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Assessing Anthropogenic Influence and Heavy Metal
Contamination in The Ona River Using Pollution and Risk

Indices
Praise Adenike Alli; Abidat Olayemi Fasasi-Aleshinloye; Solomon Ayobami Adefisoye; Oluwatunmise Peter Abolarin

Department of Civil Engineering, Faculty of Engineering and Technology Lead City University, Ibadan, Nigeria.

DOI: https://doi.org/10.51583/IJLTEMAS.2025.1410000090

Received: 12 October 2025; Accepted: 19 October 2025; Published: 12 November 2025

Abstract: Surface water contamination by heavy metals poses significant ecological and public health challenges, particularly
in rapidly urbanizing regions of developing nations. This study assessed the concentration, distribution, and associated
ecological and health risks of potentially toxic elements in the Ona River, located within the Adeoyo region of Ibadan, Oyo
State, Nigeria. Water samples were collected from three georeferenced sites and analyzed for Fe, Zn, Cu, Cr, As, Cd, Ni, Mn,
and Mg using Atomic Absorption Spectrophotometry (AAS) following APHA (2022) standard protocols. Contamination
indices including the Geo-accumulation Index (Igeo), Enrichment Factor (EF), Contamination Factor (CF), Pollution Load
Index (PLI), and Ecological Risk Index (ERI) were employed to evaluate pollution intensity, while non-carcinogenic and
carcinogenic health risks were computed using the United States Environmental Protection Agency (USEPA) model. Results
revealed that Cr, Cd, As, Pb, and Fe concentrations were within permissible limits of WHO (2022) and Nigerian Standards for
Drinking Water Quality (NSDWQ, 2015). In contrast, Ni and Mn exceeded recommended thresholds, indicating localized
anthropogenic inputs, primarily from industrial and urban effluents. Nickel exhibited the highest CF (2.86–3.86) and EF
(1735.29–3748.24), denoting considerable contamination and extreme enrichment, while PLI values below 1 suggested overall
unpolluted status. The ERI values (15.87–21.78) indicated low ecological risk; however, Ni emerged as the most significant
contributor to potential toxicity. Although the Hazard Index (HI < 1) implied minimal immediate health effects, long-term
exposure may pose latent risks. The study concludes that while the Ona River water remains largely unpolluted, elevated Ni
and Mn levels necessitate continuous monitoring, stricter effluent regulation, and sustainable watershed management to protect
aquatic ecosystems and public health.

Keywords: Heavy metals; Ona River; Water quality; Contamination indices; Ecological risk; Public health

I. Introduction

Access to safe drinking water is essential for human health and well-being; however, natural surface water bodies are
increasingly contaminated by diverse pollutants, posing serious health risks to communities (WHO 2000; Bessa 2024). The
vital importance of water to life is beyond expression, as no human activity can occur without its involvement (Obunwo and
Opurum, 2013). As noted by Obunwo and Opurum (2013), water is the source of life and fulfills various functions that nothing
else can replace. For decades, the deterioration of surface water quality has remained a critical global issue, most notably in
developing nations and in countries with struggling economies (Amoo et al., 2017). The surge in water-borne diseases across
developing nations is largely due to insufficient infrastructure for proper water treatment and distribution, leading to heightened
morbidity and mortality in recent times (Shallom et al., 2011). A significant amount of attention has been devoted to the issue
of water pollution and its resulting effects on human and animal health (Odeyemi et al., 2013; Iroha et al., 2020).

Heavy metals constitute a major environmental challenge and are of serious global concern (Zhu et al., 2020). Rapid
industrialization and urbanization have caused heavy metals to contaminate the atmosphere and it is a problem for human health
(Nour et al., 2019; Liu et al., 2019; Kahal et al., 2020).Heavy metals are naturally occurring trace elements in aquatic
environments, but their concentrations can rise due to both natural processes and human activities such as domestic, industrial,
agricultural, and mining operations. These metals are non-biodegradable and persist in the environment for extended periods.
Heavy metals are a major environmental concern because of their high toxicity and tendency to accumulate in living organisms
and ecosystems (Islam et al., 2015).

Heavy metals constitute a serious environmental hazard to both living organisms and their habitats, owing to their persistence,
stability, non-biodegradability, bioaccumulation, and inherent toxicity. (Khan et al., 2019; Ustaoğlu and Islam 2020; Zhang et
al., 2019) Heavy metals are commonly found throughout various environmental systems and possess properties such as
persistence, carcinogenicity, and the ability to biomagnify and bioconcentrate, all of which contribute to serious environmental
pollution and health risks. Although certain heavy metals are necessary for biological structure and metabolic activity, excessive
levels can have toxic effects on the human body (Anthony et al., 2022; Asare-Donkor et al., 2016; Hussain et al., 2019; Çiner
et al., 2021).

