Identification Of Nitrate And Sulfate Sources And Their Increasing Mechanism In Karst Water System Of Jinan Spring Catchment Based On Hydrochemistry And Multi-isotopes

Karst water is an important source of water supply in the karst area in north China,and it is an important guarantee to maintain the sustainable development of society,economy and Eco-environment in north China.As one of the most typical karst springs in north China,Jinan karst springs have long enjoyed a good reputation.Meanwhile,Jinan karst springs are also an important water source to ensure industrial and agricultural production and urban life in Jinan city.The historical karst water quality records show that the concentrations of NO₃^-and SO₄^2-in karst groundwater increase year by year in the process of urbanization and industrialization in the Jinan Spring Catchment,and there are multiple potential sources of contamination.However,researches on the source and biogeochemical transformation mechanism of NO₃^-and SO₄^2-in karst groundwater are mainly focused on the karst areas in southern China,while there are few reports on the sources of NO₃^-and SO₄^2-in typical karst areas in northern China,and their sources and behaviors are not clear.Therefore,the identification and quantification of nitrate and sulfate sources and their increasing mechanisms are of great significance to the protection of ecological landscape and drinking water safety in Jinan Spring Catchment.Based on this,the information of topography and geomorphology,hydroclimate,karst water flow field and land use were coupled with hydrogeological survey,hydrogeochemistry,multi-environmental isotope(δD_H2Oandδ¹⁸O_H2O,δ¹⁵N_NO3andδ¹⁸O_NO3,δ³⁴S_SO4andδ¹⁸O_SO4,δ¹³C_DICandδ¹³C_DOC)and Bayesian isotope mixing model（Simmr）to qualitatively identify and quantitatively assess the contribution of NO₃^-and SO₄^2-sources and biogeochemical processes of karst water system in the catchment.Furthermore,the increasing mechanism of NO₃^-and SO₄^2-in the karst water system of the catchment was further revealed through the historical long-term series of spring water quality and hydrological dynamic data（1958-2019）and the measured multi-isotope data,which mainly obtained the following understandings:1.The seasonal and spatial distribution of hydrochemistry in karst water system in the catchment.The hydrochemical types of surface water were Ca·HCO₃（SO₄）in the wet season,while the types were more complex and the sulfate and chloride types were increased in the dry season.Spatially,K⁺,Na⁺,Cl^-,Mg²⁺,and SO₄^2-gradually increased from upstream to downstream.Ca²⁺,HCO₃^-,and NO₃^-increased first and then decreased from upstream to downstream.Ca（Mg）·HCO₃（SO₄）is the main type of groundwater during the sampling period.Most of the main ion components of groundwater in the dry season were higher than that in the wet season.The mean concentrations of Cl^-,SO₄^2-and NO₃^-in groundwater in the direct recharge area were the highest,which may be related to the intense anthropogenic activities in exposed karst bedrock and depression.The concentrations of Cl^-,SO₄^2-and NO₃^-in the discharge area were between the surface water and the karst groundwater in the direct recharge area,which may be attributed to the mixing between them.The inorganic nitrogen of the karst water system in the catchment was mainly NO₃^--N,and18.4%and 44.2%of surface water and groundwater exceed the WHO drinking water standard,respectively.Although the SO₄^2-concentrations did not reach the upper limit of drinking water（250 mg/L）,it still showed an increasing trend.The NO₃^-and SO₄^2-concentrations in the dry season was slightly lower than that in the wet season,which was related to the surface runoff carrying NO₃^-and SO₄^2-contaminants into surface rivers or rapid leakage into karst aquifers through karst conduits and fissures caused by increased precipitation in the wet season.The SO₄^2-concentrations of surface water were higher than that of groundwater,which proves that it may be an important recharge source of groundwater.However,NO₃^--N in surface water was significantly lower than that in groundwater,indicating that there were other sources of NO₃^--N in groundwater besides surface water,such as agricultural irrigation water and domestic sewage.The SO₄^2-concentrations of groundwater had significant spatial difference,and was in the discharge area（DA）<indirect recharge area（IRA）<direct recharge area（DRA）.The highest SO₄^2-concentration in the DRA was related to exposed karst mountain area and intensive human activities.The NO₃^--N concentrations were DRA>DA>IRA>surface water in the wet season,while in the dry season was DRA>IRA>DA>surface water.The variation of NO₃^-and SO₄^2-concentrations in groundwater is the result of factors such as the underlying surface and karst development,terrain slope and human activities.2.Identification of nitrogen and sulfur biogeochemical cycles in karst water system in the catchment.The hydrochemistry and multi-isotopes