Distribution,sources,and Potential Health Risk Assessment Of Persistent Organic Pollutants(POPs) In The Soil From Selected Cities In Sierra Leone,West Africa

Contaminated soils with persistent organic pollutants（POPs）including,polycyclic aromatic hydrocarbons（PAHs）and organochlorine pesticides（OCPs）in most urban soils are increasingly becoming a global problem due to their hazardous potential effects on human health and the ecosystem.As a result,microbial degradation is considered a suitable potential mechanism to remove POPs contamination in the environmental media.Nonetheless,it’s often limited by the low bioavailability of POPs.Consequently,it is imperative to improve the bioavailability of POPs in soil bioremediation.Despite the numerous investigations on the status,transport mechanism,and distribution of POPs in the soil environmental media on a global scale,there is limited documented information on the occurrence and distribution levels of PAH and OCP in the soils of Sierra Leone which makes it challenging for policymakers to implement proper environment monitoring and pollution control and preventive measures.Therefore,this study investigates the concentration,spatial distribution patterns,migration,and health risk assessment of the two main POPs such as OCPs and PAHs in Sierra Leone’s soils,and collectively identifies their potential sources.Additionally,evaluates their ecological and human health risks,and determined the relationship between soil p H and particle-size impact on the concentration and distribution of OCP and PAH in soils from six major cities in Sierra Leone,West Africa.A total of seventeen（n=17）surface soil（0–20 cm）samples were collected from both remote and developed cities including Freetown,the capital city of Sierra Leone（Kingtom and Waterloo sampling sites）,Bonganema,Makeni,Kabala,Magburaka,and Sinikoro in the wet season between（September to October 2019）Seventeen surface soil（0–20 cm）samples were collectively analyzed for the 16united states environmental protection agency（USEPA）priority PAHs and 24 OCPs in soil.The soil samples were quantitatively analyzed with chromatography spectrometry（GC–MS,Agilent 7890 GC,and 5975MSD）for PAHs and gas chromatography（7890A,Agilent,Santa Clara,CA,USA）with a ⁶³Ni-electrom capture detector（GC-ECD）for OCPs at the State Key Laboratory of Biogeology and Environmental Geology.All the targets PAH and OCP compounds were detected in the soil profile.All statistical analyses result of PAH and OCP were achieved via Excel 2016 and SPSS25.0（IBM）,Arc GIS 10.5software（Esri）,and Origin 2022b.The total concentration and spatial distribution levels of PAHs and OCPs in the soil varied significantly across the different surveyed areas.PAH compounds were ubiquitous in Kingtom,followed by Waterloo,and OCP compounds were ubiquitous in Kingtom,followed by Kabala.The total average concentrations of the∑₁₆PAH in the surveyed soil areas were in the order;Kingtom（1142 ng/g dw）>Waterloo（265 ng/g dw）>Magburaka（79.7 ng/g dw）>Bonganema（54.3 ng/g dw）>Kabala（54.2 ng/g dw）>Sinikoro（52.3 ng/g dw）>Makeni（36.6 ng/g dw）.Similarly,the total average concentrations of the∑₂₄OCP in the soil studied areas were in the order;>Kingtom（200.02 ng/g dw）>Kabala（64.69ng/g dw）>Sinikoro（4.73 ng/g dw）>Bonganema（4.31 ng/g dw）>Waterloo（4.0 ng/g dw）>Magburaka（2.48 ng/g dw）>Makeni（0.33 ng/g dw）.The individual mean concentrations and distribution levels of the predominant PAH and OCP compounds in the soil profile were in the order:Nap（63.21 ng/g dw）>Ba P（34.77 ng/g dw）>Phe（28.37ng/g dw）>Bb F（27.49 ng/g dw）>Fluo（24.29 ng/g dw）>Chr（23.48 ng/g dw）>Bghi P（23.46 ng/g dw）>Pyr（22.73 ng/g dw）>Ind P（18.91 ng/g dw）,and p,p-DDE 24.945 ng/g dw)>Dieldrin（14.721 ng/g dw）>o,p-DDD（9.656ng/g dw）>p,p-DDT（4.994 ng/g dw）>p,p-DDD（9.493 ng/g dw）>Endrin（3.798 ng/g dw）>Heptachlor（3.424 ng/g dw）>o,p-DDT（2.255 ng/g dw）>Heptachlor-epoxide（2.008 ng/g dw）>Aldrin（1.495ng/g dw）>Endosulfate-sulfate（1.173 ng/g dw）.The PAH compound with the highest mean concentration was Nap,followed by Ba P,and the lowest was Ace.Similarly,the OCP compound with the highest mean concentration was p,p-DDE,followed by Dieldrin and the lowest was Endrin Ketone.The total mean percentage（%）composition of POPs contamination in soil across the surveyed areas varied extremely.Overall,the highest concentration of both PAH and OCP was found in Kingtom,contributing to 71.16%,and 16.51%of the total PAHs soil concentration in Kingtom and Waterloo respectively,and contributing to 77.78%and16.77%of the total OCP concentration in Kingtom and Kabala respectively.The dominant PAH-ring compounds in this study were 2-ring 4-rings,and 5-ring PAHs.High molecular weight（HMWPAHs,4-6 rings）compounds extracted 62.5%of the total PAHs,while low molecular weight（LMWPAHs,2-3 rings）compounds extracted 37.5%.In general,the migration of HMWPAHs（4-5 ring）compounds was faster in Kingtom,followed by Waterloo soils compared to Makeni,Kabala,Magburaka,Sinikoro,and Bonganema soils which exhibited faster migration of LMWPAHs,（2-3 rings）compounds.The dominant OCP isomers were DDTs,DRINs,and CHLs contributing to 61.71%,23.89%,and 8.32%of the total OCP concentration in soil respectively.The migration of DDTs followed by DRINs,and CHLs were faster in Kingtom followed by Kabala soils than in other areas.The concentration,distribution patterns,and migration of PAH and OCP compounds in the soil indicate that anthropogenic activities were the primary contributing factors for the high POPs concentration observed in KT,the soil