HPx is a multicomponent reactive transport model which uses HYDRUS as the flow and transport solver and PHREEQC-3 as the biogeochemical solver. Some recent adaptations have significantly increased the flexibility of the software for different environmental and engineering applications. This paper gives an overview of the most significant changes of HPx, such as coupling transport properties to geochemical state variables, gas diffusion, and transport in two and three dimensions. OpenMP allows for parallel computing using shared memory. Enhancements for scripting may eventually simplify input definitions and create possibilities for defining templates for generic (sub)problems. We included a discussion of root solute uptake and colloid-affected solute transport to show that most or all of the comprehensive features of HYDRUS can be extended with geochemical information. Finally, an example is used to demonstrate how HPx, and similar reactive transport models, can be helpful in implementing different factors relevant for soil organic matter dynamics in soils. HPx offers a unique framework to couple spatial-temporal variations in water contents, temperatures, and water fluxes, with dissolved organic matter and CO2 transport, as well as bioturbation processes.
An understanding of preferential flow in the vadose zone is crucial for the prediction of the fate of pollutants.
Infiltration basins, developed to mitigate the adverse effects of impervious surfaces in urban areas, are established above
strongly heterogeneous and highly permeable deposits and thus are prone to preferential flow and enhanced pollutant
transport. This study numerically investigates the establishment of preferential flow in an infiltration basin in the Lyon
suburbs (France) established over a highly heterogeneous glaciofluvial deposit covering much of the Lyon region. An investigation
of the soil transect (13.5 m long and 2.5 m deep) provided full characterization of lithology and hydraulic
properties of present lithofacies. Numerical modeling with the HYDRUS-2D model of water flow in the transect was
used to identify the effects of individual lithofacies that constitute the deposit. Multiple scenarios that considered different
levels of heterogeneity were evaluated. Preferential flow was studied for several values of infiltration rates applied
after a long dry period. The numerical study shows that the high contrast in hydraulic properties of different lithofacies
triggers the establishment of preferential flow (capillary barriers and funneled flow). Preferential flow develops mainly
for low water fluxes imposed at the surface. The role of individual lithofacies in triggering preferential flow depends on
their shapes (layering versus inclusions) and their sizes. While lenses and inclusions produce preferential flow pathways,
the presence of the surface layer has no effect on the development of preferential flow and it only affects the effective
hydraulic conductivity of the heterogeneous transect.