and setback distances.