This study reports the occurrence and risk assessment of 2,4- Dinitrophenol (2,4-DNP), Phenol (PHE), and 2,4,6- Trichlorophenol (2,4,6-TCP) in drinking water sources in three South western States in Nigeria (Osun, Oyo and Lagos). Groundwater (GW) and surface water (SW) were collected during dry and wet seasons of a year. The detection frequency of the phenolic compounds followed the trend: Phenol > 2,4-DNP > 2,4,6- TCP. The mean concentrations of 2,4-DNP, Phenol, and 2,4,6-TCP in GW/SW samples from Osun State were 639/553 µg L− 1, 261/262 µg L− 1, and 169/131 µg L− 1 respectively, during the rainy season and 154/7 µg L− 1, 78/37 µg L− 1, and 123/15 µg L− 1 during the dry season. In Oyo state, the mean concentration were 165/391 µgL− 1 for 2,4-DNP and 71/231 µgL− 1 for Phenol in GW/SW samples respectively, during rainy season. Generally, in the dry season, these values decreased. In any case, these concentrations are higher than those previously reported in water from other countries. The concentration of 2,4-DNP in water posed serious ecological risks to Daphnia on the acute scale while it was Algae on the chronic scale. Estimated daily intake and hazard quotient calculations suggests that 2,4-DNP in water pose serious toxicity concern to humans. Additionally, the concentration of 2,4,6-TCP in water from Osun State in both seasons pose significant carcinogenic risk to persons ingesting water from this State. Every exposure group were at risk from ingesting these phenolic compounds in water. However, this decreased with increasing age of the exposure group. Results from the Principal Component Analysis indicate that 2,4-DNP in water samples is from an anthropogenic source different from that for Phenol and 2,4,6-TCP. There is a need to treat these GW and SW before drinking while maintaining regular assessment of these water sources.
Research Article
Pollution and Risk Assessment of Phenolic Compounds in Drinking Water Sources in South-Western Nigeria
https://doi.org/10.21203/rs.3.rs-2233618/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 29 May, 2023
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Environment
Groundwater
2
4-Dinitrophenol
Carcinogen
Risks Assessment
Due to their carcinogenicity, neurotoxicity, mutagenicity and endocrine-disrupting abilities, surveillance has been increased on phenolic compounds in the aquatic environment. Phenolic compounds such as Phenol, 2,4-dinitrophenol (2,4-DNP), and 2,4,6-trichlorophenol (2,4,6-TCP) are listed as priority pollutants in water by United States Environmental Protection Agency (USEPA; 2014). Phenolic compounds are organic matter with special chemical structures consisting of an aromatic ring with one or more hydroxyl (-OH) functional groups. These -OH functionalities make phenolic compounds soluble in water increasing their prevalence and easy transportation around surface water and in groundwater. Their presence has been reported across international borders and in places where they have never been used. A good example is the earth's pole (Jacob and Cherian, 2013).
The presence of Phenol and 2,4-dinitrophenol in water stems from their massive utilisation in agricultural industries for the production of pesticides and fertilisers. 2,4-dinitrophenol is also used in weight control pills (Gziut and Thomas, 2022). Other sources of phenolic contamination in our water bodies are wastewater from industries such as textile, dyeing, and pharmaceutical (in the manufacture of personal care products). These phenolic compounds have the potential to bio-accumulate primarily because of their low biodegradability index (Zhou et al., 2017b). This ability to bioaccumulate is amplified in the aquatic food chain (Al-Ahmari et al., 2018; Alharbi et al., 2018).
Phenolic compounds have caused a vast array of health challenges in humans, including endocrine disruption and toxicity to reproductive organs (Wolff et al., 2015; Zhang et al., 2017), alteration in growth and development, attack on the immune system (Goulart and Mascaro, 2016), carcinogenicity and neurotoxicity (Bolton and Dunlap, 2017). Specifically, chlorophenols and nitrophenols are known to cause digestive tract infections, histopathological alterations, genotoxicity (Chen et al., 2017), mutagenicity (Zhang et al., 2017), heart diseases, sarcoma, asthma (Igbinosa et al., 2013), lung cancer in living organisms (Honda and Kannan, 2018), and thyroid-hormone disrupting effects (Hernández et al., 2020). One of such nitrophenols, 2,4-dinitrophenol (2,4-DNP), is noted to be the most noxious, having an LD50 around 30 mg kg− 1 body weights in rats (Shea et al., 1983). 2,4-DNP is toxic at certain concentrations, and prolonged exposure to it through inhalation and skin absorption could affect the bone marrow, cause breakdown of the central nervous system, affect the cardiovascular system, cause cataracts, and increased metabolism resulting in fever, headache, profuse sweating, thirst and fatigue and more. From animal studies, 2,4-DNP is teratogenic, mutagenic and carcinogenic (Grundlingh et al., 2011). For 2,4,6-TCP, it is documented that it causes endocrine disruption and induces liver cancer, lymphomas, leukaemias, and hemangio-sarcoma in rodents (Jung et al., 2004; McConnell et al., 1991; Program, 1979). On the other hand, Phenol impact water with unpleasant odour and taste, which reduces the quality and quantity of water available for human consumption (Yahaya et al., 2019). The hydrophobicity of these phenolic compounds, their ability to form free radicals, their low concentrations in water (usually at ng L− 1 – µg L− 1 levels), and the complexity of environmental matrices make them even more toxic, which makes monitoring of these pollutants necessary.
There are reports from China (Chen et al., 2021; Wang et al., 2020), Brazil (Ramos et al., 2021), Egypt (El-Naggar et al., 2022), South Africa (Yahaya et al., 2019), India (Kumar, 2015) and Malaysia (Al-Janabi et al., 2012) about the presence of these priority pollutants in their water bodies. A few countries, especially North America (USA & Canada) and Europe, have successfully tackled the problem of phenolic contamination in their water bodies. As a result, there have been very few reports of these chemical compounds in their water bodies since the last decade. In contrast, high concentration of phenolics compounds have been found in a few countries in Asia and Africa, e.g. China, India, South Africa & Egypt, indicating some level of increasing industrialisation in these countries.
In addition, several developed countries have in place permissible concentration limits for the presence of these phenolic compounds in their water especially drinking water. Countries in Europe, for example, put the maximum level of Chlorophenol in drinking water at 2 µg L− 1 (Organization, 2017), Canada at 2 µg L− 1 (Protection, 2018), India at 1 µg L− 1 (Chand, 2018), China at1 µg L− 1 (Liu, 2009), Australia at 2 µg L− 1 (Batley, 2014), and the World Health Organization (WHO) at 1 µg L− 1 (WHO, 2017). The USEPA recommends 10 µg L− 1 as the maximum concentration limit for 2,4-DNP in drinking water in the United States. It is even regarded as a priority pollutant by the Clean Water Act (Agency, 1979; Al-Mutairi, 2010). While the USEPA has placed a maximum permissible limit of 1.0 mg L− 1 for Phenol in wastewater, the WHO has 0.1 mg L− 1 as its maximum permissible limit for the same in drinking water (Sonawane and Korake, 2016). In Brazil, the Brazilian potability standard for drinking water set the maximum permissible level for 2,4,6-TCP at 200 ng L− 1 (Sartori et al., 2012). However, there is a dearth of data on the presence of these phenolic compounds in water bodies and drinking water sources in Africa, even though these compounds are vastly used in industries and particularly for agricultural purposes in the continent. Consequently, no limit has been determined for these priority pollutants in aquatic environments in West African countries.
