Nitrogen And Phosphorus Pollution In Shallow Groundwater Of Mudong River Watershed In Huixian Karst Wetland And Its Simulation Model

At present,irrational use of groundwater resources in Huixian wetland has caused a series of environmental problems that endanger the survival of local residents.Through agricultural activities,nitrogen,phosphorus and other pollutants caused by surface runoff and underground infiltration have caused serious pollution to groundwater.Therefore,it is of great practical significance to explore the degree of groundwater pollution caused by non-point source pollution and find out the migration process of nitrogen and phosphorus pollutants in the shallow groundwater of karst wetland for the management and sustainable utilization of groundwater resources in the whole region.Mudong River watershed（31.09 km²）of Huixian karst wetland in Guilin city,was chosen as the study area.Field investigation,monitoring test,indoor experiment and model simulation methods are used in the study.Through two years of shallow groundwater,surface water and nitrogen phosphorus monitoring test,analyzes the groundwater in the total nitrogen,nitrate nitrogen,total phosphorus content.The correlation between nitrogen and phosphorus pollutants was examined to reveal their spatial and temporal distribution characteristics in the study area.According to the Groundwater Environmental Quality Standard（GB/T14848-2017）,the groundwater quality grade was evaluated by using the improved SPA-TFN model method and other methods.Based on the Groundwater Modeling System（GMS）,a numerical model of groundwater flow in the Mudong River watershed of Huixian Wetland was established to simulate the water level and solute transport in the study area.The main conclusions are as follows:（1）According to the comprehensive analysis of the monitoring data of two years,in terms of time,the concentration of TP in the irrigated stage was higher than that in the non-irrigated stage,with a peak value of 0.98mg/L,the concentration of NO₃^--N and TN varied significantly between years,with higher concentration in the irrigation stage than that in non-irrigation stage in 2019,and on the contrary in 2020.The concentration of NO₃^--N did not change significantly during the two years,with an average of 4.68-6.49mg/L.In spatial distribution,the concentration of nitrogen and phosphorus pollution in shallow groundwater gradually increased from the northern and southern ends of the study area to the central watershed.The analysis of variance of repeated data showed that the concentrations of TN and NO₃^--N were significantly different in space（P=0.016,0.021）,while the concentrations of TP were not significantly different in time and space,and there was no significant difference between groups.In terms of nitrogen and phosphorus correlation,the results of the pearson correlation showed that there was a significant positive correlation between TN and NO₃^--N concentration during irrigation-non-irrigated period in 2020.（2）According to the comprehensive evaluation of the three methods,during the monitoring period from 2019 to 2020,68.75%-69.23%of the nitrogen and phosphorus indicators of the shallow groundwater of the monitoring Wells belong to classⅤwater quality standard.TN pollution was more serious than that of TP.And the water quality of shallow groundwater was worse than that of surface rivers.（3）The flow simulation results based on GMS software showed that the difference between the measured value and the simulated value of more than 80%monitoring data was less than 1.2m.The TN concentration identification model showed that the difference between the simulated and monitored values was20%～40%in 22.2%of cases,and less than 20%in 66.7%of cases,which means that the boundary conditions,source and sink terms,hydrogeological parameters and water head setting in the model were basically in line with the actual situation.Five-year simulation results show that the nitrogen and phosphorus pollutants will be mainly in the Doumen Village and the Mudong lake on the lower right of the study area,and the pollution would be more and more serious around the Fenghuang mountain on the lower left area because of the farms and fields.The average concentration of TN in the area will rise from 7 mg/L to more than 18 mg/L,with a tendency to spread eastward to Mudong Village.