Concentrations of 222Rn and other toxic metals (Ni, Cu, Fe, Cd, Zn, Pb, Cr, and Mn) were measured by alpha spectrometry using Rad7 and atomic absorption spectrophotometry, respectively, in the water samples from the Ilokun dumpsite, Ado-Ekiti, Nigeria. The physicochemical parameters of the groundwater were also assayed to determine its suitability for drinking and irrigation purposes. While the physicochemical parameters met the World Health Organization (WHO)-safety standards, Cu, Fe, Cd, Pb, Cr, and Mn exceeded permissible limits. Hazard index (HI) values for adults (8.40) and children (39.23) indicated heightened susceptibility to non-carcinogenic health risks among both age group populations. The 222Rn concentrations present in the studied samples range from 5.4 to 23.1 Bq l−1 with an average of 13.9 Bq l−1. Approximately 60% of the samples have radon concentrations above the US Environmental Protection Agency (EPA)-maximum contaminant level of 11.1 Bq l−1. The evaluated radiation doses to the stomach and respiratory tract due to radon content in the water are well below the WHO-specified individual dose criterion of 0.1 mSv year−1. This study suggests a need for groundwater resource management strategies and measures to protect human health from radiological and toxicological risks associated with groundwater consumption around the Ilokun dumpsite.

  • The study assessed the concentrations of 222Rn and toxic metals in water samples near a dumpsite.

  • The hazard index values for adults and children indicated heightened non-carcinogenic health risks.

  • Approximately 60% of the samples surpassed the USEPA-approved safety limit for 222Rn.

  • Despite elevated radon levels, the evaluated radiation doses to the stomach and respiratory tract were below the WHO dosage limit.

Paucity of potable water has often necessitated the use of groundwater as an alternative drinking water source in many areas of the world. Within the vicinity of a dumpsite, groundwater may be prone to chemically toxic substances due to infiltration of hazardous materials, thereby increasing human health risk burden in addition to the already dissolved naturally occurring radionuclides. 222Rn is one of the naturally occurring radionuclides present in groundwater that presents significant health risks to humans. Although the exposure from 222Rn due to inhalation accounts for about half of the radiation dose to human beings received from natural radionuclides (UNSCEAR 2002), the radiation exposure from radon ingestion in water has also been considered important. Radon is a radioactive gas that occurs in the 238U decay series. It exists in three radioisotopes, namely, 219Rn, 220Rn, and 222Rn. Of the three radioisotopes, 222Rn is the most important due to its relatively long half-life (3.82 days). The cancer risk from radon ingestion in groundwater has been studied by many researchers around the world (Auvinen et al. 2005; Al-Alawy et al. 2018; Divya & Prakash 2019; Naskar et al. 2023).

Apart from the radiological health concerns posed by radon in groundwater, the groundwater reservoirs near dumpsites may also expose the local residents to potential risks of chemical toxicity. Groundwater contamination caused by leachate emanating from dumpsites has been a serious environmental concern all over the world (Aboyeji & Eigbokhan 2016; Parvin & Tareq 2021; Sanga et al. 2023). Dumpsites receive various forms of waste, encompassing industrial refuse, hazardous materials, and municipal solid waste (MSW). When these wastes decompose, they produce a liquid known as leachate, which comprises a blend of toxic metals, organic and inorganic compounds, pathogens, and other pollutants (El-Saadony et al. 2023). The makeup of leachates differs based on the nature of the waste, the landfill's age, hydrological conditions, landfill operation practices, and climatic conditions (Somani et al. 2019; Lindamulla et al. 2022). The leachate generated at dumpsites has the potential to infiltrate the soil and enter the groundwater system through natural fractures, sinkholes, or porous formations, resulting in aquifer contamination. Consequently, regions in close proximity to dumpsites are susceptible to groundwater contamination due to the potential for contamination from the nearby disposal site. If such contamination persists for long periods, it could pose significant health hazards to users of the local groundwater resource and to the surrounding natural ecosystem. If contaminated groundwater is used for drinking, cooking, or irrigation, it can lead to serious human health issues. Exposure to contaminants like heavy metals and organic compounds can cause various ailments, including neurological disorders, cancers, and reproductive problems. Groundwater contamination affects not only human populations but also ecosystems. Aquatic life can be harmed by toxic substances, leading to the disruption of natural habitats and reduction in biodiversity (Bassem 2020). Contaminated groundwater can also impact surface water bodies, such as lakes and rivers, through leachate discharge.

Illnesses stemming from the pollution of drinking water pose a significant challenge to human health (WHO 2017). While approximately 80% of global diseases are attributed to inadequate drinking water quality, this factor contributes to about 50% of child fatalities on a global scale (Lin et al. 2022). Against this background, the assessment of groundwater quality from radiological and chemical toxicity perspectives in the proximity of dumpsites becomes crucial to ascertain the water suitability for human consumption. The assessment will provide valuable insights for effective planning and sustainable administration of groundwater resources in the environs of a dumpsite. The Ilokun dumpsite, situated in Ado-Ekiti, is a prominent dumpsite among many others found across the country. Similar to other dumpsites in Nigeria, waste is deposited on excavated plots without considering the impact on the underlying environment (Aduojo et al. 2018). This practice contrasts with global best practices wherein dumpsites are designed with lined walls made of clayey materials or polyethylene geomembrane liners. These measures aim to minimize the upward and outward movement of pollutants produced by the disposed substances in dumpsites, thereby reducing the potential environmental impact.

Past research endeavors (Olagunju et al. 2018; Olaseeni et al. 2018; Okunade et al. 2019; Omofunmi et al. 2020) carried out within the study locale have primarily concentrated on evaluating the impact of the waste dumpsite on both groundwater and soil quality. However, limited attention has been given to investigating the potential human health risks associated with the dumpsite. Also, there has been no assessment of the groundwater of the area from the radiological perspective. Hence, the objectives of this research are to conduct hydrogeochemical analysis of groundwater samples, assess the groundwater suitability for irrigation and domestic purposes, and determine the health risks associated with the ingestion of groundwater-borne radon and heavy metal in the vicinity of the Ilokun dumpsite in Ado-Ekiti, Nigeria.

