Study On Nitrate Transportation In Agriculture Soil And Groundwater Contamination In Chengdu Plain

Since the 1970 s,water environment problems from agriculture non-point source became serious,especially the groundwater contamination due to the abuse of fertilizers.Nitrate is easy to accumulate in groundwater in some plain areas of China due to the characteristics such as intense agriculture activities,improper management,flat terrain,high infiltration of precipitation and shallow groundwater level.In this study,a series of numerical simulation models were developed,based on the software of HYDRUS-1D and Visual Modflow,to assess the responses of groundwater nitrate assessed under different scenarios of fertilizer application.The results of vadose and saturated zones showed that—the water content increased rapidly then gradually decreased after irrigation and no obvious fluctuation was observed beneath 400 cm.After fertilization application,the nitrate concentration changed by a large margin above 100 cm.A concentration peak was observed near the depth of 30 cm,which decreased and moved down afterward.Another peak was observed in the depth of 280~ 400 cm,indicating that nitrate migrated with water and accumulated in deep soil.The total leaching loss of nitrate during a rice-rapeseed rotation cycle is 36.3569 kg/ha(N).The leaching is effected by the precipitation and irrigation,which account for 48.56% from June to September.In the study area,groundwater nitrate concentration is obviously higher in agriculture area,and nitrate concentration rise significantly during winter compared with the relatively low values in summer,even though the high leaching during summer.It is because of dilution of the heavy precipitation infiltration during summer.Based on the analysis of groundwater nitrate concentrations under different amount of fertilizer application,higher fertilization(increase 2% per year)could result in an exceeding of the groundwater standard limited value(20 mg/L).Lower fertilizer application could contribute to relieve the accumulation of nitrate in groundwater,but a higher rate in fertilizer reduction(more than 2% per year)is also needed to obtain an overall decrease in the nitrate concentration and to improve groundwater quality.