Study On The Source,Migration And Transformation Mechanism Of Nitrate In Karst Underground River Catchment

Karst underground rivers,which are widely distributed in karst areas in Southwest China,are important water resources.In recent years,with the continuous development of economic level,karst underground rivers have suffered from different degrees of pollution.Nitrate pollution is a prominent and common problem in karst underground rivers.Because of the high heterogeneity of karstic aquifers,it was difficult to reveal the distribution and formation process of nitrate by simulation.In this thesis,the typical karst underground river system in Guancun Village,Daliang Town,Liuzhou City,Guangxi was selected as study area.Using water chemistry,stable environmental isotopes and groundwater resistance time,the source,transport and fate of nitrate in ground water were discussed.The conclusions would be significant for the formation of non-point source pollution prevention strategies in karst areas.These conclusions include:?1?Nitrate nitrogen pollution in groundwater were mainly derived from ammonium fertilizer,soil nitrogen and domestic sewage or manure.During wet season,the overlaps of ammonium fertilizer and soil nitrogen abundance was the main source of nitrate.In the dry season,the distribution range of?¹⁵N-NO₃^-and?¹⁸O-NO₃^-was wide,the three sources jointly affected the Guancun underground river system.?2?The underground river system in the study area was mainly recharged by atmospheric precipitation and it was obviously affected by evaporation,compared with the dry season,the water table rose during the wet season.The head difference between the recharge area and the discharge area became larger.The increase in the water head allows the relatively deep groundwater to be drained.Therefore,the apparent age of groundwater during the wet season was significantly greater than that of the dry season.Groundwater nitrate excreted during the wet season might be affected by earlier land use.?3?There were time and space differences in the conversion of nitrates in the study area.There was nitrification in the watershed during the wet season,however,there was an obvious denitrification process in the underground river sample.nitrification and denitrification existed in the dry season,but the nitrification of the spring point was obvious.?4?The order of contribution of nitrate nitrogen source in groundwater was soil organic nitrogen>ammonium fertilizer>domestic sewage and manure in the wet season.The order of contribution of nitrate nitrogen source in groundwater was Soil organic nitrogen>domestic sewage and manure>ammonium fertilizer in the dry season.