Location, geology, and hydrogeology of the study area

The Ilokun dumpsite is positioned in Ado-Ekiti Local Government Area (LGA) (Figure 1), which serves as the capital city of Ekiti State, Nigeria. This site is situated along the Ado-Iworoko road, precisely at longitudes 5°15′15.18′′ E and 5°15′60.49.61′′ E and latitudes 7°41′09.26′′ N and 7°41′30.69′′ N. The LGA encompasses approximately 40 km2 and was home to around 313,690 residents as of the conclusion of the 2006 census, according to data from the National Population Commission. With a population growth rate of 2.6%, it is estimated that by 2025, the population of Ado-Ekiti LGA will reach approximately 507,297 individuals. The Ilokun dumpsite serves as the primary disposal site for the MSW collected from the residents of Ado-Ekiti. It is managed by the Ekiti State Waste Management Board (EKSWMB), which oversees waste management activities in the state (Akinro & Oni 2022). Being the major dumpsite in Ado-Ekiti, the Ilokun dumpsite usually receives waste on a daily basis. The dumpsite serves as a repository for a diverse range of waste materials (metal scraps, polythene and plastic materials, organic materials, paint, animal wastes, industrial wastes, used batteries, and so on) that have undergone compaction over time (more than two decades), fostering prolonged interplay between the waste and the underlying geological formations within the surrounding community. The dumpsite spans across an area of 24.7 hectares and encompasses a topography varying between 337 and 405 m above mean sea level, as reported by Olagunju et al. (2018). The prevailing climate in the area adheres to a tropical pattern, characterized by a wet season from April to October, succeeded by a dry season spanning November to March (Olaseeni et al. 2018). The yearly average rainfall amounts to roughly 1,300 mm, and the monthly temperatures span between 18 and 33 °C, coupled with considerable humidity levels (Ogungbemi et al. 2013).
Figure 1

Location map of the study area showing the Ilokun dumpsite (Inset: Map of Nigeria) (Modified after Badmus et al. (2022)).

Figure 1

Location map of the study area showing the Ilokun dumpsite (Inset: Map of Nigeria) (Modified after Badmus et al. (2022)).

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Several authors (Bayowa et al. 2014; Jayeoba & Odumade 2015; Ayodele et al. 2016; Bolarinwa et al. 2017; Ajayi et al. 2019) have researched the general geology of Ekiti State (Figure 2). The Ado-Iworoko region in Ekiti State lies above the Precambrian Basement Complex rocks found in Southwestern Nigeria. These underlying rocks show notable disparities in both mineral composition and grain size (Bolarinwa et al. 2017). The study area comprises different rock types, including granite, charnockite, migmatite gneiss, and porphyritic granite. Migmatite is the predominant rock type in the region. Groundwater is found within the fractured and weathered basement layer. The bedrock is located at shallow depths, resulting in a generally thin weathered layer. Consequently, efforts to tap into productive boreholes focus on underlying structural features like shear zones, joints, and fractures as highlighted by Rahaman (1976).
Figure 2

Geologic map of Ekiti State showing the area of study (digitized from Ogungbemi et al. (2022)).

Figure 2

Geologic map of Ekiti State showing the area of study (digitized from Ogungbemi et al. (2022)).

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Data collection/quality assurance for hydrogeochemical analysis

A total of 10 samples (S1–S10) of water were collected spatially from residences within a radius of 500 m from the center of the Ilokun dumpsite during the dry season (December 2023). To comply with the World Health Organization (WHO) and the American Public Health Association (APHA et al. 2017) water sampling guidelines, a bailer was employed to collect the samples, which were carefully transferred into 1-L plastic bottles. The sampled wells vary in depth from 9 to 35 m. Sample locations were recorded with the aid of a highly sensitive handheld GPS device (GARMIN GPS 72H). Prior to the water sample collection, the water pipelines were adequately flushed for several minutes to ensure that representative groundwater samples were collected in accordance with the measurement of water for radon protocol (ANSI/AARST MW-RN 2020). Immediately after collection, the bottles were tightly sealed to prevent aeration and then transported to the laboratory in a cooler with ice for subsequent analysis. The collected samples were stored in a refrigerator at a temperature of 4 °C and subjected to analysis within 72 h from when they were collected. The laboratory employed analytical-grade reagents for the analysis, ensuring reliability and precision by employing blanks and duplicate samples. Every specimen underwent assessment for 18 parameters, encompassing total dissolved solids (TDS), temperature, electrical conductivity (EC), and potential of hydrogen (pH), which were measured on site utilizing a portable device (Jenway 430). The analysis of cations, anions, and heavy metals was carried out on the collected samples at the chemistry laboratory of Afe Babalola University. Flame photometer (FP 902 PG model) was used to analyze calcium, potassium, and sodium concentrations, while the UV spectrophotometer (Jenway PFP7 model) was employed to analyze sulfate concentrations. Bicarbonate and chloride levels were ascertained through the titration technique. Furthermore, an atomic absorption spectrophotometer (AAS) of the Buck Scientific 211 VGP model with a detection limit of 0.001 was utilized to analyze the presence of additional elements, such as nickel, copper, lead, iron, magnesium, manganese, zinc, and chromium.

Hydrochemical analysis

The groundwater dataset was statistically analyzed with the aid of IBM Statistical Package for Social Sciences (IBM SPSS) version 22.0. Subsequently, a comparison was made between the mean, range, and standard deviation of the parameters being investigated and the standards prescribed by the WHO (2011). To characterize the hydrochemical compositions of the groundwater samples, the water chemistry data were analyzed with the aid of a Geochemist Workbench version 17.0 software and the findings were visually depicted on Schoeller and Piper diagrams.

