Transport Of Persistent Organic Pollutants (POPs) Along Altitudinal Gradient In Agricultural Soils Of Tanzania

This study investigated the persistent organic pollutants（POPs）particularly the organochlorine pesticides（OCPs）and the 16 USEPA priority polycyclic aromatic hydrocarbons（PAHs）.Thirty-two agricultural surface soils samples were collected from August to September 2019 among geographical regions from South-eastern（SE）lowlands to Central-western（CW）highlands in Tanzania.The study aimed at recognizing the status of POPs and their transportation between the high pressure zones of the Indian Ocean coast（SE）through the Eastern（E）lowlands regions and the low pressure zones in the South-western highlands（SH）and Central-western（CW）highland regions,and understanding its implications in environmental and human health.With this regard,the study specifically examined the status of OCPs and PAHs in the surface soils including the concentration levels and compound compositions,and mapped the spatial distribution among the land uses in different geographical regions.It also assessed the ecological and human health risks associated with the compounds using toxic equivalency factor（TEF）and incremental lifetime cancer risks（ILCR）.It further examined depth distribution of the compounds in relation to soil properties,explored the sources and ascertained preferential transport pathways among the lowlands and highlands surface soils using molecular diagnostic ratios（MDGs）,Positive Matrix Factorization（PMF）and Hybrid Single-Particle Lagrangian Integrated Trajectory（HYSPLIT）model.The results showed that DDT,dieldrin,and methoxychlor were the most abundant OCPs with mean concentration of 2.29±9.04 ng/g,1.572±6.58 ng/g,and 0.796±1.11ng/g,respectively.The detection frequencies of these compounds were 84%,75%,and59%,respectively.HCH was less abundant（mean concentration 0.147±0.13 ng/g）however it was the most dominant OCP（detected in 88%of the samples）compared with DDT（84%）,dieldrin（75%）,and methoxychlor（59%）.DDT contamination was dominated by p,p′-DDE and resulted from both technical DDT and dicofol while HCH contamination resulted from technical HCH and lindane（γ-HCH）as a dominant compound.With the exception of high methoxychlor in the E-sites and high DDT in the SE-sites,the OCPs predominate mostly in the SH-sites,which is the most productive agricultural zone.In both lowlands and highlands,the OCPs predominated in the upper soil except for some few compounds such as o,p’-DDE,o,p’-DDD,α-endosulfan,and aldrin.The detection frequency of the other compounds such asα-HCH,β-HCH,γ-HCH,δ-HCH,p,p′-DDT,o,p’-DDT,p,p′-DDE,p,p′-DDD,β-endosulfan,endosulfan sulfate,cis-Chlordane,trans-Chlordane,heptachlor,heptachlor epoxide,endrin,and dieldrin were100%,89%,60%,64%,60%,64%,60%,60%,83%,73%,71%,57%,83%,57%,78%,and 62.5%while their detection in the lower soil were 0%,11%,40%,36%,40%,36%,40%,40%,17%,27%,29%,43%,17%,43%,22%,and 37.5%,respectively.Despite the predominance in the upper soil,the OCPs were from old usage in most of the sites,even though there were indications of recent input shown by dominance of parent compounds at some sites.OCPs predominance in the upper soil was more evident in the highlands,while the predominance of some OCPs in the lower soil was apparent in the lowlands.It was further noted that OCPs accumulation in the surface soil was linked to low volatilization and slow degradation but preferentially leached in soils with high water saturation and low adsorption capacity.On the other hand,PAHs were dominated by low molecular weight PAHs（LPAHs）compared to high molecular weight PAHs（HPAHs）.The sources of PAHs contaminated soil was mainly of pyrogenic origin.Five factors were identified based on PMF analysis,that is,mixed natural and agricultural sources,wood and biomass combustion,industrial source associated with the use of coal,natural gas,and oil/diesel as sources of energy,petrogenic,and vehicular emission,which contributed 63%,20.9%,5.7%,8.3%,and2.2%,respectively.Based on spatial distribution,the lowland sites showed more PAHs abundance.However PAHs transportation was mainly localized,with atmospheric deposition of HPAHs occurring closer to emission sources,and leaching occurred preferentially in wetlands and irrigated soils.This reflected the influence of localized wind masses within the country that reached the highlands（SH-sites）with PAHs predominance in the upper soil but a different pattern showing PAHs predominance in the lower soil at the one highland irrigation site（IR）.There was also an influence of volatilization that occurred mostly in the coast and CW hot-environment sites associated with less detection of LPAHs in the upper soil.Accordingly,in the lowlands,both LPAHs and HPAHs predominate mainly in the lower soil,analogous to less detection of LPAHs in the upper soil in CW-sites,contrary to the predominance of LPAHs in the upper soil in SH-sites.Nonetheless,there are wind masses