topography,and atmospheric circulation associated with the high winds blowing from the Atlantic Ocean transporting dusty and hot air from the sub-Sahara flows directly into the surface soil of Kingtom which displaced the highest concentrations of PAH and OCP.Thus,contaminants can easily migrate from the Atlantic Ocean to the high-pressure mountainous areas in the surface soil profile.Based on the European soil quality criteria classification of PAH-contamination in the environment,the total PAH concentration in Kingtom soil is categorized as heavily polluted soil PAH,while the total PAH concentration in Waterloo soil is categorized as contaminated soil PAH.However,the total concentration of PAH in Magburaka,Bonganema,Kabala,Sinikoro,and Makeni soils were all grouped as no or negligible contaminated soils PAH.The average concentration and detection frequencies of the carcinogenic（∑₇PAHs）compounds in the soil profile were distributed in the following order:Ba P（34.77 ng/g dw）>Bb F（27.49 ng/g dw）>Chr（23.48 ng/g dw）>Icd P（18.91ng/g dw）>Bk F（13.01 ng/g dw）>Ba A（12.53 ng/g dw）>Dah A（3.22 ng/g dw）.Although the average concentration level for Dah A was the lowest,it was 100%detected in the soil profile and the detection frequency was in the reversing order for the rest of the carcinogens.Similarly,the mean concentrations for the sub-grouped OCP isomers in the soil profile were distributed in the following order:DDTs（27.95 ng/g dw）>DRINs（10.96ng/g dw）>CHLs（3.81 ng/g dw）>SULPHs（1.22,ng/g dw）>MAC（0.59 ng/g dw）>HCB（0.53 ng/g dw）>HCHs（0.31 ng/g dw）.Although the average concentration of HCHs and HCB isomers contributed the least concentration,they recorded the highest detection frequencies of 95.58%and 94.12%.respectively.Collectively,using the compositional pattern method,molecular diagnostic ratios,and principal components analysis（PCA）for potential source identification results revealed multiple sources of PAH emission but sources from pyrogenic and petrogenic PAH emission origin.The user the compositional pattern method result indicated that pyrogenic sources were the primary source of PAH emission in Kingtom and Waterloo,while petrogenic PAH source emissions were predominant in Bonganema,Sinikoro,Kabala,Magburaka,and Makeni.Overall,the main sources for PAH enrichment in this study revealed the predominance of pyrogenic sources（petroleum,coal&biomass combustions,and vehicle emissions）especially in Kingtom,followed by Waterloo with minimum contribution from petrogenic sources emission（fossil fuel combustion）.Similarly,the characterized ratios of HCHs,DDT,TC/CC,andα-Endo/β-Endo isomers composition were analyzed to determine the potential sources of OCPs pollution emission in the surveyed areas.The results revealed mixed sources of HCHs,DDTs,TC/CC,andα-Endo/β-Endo,however,fresh input of technical DDTs,technical HCHs（Lindane）,technical chlordane,and historical endosulfan application were the predominant sources of emission in the study area.Using the physicochemical properties of the soil to determine their influence on the distribution and concentration of PAH and OCP in soil.The result revealed that soil p H strongly influenced the concentration and distribution of OCPs and PAHs positively in this study.In general,soil p H had strong positive significant corrections on 4-,5-,and 6-rings,∑7PAHs,and HMWPAHs for PAH and on HCHs and DDTs for OCPs.Similarly,soil textures had a strong positive correlation with sand particles and a strong negative correlation with silt and clay for PAHs.However,there was no significant correlation observed between the soil texture and OCP concentrations.Using the toxicity equivalent quantity(TEQ_{Ba P})to examine the potential cancer risk to human exposure to toxic PAHs and the incremental lifetime cancer risks（ILCRs）to assess the potential cancer risk to humans exposed to potent OCP compounds in soil.The total TEQ_BAP levels in the soil varied significantly from（87-273.4 ng/g dw）with an average of 153.6 ng/g dw,and from（16.5-34.3 ng/g dw）with an average of 24.9 ng/g dw in Kingtom and Waterloo respectively.The total ILCRs concentration level varied from10^-5to 10^-5 and 10^-6 to 10^-6 in Kingtom and Kabala respectively.The result indicated that TEQ_{Ba P} levels in Kington soil posed a potential cancer risk to its residents,followed by Waterloo.However,the ecological risks associated with TEQ_BAP in Magburaka,Bonganema,Kabala,and Sinikoro posed a negligible potential ecological cancer risk to their residents.The total（ILCRs）level in Kingdom soils posed a potential cancer risk to its residents,followed by Kabala,especially in females and childhood females compared to males,but the difference was quite significant.However,the total（ILCRs）in Magburaka,Bonganema,Waterloo,and Sinikoro posed a negligible potential cancer risk.to their residents.In general,this study’s result is highly significant as it revealed a novel result on the status of PAHs and OCPs in the surface soils of six major administrative cities in Sierra Leone for the first time.Our research findings would enable policymakers and stakeholders to identify high-risk zones and establish environmental monitoring programs,pollution control,and remediation strategies.However,the content of PAH levels especially in Kingtom,followed by Waterloo,and in Kabala for OCP should be further analyzed to prevent potential risks and to ensure the levels reported in these areas are not being underestimated owing to the continuous increase in anthropogenic activities in these areas.