In addition, an evaluation of the suitability of the groundwater for irrigation was conducted employing four distinct water quality indices: total hardness (TH), sodium adsorption ratio (SAR), salinity hazard assessed through EC, and percentage sodium (. The values for SAR and %Na were calculated using Equations (1) and (2), as introduced by Richards (1954) and Wilcox (1955), respectively:
(1)
(2)

Evaluation of health risks resulting from heavy metals

Dermal contact, ingestion, and inhalation are the three principal pathways through which humans can be exposed to heavy metals. This research specifically addresses the health hazards connected to the ingestion of heavy metals, using Equations (3)–(5) as outlined in USEPA (2001):
(3)
where () represents the mean daily ingestion, represents the mean heavy metal concentration, represents the rate of ingestion, EF is the frequency of exposure , ED (years) represents the exposure duration, AT represents average time , and BW (kg) represents average body weight. The input parameters used for are outlined in Table 1.
Table 1

Exposure parameters for average daily intake (Badmus et al. 2022)

ParametersUnitChildrenAdultsReferences
EF  365 365 Ametepey et al. (2018)  
ED years 30 Alidadi et al. (2019)  
AT days   Haque et al. (2018)  
IR  Ojo et al. (2020)  
BW  15 70 Haque et al. (2018)  
ParametersUnitChildrenAdultsReferences
EF  365 365 Ametepey et al. (2018)  
ED years 30 Alidadi et al. (2019)  
AT days   Haque et al. (2018)  
IR  Ojo et al. (2020)  
BW  15 70 Haque et al. (2018)  

EF, exposure frequency; ED, exposure duration; AT, average time; IR, ingestion rate; BW, body weight.

The hazard quotient (HQ) is determined by dividing the average dose intake of a particular metal by its corresponding reference dose (RfD) as presented in Equation (4):
(4)

The reference dose indicates the highest permissible amount of a harmful substance that can be taken in over a lifetime without causing any expected negative health effects (USEPA 2022; Nativio et al. 2024). The values of the RfD considered for this study are outlined in Table 2.

Table 2

RfD values considered for the study

ParametersRfD valuesReferences
Fe 0.70 Yadav et al. (2018)  
Mn 0.14 Yadav et al. (2018)  
Zn 0.30 Yadav et al. (2018)  
Cu 0.04 Yadav et al. (2018)  
Ni 0.02 Liang et al. (2017)  
Cr 0.11 Liang et al. (2017)  
Pb 0.004 USEPA IRIS (2011)  
ParametersRfD valuesReferences
Fe 0.70 Yadav et al. (2018)  
Mn 0.14 Yadav et al. (2018)  
Zn 0.30 Yadav et al. (2018)  
Cu 0.04 Yadav et al. (2018)  
Ni 0.02 Liang et al. (2017)  
Cr 0.11 Liang et al. (2017)  
Pb 0.004 USEPA IRIS (2011)  

RfD, reference dose.

The hazard index (HI) is used to define the potential toxic risk arising from substances that might pose hazards within the same environmental medium (Amirah et al. 2013; Ayantobo et al. 2014; Hashmi et al. 2014; Myers et al. 2023). Equation (5) was employed to evaluate the non-carcinogenic health risks due to the analyzed toxic metals:
(5)

The HI value serves as a screening tool to evaluate the significant health concerns due to heavy metal exposure through groundwater ingestion (Edokpayi et al. 2018; Yadav et al. 2018).

If the HI surpasses a value of one, it indicates the potential for non-carcinogenic health repercussion within the population exposed (Edokpayi et al. 2018).

Radon measurement

For the purpose of radon measurement, 10 samples of groundwater were also collected from the same sample sites described above for hydrochemical water sampling. The water samples were obtained in accordance with the sampling guidelines provided in the American Association of Radon Scientists and Technologists (AARST)/American National Standards Institute (ANSI) document (ANSI/AARST MW-RN 2020). A Durridge Rad7 radon monitor (Serial Number: 03093) was employed for the radon concentration measurement in the samples. The instrument was used because of its versatility. It is capable of determining radon concentration in water within 30 min. For the water samples collection, 10 250 mL sample vials supplied with the RadH2O accessory kits of the Rad7 instrument were used. The water samples were prevented from air contact during the collection process and thereafter transported to the laboratory for analysis. At the laboratory, the 250 mL sample vial was connected to the Rad7 instrument in the setup explicitly described by Ajiboye et al. (2018).

The Rad7 is a solid-state alpha particle detector. It has an in-built air pump connected in a closed air loop with the 250 mL vial containing the water sample. The Rad7 was operated in the WAT250 mode following the protocols explicitly described in the manufacturer's manual (Durridge 2023). The instrument detects and counts radioactive transformations within the sample cell for four cycles of 5 min each. The count rate (at sensitivity of 6.37 cpm/(Bq/L)) as determined by the instrument is used to estimate the concentration of 222Rn due to the alpha particle emissions from its daughters: 218Po and 214Po. A typical spectrum of the instrument is shown in Figure 3. The Capture Software 6.2.1 was used for the extraction and analysis of the radon results. Because radon measurements were not taken at the point of sample collection, it is necessary to correct for radon decay. The corrected radon activity was evaluated using the following expression:
(6)
where A is the measured radon activity and is the decay correction factor given as:
(7)
where is the half-life of the radon in hours and t is the lapse time between collecting the samples and determining the measurements in hours.
Figure 3

A typical spectrum showing count rate of Rad7 radon monitor.

Figure 3

A typical spectrum showing count rate of Rad7 radon monitor.

