Numerical modeling of water infiltration and nitrate movement through vadose zone using dual porosity approach

Extensive fertilizer application on agricultural fields can be a major threat to the quality of subsurface and surface water bodies by leaching and return flow of fertilizer from the field. Our hypothesis is that water and nitrate transfer between mobile and immobile regimes of the soil can play a substantial role in the nitrate leaching to the groundwater. To improve understanding of mobile immobile exchange process sand tank experiment data of nitrate leachate were analyzed under different boundary condition with single and dual porosity model approaches. For this study a sand tank of L-60 cm × W-30 cm × D-60 cm was fabricated to conduct the laboratory experiments. A porous medium having grain size of 0.5-1.0 mm free from organic content was packed homogeneously for conducting steady state experiments. A steady flux of 150 mL/hr. having 300 ppm nitrate concentration was allowed to flow in a vertical direction. The pore water samples were collected hourly from the sampling ports and were analysed using a spectrophotometer. Numerical simulation was conducted of the same domain size incorporating single porosity model and dual porosity model having 15% and 20% mobile regimes. The observed breakthrough curves (BTCs) at different ports show the higher cumulative nitrate flux at bottom in tank setup than 15% mobile domain followed by 20% mobile domain. Similarly, the bottom equilibrium nitrate concentration is higher in laboratory setup than 15% mobile domain followed by 20% mobile domain. This higher nitrate flux and concentration at bottom is due to less retention and more restriction of vertical movement of flux in the laboratory experiment. The residual nitrate mass were computed and its shows maximum in single porosity than 20% mobile followed by 15% mobile regions during simulation of nitrate movement through the vadose zone for the entire time duration. These observations could be explained by more exchange of dissolved nitrate from immobile to mobile regimes through the study domain. Overall, the sand tank results well matched to the domain using a dual porosity approach with 15% mobile regions. Furthermore, small scale process like mass transfer between mobile and immobile regimes should be included in field scale model prediction of nitrate leaching from the agricultural lands.