Yahaya et al. (2024) investigated the contamination levels and associated risks of heavy metals in water and fish species from
the Bunza River in Kebbi State, Nigeria. The study focused on assessing heavy metal concentrations in water as well as in
tilapia (Oreochromis niloticus) and catfish (Clarias gariepinus). Results indicated that both water and fish samples contained
heavy metal levels sufficient to pose toxicity risks. Based on these findings, the study recommends the formulation and
enforcement of policies aimed at the effective decontamination of the river to safeguard environmental and public health. Bardi
et al., 2025 also study “Analysis of Heavy Metal Concentration of Surface Water Around Gas-flaring Stations in Selected Areas
of Delta State, Nigeria”. This research investigated the heavy metal concentration in surface water around gas flaring stations

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in selected areas of Delta state. Results demonstrate a progressive accumulation of heavy metals in the environment adjacent
to the water body influenced by gas flaring. Despite this, there is insufficient comprehensive research on the effects of domestic
and industrial activities on the Ona River, limiting the implementation of effective local surface water monitoring and
management strategies. Therefore, this study aimed to assess the heavy metal safety of water samples obtained from the Ona
River in Adeoyo region of Ibadan in Oyo State, Nigeria.

II. Materials and Methods

Description of Study Area

This study concentrated on strategically selected segments of the Ona River, situated within Ibadan, Oyo State, Nigeria. This
river constitutes a vital natural resource, underpinning local livelihoods through its provision of water for domestic, agricultural,
and industrial purposes, while concurrently supporting a diverse array of aquatic and riparian biodiversity. A detailed
georeferenced map in Figure 1 and Figure 2 of the sampling locations was meticulously developed to facilitate a comprehensive
analysis. This map delineates the precise geographic distribution of sampling points along the river continuum and integrates
critical contextual information regarding surrounding land use patterns, including agricultural zones, industrial facilities, and
urban settlements.

Figure 1. Map showing the Ona River (Olabamiji et al., 2023)

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Figure 1. Sample locations

Sample Collection Procedure

Water samples were collected following standard procedures using pre-labeled, acid-washed bottles and disposable gloves to
prevent contamination. The sampler faced upstream to avoid sediment disturbance, and samples were taken at mid-depth (≈30
cm) while avoiding surface films and bottom debris. Bottles for heavy metal analysis were pre-treated and filled carefully to
minimize aeration. Immediately after collection, samples were stored in ice-packed coolers (< 4 °C) and protected from sunlight
to prevent algal or photochemical changes. Analyses for heavy metal parameters were conducted within 24 hours. Field data
sheets were completed for each sampling event, recording sample ID, GPS coordinates, location, and time of collection.

Laboratory Analysis and Procedures

All laboratory procedures adhered to the Standard Methods for the Examination of Water and Wastewater (APHA, AWWA,
WEF, 24th Edition, 2022). Quantitative analysis of heavy metals was conducted to evaluate the concentration and distribution
of toxic trace elements in the river water.

Heavy Metals Analysis

River water samples were digested using a mixed acid solution of concentrated nitric (HNO₃) and perchloric (HClO₄) acids to
decompose organic matter and release metals into solution. Heavy metal concentrations (Pb, Cd, Cr, Cu, Zn, Ni, Mn, Fe, and
As) were quantified by Atomic Absorption Spectrophotometry (AAS). The target metals were selected based on their
environmental persistence and toxicological relevance. Results were evaluated against permissible limits recommended by the
World Health Organization (WHO, 2022) and the Nigerian Standards for Drinking Water Quality (NSDWQ, 2015).

Data Analysis and Indices Computation

Geo-Accumulation Index (Igeo)

The Geo-accumulation Index (I<sub>geo</sub>) was applied to quantify the degree of heavy metal contamination in surface
water relative to pre-industrial background concentrations, as expressed in Equation (2.1).

�� = ��2 (
��

1.5 ��
) 2.1

Where,

Cn = measured concentration of the metal in the sample

Bn = geochemical background concentration of the metal

The constant 1.5 accounts for possible variations in background values due to lithogenic effects.

Interpretation of Igeo Values (Müller, 1969):

Igeo ≤ 0 → Uncontaminated

0 < Igeo < 1 → Uncontaminated to moderately contaminated

1 ≤ Igeo < 2 → Moderately contaminated

2 ≤ Igeo < 3 → Moderately to heavily contaminated

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3 ≤ Igeo < 4 → Heavily contaminated

4 ≤ Igeo < 5 → Heavily to extremely contaminated

Igeo ≥ 5 → Extremely contaminated

The Igeo provides a clear indication of anthropogenic influence. Higher values in the Ona River would suggest that industrial,
agricultural, or domestic discharges are contributing significantly to metal enrichment.

Contamination Factor (CF)

The Contamination Factor (CF) was used to evaluate the level of contamination of individual heavy metals in the water samples.
It was computed using Equation (2.2):

�� =
��
��

2.2

Where:

Ci = Measured concentration of the element in the sample

Cn = Background concentration or reference value (e.g., WHO standard or local baseline)

Interpretation of CF Values

CF < 1 → Low contamination (no significant pollution)

1 ≤ CF < 3 → Moderate contamination

3 ≤ CF < 6 → Considerable contamination

CF ≥ 6 → Very high contamination

Elevated Contamination Factor (CF) values for metals such as lead (Pb), chromium (Cr), and cadmium (Cd) indicate
anthropogenic enrichment and potential ecological or health risks. In this study, CF was employed to identify metals occurring
at levels significantly above permissible limits, reflecting possible inputs from industrial or urban sources.

