| The Hulan River Basin is located in the eastern of Songnen Plain in central Heilongjiang Province of China,which is an important agricultural production area.In recent years,agricultural production activities have caused serious problems of ammonia nitrogen pollution and threatened the production and lives of people in the river basin.Therefore,researchs on the migr ation process of ammonia nitrogen under irrigation conditions has attracted widespread attention.At present,the irrigation of paddy fields in this basin is mainly based on the exploitation of shallow groundwater.However,due to the Original geological conditions,the content of iron in the groundwater is relatively high.It has been shown in the literature that the presence of iron will affect the migration and transformation of ammonia nitrogen in the environment.The characteristics of groundwater quality in the watershed and the research on the impact of high-iron content groundwater irrigation on the migration process of ammonia nitrogen in the aeration zone will provide theoretical basis for the adjustment of agricultural production activities and the rational development and utilization of groundwater in the region.This article fully investigates the hydrogeological conditions and agricultural activities in the study area,and analyzes the water quality characteristics of shallow groundwater used for irrigation in the Hulan River Basin;on this basis,soil samples from the aeration zone in the irrigation area were collected to develop indoor iron and ammonia nitrogen The static adsorption and soil column co-migration simulation experiment quantitatively characterizes the effect of iron ions in irrigation water on the adsorption and migration behavior of ammonia nitrogen in different soil media;Hydrus-1D software is used to numerically calculate the ammonia nitrogen migration process of representative soil profiles in the study area.Based on the calculation results,the risk of ammonia nitrogen migration into groundwater is evaluated.There are several conclusions:1)The test results of 68 groundwater samples in the study area showed that 55.9% of the groundwater samples did not meet the groundwater class III water quality standards,of which manganese,iron and ammonia nitrogen were the main overstandard items,and the over-standard rates were 39.7%,20.6% and 19.1%,respectively.The maximum concentrations were 3.27 mg/L,14.4 mg/L,and 1.66 mg/L,which were 31.7 times,34.6 times,and 2.3 times the standard value of Class III.2)The adsorption behavior of ammonia nitrogen in different media types in the aeration zone of the irrigation area is obviously different.The adsorption capacity of clay for ammonia nitrogen is obviously greater than that of coarse sand,the adsorption capacity is 6.53 mg/kg and 21.5 mg/kg respectively,and the clay is 3.29 times of coarse sand.When the irrigation water contains iron ions,the adsorption capacity of clay to ammonia nitrogen is weakened,and the adsorption capacity is reduced to 16.8mg/kg,which is reduced by 0.78 times;while the adsorption capacity of coarse sand to ammonia nitrogen is enhanced,and the adsorption capacity is increased to 8.89 mg/kg.,Expanded by 1.36 times.3)The migration ability of ammonia nitrogen in different soil media is as follows: the migration ability of ammonia nitrogen in clay is significantly lower than that of coarse sand,and the hysteresis factors(Rs)are 33.9 and 18.It means that the migration ability of ammonia nitrogen in clay is 0.55 times that of coarse sand.The migration ability increases with R value of ammonia nitrogen in clay becomes 30.7 under high-iron water irrigation conditions.while But the migration ability decreases with R value in coarse sand becomes 22.5 under high-iron water irrigation conditions.4)Adsorption and conversion are influencing factors of ammonia-nitrogen migration ability.When the medium is coarse sand,the migration process is mainly affected by adsorption,while when the medium is clay,the migration process is affected by both adsorption and conversion.Under high-iron water irrigation conditions the conversion ability of ammonia nitrogen has been significantly improved,and the conversion amount of ammonia nitrogen can be increased from 0.87 mg/L to 4.32 mg/L,which is expanded by 4.97 times.5)Numerical calculation results of ammonia-nitrogen migration based on Hydrus-1D show that the formation structure of different aeration zones determines the time for ammonia nitrogen to enter the groundwater.Where the clay layer is thick,it takes a long time for ammonia nitrogen to penetrate the aeration zone,while clay and subclay When the thickness of the layer is similar,the effect of the thickness of the coarse sand layer on ammonia nitrogen is also very obvious.The thickness of the coarse sand at simulation point 4 is about 2.5 times the thickness at point 3,but the final ammonia nitrogen concentration value is different by 4 times.6)According to the length of time that the concentration of ammonia nitrogen migrated to the groundwater surface reached the groundwater level III water quality standard,the risk level of ammonia nitrogen pollution in the paddy field of the study area was evaluated.The results showed that: when using no(low)molten iron for irrigation,the central,northern Southwest of Kui County is a low pollution risk area,west of Wangkui County,west and northeast of North Forest District,northeast of Lanxi County is a medium pollution risk area,southwest of Helen City,northwest of Wangkui County,east of Beilin District,Qing'an County Central and eastern Lanxi County are high-pollution risk areas,and the total area of low-risk,medium-risk and high-risk areas account for 19.8%,21.6% and 58.6% of the paddy field research area respectively;when the irrigation water is changed to high-speed groundwater,it will cause some areas Pollution risk has increased,and the southwestern part of Wangkui County has changed from a low-pollution risk zone to a medium-pollution risk zone.The total area of low-risk areas has dropped to 13.8%,while the total area of mediumrisk and high-risk areas has increased to 25.5% and 60.7%,respectively.7)In view of the problem of ammonia nitrogen pollution of groundwater in the Hulan River Basin,in order to protect groundwater resources and curb the further deterioration of groundwater ammonia nitrogen pollution,the following suggestions are proposed: for high-risk areas,the amount of nitrogen fertilizer should be reduced,the nitrogen fertilizer should be reasonably applied,and the groundwater will be reduced by ammonia nitrogen The possibility of pollution,while reducing the use of groundwater for irrigation,surface water as much as possible.Medium-risk areas try to use groundwater with lower iron content for irrigation. |
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