Persistent Toxic Substances In Soils And Tree Bark Across Chinese Agricultural Regions:Residue Characteristics And Associated Health Risks

Persistent toxic substances are a class of chemicals that are environmentally persistent, bioaccumulative and toxic, posing potential risks to both humans and ecosystem. Organochlorine pesticides, including hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs), phthalates esters (PAEs) and heavy metals are typical PTSs. With the rampant development over the past decades, large amounts of these substances were produced and applied in China. Although the application of organochlorine pesticides has been restricted for over three decades in China, they can be still detected in various environmental compartments and having adverse impacts to human health due to their persistent and long-distance migration. Phthalates and heavy metals are ubiquitous contaminants in agricultural practices and still largely introduced into agricultural environment now. Hence, it is significant to conduct studies on their residue levels and environmental fate, as well as health risk assessment. However, up to now, there has been limited information on the residue patterns and risk assessment of PTSs in soils and the air at a national scale in China, especially for the enantiomeric signatures of chiral organochlorine pesticides, In addition, there were also quite a little studies using tree bark as passive sampler for monitoring atmospheric pollutants. Therefore, in this study, surface soil and tree bark samples from agricultural fileds across China were collected. The concentrations of HCHs, DDTs, PAEs and metals in soils and HCHs and DDTs in bark were determined. We aimed to elucidate their residue status, factors influencing their distribution patterns, possible sources and potential human exposure risks, as well as the enantiomeric signatures of a-HCH and o, p’-DDT. The major results are as follows.(1) The average concentrations of a-HCH,β-HCH, y-HCH, and total HCHs in Chinese agricultural soils were 0.190,1.31,0.236 and 1.74 ng/g, respectively. Residues of HCHs likely originated from past usage of technical HCHs. High residue levels were found in central and southern part of China. The isomers of a-HCH and y-HCH tended to accumulate in the sites with lower total HCH concentrations, lower temperature, higher elevation, and less wet precipitation when compared to β-HCH. Enantiomeric analysis showed a preferential degradation of (-)-a-HCH. Human health risks via various exposure routes to HCHs in soils were further estimated. Overall, the mean hazard index (HI) linked to non-carcinogenic risks was below 1, suggesting an absence of non-carcinogenic risks of HCHs in Chinese soils. In addition, the cancer risk values were all below 10-4, which indicates low or very low risks.(2) The average contents of α-HCH, β-HCH, γ-HCH, δ-HCH and ΣCHs in bark were 1.16,2.51,1.67,0.368 and 5.71 ng/g (dry basis), respectively. Jing-Jin-Ji region was identified as the "hot-spot" of bark HCHs in China. Their residues were likely from the combined sources of historical applications of technical HCHs and lindane through long-distance transport. HCH contents were found inversely correlated with annual precipitation and temperature, but positively correlated with PM10 or PM2.5 due to the bioaccumulation of both vapor-and particle-phase HCHs by tree bark. Most bark samples preferentially accumulated (+)-a-HCH, and the enantiomeric fractions (EFs) of a-HCH were positively correlated with a-HCH concentrations and the elevations of sampling locations.(3) The average concentrations of o, p’-DDE, p, p’-DDE, o, p’-DDD, p, p’-DDD, o, p’-DDT, p, p’-DDT and total DDTs in Chinese agricultural soils were 0.137,3.29, 0.242,0.469,0.711,3.21 and 8.06 ng/g, respectively. Their residues likely originated from the historical usage and new input of DDTs and dicofol. The levels of DDTs in eastern China were higher than those in western China, and their distribution patterns were significantly influenced by soil organic matter and socioeconomic development. The mean concentrations and total inventories of DDTs in Chinese surface soils determined in this study decreased with time when compared with that in 2011. Soil organic matters played key roles in these variations. The results of enantiomeric signatures showed that (-)-o, p’-DDT preferred to accumulate in soils compared to (+)-o, p’-DDT. The value of DEVrac of o, p’-DDT also increased with temperature. Except for 2.46% and 1.64% samples posing non-carcinogenic risks and carcinogenic risks, respectively, the levels of DDTs in most soils across China were relative safe to human health.(4) The average residue levels of o, p’-DDE, p, p’-DDE, o, p’-DDD, p, p’-DDD, o, p’-DDT, p, p’-DDT and total DDTs in tree bark across Chinese agricultural regions were 0.558,1.56,0.375,0.298,1.22,1.60 and 5.61 ng/g, respectively. Their residues are likely from the historical usage and new input of DDTs and dicofol. Similar to those in soils, the levels of bark DDTs in eastern China are higher than those in western China. Jing-Jin-Ji region was identified as the "hot-spot" of DDTs in bark. The atmospheric DDTs were positively correlated with PM2.5 and socioeconomic indices. The enantiomeric signature of o, p’-DDT in bark was impacted by those in soils. In most bark samples, (+)-o, p’-DDT was preferred to be degraded and (-)-o, p’-DDT was enantioselectively accumulated.(5) The concentrations of DDTs in the air were estimated based on their residue levels in tree bark. The average atmospheric content of DDTs was 860 pg/m3. The carcinogenic risks of human exposure to airborne DDTs were all lower than 1×10-4, suggesting the absence of health risks. The results of soil-air exchange showed that the fugacity fractions (ff) of DDTs in most agricultural regions were below 0.3, suggesting their net gaseous deposition to soil. In addition, the ff values were inversely impacted by temperature and precipitation.(6) The concentrations of E15PAEs in agricultural soils were in the range of 75.0-6369 μg·kg-1. Three provinces (i.e., Fujian, Guangdong and Xinjiang, China) showed the highest loadings of PAEs. Bis(2-Ethylhexyl) phthalate (DEHP) was found as the most abundant component and contributed 71.5%　to the ∑15PAEs. The major source of PAEs in arable soils was associated with the application of agricultural plastic films, followed by the activities for soil fertility. Furthermore, the non-cancer and carcinogenic risks of target PAEs were estimated. The hazard indexes (HIs) of PAEs in all samples were below 1 and the carcinogenic risk levels were all within 10-4.(7) The concentrations of Pb, Cd, Zn, and Cu (10.1-184.2,0.082-1.31,20.2-321.9, 8.2-515.9 mg/kg, respectively) in agricultural soils were elevated above their reference values. The levels of Cr, Sb, Ni, Co, Mn, Ca and Mg in soils were in the range of 27.5-111.1,0.564-3.54,3.14-94.4,0.715-31.3,40-1681,1289-132904 and 712-22626 mg/kg. Pb, Cd, Cr, Zn, Cu and Ni followed a similar geochemical distribution pattern. The accumulation of Pb, Cd, Zn, and Cu in soils may be associated with human activities for soil fertility. Cadmium had the highest pollution index (PI) of 5.28, and the average hazard quotients (HQs) for all the metals were below 1. Hence, except for Cd, the metals in Chinese arable soils are comparatively safe.