Spatial Distribution,Vertical Migration And Risk Assessment Of Pahs In Plain Farmland Soil

The pollution of soil in plain agricultural areas is related to the safety of agricultural products quality and the stability of ecosystem,its pollution problem is of great concern.Polycyclic Aromatic Hydrocarbons(PAHs),the second largest pollutant after heavy metal pollution,pose potential threats to ecological environment safety and human health due to their strong tri-caustic effects.Therefore,it is of practical and longterm significance to rationally investigate soil PAHs pollution.As the largest logistics center of agricultural and sideline products in East-China,Zhougang District of Nanjing City is also one of the regions with rapid urbanization development.As a product of the urbanization process,this region gradually shows the composite pollution characteristics integrating industry,agriculture,transportation and life under the interaction of urban and rural areas.Meanwhile,the use and conversion of different land use types also affect the pollution and migration of PAHs in the local soil.Therefore,the study on the pollution of PAHs in soil in this region has certain reference value for the pollution control and prevention of PAHs in farmland soil in other areas.In this paper,the topsoil and soil profile of farmland in Zhougang area were taken as the research objects,and 15 kinds of optimal control PAHs stipulated by U.S.Environmental Protection Agency(USEPA)as the target pollutants,the residual status and spatial distribution of PAHs in farmland soils were analyzed.The vertical migration characteristics,its influencing factors and pollution sources of PAHs in soil profiles were also discussed and evaluated the ecological and health risks of farmland soils in the study area The main conclusions were as follows:(1)By analyzing the content and residual characteristics of PAHs in the topsoil of the study area,it was found that 14 PAHs were commonly detected in the soil(acenaphthene was not detected),and the high-ring(4-6)PAHs were the dominant components.Compared with other farmland soils in China,the content of PAHs in soils in the study area was at a medium and low level.The method of source apportionment was conducted for soil PAHs.The result performed that the mixed source was the dominant factor,of which the contribution rate of coal combustion and biomass combustion was 71% and the contribution rate of gasoline and diesel combustion was 29%,accompanied by a small amount of oil leakage,which was closely related to the level of local industrial development.(2)The horizontal distribution of PAHs content of different monomers in soil showed obvious increase and decrease regularity.From the perspective of spatial variation trend,the contents of 14 PAHs showed different rising and falling trends not only in the east and west directions but also in the south and north directions.The total PAHs showed a U-shaped distribution of "high on both sides and low in the middle" in the east and west directions,and gradually decreased from south to north in the south and north directions.From the perspective of spatial distribution characteristics,the contents of 14 kinds of PAHs in the soil were significantly different,and the overall trend was gradually increasing from southwest to northeast.(3)By analyzing the content characteristics of PAHs in soil profiles,it was found that PAHs showed different residual and migration characteristics in different soil profiles.The content levels of total PAHs in the soil profile of 0-80 cm were different,and and they showed the order as wasteland point>vegetable field>paddy No.2point >paddy No.1 point.Naphthalene(NAP)and fluoranthene(FLT)showed higher residue levels.The vertical distribution of PAHs is different in different farmland types.They mainly stay in the 0-20 cm soil in the wastelands,and migrate to the deeper soil layer in the cultivated soil such as vegetable field and paddy field.(4)The vertical distribution and migration of PAHs in soil profile was not only related to its physical and chemical properties,but also significantly affected by soil texture,organic matter content and pH value.The migration ability of different PAHs in the profile was as follows: Nap>Phe>Pyr>BaP>Inp,namely the lower cyclic PAHs component was more prone to migration.Soil texture was an important factor affecting the downward migration of PAHs in paddy field soil profile.Soil organic matter and pH were significantly correlated with the migration of PAHs in soil.(5)Comprehensive risk assessment of PAHs in farmland soil in the study area was carried out by using ecological risk assessment models and health risk assessment models.Based on TEQBaP,BaP and Chr in the study area contributed more to the total toxicity equivalent concentration,which needed to be further monitored.Based on risk quality standard method,the detection rates of BbFandBkF were 100%,and those of IPY andBPE were partially detected(82.14%).The probability of negative effects to the local ecological environment was large;The results of ILCR model evaluation showed that the order of the ILCRs produced under the threet exposure pathways was: ingestion by mistake>skin exposure exposure>breathing;According to the health carcinogenic risk standard established by USEPA,,the potential threat of soil PAHs to human health in the study area was low.On the whole,the risk of PAHs in farmland soil in the study area was low,but monitoring and prevention work was still needed in the later stage.