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Calculation of ingestion and inhalation doses

Humans are exposed to radiation from radon-borne water from both ingestion and inhalation pathways. Radiation doses to the stomach and the respiratory tract due to the ingestion of radon in water and the radon gas released into the indoor environment from the water, respectively, are estimated using Equations (8) and (9) as given in UNSCEAR (2002):
(8)
where is the radiation dose to the stomach and CVA is the annual water consumption taken as 150, 350, and 500 l year−1 for infants, children, and adults, respectively. EDC is the effective dose coefficient taken as 3.5 nSv Bq−1:
(9)
where is the radiation dose to the respiratory tract, the constant is the air–water concentration ratio, F is the indoor occupancy factor taken as 7,000 h/year, is the equilibrium factor for radon indoors, and DCF is the dose conversion factor for radon given as . Despite the 1993 ICRP report setting DCFs for ²²²Rn between 6 and 15 nSv(Bqhm⁻³)⁻¹, UNSCEAR used 9 nSv(Bqhm⁻³)⁻¹ for dose calculations in its 1993 and 2000 reports (ICRP 1993; UNSCEAR 1993, 2002). This value was used because of the limited number of domestic epidemiological radon studies providing precise numerical risk estimates for use in an epidemiological dose conversion convention. Consequently, for the current study, we still find the value of suitable for calculating average effective doses.

Physicochemical parameters and heavy metal concentrations

Table 3 presents an overview of the physicochemical parameters and heavy metal concentrations observed in the studied groundwater. It compares the descriptive statistics of these parameters with the recommended limits set by the WHO (2011). The groundwater pH varied between 6.76 and 7.69, with an average of . The observed mean pH indicates a slightly alkaline nature of the groundwater within the investigated region. The EC values varied between 114.85 and 338.61 μS/cm, with an average of . The TDS varied between with an average value of . Bicarbonate, sulfate, chloride, magnesium, calcium, potassium, and sodium have a mean concentration of , , , , , , and , respectively. All the measured physicochemical parameters of the groundwater were found to be within the acceptable limits set by the WHO for safe drinking water (Table 3).

Table 3

Comparison of statistical summary of groundwater parameter with the World Health Organization standard

SamplepHTDSECHCO3SO42−ClMg2+Ca2+K+Na+FeMnZnCuPbNiCrCd
S-1 7.69 148.50 296.21 7.36 1.35 15.24 2.87 27.20 32.75 8.35 0.37 0.41 0.78 0.48 0.11 0.00 0.76 0.01 
S-2 7.02 116.34 254.58 5.95 2.14 11.62 3.22 17.70 28.50 4.35 0.29 0.62 0.81 0.40 0.08 0.01 1.08 0.01 
S-3 6.85 155.27 310.73 9.21 3.06 22.52 1.96 24.45 41.25 12.55 0.45 0.80 0.52 0.41 0.06 0.01 0.97 0.02 
S-4 6.76 161.59 338.61 8.16 2.97 24.49 2.96 19.50 30.75 9.70 0.50 0.91 0.92 0.58 0.03 0.00 0.85 0.01 
S-5 7.33 93.75 185.25 3.33 0.91 7.27 1.98 13.85 15.60 7.40 0.19 0.29 0.27 0.32 0.05 0.00 0.49 0.00 
S-6 7.43 108.32 216.83 4.17 1.32 10.37 2.27 21.50 18.25 5.80 0.20 0.31 0.31 0.29 0.03 0.00 0.41 0.01 
S-7 7.51 76.56 145.95 3.05 1.06 8.15 6.15 33.35 50.75 17.30 0.62 0.92 0.65 0.70 0.07 0.01 0.85 0.02 
S-8 7.15 88.91 171.58 2.14 2.65 7.49 5.67 26.80 38.50 10.60 0.53 1.08 0.65 0.81 0.09 0.01 1.17 0.02 
S-9 7.60 56.12 114.85 6.84 0.58 9.59 2.32 10.45 8.20 3.65 0.22 0.32 0.21 0.25 0.02 0.00 0.29 0.01 
S-10 7.60 62.56 128.26 5.19 0.72 10.07 3.07 19.55 13.50 5.05 0.31 0.42 0.19 0.17 0.01 0.00 0.42 0.01 
6.50–9.50 1,500 1,200 – 500 250 20 – – 200 0.30 0.50 5.00 0.50 0.01 0.02 0.05 0.005 
Min. 6.76 56.12 114.85 2.14 0.58 7.26 1.96 10.45 8.20 3.65 0.19 0.29 0.19 0.17 0.01 0.00 0.29 0.00 
Max. 7.69 161.59 338.61 9.21 3.06 24.49 6.15 33.35 50.75 17.30 0.62 1.08 0.92 0.81 0.11 0.01 1.17 0.02 
Mean 7.29 106.79 216.28 5.54 1.68 12.68 3.25 21.44 27.81 8.49 0.37 0.61 0.53 0.44 0.05 0.00 0.73 0.01 
SD 0.31 36.21 75.95 2.24 0.90 5.85 1.40 6.42 12.98 4.01 0.14 0.28 0.26 0.19 0.03 0.00 0.29 0.00 
SES – – – – – – – – – – – – 10 
SamplepHTDSECHCO3SO42−ClMg2+Ca2+K+Na+FeMnZnCuPbNiCrCd
S-1 7.69 148.50 296.21 7.36 1.35 15.24 2.87 27.20 32.75 8.35 0.37 0.41 0.78 0.48 0.11 0.00 0.76 0.01 
S-2 7.02 116.34 254.58 5.95 2.14 11.62 3.22 17.70 28.50 4.35 0.29 0.62 0.81 0.40 0.08 0.01 1.08 0.01 
S-3 6.85 155.27 310.73 9.21 3.06 22.52 1.96 24.45 41.25 12.55 0.45 0.80 0.52 0.41 0.06 0.01 0.97 0.02 
S-4 6.76 161.59 338.61 8.16 2.97 24.49 2.96 19.50 30.75 9.70 0.50 0.91 0.92 0.58 0.03 0.00 0.85 0.01 
S-5 7.33 93.75 185.25 3.33 0.91 7.27 1.98 13.85 15.60 7.40 0.19 0.29 0.27 0.32 0.05 0.00 0.49 0.00 
S-6 7.43 108.32 216.83 4.17 1.32 10.37 2.27 21.50 18.25 5.80 0.20 0.31 0.31 0.29 0.03 0.00 0.41 0.01 
S-7 7.51 76.56 145.95 3.05 1.06 8.15 6.15 33.35 50.75 17.30 0.62 0.92 0.65 0.70 0.07 0.01 0.85 0.02 
S-8 7.15 88.91 171.58 2.14 2.65 7.49 5.67 26.80 38.50 10.60 0.53 1.08 0.65 0.81 0.09 0.01 1.17 0.02 
S-9 7.60 56.12 114.85 6.84 0.58 9.59 2.32 10.45 8.20 3.65 0.22 0.32 0.21 0.25 0.02 0.00 0.29 0.01 
S-10 7.60 62.56 128.26 5.19 0.72 10.07 3.07 19.55 13.50 5.05 0.31 0.42 0.19 0.17 0.01 0.00 0.42 0.01 
6.50–9.50 1,500 1,200 – 500 250 20 – – 200 0.30 0.50 5.00 0.50 0.01 0.02 0.05 0.005 
Min. 6.76 56.12 114.85 2.14 0.58 7.26 1.96 10.45 8.20 3.65 0.19 0.29 0.19 0.17 0.01 0.00 0.29 0.00 
Max. 7.69 161.59 338.61 9.21 3.06 24.49 6.15 33.35 50.75 17.30 0.62 1.08 0.92 0.81 0.11 0.01 1.17 0.02 
Mean 7.29 106.79 216.28 5.54 1.68 12.68 3.25 21.44 27.81 8.49 0.37 0.61 0.53 0.44 0.05 0.00 0.73 0.01 
SD 0.31 36.21 75.95 2.24 0.90 5.85 1.40 6.42 12.98 4.01 0.14 0.28 0.26 0.19 0.03 0.00 0.29 0.00 
SES – – – – – – – – – – – – 10 