from far sources of similar origin across the coast of Indian Ocean that reaches the lowland sites and some highland sites.This suggested that there was trans-boundary movement of the compounds between the lowlands and adjacent highlands.Based on assessment of toxicity,∑₁₆PAHs ranged from 10.43 ng/g to 288.58 ng/g（mean 70.273±74.129 ng/g）with the coefficient of variation（CV）of 105.5%showing a high variation among the sites.The∑₁₆PAHs was higher at the sites KL>NC>MBM>DM while the total of Benzo（a）pyrene（Ba P）equivalent（∑Ba Peq）that ranged from0.063 to 5.235 ng/g was higher at the site NC（1.7 to 83.6 folds）following the trend NC>MBU>DM>KL.On the other hand,∑₇PAHs（0.43 to 5.25 ng/g）was higher at the sites NC>KL>DM>MBU,and∑₇Ba Peq that ranged from 0.0056 to 5.15 ng/g was higher at the sites NC>MBU>DM>RP>KL.This showed that NC,KL,MBU,and DM were the hotspots for PAHs toxicity.Despite the abundance of Naphthalene（Nap）,the∑Ba Peq comprised of mostly Ba P following the trend Ba P>Dibenz（a,h）anthracene（Dah A）>Benzo（k）fluoranthene（Bk F）>Indeno（1,2,3-cd）pyrene（Icd P）>Nap>Benzo（a）anthracene（Ba A）>Benzo（b）fluoranthene（Bb F）>Chrysene（Chr）>Phenanthrene（Phen）>Anthracene（Anth）>Fluoranthene（Fla）>Fluorene（Flu）>Pyrene（Pyr）>Benzo（g,h,i）perylene（Bghi P）>Acenaphthylene（Acy）>Acenaphthene（Ace）;and there was a low contribution of∑₇PAHs to∑₁₆PAHs.However,PAHs can be considered as non-contaminated or safe level in the environment as did not exceed the threshold limit for∑₁₆PAHs level(<200μg kg^-1),Ba P（30μg/kg）,Nap,Phen,Flu,Anth,Ba A,Chr,Bb F,and Bghi P（100μg/kg）and their totalΣ₉PAHs（1,000μg/kg）,and∑₇Ba Peq（<600μg/kg）following Dutch regulations for unpolluted soil,Polish standards for the upper layer of agricultural surface soils,and Canadian soil quality guidelines（CCME）for the protection of environmental and human health.Likewise,the level of soil OCPs among the sites was low as the maximum concentration of the DDTs was 47.44 ng/g,dieldrin（32.42 ng/g）,methoxychlor（4.9 ng/g）,HCHs（1.54）,endrin（2.52ng/g）,and endosulfan（2.42 ng/g）.Thus,OCPs contamination can be considered as causing negligible environmental risk following reference standards for unpolluted soil based on Chinese Environmental Quality Standards for Soils and Netherland Soil Protection Guidelines.However,considering recent contamination and occurrence of the banned OCPs as well as toxicity and mobility of their metabolites,it is an alert of potential ecological risks with further implications on human health along the food chain.Consistently,based on ILCR,the cancer risk was very low but there was an indication of a potential cancer risk for the PAHs exposure through ingestion and dermal pathways that ranged from 1.79ⅹ10^-7 to2.46ⅹ10^-5 and 3.18ⅹ10^-7 to 4.37ⅹ10^-5,respectively.In general,the cancer risk from the agricultural soils was relatively higher for the PAHs with ILCR ranged from 1.9ⅹ10^-11 to 4.4ⅹ10^-5 compared to the OCPs with ILCR ranged from 1.5ⅹ10^-13 to 6.7ⅹ10^-7.Based on the findings,this study concluded that there is still occurrence of the banned OCPs,and PAHs in the agricultural soils attributed by mainly onsite sources and subject to localized transport.Though the current level appeared to be low,there are potential risks to the environment and human health in the future.The low OCPs level reflects compliance with the country’s regulations following international agreements that banned their use.However,detection of recent OCPs,and detectable concentrations of the banned and unlisted OCPs in the published list of registered pesticides may cause a high concern in the future.Similarly,PAHs is a human culprit apart from its natural origin but it is not yet regulated in Tanzania.Despite the low PAHs level,the detectable concentrations serve as a base for future research on PAHs contamination and as an alert on gaps in policies and regulations related to prevention and control of the environmental and health risks from soil contamination.Therefore,this study calls upon more mapping and strengthened monitoring of the OCPs and PAHs status within a country complemented with formulation of PAHs regulations and updating strategies,policies,and regulations related to pesticides’use and control for assuring a right to a safe and healthy environment.This study also recommends further studies on OCPs’transport from the surface soil to deeper soils,nearby surface water,and in the air particularly in areas that are susceptible to high contamination.Moreover,the toxicity and carcinogenicity of the PAHs underscores the need for further studies on environmental pollution associated with PAHs potential sources such as major industries,heavy traffic areas,coal mines,and mushrooming gasoline filling stations.Furthermore,this study recommends the need for a comprehensive investigation of PAHs transport based on the soil-air exchange using grid-sampling along altitudinal gradient based on regional zones,and its implications to ecological risks and human.