Characteristics And Risk Assessment Of Heavy Metals Pollution In Multi-media From The Cold Region River And Typical Riparian City

Heavy metals have received widespread public attention due to their toxicity,bioaccumulation,environmental persistence,and potential hazards to human health and regional ecological safety.With the accelerating process of industrialization and urbanization,the pollution sources of heavy metals have become more complex,influenced by factors such as industrial production,transportation emissions,coal combustion for heating,and agricultural activities.As a result,heavy metals are widely present in environmental media such as the atmosphere,water bodies,sediments,and soils.Therefore,studying the pollution characteristics and risk identification of a single environmental medium alone is difficult to systematically reflect the extent and pathways of human activities’impact on regional heavy metal pollution,and it cannot fully quantify the ecological and health risks of regional heavy metal pollution.Rivers,especially urban rivers,provide essential water resources for human production and daily life.However,due to their close association with human activities,they have become major gathering places for regional environmental pollutants,serving as important carriers linking regional ecological environmental pollution and human health.Therefore,when addressing water pollution in river basins,we should not limit ourselves to the rivers alone.Instead,we should focus on rivers and establish a regional-scale multi-medium environmental monitoring network,as well as a pollution risk identification and evaluation system.This will provide more comprehensive pollution prevention and control information for improving the quality of the regional ecological environment.It is of great practical significance and research value in achieving green and sustainable economic and social development and ensuring regional ecological and environmental safety.This study focuses on Songhua River,a typical river in China high-cold region,and the large riparian city.By establishing a watershed multi-medium environmental monitoring network system（including the atmosphere,snow,water bodies,sediments,and soils）,the study identifies the occurrence and pollution levels of heavy metals in various environmental media.It reveals the potential sources of heavy metal pollution and quantifies the contributions of local and non-local sources.The distribution characteristics of heavy metals among the river water,suspended particles,and sediments are clarified.Additionally,the study utilizes three-dimensional fluorescence spectroscopy techniques to investigate the relationship between the speciation of heavy metals and dissolved organic matter.Taking the actual environmental exposure pathways of heavy metals as a starting point,the study quantifies the potential exposure risks to aquatic organisms and establishes a human health risk assessment model for multiple exposure pathways,including the atmosphere,water bodies,and soils.This research provides a systematic approach for the comprehensive assessment of the characteristics of multi-medium environmental heavy metal pollution and ecological risks at the watershed scale.The main research findings are as follows:（1）The average concentrations of heavy metals in water from the mainstream of Songhua River and the Harbin section are as follows(inμg·L^-1):Zn（64.25）>Cr（12.01）>Cu（4.27）>Pb（3.02）>Ni（1.68）>Cd（0.26）for the mainstream Songhua River and Zn（63.54）>Cr（32.09）>Pb（17.29）>Cu（14.39）>Ni（13.16）>Cd（0.21）for the Harbin section.These concentrations are significantly higher than the environmental background values.The levels of several heavy metals exceed the permissible limits for aquatic organisms,and the concentrations of heavy metals in the water bodies of Songhua River and the Harbin section no longer meet the requirements of the"Standards for Irrigation Water Quality"and the"Standards for Drinking Water Quality".In surface sediments,the concentrations of Zn and Cd in the main stream of Songhua River and Cd,Pb,and Zn in the Harbin section are significantly higher than the background concentrations.This is related to the pollution inputs from the primary tributaries entering the Harbin section.The results of principal component analysis indicate that Cr,Cu,and Ni in the water bodies originate from industrial sources,while Pb and Zn come from a mixture of traffic and agricultural sources,and Cd is derived from both industrial and agricultural sources.In the sediments,Cr,Cu,and Ni originate from industrial and natural sources,Pb comes from traffic sources,and Cd and Zn come from a mixture of industrial,agricultural,and traffic sources.（2）The average concentrations of dissolved heavy metals in the water of the Harbin section from Songhua River are as follows(inμg·L^-1):Zn（21.75）>Ni（8.12）>Cu（6.61）>Cr（3.08）>Pb（2.29）>Cd（0.11）.While the average concentrations of particulate-bound heavy metals are as follows(inμg·L^-1):Zn（41.79）>Cr（29.02）>Pb（15.00）>Cu（7.78）>Ni（5.52）>Cd（0.10）.Pb and Cr have large solid-liquid distribution coefficients,which was 189.09 and 102.08,respectively,indicating a higher affinity for the particulate phase.The residual fraction of heavy metals in the sediment accounts for an average proportion of 70.18%,representing the main fraction of heavy metal occurrence.Cr and Ni are relatively stable heavy metal elements in the sediments from the Harbin section of Songhua River,with residual fractions accounting for 86.21%and 70.17%,respectively.Pb and Cu have a higher proportion of iron-manganese oxide fractions.Cd mainly exists in the residual fraction and the iron-manganese oxide fraction,with a secondary phase proportion of 15.4%.However,considering the toxicity and relatively large total amount of Cd,its potential risk should not be ignored.Additionally,the primary tributaries flowing through the urban area of Harbin show a more significant influence from human activities,resulting in a higher proportion of the secondary phase compared to other regions.