All concentrations in mg/L except EC (μS/cm) and pH (no unit). S, standard prescribed by WHO guidelines; SD, standard deviation; SES, sample(s) exceeding standard.

Among the various heavy metals studied, 60% of the analyzed samples exceeded the iron concentration limit of 0.30 mg/L set by the WHO. While iron is a vital mineral needed for various bodily functions, excessive intake of iron through water can pose health risks. Heightened concentrations of iron in drinking water have been linked to a higher probability of experiencing gastrointestinal issues, iron overload (hemochromatosis), and teeth discoloration, among other potential concerns. This study supports previous findings on iron concentrations in Nigerian groundwater, as reported by the United Nations International Children's Emergency Fund (UNICEF 2018).

Manganese concentrations varied between 0.29 and 1.08 mg/L. Approximately half of the collected samples have manganese concentrations that exceed the maximum threshold of 0.5 mg/L prescribed by the WHO. Groundwater manganese primarily originates from the natural breakdown of manganese-bearing minerals through the weathering process, while human-activities derived industrial waste, sewage, and dumpsite leachate contribute to anthropogenic manganese presence. Prolonged exposure to excessive manganese levels in adults can lead to neurological impairments characterized by hallucination, symptoms akin to Parkinson's disease, and emotional instability (Badmus et al. 2022). Furthermore, elevated manganese concentrations in children have the potential to hinder learning and adversely affect their intelligence quotients.

Copper is a vital element that plays a crucial role in maintaining human health, but it is required only in small amounts in the human diet. Excessive copper intake can lead to significant toxicological concerns, including symptoms such as nausea, diarrhea, vomiting, and stomach cramps. Approximately 70% of the assayed samples exhibited copper concentrations that aligned with the recommended limit of 0.50 mg/L set by the WHO, while the remaining 30% exceeded this limit.

The analysis of lead contents in the samples under investigation revealed that 90% of the samples surpassed the lead concentration threshold of 0.01 mg/L set by the WHO. The United States Environmental Protection Agency (USEPA 2014) reported that lead is a highly hazardous heavy metal known to cause severe health issues such as high blood pressure and renal problems in adults, and slows physical and mental development in children even in small concentrations.

Chromium is a naturally occurring element, and its presence in groundwater can be influenced by geological factors, as well as human actions. In the area under investigation, chromium concentrations surpassed the WHO's acceptable threshold of 0.05 mg/L in all the samples examined. These levels varied between 0.28 and 1.17 mg/L, with an average value of . As reported by WHO (2004), elevated levels of chromium in the human body can lead to corrosion of the intestinal tract. According to Myers et al. (2023), the detrimental impacts of chromium toxicity on humans encompasses a broad spectrum of adverse outcomes, including stomach cancer, disruption of calcium metabolism, intestinal cancer, and renal disorder.

The levels of zinc and nickel observed within the study area conform to the permissible limits established by the WHO. By contrast, cadmium concentrations in the groundwater samples ranged from 0.00 to 0.02 mg/L, exceeding the WHO recommended limit of 0.005 mg/L in 80% of the samples. Consequently, the studied samples meet the safety thresholds for zinc and nickel. However, Samples S-7 and S-8, taken from wells located farther from the dumpsite, show higher levels of chromium, lead, cadmium, manganese, iron, and copper. These concentrations surpass the WHO recommended limits and exceed those found in most samples closer to the dumpsite.

Although the dumpsite may contribute to the anthropogenic presence of heavy metals in groundwater, a plausible explanation for the elevated concentrations observed in the present study suggest a significant influence of geogenic processes such as weathering of metal-bearing minerals, hydrothermal alteration, metasomatic processes, and deep weathering profiles. These factors can collectively or individually contribute to the mobilization and concentration of these metals in groundwater. Olagunju et al. (2018) previously documented comparable results in an earlier study conducted within the same study area.

