Seasonal distribution, source apportionment and risk assessment of polycyclic aromatic hydrocarbons in groundwaters in Owo, Southwestern Nigeria

Polycyclic aromatic hydrocarbons (PAHs) are organic molecules composed of several carbon rings. They emanate from natural and anthropogenic processes including incomplete combustion of organic matter (Adekunle et al. 2017). They are found all over the environment and have been the focus of much investigation due to the harm they might do to human health and/or the natural world (Balcıoğlu 2016). Most PAHs are discharged into the environment as a result of human activities including fossil fuel combustion, industrial operations, and waste disposal (Li et al. 2015; Varjani et al. 2018). Because they are persistent, many accumulate in living beings and have been linked to cancer, and their prevalence in different aspects of the environment, including groundwater, is worrisome (Ali et al. 2021).

Groundwater is a vital drinking water resource for millions of people worldwide (Mishra 2023). It can represent a major hazard to both the environment and public health if it becomes polluted with PAHs (Hojjati-Najafabadi et al. 2022). It is critical to understand how PAHs are dispersed in groundwater, where they come from, and what hazards they represent, to control pollution properly and safeguard human health, as well as develop strategies for cleaning up contaminated groundwater and preventing further pollution.

The prevalence of PAHs in groundwater is influenced by several variables, including climate, geology, and land use (Burri et al. 2019). Seasonal variations can also have a substantial influence. Changes in rainfall, temperature, and the pace of groundwater recharge, for example, can all affect contaminant movement (Saravanan et al. 2021). PAHs can be washed out of contaminated soil and transferred to groundwater during wet seasons when there is more rainfall. Raised water levels may also result in increased subsurface water flow, potentially distributing PAHs to new regions (Jiang et al. 2022). During dry seasons, however, when groundwater levels are lower and water moves more slowly, PAHs may linger in the ground for extended periods, resulting in increased concentrations. Temperature fluctuations can also influence PAH levels in groundwater by changing the activity of soil bacteria (Alegbeleye et al. 2017). PAHs may be broken down by microbes, reducing their concentrations in groundwater (Padhan et al. 2021). When temperatures rise, microbial activity increases, resulting in quicker PAH degradation, suggesting that PAH concentrations in groundwater may decline more quickly in warmer seasons due to greater microbial breakdown (Logeshwaran et al. 2018; Adetunji & Anani 2021).

In order to minimize pollution and clean it up, it is critical to identify the sources of PAH contamination. PAHs can be found in both man-made and natural environments, and anthropogenic sources include automobile exhaust, industrial pollution, and inappropriate garbage disposal. These PAHs are released into the atmosphere, from where they can settle on the ground and eventually enter groundwater (Kozak et al. 2017; Li et al. 2021). However, in many situations, PAH pollution of groundwater results primarily from human activity (Ren et al. 2021). Profiles can be constructed to assist in pinpointing pollution sources by analyzing the precise types and concentrations of PAHs found.

Many PAHs are known or suspected causes of cancer, and being exposed to excessive quantities of them by drinking water or skin contact can be harmful. PAHs can also accumulate in aquatic creatures, altering ecosystems, and potentially entering the food chain (Okechukwu et al. 2021; Oyekunle et al. 2023; Vijayanand et al. 2023). PAH mobility in the environment is influenced by factors such as geology, how easily they adhere to soil particles, and how rapidly they degrade. Understanding how PAHs migrate makes it possible to forecast whether they will end up in sensitive places such as wells or bodies of surface water (Adeola & Forbes 2021a, 2021b).

Therefore, effective water resource management depends on PAH source identification and risk evaluation (Li et al. 2017). Many recent studies have been carried out on PAH seasonal distribution, source identification, and risk assessment in water resources (Apata et al. 2022; Aralu et al. 2023; Areguamen et al. 2023; Ogunbisi et al. 2023; Ololade et al. 2023). However, this study is the first known attempt at evaluating groundwater, the predominant water resource in the study area.

Water samples were collected from randomly selected wells around dumpsites in Owo, southwestern Nigeria, during the dry and wet seasons in 2021. Prior to analysis, the samples were pretreated with 20 mL chloroform and refrigerated, to inhibit microbial action.

All reagents were of analytical grade and were obtained in Nigeria. They included chloroform, acetone, and methanol (BDH Poole House, England), dichloromethane and nitric acid (Sigma-Aldrich), anhydrous sodium sulfate (BDH Poole House, England), and silica gel 60-200 PF₂₅₄ (MERK, Germany).

Sample aliquots of 100 mL were measured into a 500 mL separatory funnel to determine their PAH content. The samples were extracted in triplicate with 200 mL of dichloromethane, before being mixed in an amber-colored vial, which was kept at 4 °C in preparation for further clean-up. This method provides for precise determination of PAH levels in water samples.

