Nitrogen Numerical Simulation In Beijing Urban Groundwater And Monitored Natural Attenuation About Groundwater

Because of the severe groundwater pollution in the suburb of Beijing, representative contaminating materials like NO₃-N, NO₂-N and NH₄-N were selected to study their transport and transformation model in the confined aquifer. The numerical model of the transport and transformation of ammonia-nitrite-nitrates in groundwater was then formed to simulate the process. Monitored natural attenuation of the groundwater was analyzed and evaluated.The research was conducted on the software Visual MODFLOW. The flow model was established in the first instance based on the MODFLOW module. Through generalizing the hydrogeologic condition in the research zone, calculating the equilibrium item, inputting the parameters and initial flow, choosing the simulating period, the initial flow model was established. Reasonable parameters were acquired after debugging time and again and correcting the model, and groundwater flow field model in accord with the actual hydrogeologic condition of the research zone was formed.The solute transport model including NO₃-N, NO₂-N and NH₄-N three ions was established based on the accurate flow model. According to the transport and interaction response of the three solutes, the model was generalized to accord with linear isotherm sorption, convective dispersion, and the first-order irreversible decay reaction. After debugging and verifying the model time and again, the suitable response parameters were acquired to meet the model precision when used in the actual simulation.According to the simulating results, it was indicated that the concentration variety of the ammonia-nitrite-nitrates polluting substances in the aquifer was different between July 2004 and June 2008. The NO₃-N、the NO₂-N and NH4-N concentration decreased. On the basis of the simulating results, the monitored natural attenuation of the groundwater was evaluated. It is indicated that the monitored natural attenuation to the nitrogen solute of the groundwater in the research zone became weaken gradually form west to east horizontally. The monitored natural attenuation to NO₃-N of the groundwater was weak but the monitored natural attenuation to NO₂-N and NH₄-N was better in the unconfined aquifer. The monitored natural attenuation to NO₃-N was better but The monitored natural attenuation to NH₄-N was weak in the confined aquifer. Finally it was believed that the monitored natural attenuation to nitrogen polluting substances of the groundwater in the research zone had certain effect which was not sufficient to be taken as the main restoration method. The artificial restoration facility was needed to control the diffusion of the ammonia-nitrite-nitrates polluting substances in the groundwater.