Sedimentary Records Of Polycyclic Aromatic Hydrocarbons(PAHs) In The Chaihe Catchment Over The Past Century And Its Ecological Risk Assessment

Polycyclic aromatic hydrocarbons（PAHs）are persistent organic pollutants found in environmental media,including soil,water,and sediment.And they exhibit characteristics such as high toxicity,bioaccumulation,and long-distance migration.As PAHs migrate and change throughout the world,they pose threats to human health and ecosystems alike.The Chaihe catchment,a typical small agricultural catchment in the southern Dianchi watershed,had rapid economic growth and increased population pressure,causing changes in energy structure and transportation mode.The intensified anthropogenic activities in the catchment gave rise to the continuous input of PAHs into the water environment,causing pollution of the water environment.Additionally,several factors interfered with PAH migration to sediments,affecting the accumulation of PAHs in sediments.Sediment can provide accurate records of environmental changes due to its capacity to store pollutants.Therefore,the study targeted the sediment in the Chaihe catchment as the research object and utilized radionuclides(¹³⁷Cs and ²¹⁰Pb)for chronology.Meanwhile,the study also primarily examined PAH composition and distribution characteristics,investigated dynamic changes in historical PAH pollution levels,and showed the overall level of PAH pollution.Subsequently,the study analyzed the effects of geochemical indices,anthropogenic activities,and natural factors on PAHs.Finally,the study carried out the source apportionment and associated risk assessment of PAHs in the sediment.The relative results are as follows:（1）The ²¹⁰Pb_ex activities were in the range of 14.82-143.27 Bq/Kg,with an average of 66.98 Bq/Kg.Following the ²¹⁰Pb CRS model,the sediment core of the Chaihe catchment had a chronology range between 1863 and 2018.Sediment accumulation rates ranged from 0.03 to 0.87 cm/yr,with an average of 0.49 cm/yr.Over the past century,there has been a continuous rise in exogenous inputs into the water environment of the catchment.According to the sedimentary grain size analysis,the sediment was dominated by fine particles（clay and silt）,indicating a relatively stable water environment existed in the catchment.The sedimentary features of the catchment suggested that the water environment in the catchment has transformed from a near-natural state to one affected by anthropogenic activities.Especially in recent years,the inputs of carbon,nitrogen,and sulfur into the water body of the catchment have increased due to anthropogenic activities and natural factors.As indicated by the element ratios（C/P and N/P ratios）,the aquatic nutrition state of the catchment was affected by phosphorus enrichment and the significant limit on nitrogen.Simultaneously,the oxygen concentration in the surface sediment decreased,corresponding to intensified aquatic eutrophication.The stable carbon isotope(δ¹³C)and C/N ratios indicated that algae,CAM plants,and bunchless fibrous plants contributed significantly to the organic matter in the sediment.（2）Low molecular weight PAHs（LMW PAHs）occupied a considerable proportion of sediment PAH composition.Simultaneously,PAH sources were from organic conversion or low-temperature combustion.Total sulfur（TS）and TOC/TS ratios（C/S ratios）significantly influenced the concentration and distribution of LMW PAHs.However,the sediment physicochemical properties had no evident effect on those of HMW PAHs and total PAHs.Coal,oil,and gas consumption,as well as vehicle use,increased the introduction of HMW PAHs into the water body of the catchment.Simultaneously,the increased precipitation and temperature had the same environmental effect of enrichment.Additionally,increased coal consumption was a significant factor in the continued migration of LMW PAHs into the catchment sediment.And the coal,oil,and natural gas consumptions contributed to the overall amount of PAHs entering the water environment of the catchment.However,the overall decreasing trends of LMW PAH concentrations and PAH pollution were related to local anthropogenic activities,temperature,and precipitation.（3）The PAHs in the sediment were primarily derived from local sources,including biomass,coal,and oil combustion.Relative to other PAHs individuals in the sediment,Acy,Ace,Flu,Ant,Ba A,Ba P,Bb F,and Bk F gave rise to greater toxicity to organisms.As for the potential toxicity risk,Acy,Ant,Pyr,Bb F,Bk F,and Ba P played critical roles.Overall,the ecological risk and potential toxicity risk were within acceptable limits.Additionally,the comprehensive ecological risk of PAHs in the sediment was minimal.As a result of these findings,aquatic eutrophication in the catchment has aggravated.However,combining natural and anthropogenic intervention can as sist in alleviating PAH pollution in the water environment to some extent.Meanwhile,further reduction s in PAH emissions may be achieved by promoting improved fuels,new energy vehicles,and cleaner energy.Therefore,the study can provide theoretical support for priority pollutant control,hydrological management,and drinking water safety in a small watershed.