Nitrogen And Phosphorus Pollution Characteristics Of Farmland In The Hetao Irrigation District And Estimation Of Non-point Source Pollution Loads

Agricultural non-point source pollution has caused pollution of surface water as well as groundwater,leading to wetland degradation,red tides and a series of other global water environment challenges.Compared with 2007,total nitrogen and phosphorus emissions from agricultural non-point sources in China increased by 8.8%and 16.4%respectively in 2016,with Fujian,Xinjiang,Heilongjiang and Inner Mongolia were the top four provinces.Farmland irrigation in China is still dominated by large water irrigation,with farmland being the largest agricultural non-point source.The Inner Mongolia Hetao irrigation district is the largest self-made diversion irrigation district in Asia,discharges about 6900 km²of receding farmland water carrying nitrogen and phosphorus and other nutrients into the Wuliangsuhai.However,the current studies on pollution of farmland receding water in the Hetao irrigation district only focus on the loss of nitrogen and phosphorus in a specific period or overall analysis,and there is a lack of comparison between the pollution characteristics of farmland and drainage ditches and follow-up studies in different irrigation periods,and the studies on agricultural non-point source pollution only stay in the single aspect of estimating the pollutant export or the pollutant load into the lake,without closely linking the two sides.It is of great practical significance and scientific value to explore the pollution characteristics of farmland retreats in the Hetao irrigation district and to make quantitative estimates of farmland non-point source pollution loads,which can help clarify pollutant indicators,evaluate and strengthen the quantitative management of pollutants.Due to the complexity and uncertainty of agricultural non-point source pollution,model simulation has become an important tool to study agricultural non-point source pollution.Most scholars choose to use the export coefficient model for the analysis and research of agricultural non-point source pollution problems because of the characteristics of simple parameters and easy access,and the simulation accuracy is improving after improvement,calibration and rate determination.This paper analyzed the content of nitrogen and phosphorus pollutants in farmland surface water,drainage ditch water,farmland soil and drainage ditch substrate during different irrigation periods,so as to elucidate the water quality of farmland receding water in Hetao irrigation district and explore the distribution characteristics of farmland nitrogen and phosphorus pollution;used the improved export coefficient model（IECM）by the rainfall influence factor（α）and topographic influence factor（β）to estimate the farmland non-point source nitrogen and phosphorus pollution load in the area;combined the pollutant entry coefficient（γ）to explore the amount of farmland non-point source nitrogen and phosphorus entering the river（Wuliangsuhai）,and identified the critical source areas of farmland non-point source pollution,and the following conclusions were drawn:（1）The mean values of p H in farm field surface water,drainage ditch water,farm soil and drainage ditch substrate during the spring and summer irrigation periods in the Hetao irrigation district were during7.88～8.44,all showed a weak alkalinity,except for the farm field surface water during the summer irrigation period in August,the p H values at all sampling sites were farm field surface water>drainage ditch water,farm soil>drainage ditch substrate.The mean values of EC in water and soil during the spring and summer irrigation periods were during 0.75～21.26ms·cm^-1respectively,showed farm surface water<drainage ditch water,farm soil<drainage ditch bottom mud.The anions in the ionic composition of the water and soil were mainly Cl^-and HCO₃^-and the cations was showed that Na⁺is higher than Mg²⁺and Ca²⁺.Except for the cation and Cl^-content in the bottom mud of the drainage ditch during the summer irrigation period in August,which was lower than the cation and Cl^-in the farmland soil,the anion and cation contents of the rest of the sampling sites showed that the water of the drainage ditch>the surface water of the farmland,and the bottom mud of the drainage ditch>the farmland soil.（2）According to the National Environmental Quality Standard for Surface Water（GB 3838-2002）,with TN,TP and COD as assessment indicators,the water quality of farmland drainage water during the spring and summer irrigation periods in the Hetao irrigation district were in the V and poor V categories of the environmental quality standard for surface water,and the water quality of farmland drainage water during the summer irrigation period was better than that of farmland drainage water during the spring irrigation period.NO₃^--N>NH₄⁺-N in receding water and NO₃^--N<NH₄⁺-N in soil during spring and summer irrigation.TN content in water showed that farmland surface water>drainage ditch water,spring irrigation period in May>summer irrigation period in August,TN content in soil shows that farmland soil>drainage ditch bottom mud,spring irrigation period in May<summer irrigation period in August.The TP content in both water and soil during the spring irrigation period in May was farmland>drainage ditch,while the summer irrigation period in August showed the opposite.The COD content in water during the spring and summer irrigation periods showed that the surface water of the farmland<the water of the drainage ditch,while the average TOC content in the soil of the farmland and the subsoil of the drainage ditch did not vary much,at 606.11 mg·kg^-1and 609.10mg·kg^-1respectively.（3）The total non-point source TN and TP pollution load of different land use types in the Hetao irrigation district in 2021 were 4419.25 t and456.41 t,respectively.The land use type with the highest pollution load contribution was cropland,and the non-point source TN and TP export load of farmland were 3521.00 t and 407.13 t,accounting for the total TN and TP pollution load of different land use types,respectively.79.67%and 89.20%,respectively.The pattern of spatial distribution of total nitrogen and total phosphorus pollution loads was basically the same,but on the whole the distribution is uneven,showed the spatial distribution characteristics of local concentration of loads,large pollution loads in townships near water bodies and drainage,and less pollution loads in the western part of the irrigation area.（4）The total amount of total nitrogen and total phosphorus entered the river from non-point sources in farmland in the Hetao irrigation district were 387.31 t·a^-1and 16.29 t·a^-1respectively.And the generation and discharge of non-point source pollutants in Hetao irrigation district had strong spatial property.The mean values of TN and TP pollution load intensity of farmland non-point sources were 3.04 kg·ha^-1and 0.35kg·ha^-1,respectively.Combined with the pollution load intensity of pollutants,it was determined that Xinhua Town,Xin’an Town,Talhu Town,Longxingchang Town,Xixiaozhao,Tuanjie Town and Manhui Town were the critical towns of farmland non-point source pollution prevention in the Hetao irrigation district.（5）The improved export coefficient model was validated used the measured data of Wuyuan County in 2022,and the results showed that the relative errors of TN and TP pollution loads were-4.74%and 19.04%,respectively,indicating that the model simulation ability was relatively good.