| Persistent organic pollutants(POPs)are of global concern due to their persistence,high toxicity,and long-range transport(LRT)potential.Understanding the environmental distribution,sources,and mechanisms of transportation and transformation of POPs is important for the reduction and elimination of POPs and to protect the environment.The Tibetan Plateau has an average altitude of more than 4000 m and is also called the "Third Pole" of the Earth.At the same time,there are many densely populated and industrially developed areas around the Tibetan Plateau.Frequent human activities are often accompanied by the emission of a large number of pollutants(including POPs).These pollutants can migrate via LRT to the edge of the Tibetan Plateau.For example,Sichuan Basin,adjacent to the eastern edge of the Tibetan Plateau,has historically high-intensity emissions of multiple types of POPs which may threaten the ecological system of the Tibetan Plateau and the health of local residents through LRT.In addition,although the Tibetan Plateau is remote,with accelerated urbanization in recent years,local towns within the plateau have developed rapidly,and POPs emitted from the plateau's local sources also pose potential threats to the plateau ecology and residents' health.Revealing the environmental distribution of POPs and quantifying the contribution of LRT and local sources are of great significance for reducing emissions of POPs and the protection of plateau ecology and people's health.On the other hand,the eastern edge of the Tibetan Plateau is strongly undulating(with an altitude of <1000 m?>4500 m),and the corresponding climate and environmental conditions are also diverse.To date,the research on the transportation and transformation of POPs,especially the kinetics,mostly stays in the laboratory stage with single or few environmental variables,and the relevant parameters may not be suitable for direct application in the natural environment with more changeable environmental conditions.For the discussion on the transportation and transformation mechanism of POPs under different natural environmental conditions on the Tibetan Plateau,in-situ or field experiments are urgently needed to verify the results of relevant indoor experiments.In addition,the gradient of the climate environment on the eastern edge of the Tibetan Plateau also provides diverse climate scenarios for studying the response of transportation and transformation of POPs to changes in environmental conditions,which is of great significance to understand environmental fate of POPs better.Based on environmental sample collection and a field experiment,this research focused on the spatial distribution and source of POPs on the eastern edge of the Tibetan Plateau(Chapter 3)and mechanisms of transportation and transformation of POPs(Chapters 4and 5).In Chapter 3,we collected the surface soils from a long section between the Sichuan Basin and the Yellow River source on the eastern edge of the Tibetan Plateau and,analyzed 4 organochlorine pesticides(OCPs)and 25 species of polycyclic aromatic hydrocarbons(PAHs)including PAH-derivatives.Then,the concentrations and spatial distribution of these pollutants were discussed.Using Pearson correlation analysis,longrange transport potential(LRTP)analysis,ratios of characteristic compounds,and principal component analysis-multiple linear regression analysis,the sources of POPs were identified,and the contributions of different sources were quantified.Chapter 4 is based on a one-year field pot experiment(located in four sites A/B/C/D with climate gradient)with two OCPs(?-HCH and p,p'-DDT)in spiked soils.The dissipation rates of?-HCH and p,p'-DDT in soil,the rates of formation and dissipation of their transformation products,and the transformation pathway were discussed.The dissipation and transformation of ?-HCH and p,p'-DDT in response to altitudinal climate gradient(as a proxy for climate change)were also discussed.Using the concentrations of ?-HCH,p,p'-DDT,and transformation products obtained in the field pot experiment,the applicability of the empirical ratios for identifying the source of pesticides and their input time was evaluated.In Chapter 5,based on a one-year field pot experiment,the dissipation rates of four PAHs(Phe,Pyr,Ba P,and Bghi P)were studied using a biphasic first-order model.Using a surface soil model based on the fugacity theory,the dissipation rates of three important dissipation processes(volatilization,leaching,and biodegradation)were calculated and the formation rate of non-extractable residue(NER)of PAHs in soil was derived.The uncertainty of this derivation was analyzed.In addition,the effects of both physico-chemical properties of pollutants and environmental conditions on the fate of PAHs were discussed.The results and conclusions are as follows: 1)Compared with other mountainous and remote regions in the world,the concentrations of DDTs,HCHs,and HCB in the soil from the eastern Tibetan Plateau stay at a medium level,and PAHs stay at a medium or low level.The spatial distributions of Pe CB,HCB,and most PAHs are affected by residential or industrial activities,and the spatial distribution of DDTs is affected by agricultural activities and ranching.The LRTPs of HCB and Pe CB are high,of HCHs are medium,of DDTs are low,and of PAHs are extremely low.HCHs and DDTs are mainly from historical input of technical-HCH and technical-DDT,and PAHs may originate from petroleum combustion and biomass combustion.Three local sources of OCPs and PAHs accounted for a total of 88%,and two mixed sources of LRT and local emissions accounted for the rest of 12%.2)In the field experiment,the dissipation rate of p,p'-DDT is faster than that of ?-HCH,which may reflect the significant influence of the NER formation process in the soil.?-HCH and p,p'-DDT in low altitudes with high temperature and high precipitation have a faster dissipation rate,indicating the promotion of high temperature and high precipitation on the dissipation processes such as volatilization,leaching,and biodegradation.p,p'-DDT also has a faster dissipation rate on Site D at low temperature and high altitude,which may reflect photolysis of p,p'-DDT under strong solar radiation.The degradation products of ?-HCH have a faster dissipation rate than the degradation products of p,p'-DDT.The transformation pathways confirmed in this study include ?-HCH?Pe CCH,?-HCH?Te CCH and ?-HCH??-HCH as well as p,p'-DDT?p,p'-DDE and p,p'-DDT?p,p'-DDD?p,p'-DDMU?p,p'-DBP.The ?-HCH and p,p'-DDT in Sites A and D have a faster dechloroelimination rate and a slower dehydrochlorination rate,suggesting that the humid environment and potential photolysis favor dechloroelimination but suppress dehydrochlorination.The ratio of(p,p'-DDD+p,p'-DDE)/p,p'-DDT is more suitable for identifying the input time of DDT than that of p,p'-DDE/p,p'-DDT,especially in humid subtropics.3)In the field experiment,at all sites,the decline in concentrations is more pronounced for lighter than heavier PAHs;For all PAHs,the decrease of concentrations is more notable at Sites A and B than that at Sites C and D.Overall,the dissipation of PAHs is controlled by biodegradation and NER formation.In general,Ba P and Bghi P with larger molecular weights have shorter halflives in the rapid phase and longer half-lives in the slow phase(except for Phe and Pyr in Sites A and B).PAHs in low altitudes with high temperature and high precipitation have a longer half-life in the rapid phase(except for Phe and Pyr in Site D),and a shorter halflife in the slow phase.This result suggested that the environmental behavior of largemolecular-weight organic pollutants is controlled by NER formation,and smallmolecular-weight organic pollutants are dominated by both NER formation and biodegradation.The patterns of half-lives of PAHs provided a plausible explanation that NER formation as derived in this field is governed by sorption process,i.e.,the process of type I NER formation. |
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