（3）The dissolved organic matter（DOM）in the sediments from the Harbin section of Songhua River is mainly composed of humic-like substances（fulvic acid-like and humic acid-like）and protein-like components（tryptophan-like）.The upstream region of the Harbin section has relatively abundant humic-like substances,and the application of fertilizers and increased plant litter enhance the fluorescence intensity of the humic-like substances.In contrast,the downstream region has relatively abundant protein-like components.Fluorescence Index（FI）and Biological Index（BIX）indicate that domestic sewage discharge increases the endogenous load of the Hejia River tributary,where endogenous sources contribute significantly.BIX and Humification Index（HIX）show distinct variations with runoff.The upstream region is mainly influenced by terrestrial inputs,with slightly higher humification than the downstream region,which is influenced by both terrestrial and endogenous sources.Due to the inhibition by low temperatures,the sediment exhibits weak humification characteristics.Ultraviolet-visible absorption spectral indices,such asα（254）,SUVA₂₅₄,SUVA₂₆₀,E2/E3,E2/E4,and E3/E4,indicate a relatively high abundance of humic substances in the upper and middle reaches,with a relatively rich aromatic hydrophobic component.However,overall,DOM is predominantly composed of low molecular weight fulvic acid-like compounds.Correlation analysis results indicate significant correlations between organic matter content in sediments and the content of heavy metals in carbonate-bound,iron-manganese oxide-bound,and organic-bound fractions.FI and BIX can effectively indicate the speciation characteristics of Zn and Pb.（4）The average concentrations of heavy metals in snow follow the decreasing order of(inμg·L^-1):Zn（48）>Cu（42）>Cr（32）>Pb（28）>Ni（8.0）>Cd（1.0）.The concentrations of Zn and Ni in the snowmelt are lower than the Class I surface water standard limit,while the concentrations of other heavy metal are between the Class I and Class II surface water standard limits.The concentrations of heavy metals（excluding Zn and Pb）show a decreasing trend in urban,suburban,and rural areas,indicating the presence of initial fractionation and urban fractionation effects.The intensive emissions of Zn and Pb during rural heating suggest that this fractionation pattern is influenced by local emission sources.The urban site UR1 and the suburban site SU4 are two distinct areas of heavy metal enrichment.Industrial emissions in urban areas and coal combustion for heating in urban and rural areas are the main sources of heavy metals in the snow.The direct input of heavy metals from urban snow to rivers is as follows(inμg·m^-2):Zn（828.29）>Cu（761.49）>Pb（574.46）>Cr（547.74）>Ni（128.25）>Cd（20.04）,which is much higher than in suburban and rural areas.（5）The PM_2.5 concentration in Harbin reaches its peak during winter.Trajectory and cluster analysis72 h later indicate that most of the air masses causing severe pollution originate from the western and southern urban clusters of Harbin.Potential source contribution factor analysis and concentration-weighted trajectory analysis show that the Harbin-Changchun urban cluster,northern Liaoning region,and Chifeng city are the main potential source regions contributing to PM_2.5 in Harbin,with concentration weights exceeding 120μg·m^-3.The concentration weights in urban areas are significantly higher than in rural areas,and urban activities are the core factor contributing to the increase in PM_2.5concentration.The concentration range of heavy metals in PM_2.5 is 201.85 to 721.33 ng·m^-3,with the concentration of Cr exceeding the limit set by the Technical Regulation on Ambient Air Quality Index（AQI）.Compared to other cities,Harbin experiences severe atmospheric pollution during winter due to coal heating.Cd,Zn,Pb,and Cu are heavily influenced by human activities（EF>10）,with Cd（EF=1195.89）exhibiting the highest enrichment.Source apportionment using Positive Matrix Factorization（PMF）indicates that coal and biomass combustion are the main contributors to heavy metal pollution in Harbin’s winter PM_2.5,accounting for 48.6%of the total contribution,followed by transportation（29.5%）and industrial emissions（14.2%）,while the impact of natural dust is minimal（7.6%）due to the influence of snow cover.（6）The results of the Water Quality Index（WQI）and Potential Ecological Risk Index（RI）indicate that the water pollution level and ecological risk in the mainstream of Songhua River and the Harbin section are lower than those in the snow samples,suggesting that atmospheric deposition is an important source of heavy metal pollution in Songhua River.The Nemerow Pollution Index（P_N）results show that the sediment in the mainstream of Songhua River and the surrounding soil,as well as the sediment in the Harbin section,exhibit an overall"moderate pollution"level（2.5<P_N≤7）.Based on the Risk Assessment Code（RAC）using the fraction of heavy metal occurrence,it is indicated that Zn,Pb,and Cu in the sediment have a relatively high bioavailability,resulting in significant potential ecological risks.The Species sensitivity distribution indicates that the chronic HC₅ values for each heavy metal are as follows:Ni（27.49）>Pb（11.08）>Cr（9.38）>Cu（0.94）>Zn（0.89）>Cd（0.15）(μg·L^-1).More than 5%of aquatic organisms in the mainstream of Songhua River and its primary tributaries in the Harbin section are generally exposed to chronic risks caused by Cu,Zn,Cd,and Cr,particularly for organisms at the bottom of the food chain The human health risk assessment results considering the multiple pathways of atmospheric-water-soil exposure indicate that heavy metal pollution in the study area does not pose a non-carcinogenic risk（HI<1）.However,the carcinogenic risks associated with Cr and Cd(CRT>10^-6)are slightly higher than the risk threshold,with children being more significantly affected.Among the pathways,respiratory exposure is the main route for health risks,followed by ingestion,while skin contact poses the lowest risk.