To make matters worse for millions of Africans, the major sources of drinking water are some of these aquatic environments, including streams, rivers, hand-dug wells and boreholes. In fact it is estimated that > 50% of Africans use groundwater as their primary source of drinking water (https://businessday.ng/opinion/article/world-water-day-2022-where-are-we/ ). Water from these sources is not pre-treated before use. The quality of these drinking water sources from Africa and especially Nigeria, with respect to phenolic compounds is relatively unknown.
Based on the observed gaps, this study aims to provide baseline data on the occurrence of some phenolic compounds [Phenol, 2,4-dinitrophenol (2,4-DNP), and 2,4,6-trichlorophenol (2,4,6-TCP)] in groundwater and surface water samples from three States in Southwestern Nigeria (Osun, Oyo, and Lagos States). The chemical analytes of interest (Phenol, 2,4-DNP, and 2,4,6-TCP) were chosen based on their massive utilisation for agricultural purposes in this region, especially as constituents of fertilisers, pesticides, and antiseptics. Similarly, in most parts of Nigeria, especially in the Western region, groundwater and surface water are the primary potable water sources. Data from this study will be helpful to policy-makers and water treatment professionals who will need them to develop guidelines and effective treatment strategies for the abatement of these chemical contaminants in water in Nigeria and the African continent.
2.1. Chemicals
Analytical standards of HPLC grades for Phenol, 2,4-dinitrophenol (2,4-DNP), and 2,4,6-trichlorophenol (2,4,6-TCP) were purchased from Sigma Aldrich (St. Louis, MO, USA). Methanol, Ethyl acetate, Dichloromethane, and Acetonitrile were also obtained from Sigma Aldrich (St. Louis, MO, USA). Similarly, other chemicals, such as concentrated hydrochloric acid, were obtained from Sigma Aldrich (St. Louis, MO, USA) and were of analytical standard, ultrapure water was generated from a water purification system using Milli-Q Direct 8/16. The standard stock solutions of each analyte (200 mg L− 1) were prepared singly in ultrapure water and stored at 4°C. Working solutions were spiked with 200 µg L− 1 of the mixed phenolic compounds in the dilute stock solution prepared with ultrapure water.
2.2. Description of the study area
Osun, Oyo and Lagos States belong to the tropical rainforest biome in South-western Nigeria and lie between Lat. 06° 30′ N and Long. 04° 30′ E, Lat. 7° 51' N and Long. 3° 55’ E and Lat. 6° 27' N and 3° 24' 23' E, encompassing areas of approximately 14,875, 28,454 and 3,577 km2 respectively. Details of sampling sites are presented supporting information document (Section S1.0). The sampling States for this study are shown in Fig. 1.
2.3 Sample collection
Water samples from groundwater (hand-dug wells and boreholes) from 34 locations (Table 1S) were collected using a polyethene bailer just below the water level (hand-dug well) and directly into sample bottles (boreholes) from the study area for one year, covering wet and dry seasons. Similarly, surface water samples were randomly collected from 31 locations (Table 1S) cutting across major rivers (Epe and Osun) and a water dam (Asejire). Samplings were done between April 2021 to March 2022, and all samples were collected in triplicates.
At each sampling site, samples of groundwater were taken (n = 3) and made into a composite. At the same time, for surface water, grab samples (n = 3) were collected from all the sampling sites in Osun, Oyo and Lagos State, Nigeria but not made into composites.
Furthermore, the influence of seasonal variations on the amount and distribution of these analytes in water was studied. Three sampling campaigns (one for each State) were carried out during the wet season (April to September 2021) and three also during the dry season (December to March 2022).
Groundwater and surface water samples (500 mL) from the field were stored in amber glass bottles already pre-washed with methanol and ultrapure water and kept in ice packs to maintain the integrity of the samples. The samples were transported to the laboratory and stored at 4°C. Extraction of analytes from water samples was done within 24 h.
2.3. Pretreatment and sample extraction
Groundwater and surface water samples were filtered through a membrane filter under vacuum, and 200 mL of the water samples were spiked with 200 µg L− 1 of the combined phenolic compounds (Phenol, 2,4-DNP and 2,4,6-TCP). The pH of the resulting solution and that of a blank solution of ultrapure water was adjusted to 3.0 using 0.1M HCl and 0.1 M NaOH. Thereafter, the analyte was extracted using solid phase extraction technique. The cartridges (Oasis HLB, 500 mg, 6 mL, Waters, MA, USA) were initially pre-conditioned with 5 mL methanol and 5 mL of Millipore water. The samples at pH 3.0 were loaded into the cartridges at a flow rate of 5 and 9 mL/min, washed with 3 mL ultra-pure water, and subsequently eluted from the cartridges using 3 mL of Acetonitrile, Dichloromethane and Ethyl Acetate (volume ratio:1:1:1) and the eluents were dried in a vacuum oven at 50 oC. The eluents were reconstituted with 0.5 mL Acetonitrile for analysis with HPLC.
2.4. Instrumental Analysis
The quantification of the analytes in the reconstituted eluents was done using High Performance Liquid Chromatography system with a UV detector (Agilent Series 1100). Analytes were separated using a C18 column (5 µm particle size, 250 × 4.6 mm i. d), and all sample injections were done automatically by an autosampler. Methanol (HPLC grade) was used as the mobile phase at a flow rate of 0.5 mL/min [isocractic elution of water/methanol (30/70, v/v)]. The injection volume was 20 µL with a total run time of 17 min. The wavelength detector for the analytes was 220 nm.
2.5. Quality assurance and quality control (QA/QC)
Procedural blanks were used to check for possible contamination and interferences from solvents and materials used in the extraction process. This was done with every extraction batch. The concentration obtained from the procedural blanks was deducted from the known concentration of the environmental samples. Methanol blank and midpoint calibration standard were injected after each batch analysis to check for discrepancies in instrumental response and carry-over of the analytes of interest from prior injections. Quantification of analytes was carried out using the phenolic standards, and a standard calibration curve was obtained by analysing aqueous solutions containing the analytes of interest ranging from a concentration of 10 to 1000 µg L− 1.
