Numerical simulation of below ground nitrate dynamics in soil columns subjected to reclaimed water

A mathematical model was developed to simulate nitrate mass transport and transformations in soil during continuous application of reclaimed water in a laboratory scale soil column. The coupled material balance equations for both ammonia nitrogen (NH3-N) and nitrate nitrogen (NO3- -N) within the total soil volume were solved to simulate the NO3—N concentrations with time along the soil depth. The model is one-dimensional and based on the Galerkin technique of the Finite Element Method. It incorporates convection-dispersion processes of NH3-N and NO3--N, nitrification, denitrification and adsorption of ammonium on to soil grains. The adsorption of ammonium was assumed to be represented by the linear form of the Freundlich isotherm. The accuracy and validity of the developed model was examined by comparing simulated data with the experimental data. Optimization of the first order rate constants for nitrification (k1) and denitrification (k2) was conducted by varying both k1 and k2 within a wide range until the simulated NO3--N concentrations fitted properly with the corresponding measured values. Optimum k1 and k2 were found to be 0.188 d-1 and 0.0248 d-1, respectively. A sensitivity analysis of the kinetics of nitrate dynamics showed that the concentration of belowground nitrate is largely affected by the flow velocity (v), D, k1 and k2.