Geochemical Behaviors of Persistent Organic Pollutants in Tibet:Source and Transport

Author:Li Jun

Supervisor:yuan guo li


Degree Year:2019





The Tibetan Plateau was genreally referred as the global sink of persistent organic pollutants(POPs),which have potential to long-range atmospheric transport.Due to rapid development,local activities in Tibet including industrial and residential activities,also massively emitted POPs into the background environment over last decades.Once escaped from Tibet,POPs from local sources have potential to be involved in the global cycle.However,research focusing on the occurrence,behavior,and fate of local POPs is very scarce,and the current role of Tibet in the global cycle of POPs is not clear.Within the regional scale of Tibet,the glacier melting and soil erosion were significantly enhanced due to ongoing global warming.Some legacy POPs that were formerly trapped in glacier and soil may be released into surrounding environments,which is proposed as the “secondary source”.Herein,it is necessary to provide proof to validate the hypothesis and clarify the related environmental process.In this case,a comprehensive study was conducted in Tibet,combining multimedia fieldmeasurements with modeling.As the results showed,the soil concentrations of 16 polycyclic aromatic hydrocarbons(PAHs)were 52.3 ng/g in the industrial area,and 103 ng/g in the residential area,far exceeding the background level.Mining activities served as an the emerging source for PAHs in Tibet,and they annually emitted 2.7 tons of heavy molecular weight(HMW)PAHs into soils.In landfill,PAHs were closely assocated with residential biomass ash,and their fractionations over depths in soils profiles emphasized the post-deposal leaching.Based on the diagnostic ratios that were adjusted by rectification factors,the biomass burning was the dominant source of PAHs in the residential area.The annual emission flux of PAHs from biomass ash to soil was estimated to be 13.1–32.5 tons in Tibet.The studies indicated that PAHs generated from local sources would be transported within Tibet rather than involved in the global cycle.The soil concentration of polybrominated diphenyl ethers(PBDEs)was 392 ng/g in landfill.The vertical and spatial distribution patterns of PBDEs within soils plausibly revealed their main transport pathways by atmospheric dispersion and leachate seepage from landfill.These two pathways were estimated to account for 61% and 39% of the total concentrations,respectively.The dated sediment core from Lake Yamzho Yumco recorded the deposition fluxes of dichlorodiphenyltrichloroethanes(DDTs)and hexachlorocyclohexanes(HCHs)in history.In addition to the flux peaks in the 1970 s corresponding to their heavy usage,these pollutants’ fluxes displayed rebounds in the flood periods,with characteristic low ratios of DDT/DDE(α-/β-HCH).This might indicate a massive remobilization of “weathered” pesticides from catchment soil to the lake because of strong hydro-dynamics.This study confirmed the lateral remobilization as a secondary source,which contributed to 20%-42% of the total fluxes in lake sediments.In the glacier alluvial deposits,two concentration peaks were observed for DDTs and HCHs in the past century.The first peak was observed in the 1970 s,corresponding with their heavy usage in the surrounding countries and regions.The second one was in 2000 when the production and usage of DDTs and HCHs were strictly limited,which possibly indicated a significant release from melting glacier.This was further supported by the enantiomeric fraction values for o,p’-DDT and α-HCH.This study evidenced the hypothesis that the melting glacier in Tibet represents a secondary source.