Results suggests that standard deviations for data obtained were < 20% for both types of water samples from Osun and Lagos States, < 20% in groundwater samples from Oyo State, and 26% for Phenol from surface water in Oyo state. The instrumental quantification limit was determined at three times the signal-to-noise ratio using the standard deviation of the seven-point calibration intercepts divided by the slope, and the limit of quantification (LOQ) was calculated as ten times the ratio. The LOD ranged from 17 to 40 µg L− 1, and the coefficient of determination (r2) of calibration curves were 0.999 for Phenol and 2,4-dinitrophenol and 0.998 for 2,4,6-trichlorophenol. The coefficients of determination, LOD and LOQ values are presented in Table 1.
| Analyte | Linear range (µg L− 1) | r2 | LOD (µg L− 1) | LOQ (µg L− 1) | Spiked conc. (µg L− 1) | Recovery (%) ± SD | RSD (%) n = 3 |
|---|---|---|---|---|---|---|---|
| 100 | 86.0 ± 3.14 | 3.65 | |||||
| 2,4-DNP | 10-1000 | 0.9999 | 17 | 58 | 200 | 85.1 ± 2.28 | 2.68 |
| 500 | 82.1 ± 0.82 | 1.00 | |||||
| 100 | 92.2 ± 3.41 | 3.70 | |||||
| Phenol | 10-1000 | 0.9999 | 19 | 24 | 200 | 87.2 ± 4.54 | 5.21 |
| 500 | 89.1 ± 3.61 | 4.05 | |||||
| 2,4,6-TCP | 10-1000 | 0.9998 | 21 | 70 | 100 200 500 | 75.2 ± 0.72 77.6 ± 0.87 85.9 ± 2.13 | 0.95 1.12 2.48 |
2.6. Risk assessment
The basis for the ecological risk assessment of phenolic compounds in this study is obtained from the US EPA ecological risk assessment framework (Chakraborty et al., 2016; Chen et al., 2014). The risk quotient (RQ) method was used to determine the toxicity levels of these phenolic compounds. The RQ is determined as follows:
𝑅𝑄=\(\frac{\text{M}\text{E}\text{C}}{\text{P}\text{N}\text{E}\text{C}}\) (1)
where RQ is the risk quotient ; PNEC is the predicted no effect concentration of compounds (µg/L) and MEC is the measured environmental concentration of the compounds in water samples (µg/L). The PNEC is calculated for both acute and chronic tests using the EC50/LC50 and NOEC respectively, divided by an assessment factor (AF) (Wang et al., 2019a).
PNECacute = (EC50/LC50 of three acute toxicity tests)/AFacute (2)
PNECchronic = NOEC in chronic tests/AFchronic (3)
where EC50/LC50 is the median effect/lethal concentration and NOEC is the No Observed Effect Concentration.
The acute toxicity data (LC50 or EC50) and chronic toxicity data (NOEC, LOEC, EC10 and MATC) used in the current study for ecological risk assessment of Phenol, 2,4-DNP, and 2,4,6-TCP in three South-western states in Nigeria were obtained from existing literature, entailing three aquatic organisms [algae, invertebrates (Daphnia) and vertebrates (Fishes)].
The ecological risk levels were grouped into three levels according to the RQ values with RQ > 1 indicating a high ecological risk, RQ values between 0.1 < RQ < 1 signifying a median risk, and RQ < 0.1 suggests a minimal environmental risk (Wang et al., 2019b).
2.7. Statistical analysis
Statistical analyses were carried out using the Statistical Package for The Social Sciences (SPSS Statistics23; IBM Corporation, Cornell, NY, USA) and Origin (Origin lab 9.1). The variation in the concentrations between the urban and rural sampling sites in Osun, Oyo and Lagos States was determined by Kruskal-Wallis test, furthermore nonparametric Spearman correlation informed us about the relationship between the concentrations of the phenolic compounds, statistical significance was set at pvalue less than 0.05. Multivariate statistical analysis was carried out using the Principal Component Analysis (PCA) software, which was used to further interrogate significant association between the phenolic compounds. The principal component analysis is a vastly used multivariate statistical method of analysis in environmental studies (Adesanya et al., 2020; Ogunlaja et al., 2019). It has been established as an instrument for the identification of contamination sources and also used substantially to confirm the association between diverse environmental variables and/or total variability of a data set (Awolusi et al., 2018; Bolujoko et al., 2022; Ogunlaja et al., 2019).
3.1. Occurrence of phenolic compounds in surface and groundwater
From Fig. 2A-D, it is generally observed that the frequency of detection of these phenolic compounds were higher in the rainy season than in the dry season with the highest detection frequencies found in water samples from Osun State and the lowest found in those from Lagos State.
Phenol was more frequently detected in water samples from the different sampling sites in the three States (Osun, Oyo, Lagos) with as much as 100% detection frequency (Fig. 2A-D) irrespective of the season of the year (rainy or dry season) or the source of the water (groundwater or surface water). However, in most cases, Phenol had higher detection frequencies in surface water samples than with groundwater samples. 2,4,6-TCP is the least frequently detected phenolic compound in the water samples in all three States (≤ 50% detection frequencies) for both seasons. Nonetheless, from the raw data obtained (not presented), the presence of 2,4,6-TCP in groundwater samples from most sample sites in Oyo State were below limit of detection.
Figure 3 shows the box plots for the median, maximum, and minimum concentrations of the various phenolic compounds in the water samples collected from the three States. From Fig. 3, it is observed that data for the presence of Phenol and 2,4,6-TCP in water samples from Osun State during rainy season have a high level of agreement for both surface water and groundwater samples indicating that the concentrations of these phenolic compounds in samples collected from the different sites in this State, are within close range.
Other data sets with this high agreement are those for 2,4-DNP in groundwater samples collected from Oyo and Lagos States during the dry season (Fig. 3). In contrast, there is generally, reduced data variability from the dry season compared with data set from rainy season (Fig. 3).
The median concentration values for these phenolic compounds are higher in rainy season than in the dry season from samples from Osun State but the reverse is the case with the samples from Oyo and Lagos States.
Considering the mean concentration values of these phenolic compounds in water samples from the different sites in the various States, we note that 2,4-DNP has the highest mean values for both seasons (rainy and dry seasons) and in both groundwater and surface water (Fig. 4A-D). Apparently, the mean concentration of 2,4-DNP in groundwater samples were higher than those for surface water samples from the three States for both seasons (rainy and dry). The exception to these observations are water samples collected during the rainy season in Oyo State (Fig. 3). Analysis for the presence of 2,4,6-TCP in most groundwater and surface water samples during the rainy season in Oyo and Lagos States indicated concentrations that were below the detection limit. However, there is generally a decrease in the maximum concentrations of these phenolic compounds in water samples during the dry season except for samples from Oyo State where the reverse is the case. This is because samples from this State were collected a day after a heavy rainfall across the State during a season regarded to be dry.
Precisely, the mean concentration of 2,4-DNP in groundwater (639.2 µg L− 1) and surface water (553.4 µg L− 1) from Osun State were highest among the three States during the rainy season (Fig. 4A-B). It should be noted that 2,4-DNP in the groundwater samples from Osun State had a mean concentration of 154.2 µg L− 1 while Phenol and 2,4,6-TCP had mean concentrations of 77.7 µg L− 1 and 122.9 µg L− 1, respectively during the dry season. The mean concentration of 639.2 µg L− 1 for 2,4-DNP, 261 µg L− 1 for Phenol, and 169 µg L− 1 for 2,4,6-TCP in groundwater samples during the rainy season were observed from this State (Fig. 4A-B). The mean concentrations for 2,4-DNP, Phenol and 2,4,6-TCP in surface water samples in Osun are 553 µg L− 1, 261 µg L− 1, and 146 µg L− 1 respectively during the rainy season, and 25 µg L− 1, 42 µg L− 1, and 40 µg L− 1 during the dry season respectively.