Hydrochemical facies

The Piper diagram is a graphical tool used in hydrogeochemistry to represent the chemical composition of water samples. It provides valuable insights into the type of ions and their proportions present in a water sample, helping researchers and environmental scientists to understand the source and nature of contamination. The chemical composition of the groundwater samples in the study area was interpreted by plotting major ions in units of milli-equivalents per liter (meq/L) on the Piper diagram (Figure 4). Anions were positioned on the right triangle, while cations were depicted on the left triangle. In the present study, all samples fall within Zone G, indicating a chloride anion facies type on the right triangle. Bicarbonate and sulfate types were absent on the anion triangle. Among the cations, the dominant facies type was calcium, accounting for approximately 90% of all samples. Therefore, the groundwater chemical composition within the study area is notably distinguished by elevated levels of strong acids and alkaline earths . This observation aligns with the diamond diagram analysis, which revealed that an overwhelming 90% of the samples are clustered in Zone 6, signifying a calcium-chloride water type. This observation indicates that alkaline earths exceed alkalies, and strong acids exceed weak acids in the samples. None of the analyzed samples demonstrated a prevalence of weak acids and alkaline earths (calcium-bicarbonate water classification), alkali elements and strong acids (sodium-chloride water classification), or alkali elements and weak acids (sodium-bicarbonate water classification). In essence, the chemical composition of the groundwater in the study area primarily revealed calcium-chloride water type, suggesting that the water has undergone geochemical processes dominated by the dissolution of calcium and chloride-bearing minerals. This suggests that the chemical composition of the groundwater is strongly influenced by the underlying geology of the basement complex terrain of the study area, with possible minor contributions from anthropogenic sources.
Figure 4

Piper diagram showing hydrochemical facies of the groundwater samples around the Ilokun dumpsite.

Figure 4

Piper diagram showing hydrochemical facies of the groundwater samples around the Ilokun dumpsite.

Close modal

Schoeller diagram

The water (S-1–S-10) collected around the Ilokun dumpsite underwent a comprehensive chemical composition analysis. To visually represent this analysis, a Schoeller (1977) diagram was employed. This diagram (Figure 5) displays the concentrations of anions on the right side, while the concentrations of the cations were illustrated on the left side. The application of the Schoeller plot unveiled discernible and distinctive patterns. Notably, concentrations of calcium were found to surpass those of other cations, underscoring its prevalence. This could be attributed to the weathering of calcium-bearing silicate minerals (like plagioclase feldspar, amphiboles, and pyroxenes) present in the igneous (porphyritic granite) and metamorphic (migmatite) rocks of the terrain. The interaction of groundwater with these minerals, combined with natural geological processes like carbonation, can lead to elevated calcium levels compared with other cations in the studied samples.
Figure 5

Schoeller diagram showing concentrations of the major ions in the studied samples.

Figure 5

Schoeller diagram showing concentrations of the major ions in the studied samples.

Close modal

Also, chloride concentrations exhibited higher levels compared with other anions. The most likely sources of elevated chloride in the study area are fluid inclusions within crystalline rocks, remnants of ancient marine influence, hydrothermal alteration, or deep crustal brines. These sources reflect the complex geological history and processes that have affected the geology of the study area (Basement Complex) over time.

The relative abundance of major ions followed the sequence: for cations and for anions. These findings categorically highlight the prevalence of inorganic salts within the groundwater samples. The Schoeller diagram provides a clear visualization of these trends, shedding light on the dominant chemical constituents in the investigated wells around the dumpsite.

Irrigation water quality assessment

Assessing water quality for irrigation purposes is crucial to safeguard crop health, soil fertility, environmental integrity, and human well-being. By understanding the composition of irrigation water, farmers can adopt appropriate water management strategies, choose suitable crops, and employ irrigation practices that contribute to sustainable agricultural practices and overall ecosystem health. The irrigation indices considered in this study as outlined in Table 4 provide a comprehensive framework for assessing the irrigation viability. The groundwater samples' dissolved salt content is evaluated through EC and categorized according to the salinity hazard classes outlined in Table 4. The results indicate a consistently high water quality level, ranging from good to excellent, for irrigation purposes. Specifically, 60% of the samples are classified as excellent water, while the remaining 40% are categorized as good water. This assessment underscores the appropriateness of the analyzed groundwater samples for effective irrigation practices.

Table 4

Water quality indices for irrigation purposes

IndicesRangeGroundwater class% samples (n = 10)
EC (Wilcox 1955<250 Excellent 60 
 250–750 Good 40 
 750–2,250 Doubtful – 
 >2,250 Unsuitable – 
TH as (Todd 19800–75 Soft 70 
 75–150 Moderately hard 30 
 150–300 Hard – 
 >300 Very hard – 
%Na (Wilcox 1955<20 Excellent 90 
 20–40 Good 10 
 40–60 Permissible – 
 60–80 Doubtful – 
 >80 Unsuitable – 
SAR (Richards 1954<10 Excellent 100 
 10–18 Good – 
 18–26 Doubtful – 
 >26 Unsuitable – 
IndicesRangeGroundwater class% samples (n = 10)
EC (Wilcox 1955<250 Excellent 60 
 250–750 Good 40 
 750–2,250 Doubtful – 
 >2,250 Unsuitable – 
TH as (Todd 19800–75 Soft 70 
 75–150 Moderately hard 30 
 150–300 Hard – 
 >300 Very hard – 
%Na (Wilcox 1955<20 Excellent 90 
 20–40 Good 10 
 40–60 Permissible – 
 60–80 Doubtful – 
 >80 Unsuitable – 
SAR (Richards 1954<10 Excellent 100 
 10–18 Good – 
 18–26 Doubtful – 
 >26 Unsuitable – 

In the context of overall water hardness, the groundwater samples collected within the study area exhibited a spectrum of hardness ranging from . According to the criteria in Table 4, these samples falls within the category of soft to moderately hard water, confirming their appropriateness for irrigation purposes.

The categorization of groundwater based on its sodium content holds significant importance for irrigation due to sodium's potential reactivity with soil, which can impede soil permeability and adversely impact crop growth. The analysis of the groundwater samples reveals %Na values spanning 15.04 to 17.77%, all of which fall within the good to excellent water classification (Table 4). This assures the suitability and safety of the groundwater for use in irrigation.

