The Transportation And Transformation Of "Three Nitrogen" In Shallow Groundwater And The Retarding Effect Of Aquitard In Tongzhou,Beijing

Water is the source of human life.Groundwater is one of the important water sources for the production and living of residents.It is a reserve resource and strategic resource for social and economic development.With the rapid development of the industrial and agricultural economy,the problem of groundwater pollution has become more prominent,which has significantly increased the contradiction of water shortage.As the main pollutant of groundwater in agricultural areas,“three nitrogen”has become one of the most common and most polluted groundwater pollutants at home and abroad.Tongzhou District of Beijing is the axis of the Beijing-Tianjin Economic Belt.It is an important food and vegetable production base in Beijing.The local residents'production and life depend on groundwater.The preliminary work shows that the diving in the study area has been polluted to varying degrees.Therefore,the study of the transportation and transformation of"three nitrogen"in the phreatic groundwater and the retarding effection of aquitard which between the aquifer and the confined aquifer have theoretical and practical significance for the rational utilization of groundwater resources in Tongzhou,Beijing.The research was supported by:Study on The Migration Law of Groundwater Main Pollutants And Evaluation of Antifouling Performance,selected phreatic gruondwater in Tongzhou District of Beijing as the resarch objection.The problem of“three nitrogen”transportation and transformation in phreatic groundwater and aquitard retarding,based on hydrogeology and hydrogeochemistry,through the combination of field investigation and laboratory experiment,to reveal the phreatic water and soil characteristics of the study area and the transportation and transformation of“three nitrogens”in the phreatic groundwater aquifer.Furthermore,studyed the retarding effection of the aquitard by means of numerical simulation and numerical calculation.The main research results are as follows:1.Based on the characteristics of shallow soil physical and chemical test analysis,it is concluded that:?1?The shallow soil in the study area is mainly medium sand,fine sand,silt sand,silt and silty clay.The pore type is mesoporous,the specific surface area is 2³0m²/g,and the permeability coefficient of aquitard is 0.42¹4.80mm/d.?2?The soil chemical type is mainly HCO₃-Ca·Na,and the high concentrations of NO₃^-and SO₄^2-are present in some areas with serious human pollution.2.Based on the hydrochemical characteristics and genesis of phreatic groundwater,evaluated the water quality of phreatic groundwater by comprehensive appraisal and Nemero index.The results show that:?1?The phreatic water hydrochemical characteristic is mainly HCO₃-Ca·Mg?HCO₃-Ca·Na or HCO₃-Ca·Mg·Na?and HCO₃·Cl-Mg·Na?HCO₃·Cl-Na·Ca?,higher concentrations of CO₃^2-,NO₃^-and SO₄^2-in areas with more serious man-made pollution.?2?The phreatic water presents medium-weak alkalinity,pH is 6.97⁷.89,and the TDS tends to increase with the flow of groundwater,but the phreatic water is mainly of fresh water.?3?The phreatic water quality is mainly at Grade III;some at Grade?,mainly distributed in the Majuqiao and Yujiawu area;a small part is Grade IV,distributed in the south side of Yongledian Town;very few is Grade I,located at the junction of Jixian Town and Xiji Town.The main pollutants are NH₄-N,NO₃-N,F and As,reflecting the serious impact of phreatic water on agriculture and domestic sewage.?4?The soil minerals are mainly quartz,calcite,feldspar,illite,and montmorillonite,which provide ionic components for phreatic water.Under the influence of geological features and climate,the concentration of ions in phreatic water is changed by leaching and evaporation and concentration,and the concentration of pollutants is significantly increased under the influence of human activities.3.According to the adsorption theory and the theory of nitrogen transfer and transformation,static adsorption,dynamic adsorption and microbial culture experiments were carried out.The results show that:?1?The“three nitrogen”tansportation and transformation process in the phreatic groundwater aquifer group is mainly affected by adsorption/desorption,nitrification/denitrification and microbial action.?2?Static adsorption experiments show that the ability of different soil media to adsorb NH₄⁺is sub-clay>Asian sand>fine sand>medium sand;the adsorption amount of NO₃^-in different soil media is very small,and the maximum adsorption amount is not obtained in this experiment.Dynamic adsorption experiments show that the adsorption capacity and partition coefficient of NH₄⁺are different in different structural modes,which are related to soil medium type,aquifer thickness and aquitard thickness.The higher the fine particle content of the soil medium,the larger the adsorption amount and the partition coefficient.?3?The results of microbial culture experiments showed that the content of denitrifying bacteria was positively correlated with denitrification,and it was expressed in different media:subclay>fine sand>sub-sand>middle sand.Ammonia oxidizing bacteria play a major role in the nitrification process,which determines the size of the nitrification coefficient and the amount of ammonium nitrogen that is ultimately converted to nitrate nitrogen.4.According to the theory of nitrogen transportation and transformation,combined with the results of indoor experiments and the geological,hydrogeological and environmental conditions of Tongzhou,Beijing,the model concept map of“three nitrogens”transportation and transformation of phreatic groundwater in Tongzhou District of Beijing was constructed.5.Based on the geological structure characteristics,water cycle characteristics and pollution characteristics of the study area,15 representative aquifer group models in phreatic groundwater system were constructed.The Hydrus-1D numerical simulation software was used to analyze and predict the transportation characteristics of 15 modes of“three nitrogen”.The results show that:The migration velocity of NH₄⁺in the aquitard per unit thickness is 0.06⁰.36m/a,and the migration velocity of NO₃^-unit thickness in the aquifer is 27.66¹15.63m/a,which reveals that the larger adsorption and nitrification of NH₄⁺leads to the weak permeable of NH₄⁺.The migration speed in the layer is much smaller than that of NO₃^-migration,so the weakly permeable layer is more resistant to NH₄⁺than NO₃^-,which is mutually confirmed by the“three-nitrogen”migration and transformation experiments.6.Based on the basic principle of the DRASTIC model,an evaluation index system for the resistance of the confined aquifer in the shallow area was constructed,and the appropriate evaluation index was determined.The evaluation of the“three nitrogen”retarding effection on the shallow aquifer in the study area was obtained.The results show that:?1?The shallow confined aquifer layer has an enhanced tendency for the“three nitrogen”retarding from the northwest to the southeast,and has a high similarity to the distribution of NH₄⁺and NO₃^-retarding capacity in the aquifer,and the fine particle content and thickness of the aquifer.Positively related to the retarding effect.?2?The permeability coefficient of the aquifer and the surface pollution load have a significant impact on the blockage effect.The permeability coefficient is small in the Yujiawu and the pollution load is moderate,so the blockage effect is better.In the west of Majuqiao Town,the opposite effect is the worst.