However, in the dry season, analysis of groundwater and surface water samples from Oyo State for the presence of 2,4-DNP gave the highest mean concentration values of 1080 µg L− 1 and 405 µg L− 1, respectively (Fig. 4C-D). Basically, groundwater samples from Oyo State had mean concentration values of 1080 µg L− 1 for 2,4-DNP, 157 µg L− 1 for Phenol, and 216 µg L− 1 for 2,4,6-TCP during the dry season. In the rainy season, the mean concentration values of 165.3 µg L− 1 for 2,4-DNP and 71 µg L− 1 for Phenol. There were no mean concentration values for 2,4,6-TCP in groundwater samples from this State in the rainy season because results were all below detection limits for this phenolic compound. For surface water samples, the mean concentration values were 405 µg L− 1 for 2,4-DNP, 145 µg L− 1 for Phenol and 275.6 µg L− 1 for 2,4,6-TCP during the dry season. Furthermore, the mean concentration values were 391 µg L− 1 for 2,4-DNP and 171 µg L− 1 for Phenol during the rainy season.(Fig. 4A-D).
The mean concentration values for these phenolic compounds in groundwater samples from Lagos State in the rainy season are 260 and 131 µg L− 1 for 2,4-DNP and Phenol, respectively, and in the dry season: 341.6, 34.3, 154 µg L− 1 for 2,4-DNP, Phenol and 2,4,6-TCP respectively. For surface water samples collected from this State, mean concentrations for 2,4-DNP and Phenol were 203 and 177 µg L− 1 in the rainy season and 235 and 60 µg L− 1 in the dry season, respectively (Fig. 4A-D). The mean concentrations for 2,4,6-TCP in groundwater and surface water samples from this State were not reported because it was observed that ca. 98% of the samples had below detection limit concentrations of this phenolic compound.
3.3. Multivariate Statistics: Principal Component Analysis
The results of the Principal Component Analysis (PCA) for the parameters studied in groundwater and surface water samples from Osun, Oyo, and Lagos States are summarized in Table 2. The preliminary results of the Kaiser-Meyer-Olkin (KMO) test (≥ 0.5) and Barlett Sphericity tests (p < 0.001) indicate that the data qualify for structure detection and this further validates the results of the PCA. For groundwater samples from Osun State, three principal components dominated the PCA analysis, accounting for 82.7% of the total variance. The first principal component (PC1) explained 41.5% of the total variance and showed very high loadings of 0.99 and 0.98 for Electrical Conductivity (EC) and Total Dissolved Solids (TDS) respectively. Similar high loadings ≥ 0.97 for EC and TDS were also observed for other water samples (groundwater and surface water) from Oyo State (Table 2 and Fig. 5).
| Osun GW | Osun SW | Oyo GW | Oyo SW | Lagos GW | Lagos SW | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 1 | 2 | 3 | 1 | 2 | 3 | 1 | 2 | 1 | 2 | 1 | 2 | |
| DNP | 0.15 | 0.05 | 0.94 | 0.03 | -0.01 | 0.96 | 0.29 | 0.29 | 0.80 | -0.24 | 0.75 | -0.08 | 0.75 | 0.18 | 0.76 |
| PHE | -0.16 | 0.91 | 0.09 | 0.03 | 0.70 | -0.02 | 0.24 | 0.84 | 0.16 | 0.09 | 0.57 | -0.73 | 0.20 | -0.02 | 0.39 |
| TCP | 0.37 | 0.66 | -0.20 | -0.10 | -0.52 | 0.17 | 0.08 | 0.09 | -0.90 | -0.09 | 0.76 | 0.05 | -0.72 | -0.02 | -0.78 |
| pH | 0.51 | 0.27 | -0.56 | -0.14 | 0.66 | 0.24 | -0.01 | 0.85 | -0.03 | 0.30 | 0.51 | 0.76 | 0.32 | 0.70 | 0.30 |
| EC | 0.99 | 0.02 | 0.03 | 1.00 | 0.21 | 0.03 | 0.98 | 0.10 | 0.03 | 0.98 | -0.01 | 0.55 | -0.39 | 0.93 | -0.08 |
| TDS | 0.98 | 0.02 | 0.03 | 1.00 | 0.01 | -0.01 | 0.97 | 0.13 | 0.10 | 0.98 | -0.02 | 0.65 | -0.11 | 0.97 | -0.02 |
| Eigen values | 2.490 | 1.348 | 1.121 | 2.012 | 1.207 | 1.004 | 2.552 | 1.358 | 1.192 | 2.109 | 1.703 | 1.993 | 1.254 | 2.395 | 1.371 |
| % Total variance | 41.51 | 22.47 | 18.69 | 33.53 | 20.12 | 16.73 | 42.54 | 22.63 | 19.86 | 35.15 | 28.38 | 33.21 | 20.90 | 39.92 | 22.85 |
| % cumulative | 41.51 | 63.98 | 82.66 | 33.53 | 53.65 | 70.38 | 42.54 | 65.17 | 85.03 | 35.15 | 63.53 | 33.21 | 54.11 | 39.92 | 62.77 |
Extraction method: principal component analysis. Rotation method: Varimax with Kaise (Bold figures indicate values ≥ 0.5).
The pH loading of 0.51 for groundwater samples from Osun State (Fig. 5A) is not as high as that for EC and TDS, suggesting that the pH of water samples from this State is influenced by different sources. Likewise, the PCA results for Osun surface water (SW) showed that it had three principal components significantly dominated by EC and TDS in PC1. This accounts for 33.53% of the total variance with very high loadings of ≥ 0.99 indicating a strong relationship between EC and TDS (Fig. 5B) influence by a common factor.
The PC2 value for Osun State groundwater samples consist of a high loading of 0.91 for Phenol (PHE) and a relatively moderate loading of 0.66 for 2,4,6-TCP (TCP). Also, PC2 values (for surface water samples) for Osun State were dominated by pH and PHE with loadings of 0.66 and 0.70 respectively accounting for 20.12% of the total variance. The PC3 for groundwater samples is dominated only by 2,4-DNP (DNP) accounting for 18.69% of the total variance with high loadings of 0.94. This is confirmed in Fig. 5A, indicating that DNP is from a source different from those of PHE and TCP. The loading of -0.56 for pH indicated that DNP and pH have different sources. Like the groundwater samples, the PC3 for surface water samples from this State is dominated significantly by DNP with a loading of 0.96 accounting for 16.73% of the total variance (Fig. 5A).
Similarly, the physico-chemical parameters of groundwater samples from Oyo State were dominated by three principal components. The PC 1, 2 and 3 were best represented by EC and TDS, EC and PHE, and pH and DNP accounting for 42.54, 22.63 and 19.86% of the total variance respectively (Table 2). The negative high loading of -0.90 for TCP (Fig. 5C) and its large distance with DNP, indicates that it is from a different source. Nonetheless, surface water samples from this State (Oyo) were described by two principal components (PC1 and PC2). Like with Osun State groundwater and surface water samples, the PC1 was dominated by EC and TDS accounting for 35.15% of the total variance, while PC 2 was dominated by DNP, PHE, TCP and pH with loadings of 0.75, 0.57, 0.76 and 0.51 respectively accounting for 28.38% of the total variance. The relatively low loadings of PHE and pH suggests the influence of a different factor, though similar source as shown in Fig. 5D.