The assessment of soil sodicity through the SAR offers insight into water-extracted soil properties. Prolonged irrigation with high-SAR groundwater can displace soil magnesium and calcium due to sodium's presence in the water. This displacement disrupts soil aggregate stability, ultimately leading to soil tilth deterioration. The SAR values for this study ranged from 0.24 to 0.72 meq/L indicating a low sodium hazard. These results underscore the excellent suitability of the studied samples for irrigation, according to the SAR guidelines presented in Table 4.

Assessment of health risks associated with heavy metals exposure in adults and children

For this study, we utilized the USEPA models established in 2001 for conducting health risk assessments related to heavy metals. The toxicity evaluations of heavy metals via daily intake consider factors such as water consumption rate, mean metal concentrations, age, and weight. Tables 5 and 6 present the estimated average daily intake (ADI) and the corresponding HQ values for Cd, Cr, Ni, Pb, Cu, Zn, Mn, and Fe, for both adults and children, respectively. These values have been compiled for the purpose of non-carcinogenic health risks assessment.

Table 5

Average daily intake, reference dose, hazard quotient, and hazard index for adults

MetalsMean concentration ADI RfD HQHI
Cd 0.01  0.0005  8.40 
Cr 0.73  0.003   
Ni 0.00  0.02   
Pb 0.05  0.004   
Cu 0.44  0.04   
Zn 0.53  0.30   
Mn 0.61  0.14   
Fe 0.37  0.70   
MetalsMean concentration ADI RfD HQHI
Cd 0.01  0.0005  8.40 
Cr 0.73  0.003   
Ni 0.00  0.02   
Pb 0.05  0.004   
Cu 0.44  0.04   
Zn 0.53  0.30   
Mn 0.61  0.14   
Fe 0.37  0.70   
Table 6

Average daily intake, reference dose, hazard quotient, and hazard index for children

MetalsMean concentration ADI RfD HQHI
Cd 0.01  0.0005  39.12 
Cr 0.73  0.003   
Ni 0.00  0.02   
Pb 0.05  0.004   
Cu 0.44  0.04   
Zn 0.53  0.30   
Mn 0.61  0.14   
Fe 0.37  0.70   
MetalsMean concentration ADI RfD HQHI
Cd 0.01  0.0005  39.12 
Cr 0.73  0.003   
Ni 0.00  0.02   
Pb 0.05  0.004   
Cu 0.44  0.04   
Zn 0.53  0.30   
Mn 0.61  0.14   
Fe 0.37  0.70   

The results reveal that, in the adult population, the HQ values for most heavy metals were consistently below unity, suggesting relatively low risk. However, chromium (Cr) exhibited an elevated HQ value of 6.97. With regard to the children, HQ values of 2.66, 32.43, 1.67, and 1.47 were obtained for Cd, Cr, Pb, and Cu, respectively. The HQ values follow the order of for both age groups. This signifies that Cr presents the most significant non-carcinogenic risk for both the adult and children populations in the study area. This can be attributed to its high environmental mobility as reported by Zhitkovich (2011).

Considering the elevated vulnerability of children to pollutants, heavy metal HQ values above one should not be disregarded (Sudsandee et al. 2017; Kolawole et al. 2023). In this particular scenario, Cd, Cr, Pb, and Cu emerge as the primary factors driving non-carcinogenic health risks. To provide an overall assessment, the HI measures the collective non-carcinogenic risk associated with exposure to all considered heavy metals. The computed HI values (8.40 for adults and 39.12 for children) exceed the unity threshold, indicating a notable heavy metal pollution presence in the designated research area. As a consequence, the ingestion based exposure to these metals is likely to have substantial non-carcinogenic health implications for both adults and children residing in the study area. This emphasizes the need for robust interventions to mitigate heavy metal pollution and safeguard the well-being of the population, especially the children (Olujimi et al. 2015; Edokpayi et al. 2018).

Although dumpsites are sources of contamination of groundwater through effluents that seep into the groundwater system including heavy metals, among the studied samples, S-8 showed the highest concentrations of three out of the four heavy metals (chromium, cadmium, and copper, excluding lead), contributing significantly to the elevated HI values observed in the area. Notably, S-8 location is relatively farther from the center of the dumpsite suggesting that the dumpsite itself may not be the primary source of the increased heavy metal levels in the surrounding groundwater.

Radon concentration distribution in the groundwater

The distribution of 222Rn concentrations in groundwater around the dumpsite is detailed in Table 7. The table revealed a range from 5.4 to 23.1 Bq L−1, showcasing the variability in 222Rn concentrations. The average concentration across all measurements is determined to be of 13.9 Bq L−1. Six of the water samples representing 60% of the water samples exhibit radon concentrations surpassing the recommended safety threshold of 11.1 Bq L−1, as set forth by USEPA (1991). However, all the values are less than the WHO guidance level, which specifies that controls be implemented for all public water sources with radon concentrations above 100 Bq L−1 (WHO 2006).

Table 7

Radon concentration in groundwater of the site

Sample IDAverage radon conc. (Bq L−1)Std dev. (Bq L−1)RDSt (Infant), μSv year−1RDSt (Children), μSv year−1RDSt (Adult), μSv year−1RDRT, μSv year−1
S-1 11.6 3.0 6.1 14.3 20.4 29.3 
S-2 22.7 4.1 11.9 27.8 39.7 57.1 
S-3 13.5 3.2 7.1 16.5 23.6 33.9 
S-4 23.1 3.9 12.1 28.3 40.4 58.2 
S-5 9.1 2.4 4.8 11.2 16.0 23.1 
S-6 16.1 3.2 8.5 19.8 28.3 40.7 
S-7 16.7 3.2 8.7 20.4 29.1 42.0 
S-8 10.9 2.6 5.7 13.3 19.1 27.5 
S-9 5.4 2.0 2.9 6.7 9.5 13.7 
S-10 10.1 2.7 5.3 12.3 17.6 25.4 
Sample IDAverage radon conc. (Bq L−1)Std dev. (Bq L−1)RDSt (Infant), μSv year−1RDSt (Children), μSv year−1RDSt (Adult), μSv year−1RDRT, μSv year−1
S-1 11.6 3.0 6.1 14.3 20.4 29.3 
S-2 22.7 4.1 11.9 27.8 39.7 57.1 
S-3 13.5 3.2 7.1 16.5 23.6 33.9 
S-4 23.1 3.9 12.1 28.3 40.4 58.2 
S-5 9.1 2.4 4.8 11.2 16.0 23.1 
S-6 16.1 3.2 8.5 19.8 28.3 40.7 
S-7 16.7 3.2 8.7 20.4 29.1 42.0 
S-8 10.9 2.6 5.7 13.3 19.1 27.5 
S-9 5.4 2.0 2.9 6.7 9.5 13.7 
S-10 10.1 2.7 5.3 12.3 17.6 25.4 

