| Water is the origin of life,the essentials of production and the foundation of ecology.Groundwater accounts for 99 percent of liquid fresh water on the earth,which is an extremely important source of water in human's manufacture and life.With the rapid development of agriculture,industry,urbanization and tourism,nitrate pollution in groundwater has become the most common and extensive problem in groundwater pollution,which threatens the safety of water supply at home and abroad.Karst groundwater accounts for 1/4 of the total groundwater resources in China.In karst region of southwestern China over 100 million people depend on karst water supplies.Karst aquifers with zones of high permeability are extremely vulnerable to excess nitrate,which can travel rapidly through the conduits of the karst groundwater flow system with little attenuation.As a tourist attraction,the Jinfoshan Karst?JFK?World Heritage Site is visited by many tourists annually.The increased number of tourists in the JFK had caused nitrate pollution in karst groundwater.In recent years,there have been documented sources and transformations of nitrate contamination influenced by agriculture and urbanization in karst groundwater.A few studies have investigated the sources and transformations of nitrate influenced by tourism activities.At the same time,the sources and transformations of nitrate may have great temporal variability.Nonetheless,the majority of studies on nitrate sources,migration and transformations were carried out through short-term monitoring,which cannot achieve the accurate study of nitrate sources and transformations.Therefore,it is necessary to implement long-term monitoring to systematically quantify nitrate sources,migration and transformations from recharge to drainage through a subterranean river in such a setting.In this thesis,Shuifang Spring?SFS?subterranean basin which located at Jinfoshan Karst World Heritage Site,China,is the subject of this study.According to the relationship between recharge,runoff and discharge of the subterranean river,we had selected the six sampling sites,including water from Jinfoshan Holiday Hotel?JHH?,septic tank effluent,sinkhole effluent,groundwater from SFS?the resurgence of the basin?,atmospheric precipitation and soil water collection site.From April 2017 to April2019,we had obtained precipitation in the basin and discharge from SFS through automatic monitoring.In addition,we were investigated hydrochemistry and?15Nnitrate,?18Onitrate in the sampling sites every 24 days.The thesis study the seasonal and interannual variation and formation mechanism of nitrate dynamics,sources,migration and transformations by incorporating hydrology,hydrochemistry,?15Nnitrate,?18Onitrate,and the SIAR and Mix SIAR model.Coupled with hydrology,hydrochemistry and dual nitrate isotopes,the thesis analyzed the temporal and spatial variation of the nitrate and dual isotopes in the basin.The response of discharge to atmospheric precipitation was fast and the variation of discharge was large in different seasons.In terms of time,the nitrate concentrations in the basin mainly increased during the peak period of tourism.Spatially,the nitrate concentrations of septic tank and sinkhole effluent were the highest.The nitrate concentrations of water from JHH and groundwater from SFS were approximately within the range of local natural background value.In monitoring period,on balance,the ionic concentrations and?15Nnitrate,?18Onitrateitrate values of water from JHH were relatively stable.The NO3–concentrations ranged from 4.41 to 17.61 mg/L in the water from JHH,which was relatively low.Therefore,the water from JHH is pristine and relatively safe.The ionic concentrations and dual isotopic values of effluent from septic system and sinkhole were large variation range.The NO3–concentrations ranged from 0.73 to 105.34mg/L in the septic tank effluent and the NO3–concentrations ranged from 0.96 to 86.38mg/L in the sinkhole effluent.The NO3–concentrations from effluent at septic system and sinkhole corresponded well to the number of tourists.The NO3–concentrations ranged from 4.65 to 18.22 mg/L in the groundwater from SFS.The water quality of groundwater at SFS was generally better.The NO3–concentrations at groundwater from SFS were slightly elevated and the values of dual nitrate isotopes had been varied in the tourist season?summer drought and winter dry season?,indicating that water quality has deteriorated and the purification capacity was limited in the subterranean river.The thesis showed nitrate sources and its contributions in the basin.The dual nitrate isotopes in the basin ranged from?0.8‰to 21.5‰for?15Nnitrate,and from?11.6‰to35.5‰for?18Onitrate.The typical ranges of dual isotopic values for different nitrate sources were superimposed on the?15Nnitrate,?18Onitrateitrate values from the study area,indicating that the nitrate mainly originated from soil organic nitrogen and manure and sewage in the basin.The SIAR and MixSIAR model were used to estimate the contribution of each nitrate source to total nitrate in the basin,which indicated that the contribution of manure and sewage is the largest,reaching about 60%to 70%within two hydrological years.Specifically,the nitrate contributions vary greatly from season to season.In the rainy season 2018,the contribution of manure and sewage was as high as71.8%,reaching the highest value within two hydrological years.The main reason of the phenomenon is the increase in the number of tourists.In the dry season from 2018 to2019,the contribution of manure and sewage decreased to about 44%,reaching the lowest value within two hydrological years.In this period,the cableway station at the west slope was closed for maintenance,which led to a decline in the number of tourists in JFK.Compared with the quantitative results of SIAR and MixSIAR model,we found that the quantitative results of Mix SIAR model were closer to the reality of the basin by incorporating the actual situation of the basin.This thesis analyzed variation and formation mechanism of nitrate migration,transformation in the subterranean basin during the two hydrological years.Coupled hydrology,hydrochemistry,NO3–/Cl–molar ratios and relationship between?15Nnitrate,?18Onitrate values and NO3–concentrations,the results showed that nitrification under the oxygen-rich environment is the main process of nitrate transformation in water from JHH.The manure and sewage and oxygen-rich tap water were fully mixed by the time they reached the septic system and sinkhole.During the slow migration of the mixture from the septic system to sinkhole,the oxygen-rich environment contributes to nitrification.During the tourist season,denitrification have occurred in effluent from septic tank and sinkhole under the conditions of suitable temperature,anaerobic and micro-anaerobic situation and so on.The effluent was greatly diluted by groundwater in the subterranean river in wet season.During the tourist off-season?wet season?,assimilation was the dominant response to decline of nitrate concentrations in the subterranean river.During the tourist season?summer drought and winter dry season?,groundwater in the subterranean river was nitrification affected by effluent.The study presented here can provide scientific basis to sustainable development and utilization of karst groundwater and regulation of nitrate flux in karst regions worldwide.This thesis is helpful for the protection of lucid waters and lush mountains and construction of eco-civilization. |
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