revealed that the bacterial dissimilation sulfate reduction and sulfide oxidation and denitrification were limited in the karst water system.It also revealed that the NO₃^--N transformation process in the dry season was extremely complex,which was mainly related to point source pollution.Nevertheless,nitrates in the wet season were mainly controlled by nitrification in the karst water system,because the variation range of N and O isotopes was narrow and the fractionation was small.As a whole,the isotope fractionation generated by nitrogen and sulfur biogeochemical processes in the karst groundwater system are relatively conservative and not significantly alter the initial NO₃^-and SO₄^2-isotopic compositions in the catchment.The biogeochemical process is not the main reason for the increase of NO₃^-and SO₄^2-concentrations in Jinan spring karst water system.3.Qualitative identification and quantification of NO₃^-and SO₄^2-sources in karst water system in the catchment.Hydrochemistry,δ¹³C_DIC,δ³⁴S_SO4andδ¹⁸O_SO4combined with Simmr model showed the SO₄^2-in surface water mainly derived from evaporite dissolution（32.6%）,sewage（18.8%）,soil sulfate（15.4%）and sulfide oxidation（10.2%）in the catchment;Groundwater SO₄^2-mainly derived from soil sulfate（30.3%）and sewage/manure（29.6%）,while other sources contributed less.The source of SO₄^2-varied significantly in different seasons and hydrogeological regions.Meanwhile,hydrochemistry,δ¹³C_DIC,δ¹⁵N_NO3andδ¹⁸O_NO3combined with Simmr model indicated that the NO₃^--N source contribution in surface water during the dry season was not significantly different（13.5-22.8%）.In the wet season,non-point source pollution contributed mainly,such as soil nitrogen（64.2%）and ammonium fertilizer（19.2%）.The contribution of soil N to groundwater NO₃^--N was the highest in the wet（38.4）and dry seasons（37.4%）.The second was ammonium fertilizer（27.0and 18.9%,respectively）,but the significant increase of ammonium fertilizer in the wet season was related to the leakage of NH₄⁺fertilizer into karst aquifers by surface runoff formed by increased precipitation.The contributions of sewage,manure,nitrate fertilizer and precipitation were low,and in the wet season（14.6,13.6 and 6.6%,respectively）were slightly lower than that of the dry season（20,14.5 and 9.1%,respectively）,which was related to the dilution effect of precipitation and the enhancement of N biogeochemical processes.The uncertainty index(UI₉₀)revealed that the contributions of soil nitrogen（67.4%）and soil sulfate（56.5%）,sewage/manure（62.5%）,and ammonium fertilizer（67.6%）were the most variable,while other sources were relatively stable.4.Mechanism of increasing NO₃^-and SO₄^2-of spring water in the discharge area of the catchment.Long time series of spring water quantity and water quality monitoring data（1958-2019）revealed that NO₃^-and SO₄^2-concentrations increased over time in the catchment.Before 1980,the concentrations of NO₃^-and SO₄^2-in spring water were low and varied gently,ranging from 2.3-20.4 mg/L and 0.2-14.6mg/L respectively,and decreased with the increase of spring discharges.Afterwards,the concentrations of NO₃^-and SO₄^2-in spring water increased significantly and their dynamic changes were dramatic,ranging from 20.4-121.3 mg/L and 14.6-49.3 mg/L respectively,and they increased with the increase of spring discharges.The results showed that the spring water quality was markedly controlled by natural factors in the early period,with the rapid increase of population,agricultural intensification and rapid urban expansion,leading to the variation of karst water exploitation quantity,urban/suburban wastewater discharge,agricultural fertilization/sewage irrigation and other anthropogenic activities gradually become the dominant factors influencing the karst water quality in the catchment.The source complexity of NO₃^-and SO₄^2-in karst aquifer system of the catchment are the result of the coupling of natural factors and anthropogenic pressure,such as the heterogeneity of karst aquifers,biogeochemical cycle and hydroclimatic change.The main NO₃^-sources of surface water and groundwater in the catchment were soil nitrogen,agricultural fertilizer and sewage.The NO₃^-and SO₄^2-from agricultural fertilizer,sewage and eroded soil were the main potential sources of surface water and groundwater in the catchment.Hence,it is necessary to strictly control industrial and agricultural wastewater discharge,improve domestic sewage treatment infrastructure,improve fertilizer utilization efficiency,and develop three-dimensional ecological agriculture to reduce soil erosion and nutrient loss.This study has deepened the understanding of the sources and dynamics of NO₃^-and SO₄^2-in karst aquifers in north China,and the results have great guiding implications for NO₃^-and SO₄^2-contamination control in karst spring catchment or similar areas,protection of water quality and sustainable utilization of karst water resources with multiple contamination sources.