Experimental and numerical study of transient water flow and solute transport in layered saturated-unsaturated porous media

The field problem under study is related to the effects of changes in river stage on the migration of a chemical from buried and surface sources where these sources are located in the bank zone of the shallow groundwater flow system. The main objective was to investigate experimentally and numerically the transport processes in a layered flow system in which solute was introduced from external and internal sources under varying hydraulic heads.; Conductivity studies, using saltwater as a tracer, were performed in a laboratory sand tank (6.80 m x 1.25 m x 0.15 m) to determine the characteristic features of breakthrough curves and plume migration in a layered system subjected to a rapid falling-rising-falling head condition for (i){A0}a plane source at the inlet end of the flow system, (ii){A0}a surface source, (iii){A0}an internal source located within the coarse sand layer, and (iv){A0}internal source at the interface between the fine and coarse sand layers.; The standard Galerkin finite element model with upstream weighting and artificial dispersion options was adopted for the case under study to simulate solute transport of a conservative substance in the saturated-unsaturated flow field. An alternative numerical method based on power series has been proposed for reducing some of the numerical difficulties normally encountered in such studies.; Comparison of the numerical results with the experimental results indicates that a good match for pressure head and concentration profiles are achieved when accounting for the variations in the spatial and temporal changes in the hydraulic properties of the media. For example, it was observed that the effect of small amounts of trapped air in the flow reduces the hydraulic conductivity and can have a significant effect on the advective transport processes. The plume shape was highly distorted due to layering and a clear separation of the plumes was observed and computed. Generally, the growth of the saltwater plume was much more pronounced in the longitudinal direction then in the vertical direction as the migration was dominated by advective transport.; Based on a sensitivity analysis, the longitudinal and transverse dispersivities for porous media in the sand tank were between 0.50 and 1.00 cm and between 0.01 and 0.02 cm, respectively. The sensitivity analysis of the dispersivity parameters showed that the numerical model could significantly overestimate the saltwater affected area when the higher values of these parameters were used in the simulations.