Modelling The Effects Of Land-use Change On Groundwater Non-point Nitrate Contamination

In recent years,the excessive fertilizer application in agricultural activities results in fertilizer leaching down to groundwater,which increases the concentration of nitrate in groundwater.The high level of nitrate in groundwater is particularly in some plain areas of China due to the characteristics of agricultural activities,flat terrain,low surface runoff,high rainfall infiltration and shallow groundwater table.Based on a long term recorded data,high level of groundwater exploitation and utilization,and relatively compete hydrogeological data,Jingyang District of Deyang City was chosen as a study area.The thesis studied the historical land use data and carried out the temporal and spatial analysis on nitrate loads with ArcMap and groundwater model methods.A migration model of groundwater pollution was built and calibrated to predict the impact of nitrate in groundwater under different land use scenarios,revealing the effect of land use change to tackle the agricultural non-point sources pollution.The results of the study include:1.From 2000 to 2016,the nitrate loads in the whole study area increased year by year and an obviously regional characteristic was observed.The nitrate contamination evolved from south to north.The groundwater flows from Dexin to Huangxu.The area of nitrate plumes in Huangxu Town increased by ten times during the past ten years.In the vicinity of the urban area,the agricultural pollution load is less than 230 kg/km2,and the shorter route of the groundwater recharge near the Mianyuan River and Shiting river benefit to the dilution of the nitrate pollution in the groundwater,making the nitrate pollution in urban and south area reduce.2.The nitrate loads from agriculture in the study area varies greatly between seasons.Heavy rainfall in August and July accelerated nitrate to leach and also dilute the nitrate concentration in the groundwater.The concentration of nitrate leaching into the aquifer increased due to the higher average monthly rainfall in September and October and the monthly average nitrate loads.In December and January the situation is reversed.In February,even the fertilization load is high,the amount of nitrate leaching into the aquifer was limited by the low rainfall rate in the month.3.Based on the synthetical forecast analysis of land use change in the study area,simply increase of the area of cultivated land will lead to a larger contaminated area,with local groundwater nitrate concentration higher than 20 mg/L.Appropriate fertilizer application can reduce agricultural non-point pollution load and significantly reduce the area of contamination in the region,promoting the method of soil testing formula.However,in generally,the pollution of nitrate in some areas was serious compared with that in 2016,the area of contamination plume increases 9.33%.This study shows that the land use change has strong temporal and spatial distribution.The groundwater numerical simulation combined with ArcGIS spatial analysis can solve the problem of the complexity and randomness of the non-point contamination.It is helpful to provide a theoretical basis for controlling the non-point pollution of groundwater agriculture.