Nitrogen Transport And Fate Under Lake Water And Groundwater Interaction Driven By Lake Reclamation

Reclaiming land from lakes effectively solves the problem of land shortage.However,the natural evolution and ecological process of the lake has been changed dramatically in a short time.The original equilibrium of lake water and groundwater interaction has been broken.The new physical,chemical,and biological gradients have been formed to stimulate the reaction process of organic matter from natural sources.The input of exogenous nitrogen pollutants may increase and the nitrogen cycle in lake wetland suffers unprecedentedly strong disturbance.However,it is still unclear how the hydrodynamics,redox,and material fluxes influence nitrogen biogeochemistry under the lake water-groundwater interaction driven by lake reclamation.Many lakes in Jianghan Plain have a long history of reclamation.Serious environmental problems,such as hydrological connectivity changes,regulation and storage functions weakening,and water quality deterioration,have appeared one after another.Water resources and water ecological environment are facing unpredictable changes.This study focuses on the changes in nitrogen transfer and transformation in groundwater and its effect on lake eutrophication after the lake water-groundwater interface moves by lake reclamation.Hubei Chen Lake Wetland was selected as a typical research area for carrying out the following research:Identity the lake water-groundwater interaction driven by lake reclamation,reveal spatio-temporal variability and controlling factors of inorganic nitrogen in lake water and groundwater after lake reclamation,compare the difference of nitrogen occurrence in aquitard and aquifer sediments,and discuss the control mechanism on the redistribution of inorganic nitrogen in groundwater by the difference of the aquitard and aquifer sediments.This research has important scientific and practical significance for filling in the blank of the impact of lake reclamation on groundwater,deepening the understanding of lake reclamation on water resources,water environment and water ecology,and ensuring the sustainable development of lake wetlands.The main conclusions and understandings obtained in this research through field monitoring and sampling,indoor experiment and simulation,multivariate statistics and analysis are as follows:1.Identification of lake-groundwater interaction after lake reclamationA monitoring profile was established in Chen Lake Wetland to monitor the level and collect samples from March 2019 to January 2020.A total of 340 sets of water level monitoring data and 155 water samples were obtained.The connection between lake water and groundwater was determined by water level dynamic analysis,cross-correlation analysis and regression analysis.The water chemical similarities between lake water and groundwater hydrochemistry were demonstrated with hydrochemical type,hydrogen and oxygen isotope analysis,and clustering analysis.Through indoor seepage simulation test to carry water infiltration after the change of groundwater.The change of water flow from lake to groundwater was determined by a seepage simulation test.Lake water is an important factor for groundwater recharge and water level maintenance in the lake area.The highly consistent water level dynamics and the similarity of water chemistry indicate that there are frequent exchanges between lake water and groundwater.However,there is an obvious time lag between groundwater and lake water levels in the reclaimed farmland.In which,the response of the water level of aquitard and aquifer in reclaimed farmland to lake water level was delayed by 2-4months and 1-3 months,respectively.The recharge of lake water to the aquitard in reclaimed farmland is greatly reduced after lake reclamation and the hydraulic connection between lake water and aquifers is more frequent.The recharge from lake to groundwater after lake reclamation has changed from two interfaces of lakeshore and lake bottom to one interface of the lake bottom.With the increase of lakeshore inclination,the seepage from the lakeshore interface decreased gradually but increased from the lake bottom interface.2.Elucidation of the response of nitrogen transfer and transformation to lake reclamation in lake water-groundwater interactionKruskal-Wallis test was used to clarify the difference in inorganic nitrogen in groundwater between lake areas and farmland areas.The sources and influencing factors of inorganic nitrogen in lake water and groundwater were identified by isotopic and linear regression analysis.The importance of influencing factors was evaluated and distinguished by Random Forest Regression（RFR）Models.The lake water quality in the study area isⅤ,with NH₄⁺-N and NO₃^--N pollution.The pollution status in lakeshores is more serious than inner lake,which is closely related to the non-point source pollution such as long-term lake aquaculture,farming,and domestic sewage.The main nitrogen form in groundwater is NH₄⁺-N.There are significant differences in the distributions of NH₄⁺-N in groundwater between lake areas and reclaimed farmland.The difference in NH₄⁺-N in aquitard is strong than in aquifer.NH₄⁺-N in groundwater is attributed to organic matter（OM）mineralization.Therefore,the average concentration of NH₄⁺-N in groundwater decreased along the lake water infiltration path.The distribution of NH₄⁺-N in groundwater is controlled by the redox gradient and OM content.The importance of the OM to the concentration of NH₄⁺-N in groundwater decreased,while the importance of local redox conditions increased after lake reclamation.The importance of exogenous input increased in aquitard strongly affected by lake reclamation.The concentration of NO₃^--N in groundwater is accumulated along the lake water infiltration path.NO₃^--N concentration in groundwater is closely related to agricultural cultivation,and the interaction between lake water and groundwater promoted the migration of NO₃^--N in groundwater.3.Revealing the changes in the nitrogen occurrence in aquitard and aquifer sediment and its relationship with the nitrogen distribution in groundwater126 sediment samples of aquitard and aquifer from different boreholes were collected in lake areas and field areas.The difference in inorganic nitrogen in aquitard and aquifer sediment was determined by the Wilcoxon sign rank test.The sources and controlling factors of inorganic nitrogen in aquitard and aquifer sediment were determined by the isotope method and redundancy analysis.The importance of the main controlling factors was evaluated by RFR.The content of NO₃^--N in borehole sediment is low.The spatial distribution pattern of NO₃^--N in borehole sediment is different and the vertical leaching is obvious.After lake reclamation,the sediments rapidly converted into the soil,and the oxygen-rich environment,pesticides and fertilizers,and crop residues promoted the generation and release of NO₃^--N.There is a significant difference in the distribution of NH₄⁺-N in the aquitard and aquifer sediment between the lake area and reclaimed farmland,which is mainly reflected in the range of 0-5 m.Due to the nitrification rate was inhibited and net mineralization accelerated under high water content.The distribution of NH₄⁺-N in aquitard and aquifer sediment is mainly controlled by buried organic matter,that is,the mineralization process plays a decisive role.However,the water content decreases and the change in the sedimentary environment weakens the mineralization rate after the reclamation of the lake.The exchangeable NH₄⁺-N content in aquitard and aquifer sediment represents NH₄⁺-N in groundwater.The NH₄⁺-N generated by mineralization of buried organic nitrogen balances the nitrogen loss from ammonium dissociation in aquitard and aquifer sediment and groundwater.Further,NH₄⁺-N is transported to lake water through lake water-groundwater interaction,which aggravates lake eutrophication,deteriorates water quality,and poses a great threat to the sustainable development of lake ecology.