Comparative Study On The Source And Transformation Process Of Nitrate In Karst Underground Rivers

Water is the source of life,and it is important for human survival and development;Water is a good pollutant transmission medium either.With the increase of human pollution activities,water pollution is becoming increasingly serious.Carried by water,pollutants enters rivers,lakes,groundwater and other water storage units,in the form of point source and non-point source discharge.These water storage units are the most important water source for human beings,and the health and safety of water quality is related to the physical health of the vast majority of the people.Due to the significant differences in the way water is transported and stored under different geological conditions,these differences will also affect the process of groundwater pollution.Karst areas have very unique hydrogeological conditions,with high connectivity between the ground and the underground,providing good conditions for the circulation of pollutants,which makes the problem of groundwater pollution in karst areas more serious than in other areas.Nitrate pollution is one of the most common groundwater pollutants,closely related to human life.Nitrate pollutants are generated by many human activities,such as agricultural,industrial,and tourism activities.In karst areas,these pollutants enters the groundwater system very quickly,while the cycle of self purification of pollutants is very long,resulting in a high degree of vulnerability of the karst groundwater system.Only through artificial regulation and governance can sovle the problem of karst groundwater pollution.Theoretically,the quantitative analysis of nitrate pollution sources,source uncertainty analysis,and the study of the transformation process of groundwater nitrate,are so conducive to implement the accurate nitrate discharge policies by relevant departments,and realize the accurate control of groundwater nitrate pollution.This paper selects two representative karst geomorphic units in the southwest China Karst region,namely,the Shuifangquan underground river basin in Jinfoshan,Chongqing,and the Qingmuguan underground river basin in the Qingmuguan rock karst valley.A comparative analysis of nitrate pollution characteristics in the above two watersheds was conducted.According to the relationship between recharge,runoff,and discharge,six sampling sites were determined in the spatial order of upstream,midstream,and downstream,and water samples were collected monthly for a hydrological year.The sampling sites are respectively set up as follows:the septic tank upstream of the Shuifangquan underground river,the sewage from the middle reaches of the Yaochiba dam,the downstream Shuifangquan,the upstream Guoyuan sewage from the Qingmuguan underground river,the middle reaches of the Yankou sinkhole and the downstream Jiangjiaquan.The main hydrochemical indexes and nitrate nitrogen and oxygen isotopes were systematically measured(δ¹⁵N_-nitrateandδ¹⁸O_-nitrate),water oxygen isotope(δ¹⁸O_-water)composition,analyzed the source and transformation process of nitrate and analyzed the spatiotemporal distribution proportion and contribution probability of different pollution sources using Bayesian MixSIAR model and PMF model.The results show that:within a hydrological period,it was found that there were significant differences in the chemical characteristics of the underground water in Qingmuguan and Shuifangquan.Both basins detected as Ca²⁺and HCO₃^-type water,but the Shuifangquan basin also detected as HCO₃^-,Ca²⁺and Na⁺type water;The relationship between NO₃^-/Cl^-and NO₃^-molar ratio shows,it was preliminarily identified that the main pollution sources in the Qingmuguan and Shuifangquan watersheds come from agricultural activities and manure wastewater,respectively.The quantitative calculation results of the MixSIAR model indicate that,in terms of seasonal changes,the proportion of nitrate sources in the Qingmuguan watershed is stablely,with only slight fluctuations in fertilizer and soil organic nitrogen sources;The proportion of nitrate sources in the Shuifangquan watershed is not very stable,and each source fluctuates to some extent with seasonal changes.In terms of spatial changes,the proportion of nitrate sources in Qingmuguan and Shuifangquan underground rivers is relatively stable.To verify the accuracy of the quantitative results,we also use the PMF model to accurately quantify the sources of nitrate in the two watersheds during this hydrological year.Comparing the quantitative identification results between the MixSIAR model and the PMF model,it was found that the contribution rates of soil organic nitrogen sources to the Qingmuguan Underground River were 69.9%and 74.6%,respectively,the contribution rates of manure and sewage were 29.2%and 23.4%,the contribution rates of fertilizer sources were 0.7 and 2.72%,respectively,and the contribution rates of atmospheric precipitation were 0.3%and 1.28%,respectively;The contribution rates of soil organic nitrogen sources in the Shuifangquan underground river are 70.8%and 75.02%,respectively,the contribution rates of manure and sewage are 28.9%and 24.48%,and the contribution rates of atmospheric precipitation are 0.3%and 1.66%,respectively.The error of the quantitative results of the two models is within0.006,indicating that the quantitative results are relatively accurate.Accurately identify the uncertainty of nitrate sources based on the probability distribution calculated by the MixSIAR model.The uncertainty of nitrate source in Qingmuguan watershed is smaller in seasonal variation than that in Shuifangquan watershed,but the spatial uncertainty is slightly larger than that in shuifangquan watershed;By calculating the uncertainty index of nitrate sources in the two watersheds,it is found that the uncertainty of nitrate sources in the shuifangquan watershed is slightly higher than Qingmuguan watershed.Nitrification is the main process in both underground rivers,but denitrification,assimilation,and dissimilatory reduction to ammonium also occur locally and spatially.For example,the dissimilatory reduction of nitrate to ammonium may occur in Guoyuan sewage and Yaochiba sewage.Compared with previous studies,it is found that the export of Jiangjiaquan,Qingmuguan underground river,has undergone assimilation in 2018.During the period from June2021 to May 2022,nitrification is the main biogeochemistry process,accompanied by a small amount of denitrification.It is speculated that the nitrate biogeochemistry process in Jiangjiasping may have an inter-annual change;The biogeochemistry process in Shuifangquan basin is generally stable.nitrification is the main nitrate biogeochemistry process,and denitrification,assimilation and dissimilatory reduction to ammonium are the secondary nitrate biogeochemistry process.In summary,the Qingmuguan underground river basin has a wider scope and more complex human activities.The manure,sewage,fertilizer,and soil organic nitrogen in the underground river are mostly discharged through a larger range of non point source pollution,resulting in a much higher uncertainty in the contribution rate of various nitrate sources than in the Shuifangquan underground river.At the same time,the biogeochemical processes of nitrate in the Qingmuguan and Shuifangquan watersheds are relatively complex.Therefore,it is necessary to organically combine nitrate source control and nitrate transformation process control,to effectively solve the problem of groundwater nitrate pollution.