| Organochlorine pesticides (OCPs) are priority pollutants and typical persistent organicpollutants (POPs). Because of their persistence, half-volatility and high toxicity in theenvironment, and biological accumulation through the food web, OCPs have great harm to theenvironment and human health. Because they were used largely and are difficult to degraded,OCPs are still the most widespread POPs in the environment despite their ban or restricted use indeveloped countries in 1970s and in China in 1983.OCPs are transported into water through different pathways, such as industrial and domesticwastewater, agricultural runoff, atmospheric deposition and soil erosion. Because of theirhydrophobia, OCPs have a strong affinity for suspended particles and subsequently settle downinto sediments under gravity. Therefore, sediments are thought to be one of the major sinks.However, OCPs in sediments can release into water by diffusion and resuspension and lead tothe recontamination of water. The study on transferring and transforming pathways of OCPs inwater and sediments is very important and is a hotspot.Lakes have important function on adjusting river runoff, irrigating farmland, shipping andaquiculture. In the recent years, with the economical development and irrational exploitation andapplication on lakes, the ecologic environment of many lakes has worsened gradually. OCPscontamination is one of the environmental problems. At present, the studies on OCPs are mainlypaid attention to bays and estuaries, and very few studies on OCPs in lakes. Honghu Lake is thelargest lake in Hubei province, and is an important aquiculture base. However, no data areavailable for OCPs in Honghu Lake. Therefore, Honghu Lake was selected as our case study ofOCPs contamination and kinetics in water and sediments. Through the discussion about theorigin, tendency and release of OCPs, we can know the transferring and transforming pathwaysof OCPs in the water and sediments in Honghu Lake, which may provide theoretical foundationfor controlling and treating organic pollution of lakes.Firstly, the concentrations and distribution of OCPs in surface water of Honghu Lake in different seasons were studied. The surface water samples were collected from two sectionplanes of Honghu Lake in both low and high water seasons. The results showed that theconcentrations of OCPs in surface water of Honghu Lake in low and high water seasons rangedfrom 1.22 to 8.02 ng/L and 1.15 to 4.69 ng/L, respectively. The average concentrations of OCPsin surface water of Honghu Lake in low and high water seasons were 3.47 ng/L and 2.87 ng/L,respectively, and the average concentration of OCPs in low water season was higher than that inhigh water season. HCHs and DDTs were the most dominant compounds in OCPs in the water.The total HCHs (the sum of a-HCH,β-HCH,γ-HCH andδ-HCH) in low and high waterseasons ranged from 0.94 to 7.04 ng/L and 0.79 to 4.0 ng/L, respectively. The averageconcentrations of HCHs were 2.97 ng/L and 2.36 ng/L, respectively. The total DDTs (the sum ofp,p'-DDT, o,p'-DDT, p,p'-DDD and p,p'-DDE) in low and high water seasons ranged from 0.06to 0.48 ng/L and 0.15 to 0.83 ng/L, respectively. The average concentrations of DDTs were 0.24ng/L and 0.41 ng/L, respectively. Compared with other regions in the world, the concentrationsof HCHs and DDTs in Honghu water were lower than other water areas. The concentrations ofOCPs in Honghu surface water were higher when the sample sites were nearer the mouth ofrivers flowing into the lake and the bank of the lake. It showed that OCPs in Honghu water weremainly derived from the input of surface runoff and pollutants along the bank of the lake. Thisdistribution tendency in high water season was clearer than that in low water season. Accordingto the data analysis, OCPs in low water season were mainly derived from the release of thesediments. And OCPs in high water season were mainly derived from the input of surface runoff,because residual OCPs in soils can transfer to water by rush and erosion of surface runoff. Inaddition, the OCPs concentrations near the fishery were high because there are a lot ofsuspended particles that have a strong affinity to OCPs.γ-HCH andβ-HCH were the mostdominant compounds in HCHs in Honghu surface water, which indicated that there are still newLindane inputs in Honghu Lake, but technical HCHs were old. DDE andβ-endosulfane were themost dominant compounds in respectively DDTs and endosulfane, which indicated that they areboth old residues.Secondly, the distribution and composition of OCPs in different depth water of HonghuLake were studied. The concentrations of OCPs in bottom water in mostly sample sites werehigher than that in surface water, which indicated that OCPs in bottom water were mainlyderived from the release of the sediments. Moreover, the concentrations of OCPs in porewaterwas much higher than that in surface and bottom water, reflecting possible long-term depositionand accumulation of OCPs in the environment and the strong affinity of OCPs for colloids inporewater. The clear gradient in OCPs concentrations between porewater and overlying watersuggests that OCPs will be transported from sediment porewater to overlying water by processessuch as diffusion. OCPs compositions were almost same between surface and bottom water.Thirdly, this dissertation divided the surface sediments of Honghu Lake into upper layer anddown layer, and studies the OCPs concentrations and distribution of them. The concentrations of DDTs are highest among OCPs in the sediments. The concentrations in upper and down layersranged from 2.39 to 25.79 ng/g and 1.22 to 27.5 ng/g, and the averages were 9.19 ng/g and 8.73ng/g, respectively. HCHs had higher concentrations in the