Solute Mass Transfer Effects in Two‐Dimensional Dual‐Permeability Modeling of Bromide Leaching From a Tile‐Drained Field

Abstract

Preferential flow (PF) depends on processes and structures in soil at the small scale and can affect flow and transport processes at much larger scales. For studying PF processes, the discharge and solute effluent from subsurface drained experimental fields has frequently been used as a field‐integrated signal that included combined effects of macropore flow and lateral transport toward the drain. The objective of this study was to better understand effects of the mass transfer coefficients on bromide (Br) leaching in a two‐dimensional (2D) dual‐permeability concept. The Br leaching was simulated for data of a Br tracer irrigation experiment on a drained field (5000 m2 area) at Bokhorst (Germany), where soils developed from glacial till sediments. Flow and transport in 2D vertical cross‐sections was described using a numerical 2D dual‐permeability model. For applied Br, influx of Br only in the soil matrix (SM) domain, only in the preferential flow (PF) domain, and proportional to the water influx in both domains was considered to assess the impact of small‐scale redistribution processes occurring at the structured soil surface on field‐ and plot‐scale transport. Three values of the water and four of the solute mass transfer rate coefficients were tested to imitate local effects (e.g., of clay‐organic coatings) on the inter‐domain mass transfer. The local‐scale solute mass transfer between PF and SM domains had a clear impact on Br concentrations in drain effluent at the field scale; concentrations mainly increased more rapidly with smaller values of the diffusive mass transfer coefficient. In the 2D flow domain, representing the plot scale, mass transfer rates were temporally and spatially variable with varying importance of diffusive and advective components depending on the influx rates; local effects were still significant at the field scale. Small‐scale properties and processes such as domain‐specific infiltration and mass transfer in structured soil seem important for improving the description of larger‐scale flow and transport processes.