Likewise, Table 2 summarized the results of the PCA for groundwater and surface water samples obtained from Lagos State. Data from the PCA also showed that PHE did not contribute significantly (p ≤ 0.05) to the total variance observed in the parameters studied.
The groundwater PCA result was represented by two principal components responsible for a cumulative variance of 54.11% with pH, EC and TDS dominating the PC 1 accounting for 33.21% of the total variance. EC and TDS were also correlated in PC 1 even though not as strong as the association seen in water samples from Osun and Oyo States (Fig. 6A). The PC 2 for Lagos groundwater is dominated significantly by DNP with a loading of 0.75 accounting for 20.90% of the total variance. The surface water PCA result was equally represented by two principal components responsible for a cumulative variance of 62.77% with pH, EC and TDS dominating the PC 1 accounting for 39.92% of the total variance, and 2,4-DNP with a loading of 0.76 dominating the PC 2 accounting for 22.85% of the total variance. (Fig. 6B).
3.3.1. Hierarchical cluster analysis (CA)
Cluster analysis (CA) was performed on the selected physicochemical parameters and phenolic compound concentrations in both groundwater and surface water samples. The results are represented by the dendrograms (Figs. 7 and 8). The degree of association between the variables (physicochemical parameters and phenolic compound concentrations) is depicted by the distance between clusters in the dendrogram. The dendrogram further corroborates the results of the PCA.
Figures 7 (i-ii) shows the CA results for Osun groundwater and surface water samples were grouped into two main clusters (A and B). For Osun groundwater, Cluster A contains two lower clusters, A1 (TDS, pH and TCP) and A2 (EC and PHE). pH and TCP were joined together at a shorter distance, suggesting a common origin, but pH and TCP were joined together at a relatively high level confirming the influence of another factor. Similarly, A2 (EC and PHE) was joined together at another relatively higher level than A1 (TDS, pH and TCP), suggesting the influence from similar sources. Furthermore, the CA result for Osun groundwater also contained a stand-alone cluster B (DNP) that joined cluster A at a higher distance. Similarly, the proximity in the dendrogram between EC and TDS in Figs. 7 (ii-iv) suggests a form of similarity in distribution patterns in water samples and corroborated the PCA results.
In addition, Fig. 7iv shows that the studied variables in Oyo surface water samples were grouped into four main clusters. However, the clustering pattern was less distinct in comparison to others. Furthermore, Fig. 7 (i-iv) showed a consistent wide distance between DNP and every other studied variable, indicating the influence of a different anthropogenic source.
Although Fig. 8 (ii) showed significant associations between the physico-chemical properties and phenolic compounds in surface water samples from Lagos State at higher distances compared to Fig. 8(i), the similarity in the cluster pattern suggests a similar anthropogenic source of contamination for both groundwater and surface water in Lagos State except for DNP with large distances from other two phenolic compounds. This indicates the influence of an entirely different anthropogenic source of DNP.
3.4 Ecological Risk Assessment
The ecological risk assessment for 2,4-Dinitrophenol, Phenol and 2,4,6-Trichlorophenol was calculated based on the risk quotient for minimum and maximum measured environmental concentrations for surface water and groundwater samples from the three States. Data obtained are presented in Tables S1.0 and S2.0.
In Osun State, the ecological risk to three trophic levels: Algae, Daphnia, and Fish from the concentration of 2,4-DNP, Phenol and 2,4,6-Trichlorophenol in groundwater and surface water is high and even much higher during the rainy season than in the dry season. This trend is vice-versa for water samples from Lagos State as seen from the RQacute and RQchronic values in Table 3. Samples from Oyo State show a similar ecological risks trend to Algae, Daphnia, and Fish like that from Lagos State.
Comparing the risk from the three phenolic compounds, 2,4-DNP pose the highest toxicity risk to the three aquatic lives, followed closely by 2,4,6-TCP, with Phenol being the least with moderate ecological risk. On the acute scale (RQacute) for ecological risk, Daphnia is more at risk than either Algae or Fish (Tables S1.0 and S2.0) in both rainy and dry seasons for groundwater and surface water. Conversely, on the chronic scale (RQchronic), Algae and Fish are more susceptible to toxicity from the three phenolic compounds in both groundwater and surface water during the rainy and dry seasons.
In comparing the RQ values of water samples during both seasons, groundwater samples from Oyo and Lagos States show higher RQ values compared to the RQ values obtained from surface water in the same States during the dry season. The reverse is the case for samples from Osun State in the same season. However, during the rainy season, the RQ values obtained from groundwater samples in Osun and Lagos States were higher than those obtained for the surface water in these two States. Nonetheless, samples from Oyo State showed a reverse trend in the rainy season.
In general, it is observed that 2,4-DNP toxicity impacts Daphnia more than Algae and Fish on the acute scale (RQacute) while Algae is the most impacted on the chronic scale (RQchronic). 2,4-DNP in water samples collected around these Southwestern States in Nigeria pose more ecological risk to aquatic life-Algae, Daphnia, and Fish, than either Phenol or 2,4,6-TCP. This can be seen from the higher RQ values for 2,4-DNP compared with those for Phenol or 2,4,6-TCP.
3.4 Human Exposure and Cancer Risk Assessments
Groundwater is a major source of drinking water for the populace in Nigeria (Danert and Healy, 2021). In some impoverished areas in these States, which are rural settings, surface water suffices as a source of drinking water. Hence, it is crucial to determine the amount of human exposure to these phenolic compounds when ingesting water from these sources. The human exposure to phenolic compounds in this study was assessed using the estimated daily intake (EDI; µg kg− 1 bw day− 1) based on the United States Exposure Factors Handbook (USEPA, 2011). The population was grouped into infants (< 1 year old), toddlers (1–3 yrs old), children (4–11 yrs old), teenagers (12 − 21 yrs old) and adults (≥ 21 yrs old).
EDI water (µg kg− 1 bw day− 1) = (C × D)/BW……………………………………… (4)
Where C is the concentration of the analyte in water (µg L− 1), D is the daily water consumption rate (L day− 1), and BW is the body weight (kg). The median and the maximum concentration were used to calculate the EDI. The daily water consumption rate (D) and body weight used for calculating the EDI were: infants (1 L day− 1, 9.2 kg), toddlers (0.9 L day− 1, 13.8 kg), children (1.3 L day− 1, 31.8 kg), teenagers (2.4 L day− 1, 71.6 kg) and adults (3.1 L day− 1, 80 kg) respectively (USEPA, 2011). The Environmental Protection Agency of the United States gave a Provisional Peer Reviewed Toxicity Value of 0.002 mg kg− 1 day− 1 for 2,4-DNP in drinking water. The non-fatal lowest-observed-adverse-effect level (LOAEL) for 2,4-DNP is put at 0.9-2.0 mg kg− 1 day− 1 while its fatal doses is given as 3.0–7.0 mg kg− 1 day− 1 (Przybyla et al., 2021). However, because 2,4,6-TCP is considered a carcinogenic compound, a no significant risk level (NSRL) of 10 µg day− 1 was assigned to it (https://oehha.ca.gov/chemicals/246-trichlorophenol ). Phenol is not known to be toxic but can alter the taste of water.