Clean-up was based on chromatography and involved mixing stationary (silica gel) and mobile phases (1:2:2 acetone–dichloromethane–ethanol). A small quantity of glass wool was used as a plug to prevent the loss of the stationary phase before the addition of silica gel. Anhydrous sodium sulphate was added on top of the silica gel. The solvent mixture was introduced first into the packed column to prevent contaminant interference.

Clean-up is essential to remove or reduce impurities that might be present in the eluate. The eluate was left to dry completely, then reconstituted with 1 mL of dichloromethane and stored in amber-colored vials prior to GC-MS determination.

The recovery analysis result is presented in Table 1. The proportional recoveries ranged between 89 and 108%, well within the 70–110% recovery range stipulated, for instance, by the European Union (2006).

The PAHs were determined by a gas chromatograph (Agilent Model 7890B) coupled with a Pegasus 4D mass spectrometer (GC-MS). An Agilent DB-7890 capillary GC column, 30 m × 0.25 mm id × 0.25 μm film thickness, was used at 340 °C. The operating conditions were: splitless (1 μL) injection, injector temperature 250 °C, helium carrier gas (99.99% purity) flowing at 0.9 mL min⁻¹ with column head pressure 7.4 psi. The oven temperature was kept at 70 °C for 2 min, and then programed to rise to 130 °C at 25 °C min⁻¹, and on to 220 °C at 2 °C min⁻¹, and finally to 280 °C at 10 °C min⁻¹. The final temperature was maintained for 4.6 min.

The MS settings were electron impact ionization mode with 70 eV electron energy, scan mass range 100–400 at 0.62 s/cycle, ion source temperature 230 °C, MS quad temperature 150 °C, EM voltage 1450, and solvent delay 4 min. The MS system was routinely operated in selective ion monitoring (SIM) mode with electron ionization. The PAH compounds were identified on the basis of a comparison of peak retention times with those of standard PAHs, after which an internal standard method was used for PAH quantification (Oyekunle et al. 2022).

where C_PAHs is the PAH concentration in water (ng/L), C_QV is the corresponding PAH risk standard value (ng/L), C_QV(MPCs) is the PAH MPC value, C_QV(NCs) is the PAH NC value (ng/L), and RQ the PAH ecological risk quotient.

The descriptive seasonal distribution statistics of the PAHs in the groundwater in the dry and wet seasons are presented in Table 2. During the dry season, total PAH concentrations ranged from 182 to 2.36 × 10⁴ ng/L. However, total PAH concentrations during the wet season ranged from 1.55 × 10³ to 1.49 × 10⁵ ng/L.

Table 2

Descriptive statistics of PAHs in groundwater (ng/L) during dry and wet season

Type .	Nap .	Acy .	Ace .	Flu .	Phe .	Ant .	Fla .	Pyr .	BaA .	Chr .	BbF .	BkF .	BaP .	Dba .	BPy .	InP .	.
Dry season
Min	1.15	19.1	29.3	125	ND	7.84	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	182
Max	2.51 × 103	3.79 × 103	3.45 × 103	3.36 × 103	2.31 × 103	258	2.63 × 103	195	88.4	412	1.79 × 103	529	1.81 × 103	316	30.8	165	2.36 × 104
Wet season
Min	138	526	301	355	158	12.4	ND	ND	ND	2.45	10.1	39.8	ND	2.62	ND	3.99	1.55 × 103
Max	3.43 × 103	1.42 × 104	2.82 × 104	7.74 × 103	8.30 × 103	2.36 × 103	1.42 × 103	482	3.02 × 103	6.24 × 103	2.12 × 104	1.49 × 104	3.69 × 104	273	640	626	1.49 × 105

Type .	Nap .	Acy .	Ace .	Flu .	Phe .	Ant .	Fla .	Pyr .	BaA .	Chr .	BbF .	BkF .	BaP .	Dba .	BPy .	InP .	.
Dry season
Min	1.15	19.1	29.3	125	ND	7.84	ND	ND	ND	ND	ND	ND	ND	ND	ND	ND	182
Max	2.51 × 103	3.79 × 103	3.45 × 103	3.36 × 103	2.31 × 103	258	2.63 × 103	195	88.4	412	1.79 × 103	529	1.81 × 103	316	30.8	165	2.36 × 104
Wet season
Min	138	526	301	355	158	12.4	ND	ND	ND	2.45	10.1	39.8	ND	2.62	ND	3.99	1.55 × 103
Max	3.43 × 103	1.42 × 104	2.82 × 104	7.74 × 103	8.30 × 103	2.36 × 103	1.42 × 103	482	3.02 × 103	6.24 × 103	2.12 × 104	1.49 × 104	3.69 × 104	273	640	626	1.49 × 105

Nap, naphthalene; Acy, acenaphthylene; Ace, acenaphthene; Flu, fluorene; Phe, phenanthrene; Ant, anthracene; Fla, fluoranthene; Pyr, pyrene; BaA, benz [a] anthracene; Chr, chrysene; BbF, benzo[b]fluoranthene; BkF, benzo[k]fluoranthene; BaP, benzo[a]pyrene; Dba, dibenzo[a,h]anthracene; BPy, benzo[g,h,i]perylene; InP, indeno[1,3,3-cd]pyrene; Min, minimum; Max, maximum; ND, not detected.

