Formation Process Of Inorganic Nitrogen Pollution In Shallow Groundwater Of Songhua And Naoli River Basin,Sanjiang Plain

The inorganic nitrogen pollution of shallow groundwater in China’s important commodity grain production base,Sanjiang Plain,will inevitably restrict the sustainable development of local society,economy and environment,also has some adverse effect on the grain security of the country.Focused on the Songhua and Naoli River basin of Sanjiang Plain,this assay analyzed the character of inorganic nitrogen pollution in shallow groundwater of 2011-2015,then conducted some field surveys both in irrigation area and regional scale.The distribution of inorganic nitrogen in shallow groundwater has been described,and multivariate statistical analysis has been adopted to reveal the main factors governing the distribution.Chemical ion and Stable isotope were combined to identify the sources of nitrogen pollution.This assay has also analyzed and modeled the formation progress of pollution,then estimated the nitrogen pollution risk of regional groundwater,which will provide scientific basis for pollution treatment.Some conclusions have been drawn as below:(1)The land use type and sampling well depth have a great influence on the distribution of inorganic nitrogen.The performance is as follows: the nitrate content of groundwater in residential land is significantly higher than that of farmland,and the ammonium is reversed;the samples from deeper sampling wells have higher ammonium content but lower nitrate content.With regards to spatial distribution,the Naolihe River Basin is dominated by ammonium pollution,while the Songhuajiang River Basin is dominated by nitrate pollution.The sanples of highest concentration ammonium are distributed in the districts with a large amount of fertilizer and agricultural development,and the nitrate pollution mainly in densely populated areas.From August(crop growing period)to October(crop ripening period),the nitrogen pollution in the whole area generally slowed down,but the single well dynamics in typical irrigation areas showed inconsistent trends,which may be affected by fertilization activities,rainfall,changes in soil redox environment after paddy drainage,and changes in regional groundwater flow after irrigation periods.(2)The factors governing the distribution of ammonium in the research area are the thickness of the surface clay layer and the watershed,for nitrate are the domestic waste,the redox environment of the aquifer,and the watershed.For the typical irrigation area of the Songhuajiang River Basin,the governing factor of ammonium distribution is the redox environment of the aquifer,for nitrate are the distribution of surface pollution sources,the redox environment of aquifer,and the sampling well depth.For the typical irrigation area of the Naolihe River Basin,the governing factor of ammonium pollution is agricultural pollution,for nitrate pollution are domestic waste and aquifer redox environment.(3)The ammonium pollution of shallow groundwater mainly comes from the fertilizers such as ammonium fertilizer and urea applied in paddy fields.The nitrate mainly comes from rural domestic pollution,livestock manure,organic nitrogen fertilizer in the vegetation field,nitrogen fertilizer in dry land,and urban landfill leachate,sewage pipe leakage.(4)Based on land use types(paddy fields,dry fields,residential land),vadose zone environment(oxidation,reduction),aquifer environment(oxidation,reduction),the formation process of nitrogen pollution in groundwater is generalized into 12 modes.The pollution risks of ammonium and nitrate in shallow groundwater under each model were analyzed.It is considered that the mode of “Paddy field – Reduction vadose zone – Reduction aquifer” has the largest ammonium risk,and the mode of “Residential land – Oxidation vadose zone – Oxidation aquifer” shares the largest nitrate pollution risk.(5)The Hydrus-1D numerical model was used to simulate the 12 kinds of pollution formation modes.According to the simulation results,the risk of ammonium and nitrate pollution of each mode was divided into four levels.The pollution risks of ammonium and nitrate in groundwater of the study area were mapped.