Explicit simulation of reactive microbial transport with a dual-permeability, two-site kinetic deposition formulation using the integrated surface-subsurface hydrological model HydroGeoSphere (rev. 2699)

Abstract. Assessing the transport behavior of microbes in surface water-groundwater systems is important to prevent contamination of drinking water resources by pathogens. While wellhead protection area (WHPA) delineation is still predominantly based on dye injection tests and advective transport modeling, size exclusion of colloid-sized microbes from the smaller and usually less conductive pore space causes a faster breakthrough and thus faster apparent transport of microbes compared to that of solutes. To provide a tool for better assessment of the differences between solute and microbial transport in surface water-groundwater systems, we here present the implementation of a dual-permeability, two-site kinetic deposition formulation for microbial transport in the integrated surface-subsurface hydrological model HydroGeoSphere (HGS). The implementation considers attachment, detachment and inactivation of microbes in both permeability regions and allows for multispecies transport. The dual-permeability, two-site kinetic deposition implementation in HGS was verified against an analytical solution for dual-permeability colloid transport and the suitability of the model for microbial transport at the wellfield scale is illustrated in a multi-tracer flow and transport study of an idealized alluvial riverbank filtration site. In this illustrative example, the transport of reactive microbes, conservative ⁴He, and reactive ²²²Rn was simulated in parallel, allowing mixing ratios, tracer breakthrough curves and travel times to be assessed via multiple approaches. The developed simulation tool is the first integrated surface-subsurface hydrological simulator for reactive solute and microbial transport, and marks an important advancement to unlock and quantify governing microbial transport processes in riverbank filtration settings. It enables meaningful WHPA delineation and risk assessments even under extreme hydrological situations such as flood events.