Visual Modflow Model Based ArcMap Correction Simulation On Nitrate Pollutants From Agricultural Non-point Sources In Linzhang County

In recent years,China’s agriculture has developed rapidly.At the same time,due to long-term cultivation,a large number of chemical fertilizers have been applied to agricultural production in China in order to increase agricultural production in the case of declining farmland soil fertility.This has caused excessive nitrogen in fertilizers that could not be absorbed and fixed to seep into groundwater under the leaching effect of precipitation and irrigation,resulting in an increasing nitrate concentration in groundwater.This phenomenon is particularly prominent in the plain areas of China.In this paper,Linzhang County,a plain area with developed agriculture,is taken as the research area,and the main research contents include the following three parts.First,based on its relatively complete hydrogeological data as a reference,Visual Modflow is used to establish a 3D hydrodynamic model of groundwater.Second,the interpolation method was used to complete the monitoring data of nitrate concentration in groundwater and describe the initial distribution of nitrate concentration in groundwater.Based on the Arc Map spatial analysis module and combined with the analysis of land use data in recent 10 years and the nitrogen fertilizer utilization of main crops in the study area,the temporal and spatial estimation of surface nitrate concentration load was carried out.Since the formation parameters in Linzhang County are relatively single,the surface salinity is evenly distributed,and the influence of geological processes on the infiltration of nitrate in the region is basically the same,the surface nitrate concentration can be used as the benchmark to compare the initial concentration of nitrate in groundwater,so as to calibrate the groundwater nitrate monitoring data supplemented by the interpolation method.Thirdly,the solute transport model of groundwater was established by using the corrected water quality data,and on this basis,the influence degree of different land use change scenarios on groundwater nitrate was predicted,so as to quantitatively analyze the relationship between agricultural production activities and groundwater nitrate.Through the simulation of groundwater nitrate nitrogen in the study area,it is found that if the existing cultivated land area and the annual conventional fertilization amount remain unchanged,after five years,the nitrate concentration value of local groundwater in Linzhang County will be close to or even exceed 20mg/L,which cannot meet the Class III drinking water standard for groundwater.The following conclusions and prevention countermeasures are made for the problem of groundwater nitrate concentration exceeding the standard:(1)Under the condition of keeping the annual fertilizer use area unchanged,the infiltration amount of nitrate nitrogen was reduced by using data simulation.It was found that the nitrate concentration in groundwater decreased significantly with the decrease of nitrate concentration.It shows that groundwater pollution can be reduced by reducing the infiltration of nitrate from agricultural non-point sources;(2)Reducing the infiltration concentration of nitrate from 9.34mg/L to 5mg/L can ensure that the underground nitrate concentration remains at the Class II drinking water standard.To reduce the infiltration concentration of nitrate,firstly,high nitrogen fertilizer can be used to improve the nitrogen absorption rate of crops while reducing the application amount of chemical fertilizer;Second,improve the soil environment,increase the content of organic matter and the number of nitrifying bacteria in the soil.This method will enhance the decomposition,transformation and fixation of nitrogen in the soil aeration zone,thus reducing the infiltration of nitrate.This study innovatively uses Visual Modflow to establish a groundwater solute transport model in the study area,which quantitatively reflects the impact of agricultural non-point source pollution on groundwater nitrate concentration in Linzhang County,and provides a theoretical basis for subsequent control of agricultural non-point source groundwater pollution;In addition,Arc Map model was used for spatial analysis in the research process,which solved the precision problem of model establishment caused by incomplete groundwater monitoring data in the long-term research process.