A modelling method for simulating nitrogen dynamics under the hydrodynamic context of river network

Abstract

River network comprised by varieties of interconnected rivers is an essential form of lotic environments. Unravelling the biogeochemical processes are essential for promoting the water quality and ecological health of river networks. However, biogeochemical processes in river networks are difficult to be quantified, since they are controlled by multi-directional mass transport processes and transformation processes associated with varieties of microbial metabolic pathways. To address this gap, the present research combined turbulence model and gene-centric model to establish a systematic modelling method to quantitively describe the nitrogen dynamics in a river network. The proposed model can capture the mass transport and microbial metabolic processes, and can accurately simulate the distribution characteristics of chemical materials and functional genes. The relative errors between simulated results and in-site measured data were low than 40%. With the help of the modelling method, this study further investigated the nitrogen dynamics in the river network. The results illustrated sulfide oxidation and nitrate reduction pathway and anaerobic ammonium oxidation pathway, participated by genes hzo and nap respectively, were the main pathways of NO3– and NO2– reduction. Besides, these two pathways exhibited high rates in all of the six confluence hydrodynamic zones (CHZs) of the river network and further increased the nitrogen removal efficiency. Moreover, the high-rate nitrogen removal processes continued in the downstream areas after CHZs, and the scope of such continuous influences increased with the discharge ratio between tributary and main stream. The modelling method and results in this research can be useful for prediction of water environmental quality and design of river connection engineering.