Tracer Transport in Soils and Shallow Groundwater: Model Abstraction with Modern Tools.

Vadose zone controls contaminant transport from the surface to groundwater, and modeling transport in vadose zone has become a burgeoning field. Exceedingly complex models of subsurface contaminant transport are often inefficient. Model abstraction is the methodology for reducing the complexity of a simulation model while maintaining the validity of the simulation. The objective of this work was to apply model abstraction to characterize and understand flow and transport in soils in presence of shallow groundwater. To develop a case study, we carried the field experiment on the surface release of tracer with irrigation water and following irrigation to facilitate the subsurface transport. Soil moisture, soil water potential, tracer concentrations in groundwater and groundwater levels, and weather variables were monitored for four months. Fine-scale ground penetration radar survey, electric resistivity monitoring, and dilution tests complemented the borehole log data and laboratory hydraulic measurements to characterize soil heterogeneity. Preliminary simulations were done to evaluate the possible losses to runoff using ROSETTA and SWMS2D coupled with FEMWATER. Locations of observation wells and the expected experiment duration were estimated using HYDRUS3D. The HYDRUS3D was also used to build the 3D model of subsurface flow and transport. Then this model was abstracted to the 2D model that indicated that the field scale heterogeneities prevent a substantial part of soil material from active participation in flow and transport. Further abstraction of this model to 1D transport model was done with STANMOD that elucidated the directional differences in the tracer plume development. Abstractions of aggregation and pedotransfer have been also applied in HYDRUS3D to evaluate the desirable accuracy of soil material characterization. Overall, this work demonstrates that characterization of the vadose zone to understand flow and transport features can be greatly facilitated by the conceptual models and software suite that have been developed during last 20 years.