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The PAHs exhibited different compositional profiles in the dry and wet seasons. While acenaphthylene had the highest concentration (3.79×10³ ng/L) in the dry season, benzo[a]pyrene had the highest (3.69 × 10⁴ ng/L) in the wet season. Apart from fluoranthene, the concentrations of the other PAHs in the groundwater were relatively higher in the wet than in the dry season. One-way analysis of variance (Table 3) indicated that the concentrations of naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, benzo[g,h,i]perylene, and indeno[1,3,3-cd]pyrene were significantly higher (p < 0.05) in the wet season than the dry one. The increased wet season PAH concentrations could be due to the promotion of soil permeability by rainfall when PAHs in soils are dissolved and subsequently migrate into groundwater (Yang et al. 2012; Sun et al. 2019). In essence, the difficulty associated with the migration of these PAHs during the dry season might explain why their concentrations are relatively lower then (Perrette et al. 2013), as the relative increase in soil moisture during the wet season ensures the carriage of PAHs into groundwater by colloids and dissolved organic matter (Schwarz et al. 2011).

Unsurprisingly, in both dry and wet seasons, lower molecular weight PAHs were predominant. This has a direct link with PAH physical and chemical properties, as these relatively light molecules have low hydrophobicity and high-water solubility, while the heavier PAHs are relatively more resistant to degradation (Montuori et al. 2016; Li et al. 2017; He et al. 2020). The PAH concentrations found in this study are relatively higher than those reported elsewhere (López-Macias et al. 2019; Ibigbami et al. 2022).

Factor 2 accounted for 40% of data variance, with high factor loadings from benzo[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, and benzo[g,h,i]perylene. It comprises PAHs with 4–6 aromatic rings, which are predominantly fossil fuel combustion products (Tian et al. 2019) and can be ascribed to traffic-related vehicular emissions.

Factor 3 had the lowest data variance contribution – 29% – and arose from predominant loadings from acenaphthylene, anthracene, benzo[b]fluoranthene, dibenzo[a,h]anthracene, and indeno[1,3,3-cd]pyrene, high molecular weight PAHs implicated as tracers of incomplete PAH combustion from internal combustion engines and biomass (Li et al. 2017). Based on the foregoing, it is clear that the combustion of coal/biomass and petroleum are the chief PAH sources in the groundwater.

The possible health risks associated with PAH ingestion through drinking groundwater were assessed. The estimated ILCR and risk index (RI) for adults and children are presented in Table 4. In both dry and wet seasons, the carcinogenic PAH ILCRs were relatively higher for children than adults, except for chrysene. Regardless of season, benzo[a]pyrene contributed the greatest risk of cancer and chrysene the least. ILCR values ≤ 1 × 10⁻⁶ have been classified as acceptable, while those ≥ 1 × 10⁻⁴ have been described as indicating high risk (Huang et al. 2016). Most ILCRs exceeded the USEPA's acceptable level (1 × 10⁻⁴), indicating that drinking groundwater carries potential carcinogenic risk (Qiao et al. 2021).

The study's aim was to investigate the seasonal distribution and likely sources of 16 PAHs in groundwater. There were considerable changes in PAH concentrations between the dry and rainy seasons. The major influence of seasonal fluctuations on PAH concentrations was produced by rainfall, with higher PAH concentrations found during the wet season due to increased soil permeability, which allows PAHs to migrate into groundwater.

The study also showed that the PAH mix differed between the dry and rainy seasons. Because of their relatively low hydrophobicity and higher water solubility, lower molecular weight PAHs were more abundant than those with higher molecular weights in both seasons.

Diagnostic ratios and PMF were used to identify PAH origins in groundwater. The diagnostic ratios indicated that coal/biomass combustion and petroleum were the primary PAH sources. PMF identified three sources: low molecular weight PAHs from oil spills, higher molecular weight PAHs from car emissions, and another source associated with incomplete combustion from internal combustion engines and biomass. This emphasizes the need to consider both temporal fluctuations and probable sources when assessing PAH contamination in groundwater.

Risk assessment indicated the potential for cancer risks from drinking groundwater, as well as potential medium and high ecological risks. The findings have human health and environmental consequences, emphasizing the need for focused initiatives to minimize the effects of PAH pollution, particularly during rainy seasons.

The authors sincerely acknowledge the management of their respective institutions for providing an enabling environment for the research.

All authors have approved the final version of the manuscript for publication.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.