Two‐dimensional simulation of bromide transport in a heterogeneous field soil with transient unsaturated flow

Summary As a consequence of spatial heterogeneity, solute transport and associated phenomena of soil in the field are typically complex and often poorly described by current one‐dimensional models. Therefore, better descriptions should be obtained if the specific variation of the soil is explicitly accounted for. To test this hypothesis, we simulated the two‐dimensional transport of a bromide trace in pore‐water velocity fields which varied in both space and time. We used data obtained in a field experiment in which we studied bromide transport as input parameters and boundary conditions for the simulation. We conceptualized heterogeneity at two spatial scales. Large regions with constant spatial properties were defined by the measured geometry of the soil’s horizons. On these we superimposed small structures derived from a correlated random field of scaling factors and conditioned on measured hydraulic properties. Simulations with homogeneous and heterogeneous horizons show that both coarse and fine hydraulic variation contribute to characteristic experimental phenomena such as concentration ‘hot spots’, especially near irregularities of horizon boundaries, and early breakthrough of solute leading to large dispersion at the field scale. The comparison between simulation and experimental results demonstrates that these features are reproduced if local hydraulic variation is included in the description of the transport. As expected, the quantitative agreement is limited, since the concentration distribution is strongly sensitive to local hydraulic conditions which were only partly accounted for by the measurements.