Pollution Status And Risk Assessment Of Heavy Metals And PAHs In Farmland Soils At Regional Scale

Soil pollution is a key issue which threatens the ecosystems,food safety and human health.Research on combined pollution in farmland soils is becoming a current concern.Heavy metals(HMs)and polycyclic aromatic hydrocarbons(PAHs)are the two most typical types of pollutants in contaminated farmland soil with different chemical transformation behaviors.Meanwhile,complex interactions would happen when these two pollutants coexist.Previous studies mainly focused on environmental behavior and the exposure risk by single type of pollutant,usually heavy metals or PAHs,while studies on the combined pollution characteristics and risk assessment of soil metals and PAHs were little concerned.Thus,on the basis of soil samples collected from paddy soils in a county-scale area where e-waste recycling activities prevalent before,we employed the geostatistical method,multivariate statistical and risk assessment model to reveal the current pollution status of heavy metals and PAHs in paddy soils of study area.The associated ecological risk and human health risk were assessed.Afterwards,we explored the spatiotemporal variation of soil heavy metals and PAHs and then predicted future concentrations and risks of soil heavy metals under different assumed scenarios.Based on the above research results,we explored and identified the different risk zones of soil heavy metals and PAHs by a novel regional risk zoning method.The main results are as follows:(1)The pollution status and exposure risk of soil heavy metals.The concentrations of heavy metals(Cd,Cu,Zn,Pb,Ni,As,and Cr)in paired soil-rice grains were analyzed.Results showed that the soils were contaminated by Cd,Cu and Zn after the unregulated e-waste recycling prohibited.Rice grains were mainly contaminated by Cd,with 20.7% samples exceeding the threshold value.The spatial distribution of soil Cd,Cu,Zn and Pb shared very similar patterns with higher concentrations in northwest,which was mainly caused by e-waste.While,the concentrations of soil Cr,Ni and As increased from west to east and mainly from natural source(e.g.rock weathering).The potential ecological risk assessment indicated that the ecological risks posed by heavy metals ranked from low to high level.The human health risk assessment based on USEPA multi-exposure model concluded that the farmers might suffer from carcinogenic risk due to the exposure by Cd and non-carcinogenic risk of Cr.Combining the prediction maps of heavy metals in soils and rice grains,the potential ecological risks,together with that of human health risks,the priority areas were proposed for further target remediation.(2)The spatiotemporal variation and risk warnings of soil heavy metals.The spatiotemporal variations of soil heavy metals(Cd,Ni,Zn,Pb and Cu)in study area during 2006-2016 were analyzed.Results revealed significant and diverse spatiotemporal changes of soil heavy metals.The joint area,where Cd,Ni,Zn,Pb and Cu all increased during 2006-2016,was located in northwest area.Besides,we adopted a scenario-simulation model to predict future dynamic concentrations of soil heavy metals under optimistic(the pollution inputs are zero under strict environmental policy)and default(the pollution status maintain constant)conditions.Soil heavy metal concentrations as well as the associated ecological risks would decline gradually under optimistic scenario,while sharply increase when no control acts taken over long term.The percentage of soil Cd and Cu that exceeding their corresponding risk screening value(RSV)under default condition would be 1.6 and 1.3 times higher over optimistic scenario 10 years later.The probability of high potential ecological risk in default condition would be twice higher than that under optimistic scenario in 2026.Overall,soil quality would be significantly improved under optimistic condition that all sources of pollution were assumed ceased,while deterioration further in default condition.Strengthening the control of pollution sources and more strict environmental policy are very important for soil heavy metals contamination prevention and control.(3)The pollution characteristics and exposure risk of soil PAHs.By analyzing the concentrations of PAHs in soil samples,the results showed that 16 priority PAHs were frequently detected in paddy fields with different detection frequency,and dominated by high molecular weight PAHs.The mean concentration of 16 PAHs was 407.3 ?g/kg.The study area was contaminated by PAHs,mainly with lightly to moderately pollution level.The spatial distribution map of soil PAHs concentrations showed highest values in southwest area.Principal component analysis together with diagnostic ratios revealed the combustion of coal in unregulated e-waste dismantling process was the main pollution source of PAHs in the study area.The ecological risk assessment results concluded that 33% of the soil samples in this area exceeded the ecological risk guideline value.ILCR model was adopted to assess the human health risk posed by PAHs.Both deterministic and probabilistic assessments demonstrated potential health risk of the residents might be suffered from highest risks in southwest area.With the Monte Carlo simulation of selected parameters,the health risk of soil PAHs varying 3 to 4 times.Sensitivity analysis revealed that exposure frequency and concentration of PAHs were the two most influential parameters for the total variance in the risk assessment model.It is an effective way to reduce the exposure risk of PAHs by reducing the exposure frequency and the content of PAHs in the polluted area.Comparing with historical data in 2011,results implied that the more stringent environmental policy and regulation could lead reductions in soil contamination with PAHs.(4)Combined risk zoning and control of soil heavy metals and PAHs.By adopting the bivariate local Moran's I,we analyzed the spatial interaction between heavy metals and PAHs,and try to reveal the causes of spatial interaction pattern.The results revealed that heavy metals and PAHs showed different spatial interactions with the “high-high” cluster in southwest area,mainly caused by same source and sink.And then,different risk zones were identified in study area by combining spatial interaction with health risk assessment.The combined risk zones of both heavy metals and PAHs are located in southwest and northeast edge and occupied with 5.6% of total area.Corresponding risk control measures were recommended in different risk zones to ensure the food safety and human health.Above results showed that the paddy soils in the study area were mainly polluted by heavy metals and PAHs.Heavy metals and PAHs both caused potential ecological and human health risks for local residents.The combine risk zones of heavy metals and PAHs were located in southwest and northeast area,caused by same source and sink effect.Different temporal changes of heavy metals and PAHs with environmental policy adjustment indicated their characteristics difference.Novertheness,strict environmental policy plays positive effect on soil heavy metals and PAHs pollution control.Strict environmental policies and risk zoning measures should be adopted to reduce the pollution level and exposure risk of heavy metals and PAHs.