Finite element model of nitrogen species transformation and transport in the unsaturated zone
A two-dimensional finite element model based on Galerkin's weighted residual approach and incorporating an upstream weighting technique was developed to predict the simultaneous transformation and transport of nitrogen species. The nitrogen cycle used in the analyses assumes first-order rate coefficients for nitrification, denitrification, immobilization and mineralization between the nitrogen species NH +4, NO -3, organic-N and denitrified fractions. The NO -3-N component in the soil solution was assumed to have negligible adsorption on exchange sites while adsorption of NH +4 was assumed to be represented by a Freundlich type nonlinear isotherm ( S = kdCp). The accuracy and validity of the proposed model was examined by comparison with analytical model results and available field data. The results showed improved accuracy and stability with the upstream weighting approach in comparison to the standard weighted residual method. A sensitivity study on the kinetics of the nitrogen cycle showed that both concentration and cumulative mass distribution of different nitrogen species are substantially affected by the nitrification rate but to a lesser extent by the distribution coefficient kd for NH +4 adsorption and less so by nonlinear coefficient p. To evaluate the applicability of the model in actual field situations, data on groundwater nitrogen concentration beneath a functioning drainfield site were compared with the results of numerical simulations. Predicted results agreed with the observed data within the bounds of experimental errors.