An Analysis Of Nitrate Sources In The Upper Reaches Of The Zihe River Watershed

Nitrate(NO3-)is the main form of nitrogen pollutant present in watershed water,and water pollution caused by NO3-is a matter of great concern.Excessive NO3-concentration leads to water quality deterioration,damages the ecological environment,and interferes with human production and life;if consumed for a long time,it will pose a great threat to human health.The analysis of NO3-source in the waters of the watershed can determine the main control factors of pollution,so as to cut off the source of pollution and prevent the intensification of pollution,which is of key guidance for the effective protection and management of regional drinking water resources.The upstream agricultural area of Zihe River in Shandong Province is situated in the mountainous hilly area of central Lu,which is a significant drinking water source in Boshan District,Zibo City,whose karst groundwater is the supply source of domestic water for Boshan urban residents.However,in recent years,the nitrate nitrogen(NO3--N)content in the watershed has exceeded the drinking water limit standard(10 mg/L)set by WHO.It is crucial to investigate pollution sources in watershed waters with the objective of guaranteeing the quality and safety of drinking water for locals and supporting the effective management of water resources.In this paper,surface water(7 sampling points)and groundwater(43 sampling points)samples were collected during the wet water period(August 2021),normal water period(December 2021),and dry water period(May 2022)in the agricultural area of the upper Zihe River in Shandong Province,and the water chemistry characteristics and spatial and temporal distribution of NO3-in the upper reaches of the Zihe River watershed were systematically analyzed using water chemistry genesis analysis and spatial interpolation analysis.The water chemistry analysis,principal component analysis(PCA),and multiple stable isotope tracing techniques(δ2H-H2O,δ18O-H2O,δ15N-NO3-,δ18O-NO3-)were used to qualitatively identify the sources of NO3-,and the positive matrix factorization model(PMF),and Bayesian mixing models in R(MixSIAR)were executed to quantitatively analyze the sources of NO3-in water bodies in the upper reaches of the Zihe River watershed,and to compare the differences in the sources and contributions of NO3-in different land use types(livestock farmings,woodlands,vegetable fields,grain fields,and kiwifruit orchards)in the study area.The differences in the sources and contributions of NO3-in groundwater between different land use types(livestock farmings,woodlands,vegetable fields,grain fields,and kiwifruit orchards)in the study area were compared.Soil profile samples were collected after crop harvest(October 2022)from four land use types:woodlands,vegetable fields.grain fields,and kiwifruit orchards,and the distribution characteristics of NO3-in soil profiles under different land use types were studied.The results of the study will supply fundamental data for the prevention and control of NO3pollution in watershed water bodies.The main research findings were as follows:(1)Water chemistry characteristics and spatial and temporal distribution pattern of NO3in the upstream water bodies of the Zihe River watershedThe overall water bodies in the upper reaches of the Zihe River were aerobic and weakly alkaline:most of them were of the "Ca2+·Mg2+-HCO3-" type and a few were of the"SO42-·NO3-·Cl--Ca2+·Mg2+" type.The contents of ammonium nitrogen(NH4+-N)and nitrite nitrogen(NO3--N)in groundwater were low.but the contents of NO3--N were relatively high.and the percentages of groundwater NO3--N exceeding the WHO standard limit(10 mg/L)were 39.53%.34.88%and 37.21%in the wet.normal and dry periods,respectively.The tri-nitrogen contents of surface water in the three periods did not exceed the standard limit for drinking water quality(GB 5749-2022).From the time dimension,the average content of NO3--N in surface water:dry period(6.1 8 mg/L)>wet period(4.83 mg/L)>normal period(3.91 mg/L);the average content of NO3--N in groundwater:dry period(12.13 mg/L)>wet period(12.04 mg/L)>normal period(11.40 mg/L).all exceeding the WHO limit(10mg/L).From the spatial scale,the NO3--N content of surface water showed a phenomenon that the downstream was higher than the upstream:the results of groundwater interpolation analysis showed that the NO3-N content of groundwater in the radiation circle of Zijing.Guozhuang,and Xigao villages in the study area was the hihhest.The NO3--N contents of groundwater varied greatly among different land use types.and the average NO3--N content of groundwater in all three sampling periods showed that:livestock farmings>vegetable fields>kiwifruit orchards>grain fields>woodlands.(2)Qualitative identification of NO3-sources in water bodies in the upper reaches of the Zihe River watershedThe results of water quality index correlation analysis showed that the water body Ca3+,Na+,Mg2+,Cl-,and NO3-were positively correlated in the three sampling periods;according to the range of changes in NO3-/Cl-and Cl-molar concentration in the watershed water body,it was known that NO3-in the watershed water body was influenced by mixed sources;combining the findings of spatial interpolation analysis of water quality indicators during abundant,flat,and dry seasons with the results of PC A.it was discovered that the main sources of NO3-in the watershed waters were manure effluent and fertilizer application.The results of nitrogen and oxygen stable isotope tracing showed that NO3-in the watershed was mainly influenced by the combination of ammonium nitrogen fertilizer.soil organic nitrogen,and manure/sewage in the three monitoring periods.(3)Quantitative calculation of NO3-source in the upper Zihe River watershed water bodiesThe relationship between physicochemical indicators of water bodies and nitrogen and oxygen isotopes and hydrogen and oxygen isotopes indicated that nitrification was the main nitrogen transformation process in water bodies,and denitrification was not obvious.The calculation results of the PMF model showed that four sources were obtained from the analysis,and the source of fecal sewage accounted for the highest percentage(80.72%),followed by primary geological action(11.20%)and fertilizer application(6.53%),and the percentage of uncertain factors was 1.55%.The results of MixSIAR model calculations showed that the highest contribution of manure/sewage in the basin water body during the normal water period(49.50%);the highest contribution of soil organic nitrogen during the dry water period(48.60%);and the highest contribution of manure/sewage during the wet water period(47.70%).Manure/effluent presented a high contribution to groundwater NO3-in the vegetable field and kiwi fruit orchard.(4)Distribution characteristics of NO3-in soil profilesThe results of soil profile tests in kiwifruit orchards,woodlands,vegetable fields,and grain fields showed that the accumulation of NO3--N in the soil profiles of vegetable fields and kiwifruit orchards were significantly higher than that in grain fields and woodlands,and the profile NO3--N contents showed that vegetable fields>kiwifruit orchards>grain fields>woodlands.The higher groundwater NO3--N contents in Yuanquan Town of Boshan District,a concentrated kiwifruit plantation area,may be related to the heavy application of manure/organic fertilizer in kiwifruit plantation management,and the higher sand contents in kiwifruit plantation may accelerate the leaching of NO3--N to deeper soil layers.Comprehensive spatiotemporal test data of watershed water bodies and soil profile test results,in the context of vigorous development of the kiwifruit industry in the study area,reduce fertilizer inputs and soil nitrogen accumulation,which can improve fertilizer utilization and reduce NO3-leaching by adopting measures such as water and fertilizer control;strengthening soil fertility and nutrient diagnosis,developing new fertilizers,and guiding farmers in scientific fertilization,thus ensuring regional drinking water safety.In addition,it is of great significance to carry out planting structure adjustment,reasonably control the scale of kiwifruit orchard and vegetable field planting,and reduce the input of pollution sources to protect water resources in the watershed.