A Study On The Response Of Nitrogen To Rainfall In A Typical Trough-Valley Karst Water System In Central And Southern Area Of Hunan Province

Groundwater resource is an important support for the social and economic development of karst area in the Central-South Hunan Province,which is dry and short of water.However,with the advancement of urbanization and the progress of industrial and agricultural activities,the quality of karst groundwater is deteriorating,especially the "Three Nitrogen" pollution.Karst hydrological system is a unique hydrological system developed in the soluble rock aquifer.Compared with other hydrological systems,it is highly sensitive and fragile.The migration of pollutants,especially nitrogen,in karst areas has always been a hot topic in academia.The transport of nitrogen in karst system is closely related to many factors,among which rainfall condition is one of the important factors.The study on the response and its inner reason of nitrogen in karst groundwater to rainfall can deepen the understanding of the law of nitrogen change in Karst Agricultural Watershed,and provide technical support for nitrogen application and nitrogen pollution control in karst area.In this paper,through long-term field monitoring of groundwater chemistry and water flow(or water level)changes at rainfall scale and monthly scale in underground rivers,epikarst springs and trough valley areas in Yanzhu karst basin,seven rainfalls with typical previous rainfall and rainfall situation are selected,and the hydrological information(conductivity,turbidity,ammonia nitrogen,nitrate nitrogen and nitrite nitrogen)of each monitoring point in different rainfall processes are analyzed The response of salt and nitrogen to rainfall,and the change rule of monthly(or seasonal)scale of each monitoring point were analyzed to explore the response of nitrogen in karst groundwater system to rainfall and its internal reasons.The main conclusions are as follows:(1)The response of karst groundwater dynamics to rainfall in the study area is affected by rainfall and rainfall situation in the early stage of the stratum.In terms of water quality,the influence of early-stage rainfall on karst underground rivers and epikarst springs is similar.With the increase of early-stage rainfall,the conductivity of the two increases.However,the response of pore water in trough valley area to early-stage rainfall is shown in the variation of water level depth,which shows a negative correlation.The discharge of karst underground river,the discharge of epikarst spring,the variation of pore water level in trough and valley areas all show positive correlation with rainfall(rainfall intensity);In addition,for groundwater conductivity,rainfall events mostly show dilution,and the dilution has positive correlation with rainfall intensity.(2)The response of groundwater nitrogen to multi-scale rainfall process in the study area is affected by rainfall types and hydrogeological conditions of various types of groundwater.In the field rainfall scale(On the scale of rainfall),the response of ammonia nitrogen and nitrate nitrogen in the pipeline flow of karst underground river to the antecedent rainfall is different.With the increase of the early rainfall,more ammonia nitrogen enters the karst underground river,while nitrate nitrogen is the opposite.In general rainfall events,the peak value of nitrate nitrogen and ammonia nitrogen in karst underground river increases with the increase of rainfall(rainfall intensity),which is determined by the storage conditions of nitrogen and hydrogeological conditions of karst underground river.The change of nitrate nitrogen content in epikarst springs is more sensitive to rainfall events with the increase of rainfall in the previous period.The response of nitrogen in the pore water of the valley area to the early rainfall is mainly reflected in the change of nitrate nitrogen content.The larger the early rainfall,the more obvious and timely the response of nitrate nitrogen in the pore water of the valley area to the rainfall;with the increase of rainfall(rain intensity),the more obvious the decrease of ammonia nitrogen content;while the peak value of nitrate nitrogen in the pore water of the valley area increases with the increase of rainfall(rain intensity).In addition,when heavy rainfall events such as heavy rain occur,the dilution effect of rainfall on karst groundwater is obvious.The main pollution sources of karst underground river in dry season are domestic sewage and aquaculture sewage,which are affected by residents' life and agricultural production activities at the same time in rainy season;the dynamic change of nitrate nitrogen in surface karst spring is mainly affected by climate factors and agricultural production activities,and the dynamic change of ammonia nitrogen in seasonal scale is not obvious,because ammonia nitrogen is often adsorbed on the surface of clay particles and stored in surface soil,which is difficult to be realized Transport with slow flow.The monthly scale dynamics of nitrate nitrogen content in the pore water in the valley area is similar to that in the surface karst spring,the peak value appears in the early rainy season,and the minimum value appears in the middle rainy season;the change of nitrite nitrogen content is mainly affected by the redox environment.(3)The results of the spatial variation and traceability of groundwater nitrogen in the study area show that the source of groundwater nitrogen in the area is complex and the content of nitrogen in different types of groundwater is different due to the spatial and temporal changes of land use patterns and the influence of people's production and life.The data of hydrogen-oxygen stabilized isotope test show that the evaporation effect experienced by different monitoring point water sources is different,and further indicated that different types of karst groundwater sources and their transport ation paths are different in the study area,the reservoir underground river recharge water is mainly fast flow,and the surface karst spring and trough valley pore water is mainly slow flow.Nitrogen-oxygen isotope testing data show that the karst underground river in the study area is significantly affected by the resident sewage in the dry season,and its ammonia nitrogen content is high,in the rainy season,it is affected by both domestic sewage discharge and agricultural fertilizer and manure use;surface karst spring nitrogen in the study area mainly exists in the form of nitrate nitrogen,the main source is synthetic fertilizer,and the influence of pore water(people's wells)in the trough valley area of the study area is more prominent affected by human and animal excreta.