The objective of this study was to develop a realistic model to simulate the complex processes of flow and tracer transport in variably saturated soils and to compare simulation results with the detailed monitoring observations. The USDA-ARS OPE3 field site was selected for the case study due to availability of the extensive database on geophysical, biophysical, remote sensing, soil water and groundwater monitoring accumulated in past 10 years. High soil spatial heterogeneity and shallow groundwater were encountered at the site. A pulse of KCL solution was applied to the irrigation plot with an area of 13x14m, and then the plot was irrigated twice a day for three months using the tap water. The chloride concentration was measured at three sampling depths of 12 observation wells at 7 m and 14 m distances from the irrigation plot. Two three-dimensional flow and transport models, HYDRUS-3D and TOUGH2 codes, were applied in this study to simulate flow and chloride transport for the tracer experiment. Respectively, these models represent two conceptual approaches for flow and transport: (a) transport through all of the domain (b) preferential transport via a network of well-connected pathways (fractures) that comprise only part of the bulk soil porosity (matrix) at the field scale, assuming mass exchange between fracture and matrix continua. The atmospheric boundary condition was applied at surface of the model domain and no flux boundary was set at the bottom. The spatially variable and temporally constant pressure heads were specified along the lateral boundaries. The trial-and-error calibration based on zonation of the flow and transport domain was reasonably successful. The maximum concentration levels and the 50% concentration arrival times were described satisfactory. However, with HYDRUS3D we could not accurately reproduce the shape of the breakthrough curves, unlike TOUGH2 that better simulated the front and tailing part of observed breakthrough curves. The simulation results will be used to provide the information for improving the monitoring of flow and transport in variably saturated soils.
|State||Published - 1 Dec 2009|
- 1847 HYDROLOGY / Modeling
- 1875 HYDROLOGY / Vadose zone