Experimental Study On Nitrate Nitrogenmigration Mechanism In Riverside Farmlandunder The Combined Action Of Surface Waterand Groundwater

With the rapid development of industry and agriculture,the problem of groundwater nitrate pollution has attracted more and more attention.Riverside farmland is one of the most common types of farmland.Nitrate is easy to enter surface water and groundwater after fertilization,which pollutes surface water-groundwater system.In this paper,the problem of nitrate pollution in the Dagu River Basin was investigated.Four groups of laboratory experiments were designed to study the effects of different factors on the migration of nitrate into the surface water-groundwater system.The main conclusions were as follows:(1)The soil,surface water and groundwater in the Dagu River Basin have been contaminated with different degrees of nitrate,and groundwater nitrate pollution is the most serious.(2)The results of pure leaching experiments show that the higher the rainfall intensity and the greater the porosity of the medium,the faster the infiltration water migration rate and the deeper the migration depth,the easier the nitrate migrates downward.In the pure leaching experiment,the infiltration water flow mainly migrates vertically in the vadose zone,and the nitrates in the sand tanks enter the groundwater almost at the same time,and the diffusion rate is very slow after entering the groundwater.(3)Experiment of surface water recharge groundwater shows that there is a uniform infiltration curve between surface water and groundwater when the hydraulic gradient of surface water and groundwater is 0.3,0.5 and 0.7,but when the hydraulic gradient increases to 1,the surface water and groundwater will be out of joint.When the water level of the river is constant,the increase of the water level in the pumping well makes it easier for nitrate to accumulate in the vadose zone.When the surface water replenishes the groundwater and the two are not disconnected,the nitrate contaminates the entire groundwater through two processes.The first process is the migration in the vadose zone until it enters the groundwater,and the second process is the migration in saturated zone until entering the pumping well.When the rainfall intensity is the same,the time required for nitrate to migrate in the aeration zone is mainly affected by surface water level,groundwater level and sandy species;the time required for migration in the saturated zone is mainly affected by hydraulic gradient and sandy species.(4)Groundwater recharge surface water leaching experiment results show that when the hydraulic connection for groundwater recharge surface water,groundwater level will be high,which makes even small rainfall can also allow nitrate migration into groundwater.Compared with the first two groups of experiments,the infiltration water in this group of experiments migrates slowest in the aerated zone,and the residence time is longer,and nitrate accumulates in the aerated zone.(5)The results of the reciprocal recharge experiment between surface water and groundwater show that the nitrate content in the sand trough can reach 91.36 mg/kg after 6 times of continuous operation,the average nitrate content in the surface of the sand trough can reach 89.37 mg/kg,and the average nitrate content in the sand layer at10 cm depth can reach 85.5 mg/kg.The greater the hydraulic gradient is,the easier the nitrate will enter the groundwater.As the soil colloids repel nitrate,the ability of sand layer to accumulate nitrate becomes less and less with the increase of the number of experiments.(6)According to the above experimental data,the infiltration water can be divided into three stages from the beginning of rainfall to the pumping well when the sand is uniform:1 pure infiltration stage.2 semi-infiltration semi-transportation stage.3 pure with the flow migration stage.