The median and maximum concentrations for these phenolic compounds in groundwater and surface water samples collected from the different States are provided in Table S1.0.
Considering Fig. 9 obtained from maximum concentrations of the phenolic compounds, the EDI values of 2,4-DNP in groundwater samples from the different States both in the rainy and dry seasons are below the LOEAL and fatal doses already prescribed but are higher than the Provisional Peer Reviewed Toxicity Value of 0.002 mg kg− 1 day− 1 (Fig. 9A &D). Likewise, the EDI values for 2,4,6-TCP in the groundwater samples from Osun and Lagos States during the rainy season are far higher than the prescribed threshold limit of 10 µg day− 1 (Fig. 9C). This is similarly true for groundwater samples from Oyo State during the dry season (Fig. 9F).
The trend observed for EDI values of 2,4-DNP in surface water for the different States and in rainy and dry seasons is similar to that observed for groundwater as earlier described. However, the EDI values of the former are lower than the latter (Figs. 9–12). Just like with groundwater, the EDI values for 2,4,6-TCP in surface water are higher than the prescribed threshold limit of 10 µg day− 1 in most cases and this is more so in the dry season than in the rainy season when maximum concentrations of the phenolic compound (2,4,6-TCP) was used for the calculation of EDI values (Figs. 11C & F).
When the median values of the concentrations of phenolic compounds were used to calculate the EDI values for surface water, it is observed that EDI values calculated for Infants and Toddlers for Osun State were higher than the Provisional Peer Reviewed Toxicity Value of 0.002 mg kg− 1 day− 1 during the rainy season (Fig. 12A) as well as those for Lagos and Oyo States in the dry season (Fig. 12D). However, the EDI values were lower than the LOAEL and fatal doses prescribed. These same population group (Infants and Toddlers) are also at risk with the concentration of 2,4,6-TCP in surface water from Osun and Oyo States during rainy and dry seasons respectively (Figs. 12C & 12F). Generally, the EDI values for these phenolic compounds in surface water decreased in the dry season (Fig. 12D-F) than in the rainy season (Fig. 12A-C)
It is observed that EDI values decreased with the increasing age of the exposure group. The EDI values were highest for Infants and lowest for teenagers across the three States. However, there is no statistical difference between EDI values for Children, Teenagers, and Adults. Water samples from Osun State provided the highest EDI values across all exposure groups for 2,4-DNP, Phenol, and 2,4,6-TCP during the rainy season (Figs. 9–12). During the dry season, the EDI values for all three phenolic compounds decreased for Osun and Lagos States but increased for Oyo State. This is likely because of the unusual event earlier mentioned. This highlights the impact of climate change on seasonal variation.
The Hazard Quotient (HQ) and human exposure assessment results (Table 3) showed that human exposure to these organic contaminants needs to be further controlled by regular monitoring and assessment of the surface and groundwater quality in these areas.
The human risk is categorized into non-carcinogenic risk and carcinogenic risk.
The non-carcinogenic risk is calculated based on the hazard quotient:
HQ = CDI/RfD (5)
Where; CDI = (C × DI × RSC × ED)/(BW × AT), (6)
Moreover, the risk of cancer is calculated using the:
CR = (C×DI × ED ×CSF)/(BW×AT) (7)
Where CR is the cancer risk, C is the highest phenolic compound concentration found (µg L− 1), DI is daily input (2.4 L day− 1), ED is the exposure duration (183 days), RSC is the relative source contribution, RfD is the oral reference dose, CSF is the cancer slope factor (mg kg− 1 day− 1), BW is body weight (80 kg), and AT is the averaging times in days (ATAdult = 10950 days) (EPA, 1980; EPA, 2015). The HQ of the non-carcinogenic risk is categorized as HQ > 1, indicating a high non-carcinogenic risk, while HQ < 1 indicates low non-carcinogenic risk. The carcinogenic risk, however, was categorized into negligible risk (CR < 10− 6 ), possibility of cancer (CR > 10− 6), and unacceptable risk (CR > 10− 4 ) (Zhou et al., 2017a). The CR was calculated for all the phenolic compounds with CSF for the compounds in water as found in the literature ((EPA), 2002; IB and Assessment, 1993). Table 3 shows all results from the CR calculation.
| State | Season | PC | NCR | HQ | CR | HQ |
|---|---|---|---|---|---|---|
| Osun | Rainy | 2,4-DNP | 6.29 | High | ** | ** |
| Phenol | 0.057 | Low | ** | ** | ||
| 2,4,6-TCP | 0.53 | Low | 0.0020 | High | ||
| Dry | 2,4-DNP | 21.91 | High | ** | ** | |
| Phenol | 0.023 | Low | ** | ** | ||
| 2,4,6-TCP | 0.19 | Low | 0.0015 | High | ||
| Oyo | Rainy | 2,4-DNP | 10.53 | High | ** | ** |
| Phenol | 0.026 | Low | ** | ** | ||
| 2,4,6-TCP | ** | ** | ||||
| Dry | 2,4-DNP | 65.82 | High | ** | ** | |
| Phenol | 0.092 | Low | ** | ** | ||
| 2,4,6-TCP | 0.68 | Low | 0.0022 | High | ||
| Lagos | Rainy | 2,4-DNP | 32.28 | High | ** | ** |
| Phenol | 0.046 | Low | ** | ** | ||
| 2,4,6-TCP | ** | ** | ||||
| Dry | 2,4-DNP | 29.98 | High | ** | ** | |
| Phenol | 0.020 | Low | ** | ** | ||
| 2,4,6-TCP | ** | ** | ** | ** | ||
| §NCR = Non-carcinogenic risk; CR = Carcinogenic risk; PC = Phenolic compound | ||||||
| State | Season | PC | NCR | HQ | CR | HQ |
|---|---|---|---|---|---|---|
| Osun | Rainy | 2,4-DNP | 43.41 | High | ** | ** |
| Phenol | 0.057 | Low | ** | ** | ||
| 2,4,6-TCP | 0.32 | Low | 0.0012 | High | ||
| Dry | 2,4-DNP | 1.75 | High | ** | ** | |
| Phenol | 0.037 | Low | ** | ** | ||
| 2,4,6-TCP | 0.25 | Low | 0.00041 | High | ||
| Oyo | Rainy | 2,4-DNP | 40.20 | High | ** | ** |
| Phenol | 0.071 | Low | ** | ** | ||
| 2,4,6-TCP | ** | ** | ||||
| Dry | 2,4-DNP | 2.25 | High | ** | ** | |
| Phenol | 0.043 | Low | ** | ** | ||
| 2,4,6-TCP | 0.74 | Low | 0.0030 | High | ||
| Lagos | Rainy | 2,4-DNP | 25.21 | High | ** | ** |
| Phenol | 0.073 | Low | ** | ** | ||
| 2,4,6-TCP | ** | ** | ||||
| Dry | 2,4-DNP | 25.58 | High | ** | ** | |
| Phenol | 0.046 | Low | ** | ** | ||
| 2,4,6-TCP | ** | ** | ** | ** | ||
| §NCR = Non-carcinogenic risk; CR = Carcinogenic risk; PC = Phenolic compound | ||||||
The results in Table 3a & 3b show that 2,4-DNP in groundwater and surface water samples is the pollutant with the highest non-carcinogenic risk ( > > 1.0) in all three States and in both seasons (dry and rainy) when compared with Phenol and 2,4,6-TCP. The non-carcinogenic risk (NCR) of 2,4-DNP is generally higher during the dry season than in the rainy season for groundwater (Table 3a). The NCR is however higher during the rainy season than in the dry season for surface water (Table 3b). In Osun State, 2,4-DNP in groundwater pose the least NCR in the rainy season but the highest NCR in surface water during the same season.