Assessment of equivalent radiation doses to the stomach and respiratory tract

The estimation of the equivalent radiation dose to the stomach resulting from water ingestion by different age groups is presented in Figure 6. In the figure, the error bars indicate the standard deviations for the equivalent dose values for the age groups. For infants, the equivalent radiation dose ranged between 2.9 and 12.1 μSv year−1 with an average of 7.3 μSv year−1. For children, the range and average were 6.7–28.3 and 17.0 μSv year−1, respectively. However, for adults, the values were 9.5–40.4 and 24.4 μSv year−1, respectively. Furthermore, the evaluated equivalent radiation dose to the respiratory tract revealed a range of 13.7–58.2 μSv year−1 and an average value of 35.1 μSv year−1. All computed radiation dose values remain comfortably below the threshold of 0.1 mSv year−1 specified by the WHO (2022).
Figure 6

Radiation dose to the stomach for different age groups.

Figure 6

Radiation dose to the stomach for different age groups.

Close modal

The range of radon concentrations found in this study aligns with findings in similar geological contexts, as reported by Ajiboye et al. (2018). Similar concentration ranges were also documented by Adagunodo et al. (2023) in their groundwater radon study in Ibadan, Nigeria. These findings suggest that the radon concentrations observed in this study are likely due to background levels in the study area.

Correlation of groundwater radon and heavy metal concentrations

The Pearson's correlation coefficient defines the relationship between pairs of measured parameters. The Pearson's correlation coefficient matrix between radon and heavy metals is presented in Table 8. All tested pairs correlated positively. Radon and zinc conspicuously exhibit high correlation (). However, the correlation of radon with other heavy metals ranges from negligible to moderate correlation (typically 0–0.7).

Table 8

Correlation of radon with heavy metal contents of groundwater

FeMnZnCuPbNiCrCdRn
Fe         
Mn 0.915        
Zn 0.625 0.648       
Cu 0.848 0.865 0.699      
Pb 0.396 0.361 0.632 0.625     
Ni 0.596 0.715 0.403 0.587 0.526    
Cr 0.692 0.828 0.797 0.770 0.709 0.806   
Cd 0.784 0.765 0.318 0.619 0.322 0.748 0.595  
Rn 0.338 0.435 0.731 0.317 0.139 0.300 0.524 0.123 
FeMnZnCuPbNiCrCdRn
Fe         
Mn 0.915        
Zn 0.625 0.648       
Cu 0.848 0.865 0.699      
Pb 0.396 0.361 0.632 0.625     
Ni 0.596 0.715 0.403 0.587 0.526    
Cr 0.692 0.828 0.797 0.770 0.709 0.806   
Cd 0.784 0.765 0.318 0.619 0.322 0.748 0.595  
Rn 0.338 0.435 0.731 0.317 0.139 0.300 0.524 0.123 

Between the heavy metals, iron and manganese clearly exhibit a very high correlation . Chromium shows a strong correlation (typically 0.7–0.9) with all the other heavy metals analyzed, suggesting they may share a common source or similar environmental pathways. In a previous study by Ajiboye et al. (2022) conducted in Southwest Nigerian cities, chromium concentrations in groundwater ranged from 0 to 195 ppb. However, in the current study, chromium levels are notably higher, ranging from 0.29 to 1.08 ppm (290–1,080 ppb). This significant increase, combined with the high correlation values, suggests that the elevated chromium levels may be due to environmental oxidation of chromium-bearing minerals, such as chromite, leading to the formation of hexavalent chromium. The high solubility and mobility of hexavalent chromium may explain its widespread presence and strong correlation with other heavy metals in the environment. Generally, all other pairs of heavy metals exhibit low to moderate positive correlation.

The present research focused on the comprehensive hydrogeochemical analysis of groundwater, with specific emphasis on assessing potential health risks associated with radon and heavy metal ingestion in the proximity of the Ilokun dumpsite. All major ionic concentrations fell within the prescribed limits set by the WHO. The hydrochemical composition demonstrated a predominance of alkaline earths over alkalies, while strong acids surpass weak acids, suggesting calcium-chloride water type as the predominant water type in the study area. The irrigation indices suggest that the studied groundwater is suitable for irrigation purposes. Chromium, cadmium, lead, and copper were found to be present at concentrations exceeding the acceptable limits in the analyzed samples pointing to potential contamination arising largely from geogenic contributions. The HI values of 8.40 and 39.12 were estimated for adults and children, respectively, indicating potential health risks of non-carcinogenic effects primarily driven by chromium, cadmium, lead, and copper. Radon and zinc exhibited high correlation in the groundwater samples. These findings suggest that the heavy metal and radon concentrations observed in the investigated samples are likely due to background levels in the area. The insights from this study may help in the development of strategies for managing and protecting groundwater resources in the study area from radon and heavy metal contamination.

The authors declare that no funds, grants, or other support were received during the preparation of this article.

G.O.B., O.S.O., and Y.A. conceptualized and designed the study. G.O.B., O.S.O., and A.P.J. acquired the data. G.O.B., O.S.O., Y.A., and O.A.A. managed the literature searches. All authors analyzed and interpreted the data. All authors drafted and revised the article. All authors read and approved the final version of the article.

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

The authors declare there is no conflict.

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