| The Yanqi Basin in Xinjiang is a typical dry inland river irrigated area,which is mainly planted with wheat,corn,processed tomatoes and pigmented peppers.The irrigated area in the basin grew from4.40×10~4hm~2 in 1956 to 14.19×10~4hm~2 in 2016,due to limited The surface water is difficult to maintain the existing irrigated area,thus resulting in over-exploitation of groundwater resources.The amount of groundwater resources in the basin is 9.20×10~8m~3.In 2016,the actual exploitation volume has reached7.97×10~8m~3,which is much larger than the recoverable volume of 4.00×10~8m~3.In recent years,with the excessive utilization of groundwater in the Yanqi Basin and the large amount of nitrogen fertilizer application,the nitrate nitrogen content in groundwater is rapidly increasing.In recent years,with the excessive utilization of groundwater in the Yanqi Basin and the large amount of nitrogen fertilizer application,the nitrate nitrogen content in groundwater is rapidly increasing.Based on this,the study focused on the migration and conversion of nitrate nitrogen from soil to groundwater.Field experiments and field investigations were conducted to analyze the migration process of nitrate-nitrogen in the aeration zone and related influencing factors for leaching,and to establish the model of solute transport in groundwater system MODFLOW water flow model and nitrate the coupling based on the further understanding of characteristics of migration in aquifer nitrate,to simulate scenarios of different mining methods and fertilization systems..The results are as follows:(1)Winter irrigation and farmland planting types have important determinants for the vertical migration of nitrate in soil.There is a difference between the nitrate content and the distribution characteristics between different soil profiles,and the trend is decreasing with the increase of soil depth.At the end of the fertility period,the nitrate nitrogen mainly accumulated in the soil layer of 0?60cm.the effect of different irrigation quota on soil nitrate nitrogen was most obvious in the surface layer(0?10cm).The effects of nitrate nitrogen in 60?100cm were obvious,and the rate of tomato,pepper and maize was 48.5%,27.7%and 42.5%respectively.In the whole 0?100cm soil layer,the soil leaching rate of tomato soil was 52.4%,which was easy to move to the depth of nitrate nitrogen.(2)The leaching process of nitrate nitrogen in soil profile is more complicated,which is influenced by the lithology of the soil texture,depth of water level,irrigation and nitrogen application.The annual increasing nitrogen fertilizer is an important factor affecting the deterioration of the quality of the diving water.It is growing rapidly at an average annual rate of 17.3kg/hm~2,which has caused the interannual rapid growth of the nitrate nitrogen concentration and the interannual variation of nitrate nitrogen concentration in the confined water.(3)The shallow groundwater in the irrigation area has been contaminated.The content of nitrate nitrogen in the dive has increased from less than 1mg/L in 1996 to the current 15.75 mg/L,and the rate of increase has accelerated.The exceeding standard rate(overrun value>10mg/L)reaches 21.5%,and the severe exceeding standard rate(>20 mg/L)reaches 8.1%,which is mainly distributed in the gravel area of the vadose zone.Although the concentration of nitrate nitrogen in middle deep groundwater is not over standard,it has the potential pollution risk.(4)The migration characteristics of nitrate nitrogen in different aquifers showed that the concentration of nitrate nitrogen in dive showed a linear upward trend,in 2030 the concentration increased to 11.43 mg/L,exceeding class II groundwater quality standard,and the concentration of nitrate nitrogen in shallow layer also continued to rise,and the concentration reached 4.00 mg/L in 2030,which aggravated the risk of nitrate pollution in groundwater.The concentration of nitrate nitrogen in the middle deep confined water is basically maintained in class I groundwater quality standard.(5)Through the four scenarios of the combination of different mining methods and fertilization systems,it is concluded that the degree of groundwater quality degradation is positively correlated with the scale of mining and fertilizer application,and the amount of groundwater extracted under the control of total amount of water used under scenario 2 is used to control the appropriate amount of fertilizer.It is a reasonable and feasible solution for developing water-saving and low-fertilizer technologies and maintaining groundwater quality in irrigation areas. |
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