sediments. The concentrations ofHCHs in upper and down layers ranged from 2.05 to 18.95 ng/g and 0.66 to 11.25 ng/g, and theaverages were 6.91 ng/g and 4.73 ng/g, respectively. The HCHs concentrations in Honghusurface sediments were lower than Haihe River in Tianjin and higher than that in other areas; andthe DDTs concentrations were lower than Merhei Lake in Rumanian and Haihe River in Tianjin,but were higher than other areas. The OCPs concentrations in Honghu surface sediments tendedto increase gradually from the middle of the lake to the bank or the mouth of rivers flowing intothe lake, which was similar with the surface water. Therefore OCPs in the surface sedimentswere mainly derived from the inputs of runoff or the bank. In addition, the OCPs concentrationswere higher near the fishery because of high organic substance. OCPs in upper sediments hadsimilar distribution tendency with the down sediments, but the concentrations of OCPs in upperlayer were higher than down layer. So OCPs in the down sediments might be derived mainlyfrom penetration of the upper sediments.γ-HCH andβ-HCH were the most dominantcompounds in HCHs in the surface sediments, which indicated that HCHs were mainly derivedfrom the application of Lindane and the residues of technical HCHs. DDE was the mostdominant compound in DDTs, andβ-endosulfane and endosulfane-sulfate were the mostdominant compound in endosulfane, which indicated that they were both old.Fourthly, release kinetics of OCPs in the sediments were studied by experiments. Therelease rules were studied under static, suspended and washing by exchanging water conditions.The results showed that there were faster release speeds at the beginning of the experiment, andthe release speeds slow down gradually with the increasing time. The release intensities of OCPsin the sediments were influenced mainly by the water-solubility of OCPs. Generally, a compoundwith high water-solubility has high release intensity, such asα-HCH; and a compound with lowwater-solubility has low release intensity, such as HCB. Moreover, temperature has importantinfluence to the release of OCPs. With the increase of the temperature in the experiment, thereleases of OCPs in the sediments were enhanced. Under the static condition, the release speedof OCPs in the sediments was very slow, and the balances between desorption and adsorptionmight reach after a long time. In addition, OCPs in the surface sediment had the down tendencyby diffusion. Compared to the static condition, the release speeds of OCPs in the sediments werefaster and the release intensities were stronger under the suspended condition, and it took onlyseveral days to get desorption and adsorption balance. In the experiment exchanging water, theconcentrations of OCPs in the water decreased gradually with the increase of times exchangingwater at begin. However, the concentrations of OCPs in the water kept stabilization after fourtimes exchanging water. Therefore, exchanging water may decrease the OCPs concentrations inwater, but it cannot remove completely OCPs from water. At last the OCPs concentrations in theoverlying water will keep stable levels because of the release from the sediments. A desorption kinetic model of OCPs from aquatic sediments was set up on the basis of these data. The testresult showed that the simulation data by the kinetic model can well fit the measure data, so themodel can be used to describe the release kinetic process of OCPs from the sediments to water.Fifthly, anaerobic degradation kinetics of OCPs in the sediments were studied. The resultsshowed that most OCPs might degrade slowly in natural sediments under anaerobic condition.The degradation ofγ-HCH, p,p'-DDT and o,p'-DDT was obvious, but the degradation ofβ-HCHand p,p'-DDE was very slow. The anaerobic degradation ofα-chlordane,α-HCH, HCB andp,p'-DDE fitted pesudo-first-order kinetics equation, with kinetic constants ofα-chlordane,α-HCH, HCB and p,p'-DDE were 0.0137d-1, 0.0078d-1, 0.0042d-1, 0.0005d-1, respectively.Sixthly, the processes of moving downwards of OCPs in the sediments were studied.Through the simulation experiment of penetration, the variable tendency of the OCPsconcentrations was studied in filtrate and sediments with different depth. The results showed thatthe concentrations ofβ-HCH in filtrate decreased gradually with the increase of time. However,the highest concentrations of HCB and p,p'-DDE in filtrate had obvious lags. Moreover, inpenetration process, the concentrations of HCHs and DDTs in 0-5cm sediments decreasedobviously, and the concentrations of HCHs and DDTs in the sediments with 5-15 cm depthincreased obviously. It indicated that HCHs and DDTs in surface sediment might movedownwards, but the moving distance was in the 15 cm depth because of sorption of TOC.Besides, HCHs in the sediment with the 15-20 cm depth might move continually down to thedeeper sediment, but the moving tendency of DDTs under 15 cm sediment was not obvious.Finally, the water quality model about lakes was studied. According to the qualityconservation rule, a new lake water quality model including a variety of influence factors was setup. This model considers correctly the adsorption and desorption of OCPs between sedimentsand water, and the desorption kinetic equation was used in the new model. At last, Honghu Lakewas regarded as a case, and the water quality model was analyzed and used. The results showedthat the model reflected basically the pollution variation of water. Therefore, it indicated that thethought and method of modeling were advisable, and the static release kinetic equation usedcould reflect basically exchanging fluxes of pollutant between sediments and overlying water.
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