There is a high cancer risk probability from ingesting groundwater and surface water containing 2,4,6-TCP from both Oyo and Osun States over time in both dry and rainy seasons, given the high CR values, which were > 10− 4.
The concentrations of phenolic compounds obtained in this study are significantly higher than those previously reported from other countries worldwide, as shown in Table 4. In some cases, 2,4,6-TCP concentrations were higher than WHO set limits of 2 µgL-1 for its presence in drinking water (Angelino and Gennaro, 1997; EPA and Technology, 2015; Organization, 1994)(EPA (2019). In fact, the concentrations of 2,4-DNP and 2,4,6-TCP reported in this study are > 200 fold more than those found in the Buffalo River of Eastern Cape South Africa (Yahaya et al., 2019), one of the very few reports from Africa.
Considering the mean concentrations of 2,4-DNP, Phenol, 2,4,6-TCP shown in Fig. 4, it is evident that 2,4-DNP is the most prevalent and with the highest concentrations in water samples from the three States, with Osun State providing the highest concentration values in the rainy season. This can be explained to be because of the increase in farming activities in Nigeria during this season which requires the application of fertilisers, pesticides and herbicides (Luo et al., 2014; Ramos et al., 2021). Furthermore, during this season, there is an increase in stormwater discharge and higher atmospheric precipitation that increases 2,4-DNP in water (Yahaya et al., 2019).
Since 2,4-DNP is an essential constituent of pesticides and foliage fertilisers applied to farmlands during the rainy season, it is only apparent that the residues from the pesticide and fertilisers will be washed off into water bodies and into soils where they subsequently percolate through the soil into groundwater (Zhong et al., 2018). A further reason for these high amounts of 2,4-DNP in most water samples collected from the three States relative to Phenol and 2,4,6-TCP is the prominence Nigeria is currently giving to the use of fertilizer to boost agricultural production through the establishment of seventy fertiliser blending plants in the country (https://www.channelstv.com/2022/04/05/our-policies-have-made-nigeria-a-fertilizer-powerhouse-in-africa-buhari/). However, 2,4-DNP was found in less amounts in water samples from Oyo and Lagos States with the latter being the least. This is especially so because Lagos State is a metropolitan city with very few agricultural activities.
Although 2,4-DNP had the highest mean concentration values in the water samples studied, the concentrations of Phenol in these samples were also significantly high. This might not be unconnected with the increasing use of personal care products by millions of Nigerians, most especially as disinfectants and sunscreens, hair dyes and relaxers. These products are known to contain Phenol. Furthermore, the biodegradation of substituted phenolic compounds by microorganisms in aqueous soil phase may result in the formation of Phenol (Preda et al., 2018).
Even though the concentrations of phenolic compounds in groundwater samples were higher in urban areas than in rural areas, statistical analysis (not shown) indicate that there is no significant difference (p < 0.05) in these concentrations, which suggests that people living in both urban and rural areas in these States are equally exposed to any potential risk associated with the concentration of these phenolic compounds in their water bodies when ingested.
There was an impact of seasonal variation on the quality of water samples collected. Groundwater samples from Oyo and Lagos States show more toxicity to Algae, Daphnia and Fish in the dry season (Fig. 3) than in the rainy season due to the higher maximum concentration of these phenolic compounds. In contrast, samples from Osun State show a reverse trend. This is likely because, from our observation, there are more hand-dug wells in Osun State than in Oyo and Lagos States. It is known that surface runoff readily contaminates hand-dug wells (which are usually shallow). Consequently, these phenolic compounds can easily be transported to hand-dug wells during the rainy season (Ibrahim et al., 2021). However, groundwater samples from Oyo and Lagos were from boreholes, often below the water table and not easily polluted by transported chemicals.
High PCA loadings found in water samples (both groundwater and surface water) in Osun and Oyo States suggests a very strong association between EC and TDS, and hence similar sources and factor contributing to the values of both EC and TDS from water samples from the two States. This is expected because of the relationship between EC and TDS, and previous reports which show similar strong correlation between EC and TDS (Thirumalini and Joseph 2009, Bakhtiar Jemily, Ahmad Sa’ad et al. 2019). For groundwater samples from Osun State, there is an association between Phenol and 2,4,6-TCP in the PC2 analysis shown in Table 2. This suggests a quasi-independent behavior within the group, which is further corroborated by a large distance between the two phenolic compounds (PHE and TCP) in Fig. 7A and confirming they are from different sources. There is also an association between Phenol and pH in Osun surface water and Oyo groundwater samples which suggests that this two components (phenol and pH) have a similar source in water samples from these States. In Oyo surface water samples, there is a close association between 2,4-DNP and 2,4,6-TCP and this is suggestive of the fact that they are both from similar origin and have same influencing factors. The similarity in the PCA with reference to pH, EC and TDS between Osun groundwater and surface water also suggests similarity in the contributing factors, and hence similar sources of pH, EC and TDS (Figs. 8A and B). Although data from the PCA showed that groundwater from Osun and Oyo States had more multiple sources of contamination than other samples, the analysis showed that 67 to 100% of groundwater samples had more multiple sources of contamination compared to surface water samples from the three States. However, for Lagos groundwater and surface water samples, there was no correlation between the phenolic compounds and the physico-chemical properties (pH, EC, and TDS) as seen from the loadings. This shows that the physico-chemical factors were not the determining factors for the presence of phenolic compounds in Lagos water samples but other variables (Fig. 8). 2,4-DNP was not associated with any other phenolic compound and physico-chemical properties except in Oyo surface water.
Considering the ecological risk from the presence of Phenol, 2,4-DNP and 2,4,6-TCP in the water samples collected from the three States, toxicity to the three aquatic lives is more pronounced with water samples from Osun State than in the other two States during the rainy season, while water samples from Oyo pose more toxicity risk to these aquatic lives during the dry season. This is seen from the high frequencies of RQacute and RQchronic values > 1.0 (Table S2). This may suggest that the presence of these chemicals may, on the one hand, hinder algal growth, which appears to be a positive development, but on the other hand, harm the health of people drinking from such polluted water in the long term. The higher ecological risks posed by 2,4-DNP compared to that of Phenol or 2,4,6-TCP is in tandem with its high mean concentrations in water samples collected across the three States.
