Simulation of fate and transformation of nitrogen compounds in groundwater using meshless weak strong (MWS) form...

In this study, a two-dimensional coupled flow and multi-species reactive transport model involving the biogeochemical processes is presented for the investigation of transformation and prediction of concentrations of nitrogen species in groundwater. The kinetic equations of the nitrification, denitrification and oxidation processes combined with the groundwater reactive transport equation are solved using the Meshless Weak Strong (MWS) form method. The fate of ammonium‑nitrogen (NH₄), nitrite‑nitrogen (NO₂), nitrate‑nitrogen (NO₃), and nitrogen (N₂) subject to reactions with dissolved oxygen and dissolved organic carbon (DOC) in the presence of three bacterial species is simulated using the MWS model. This novel MWS model is developed in MATLAB by coupling the equivalent algebraic formulations of flow and reactive transport by Meshless Local Petrov Galerkin (MLPG) method and Radial Point Collocation Method (RPCM). The flow simulation is verified using the well-established MODFLOW model. The MWS transport model is validated with column experiment and field data available in the literature, indicating the strong prediction accuracy of MWS and emphasizing the importance of accounting for biogeochemical reactions. Further, the MWS model is applied for the investigation of nitrogen transformation and transport process in a large aquifer, by introducing hypothetical NH₄ contamination in a real field aquifer. Results of nitrogen transport modeling demonstrate the eventual presence of NO₂, NO₃ and N₂ in the aquifer, despite their initial absence and zero injection. Thus, the case studies highlight the effectiveness of the presented MWS model in the simulation of biogeochemical processes of nitrogen species in aquifer environments.