Abstract: The Weihe River is one of the most polluted rivers in Haihe River Basin, and the effect of pollution on nearby shallow groundwater has been a great concern for many years. However this key issue has not been studied quantitatively. To explore this issue, Hydrus2d numerical models were constructed to simulate transport and transformation of nitrogen in the riverunsaturated zone, while RT3D module in GMS software was used to simulate nitrogen transport in saturated aquifers. Concentrations of pollutants leached at the bottom of vadose zone were regarded as the upper boundary condition of solute transport model in saturated zone. A joint simulation of nitrogen transport in riverunsaturated zonesaturated aquifer network was performed for the first time, which successfully delineated the influence of the polluted river on shallow groundwater. The simulation results show that, due to adsorption, nitrification and denitrification, the thickness of the vadose zone increases from the upper reaches of the river to the lower reaches, causing a decrease of NH4N and increase of NO2N and NO3N concentration in the aquifer. As the infiltration time increases, NH4N, NO2N and NO3N concentrations in the saturated aquifers gradually increase and stabilize eventually. The pollutant enters the vadose zone vertically with the infiltration water, while horizontal migration is the main form of transport in the saturated aquifer. The pollution plume disperses in the direction of groundwater flow, extending its influence as far as 500 m away from the river.

Key words: nitrogen, rivergroundwater system, numerical simulation, Hydrus-2d, GMS, Weihe River