The susceptibility of Daphnia to acute toxicity from 2,4-DNP found in these water samples when compared to Algae and Fish is expected because of its organisation level. Lower organisms do not have a well-developed mechanism to ameliorate toxicity imposed on them from the environment. The potential acute and chronic toxicity risks posed by groundwater and surface water bodies reported in this study are likely to increase if water treatment involves chlorination (Michałowicz and Duda, 2007), as is still the case in many developing countries in the world. Although, a published study reported by Michałowicz & Duda suggests that the concentrations of Phenol in water samples posed the most negligible toxicity to algae, Daphnia and Fish when compared with 2,4-DNP and 2,4,6-TCP (Michałowicz and Duda, 2007), yet this is not obvious from this study. It, thus, implies that the concentration of Phenol may determine its level of toxicity to aquatic life since the concentration of Phenol in water samples in this study far exceeds those reported by Michałowicz & Duda (2007).
Results from EDI calculations (Figs. 9–12) suggests that the concentrations of 2,4-DNP in both groundwater and surface water can lead to serious toxicity issues for most population groups studied especially those below the age of 20 years. This is supported by results from hazard quotients in Tables 3a and 3b which suggests high non-carcinogenic human risk with the concentrations of 2,4-DNP in water samples from the three States. The concentration of 2,4-DNP in water samples from Osun State is singled out as posing the greatest threat to human health because they show the highest EDI values. Furthermore, the carcinogenic risk from the concentration of 2,4,6-TCP in groundwater samples from Osun and Lagos States especially during the rainy season is high as the EDI values suggests that they exceed the recommended daily intake of 3 µg/kg of 2,4,6-TCP that is not likely to induce carcinogenic changes in humans (Michałowicz and Duda, 2007).
The EDI range of the phenolic compounds in this study also indicate a strong and urgent need for government to be deliberate in determining phenolic compounds in drinking water. WHO has recommended the need to monitor such pollutants in water. This study show that infants are highly vulnerable to these phenolic compounds in water, hence families may require point-of-use treatment facilities since boiling (commonly used in treating water used for babies) might not be sufficient.
Nevertheless, from this study, it is observed that 2,4-DNP in groundwater is more toxic to all exposure groups (Infants, Toddlers, Children, Teenagers, and Adults) from the three States (Osun, Oyo, Lagos) than other phenolic compounds studied judging from its higher EDI values in comparison with the other phenolic compounds (Figs. 9 and 10). Furthermore, the Hazard Quotient shown in Table 3 shows that 2,4-DNP in both groundwater and surface water has the potential to cause non-carcinogenic health issues including breakdown of central nervous system, negative impact on cardiovascular system, cataracts, and increased metabolism resulting in fever, headache, profuse sweating, thirst and fatigue etc. The maximum EDI values for 2,4-DNP, Phenol, and 2,4,6-TCP in groundwater water for all exposure groups were higher than the prescribed acceptable daily intake (ADI) of 2.0, 1.0, and 1.5 µg kg− 1 bw day− 1 respectively for drinking water (EPA and Technology, 2015).
The lack of awareness or policy statement for the abatement of phenolic chemicals in water, as well as the lack of current technologies for water treatment in Nigeria, are triggering the increase of these contaminants in water systems in Nigeria. The higher detection frequency of these phenolic compounds observed in the wet season is likely because the rains potentially wash off these phenolic compounds from improperly disposed storage containers, soils, and household sewage into the aquatic environment.
Most of the populace in the three States (Osun, Oyo and Lagos) use groundwater as their source of drinking water. At the same time, the poor rural area also utilizes surface water around them without treatment. The presence of these phenolic compounds in these water systems calls for severe concerns. Thus, it is important for the government to prioritize water treatment and ensure that modern treatment facilities that can take care of phenolic compounds are deployed.
Following the thorough one-year monitoring of water sources in three South-Western States in Nigeria, the three phenolic compounds (2,4-DNP, Phenol, and 2,4,6-TCP) were present in most of the sampling sites both in surface water and groundwater, which of course, are the major sources of drinking water in Osun, Oyo and Lagos States, Nigeria. From the mean concentrations of these phenolic compounds in water samples collected, it is evident that these are among the highest reported in recent times. Concentrations reported in this study are 200 times more than the standard limits by WHO, USEPA and Brazilian Environment Protection Agency. The most frequently detected phenolic compound was Phenol, while 2,4-DNP was reported as having the highest mean concentration (> 90% of samples) in water samples. Generally, the concentrations of these phenolic compounds decreased during the dry season compared with the rainy season except for samples collected from Oyo State, which showed a reversed trend because samples were collected from the State a day after heavy rainfall during a season expected to be devoid of rain. This underscores the impact of climate change on the environment. However, there was no significant difference between concentrations of 2,4-DNP, Phenol, and 2,4,6-TCP found in water samples from urban and rural sites. Ecological risks assessment suggests that of the three aquatic lives (Algae, Daphnia and Fish), Daphnia and Algae are more susceptible to toxicities from 2,4-DNP, respectively, in the surface water and groundwater environment from the different States. The risk assessment shows how fatal these phenolic compounds are, especially 2,4,6-TCP if taken orally at the concentrations discovered in this study. Results from the PCA analysis indicate that groundwater samples had multiple sources of contamination compared to surface water samples.
Safe practices such as controlled discharge of treated sewage, hospital and industrial wastewater are therefore recommended. Efficient technologies should be adopted to decrease the contamination of water bodies by these phenolic compounds. Furthermore, funding should be provided by the government and funding agencies for thorough and substantial research on these emerging contaminants to understand their impact on human health further and aid the government in making sound policies for their abatement in the environment.
The most detected compounds were 2,4-DNP and Phenol in both surface water and the groundwater. Results showing the concentration variations of these phenolic compounds both in the surface water and groundwater showed that rainfall and precipitation did not affect the concentration of these phenolic compounds are maximum concentrations were recorded during the rainy season for Osun and Lagos state, however for Oyo state, there was a significant decrease in the concentration recorded during the wet season. This is a clear indication that the presence of these phenolic compounds vary by seasons. The Principal Component Analysis (PCA) of data obtained showed that there was a strong correlation between the concentrations of 2,4-DNP and Phenol in the water samples which suggests that 2,4-DNP and Phenol in the water samples are from similar sources. The statistical results from PCA indicated that similar factors are responsible for the presence of 2,4-DNP and Phenol in the water samples.
Ethics Approval
Not applicable
Consent to participate
All the authors approved of participating in this publication
Consent for publication
All the authors approved the manuscript for publication.
Funding
The authors EIU, OBO, ODO, AO and OOO acknowledge, with much thanks, funding from The World Academy of Sciences-Islamic Development Bank (TWAS-IsDB) collaborative research grant (Grant number: 4500420197) to carry out this study. The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.
Competing Interests
The authors have no relevant financial or non-financial interests to disclose
Author Contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Oluwafanremi Otitoju, Moses Alfred, and Chidinma Olorunisola. The first draft of the manuscript was written by Oluwafanremi Otitoju and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Data Availability
Yes
Materials Availability
Yes
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