Sources And Enrichment Mechanisms Of Inorganic Nitrogen In Quaternary Aquifer System In Jianghan And Dongting Typical Areas

At present,the nitrogen（N）load of terrestrial and aquatic ecosystems increases gradually,resulting in continuous enrichment of inorganic N in water-bearing systems,affecting the natural N cycling process and threatening human health and safety.Anthropogenic activity is generally considered to be the fundamental factor of inorganic N enrichment in aqueous system.However,the release of ammonium（NH₄-N）from natural sources has been found to be one of the important factors causing N pollution in recent years,and the synergistic effect of NH₄-N from natural sources intensifies the deterioration of groundwater and pore water quality.NH₄-N from natural sources is very easy to accumulate in fluvial and lacustrine sediments of the Quaternary.Its occurrence mechanism is controlled by the sedimentary environment,its release mechanism is affected by the recharge and discharge conditions,and its transformation mechanism is closely related to the REDOX environment of water.Based on the Han-river Gorge way groundwater recharge area of Jianghan plain and Dongting plain water Delta discharge zone as the typical research area,based on the Quaternary sedimentary characteristics,sedimentary environment evolution and organic matter（OM）content source identification in-depth analysis,reveals the influence on the occurrence characteristics of N,sedimentary environment through different drainage conditions on N source recognition,transformation and enrichment mechanism research.The N cycling model of inorganic N in aquifer system was described in order to provide auxiliary strategy and theoretical basis for the cause of inferior water quality and water and soil environmental protection in plain area.The main cognition and conclusions of this paper are as follows:1.Crossing use of chronology,sedimentology,geochemistry,mathematics and other subjects,using OSL dating,granularity,Fisher discriminant,mineral composition,OM,and the main methods of trace elements content analysis,discusses the typical area of Han-river Gorge and Zi-river Delta sedimentary environment characteristics and source of OM content.The framework of sedimentary environment evolution since Quaternary has been identified,examined and constructed.The sedimentary characteristics of Gorge area of Han-river are similar to those of Zi-river Delta.The water-bearing medium is mainly composed of coarse particles with poor sorting.The sedimentary facies mostly belong to alluvial-diluvial and lacustrine facies,and the hydrodynamic strength alternates frequently and changes complicated.The discriminant formula of the new Fisher model has successfully identified and verified the Quaternary fluvial and lacustrine sedimentary environment in the Gorge area of the Hanjiang River and has been verified in the Zi-river Delta,which is suitable for the discriminant of the quaternary alluvial and lacustrine sedimentary environment.The source of OM is consistent.Phytoplankton and terrestrial C₃ clastic contributed to the formation of shallow OM.In the Zi-river Delta area,the OM is more complex,and the lake aquatic plants and terrestrial C₃ and C₄ clastic contribute to the formation of OM.The sedimentary environment evolution characteristics of Jianghan and Dongting study areas since Quaternary are controlled by geological structure and climate.2.Based on the analysis of hydrodynamic index and N content,the relationship between the Quaternary hydrodynamic conditions and the occurrence of various forms of N was discussed,and the relationship between the percentage content of each grain size and C and N was expounded.The accumulation of N in different hydrodynamic stages was negatively correlated with hydrodynamic intensity.Affected by the percentage of sediment grain size,the weaker the hydrodynamic force,the more easily N was enriched.The fine particles in the sediments of the Gorge area of the Han-river have stronger adsorption and enrichment capacity for N,and the lacustrine sediments contain more TON than the fluvial sediments,while the fluvial sediments contain bioclasts that are not fully decomposed,resulting in a positive correlation between the percentage of coarse particles and TOC.TN and TOC in Zi-river Delta are positively correlated with clay composition,but negatively correlated with sand composition,indicating that fine-grained sediments have a strong ability to enrich OM.In conclusion,the fluvial and lacustrine transitional environment enriched abundant OM sources,especially in the low-energy and reductive slow-flow environment,which created conditions for the enrichment of NH₄-N in the basin sediments from natural sources.3.By comprehensive use of hydrochemistry,total N analysis,OM content analysis,multiple isotopes and Bayesian model calculation methods,the source,transformation and enrichment mechanism of inorganic N in the water-bearing system in the Typical study area of Han-river Gorge Road（recharge area）was interpreted,and the N cycle model of underground water-bearing medium in the"recharge area"was constructed.The aquifer system is co-contributed by human activities and natural sources.In the shallow oxidizing system,the high concentration of nitrate was traced to be significantly contributed by manure and sewage.Inorganic N in deep water-bearing system mainly comes from natural sources,including soil N and atmospheric precipitation.In addition,inorganic N in sediment pore water is indirectly contributed to the content of inorganic N in groundwater by NH₄-N release from aluminosilicate K ore.The mechanism of N transformation and enrichment in the Han-river Gorge area is complex,and the mineralization efficiency gradually decreases from shallow to deep layers.Heterotrophic denitrification is also indicated to be more efficient in shallow layers by hydrochemistry and N and oxygen isotope indexes.Adsorption is considered as a supplementary factor for the enrichment of NH₄-N in shallow groundwater and pore water.The reduction of dissimilated nitrate to ammonia also contributed to the enrichment of NH₄-N.The quantitative calculation results of Mix SIAR and SIAR models have the same trend,indicating that manure and sewage do pollute the shallow aquifer system.Paleo-atmospheric precipitation contributed the most to nitrate in sediment pore water.There is no significant difference in the contribution of soil N between shallow and deep strata.4.Based on the N research method in the typical area of Han-river Gorge,the data analysis of mineral content and NH₄-N isotope were supplemented by the measured data,a more detailed to identify the Dongting Zi-river Delta（discharge zone）typical inorganic N source of the aquifer system,discussed and compared under different oxidation-reduction characteristics of N transformation and enrichment mechanism of difference,The N circulation model of underground water-bearing medium in"discharge zone"area was constructed.The evidence of hydrochemistry and soil geochemistry shows that the enrichment of nitrate and ammonia N in shallow groundwater was induced by human activities in the typical study area of Zi-river Delta.The study of isotope identification shows that the excess NH₄-N in sediments mainly comes from the synergistic contribution of human activities,mineralization and aluminosilicate K mineral release.In the BMS01 hole in oxidized environment,ammonia oxidation process and hydroxylamine oxidation reaction are important biochemical processes that ammonia and nitrate do not accumulate in groundwater,and nitrosation reaction is an important mechanism of N loss in groundwater.However,the release of NH₄-N into groundwater in BMS02 hole in reductive environment may have caused N loss,and the anammox denitrification process is the most important cause of loss in the release process of NH₄-N.The quantitative identification of SIAR and Mix SIAR models shows that manure and sewage are the main contributions of inorganic N in regional groundwater.In order to control the further deterioration of groundwater quality,anti-seepage measures and sewage treatment devices are necessary.