Solute transport with time-variable flow paths during upward and downward flux in a heterogeneous unsaturated porous medium

To acquire knowledge of solute transport through the unsaturated zone in the shallow subsurface is decisiveto assess groundwater quality, nutrient cycling or to plan remediation strategies. The shallow subsurface ischaracterized by structural heterogeneity and strongly influenced by atmospheric conditions. This leads tochanging flow directions, strong temporal changes in saturation and heterogeneous water fluxes during infiltrationand evaporation events.Recent studies (e.g. Lehmann and Or, 2009; Bechtold et al.,2011) demonstrated the importance of lateral flowand solute transport during evaporation conditions (upward flux). The heterogeneous structure in these studieswas constructed using two types of sand with strong material contrasts and arranged in parallel with a verticalorientation. Lateral transport and redistribution of solute from coarse to fine media was observed deeper in thesoil column and from fine to coarse close to the soil surface. However, if boundary conditions are reversed due toprecipitation, the flow field is not necessarily reversed in the same manner, resulting in entirely different transportpatterns for downward and upward flow. Therefore, considering net-flow rates alone is misleading when describingtransport under those conditions. In this contribution we analyze transport of a solute in the shallow subsurface toassess effects resulting from the temporal change of heterogeneous soil structures due to dynamic flow conditions.Two-dimensional numerical simulations of unsaturated flow and transport are conducted using a coupled fi-nite volume and random walk particle tracking algorithm to quantify solute transport and leaching rates. Followingprevious studies (Lehmann and Or, 2009; Bechtold et al., 2011), the chosen domain is composed of two materials,coarse and fine sand, arranged in parallel with a vertical orientation. Hence, one sharp interface of strong materialheterogeneity is induced. During evaporation both sands are assumed to stay under liquid-flow dominatedevaporation conditions (“stage 1”). Simulations considering dynamic (infiltration-evaporation) and steady (solelyinfiltration) boundary conditions are carried out. The influence of dynamic boundary conditions (intensity andduration of precipitation and evaporation events) is examined in a multitude of simulations. If flow rates smallerthan the saturated hydraulic conductivity of both materials are chosen to be applied as boundary condition,simulation results indicate that the flow field within the domain is exactly reversed. However, if applied flow ratesexceed the saturated hydraulic conductivity of one material, the flow field is not just reversed, but different flowpaths during downward and upward flow are observed. Results show the tendency of faster solute leaching underdynamic boundary conditions compared to steady infiltration conditions with the same net-infiltration rate. We usea double domain transport method as an upscaled model to reproduce vertically averaged concentration profileswith net flux only and compare the model parameters for information about flow dynamics and soil heterogeneity