Study On Pollution Levels In Air And Soil And Environmental Behavior Of HCB In Gansu And The Surrounding Areas

HCB(Hexachlorobenzen)is one of the 12 persistent organic pollutants(POPs)whose uses are strictly prohibited or restricted in the Stockholm convention because of their persistent,strong volatilization,highly toxic,carcinogenic and uneasily degradation.HCB can accumulate through food chain,and then make a biomagnification in living tissues.The sources of HCB are very extensive,including using as pesticide,the emissions of combustion and major industrial manufacturing.The specific geographical environment in northwest of China,contributed the weaker development of production.Public had paid little attention on POPs,and there were scarcely any studies on it,presently.In this study HCB was taken as the main object,the samples including air and soil from 32 sites were collected respectively in Gansu and the surrounding areas from August of 2010 to August of 2011.The seasonal and spatial pollution levels of HCB in air and soil were investigated,meanwhile,the source and soil-air exchange were discussed,in addition,the transfer and fate of HCB in 5 main environment phases of Lanzhou area were simulated by using the III-level Multimedia Fate and Transport Model(MSTM-III).This work would be benefit to estimate the HCB pollution levels in air and soil,exhibte multimedia environmental behavior,and lay scientific basis for ecological risk assessment and pollution controlling in the study areas.The study results are summarized as follows:(1)The seasonal and spatial pollution characteristics of HCB in air.The detection rate of HCB was as high as 95.83%,the annual average concentration of HCB was 70pg/m3,and the concentration levels were in the range of 9pg/m?489pg/m.The pollution levels showed a significant seasonal variation as follows:winter>autumn>spring>summer,inferring that the sources of HCB come from the incomplete combustion of coal during heating period according to the characteristics of agricultural manufacturing and lifestyle in Gansu and the surrounding areas.Meanwhile,the spatial pollution levels of HCB showed obvious differences,urban>rural>background,and the pollution levels of Lanzhou and its surrounding cities were higher than those of Gansu Corridor relatively.As we knew,Lanzhou is the major industrial center in North-West,so the high pollution levels of HCB in the surrounding cities of Lanzhou may be affected by the emissions of chloride industrial from Lanzhou.At the same time,the differences in coal combustion might have some impacts on it.Compared with the relevant studies of domestic and abroad,we found that the pollution levels of HCB are higher than those of abroad slightly,but much lower than the northern areas such as Beijing and Tianjin.Actually,the pollution levels matched well with the southwest areas and the Pearl River Delta regions.(2)The spatial pollution characteristics of HCB in soil.The detection rate of HCB in the study areas was as high as 96.87%,the annual average concentration was 1.211ng/g,and the concentration levels were in the range of 0.018ng/g?11.707ng/g.There were some variations in each sampling site,in which the HCB concentrations in soil of urban were higher than those of rural,and the lowest of background.The soil concentration of HCB in rural was much lower than the agricultural soil standard value 50ng/g in Canadian soil quality guidelines.From the study results,we can deduce that the HCB may come from chloride industrial pollution emissions.This study also showed that the content of soil organic matter is clearly an important influence factor,and the soil concentrations of HCB would increased with the CSOM.Furthermore,the HCB concentration in soil of Gansu and the surrounding areas was at a very low level as a whole compared with the relevant studies from domestic and abroad.(3)The soil-air exchange characteristics of HCB.The HCB volatilized mainly from soil to air as a whole,and soil became a secondary emission source of HCB.The HCB volatilize degree showed obvious variation in different seasons,while summer>spring>autumn>winter,speculating that temperature is a significant factor which affected the progress of soil-air exchange.There was obvious spatial pattern emerging,for the volatile intensity in rural was higher than that in urban,and the lowest of background.(4)MSTM-? model in Lanzhou showed that the concentrations of HCB in air,water,soil,sediment and vegetation were 1.49×10-1212mol/m3,1.03×10-9mol/m3,1.31×10-9mol/m3,1.90×10-5mol/m3 and 2.35×10-14mol/m3,respectively,and the order of the corresponding concentration was sediment>air>soil>water>vegetation.The quantity of sediment retention was 54.1kg,accounted for 98.8%of the total retention.Moreover,the simulating concentrations matched well with the observed data.In other word,the MSTM-III model had a good applicability in Lanzhou area.In addition,the main transport processes in 5 environmental phases were:air-vegetation,air-soil,sediment-water,water-sediment and air-water.Simultaneously,the degradation fluxs showed that sediment,air and soil were the primary degradation media for HCB in Lanzhou area.