Enrichment Factor (EF)

The Enrichment Factor (EF) is employed to assess the extent of anthropogenic influence on heavy metal concentrations in
surface water relative to natural crustal inputs. It involves normalizing the concentration of each metal to that of a reference
element—commonly Fe, Al, or Ti—as expressed in Equation (2.3):

�� =
(

��
��

)��

(
��

��
)��

2.3

Where:

Cx = concentration of the target heavy metal in the sample

Cref = concentration of the reference element in the sample

Bx = background concentration of the target heavy metal

Bref = background concentration of the reference element

Interpretation of EF Values

EF ≈ 1 → Metal originates mainly from crustal materials (no enrichment)

1 < EF < 3 → Minor enrichment

3 ≤ EF < 5 → Moderate enrichment

5 ≤ EF < 10 → Significant enrichment

10 ≤ EF < 25 → Very high enrichment

EF > 25 → Extremely high enrichment

An Enrichment Factor (EF) above 5 in the Ona River samples signifies substantial anthropogenic contribution, likely arising
from industrial effluents, vehicular emissions, or agricultural runoff.

Pollution Load Index (PLI):

The Pollution Load Index provides a cumulative indication of the overall level of heavy metal pollution at a site. It is calculated
as the nth root of the product of n contamination factors (Equation. 2.4):

�� = √��1 �� 2 �� 3 �� … . ��
�� 2.4

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Where;

CF1, CF2, CFn = contamination factors for n different metals

Interpretation of PLI Values

PLI = 1 → Baseline level of pollutants (no pollution)

PLI < 1 → No or low pollution

PLI > 1 → Pollution exists and its severity increases with the value

The PLI allows comparison between locations or samples. A PLI greater than 1 in the surface water samples would suggest
cumulative pollution, warranting immediate attention for source identification and remediation.

Human Health Risk Assessment (HHRA)

Human Health Risk Assessment (HHRA) evaluates the potential health risks associated with human exposure to heavy metals
through ingestion, dermal contact, and inhalation. It is divided into non-carcinogenic risk and carcinogenic risk.

Non-Carcinogenic Risk (HQ)

The Average Daily Dose (ADD) for each exposure pathway is calculated using standard USEPA (1989, 2011) models as shown
in Eqn 2.5.

�� =
��

��
2.5

Where;

C = concentration of metal in water/soil (mg/L or mg/kg)

IR = ingestion rate (L/day or mg/day)

EF = exposure frequency (days/year)

ED = exposure duration (years)

BW = body weight (kg)

AT = averaging time (days)

The Hazard Quotient (HQ) is then derived ass shown in Eqn. 2.6

�� =
��
��

2.6

Where;

RfD = Reference dose (mg/kg/day)

The Hazard Index (HI) is the sum of HQs across all metals and pathways;

�� = ∑ �� 2.7

Interpretation of HL Values

HI < 1 → No significant non-carcinogenic health risk

HI ≥ 1 → Potential adverse health effects

2.10.2 Carcinogenic Risk

Carcinogenic risk is assessed using the Cancer Risk (CR) mode

CR = ADD x SF 2.8

Where;

SF = Slope factor (mg/kg/day)-1

The cumulative cancer risk (TCRTCRTCR) is calculated by summing CR across metals and pathways:

�� = ∑ �� 2.9

Interpretation (USEPA guidelines)

1 × 10−6 to 1 × 10−4 → Acceptable risk range

TCR > 1 × 10−4 → Potentially unacceptable cancer risk

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In the context of Ona River, elevated HQ or HI values would suggest potential non-carcinogenic risks (e.g., kidney or
neurological damage), while high CR or TCR values would indicate long-term cancer risks due to chronic exposure to toxic
metals such as arsenic, cadmium, or chromium.

Ecological Risk Index (Er)

The Ecological Risk Index assesses the potential ecological risk posed by individual heavy metals. It combines the
contamination factor with the toxic response factor (Tr) of each metal (Eqn. 2.10):

ERI = CF x Tr 2.10

Where:

Tr = Toxic response factor, which varies by metal

(e.g., Cd = 30, As = 10, Pb = 5, Cu = 5, Cr = 2, Zn = 1, Ni = 5)

Interpretation of ERI Values

ERI < 40 → Low potential ecological risk

40 ≤ ERI < 80 → Moderate risk

80 ≤ ERI < 160 → Considerable risk

160 ≤ ERI < 320 → High risk

ERI ≥ 320 → Very high risk

Calculating the Ecological Risk Index (ERI) for each metal identifies pollutants that not only occur at elevated concentrations
but also exert significant ecological impact based on their toxicity coefficients. For instance, cadmium (Cd) and arsenic (As)
may exhibit high ERI values even at low concentrations due to their pronounced toxicity.

III. Results and Discussion

Tests and Results

This chapter delineates the findings of heavy metal assessments of water samples obtained from the Ona River during wet
seasons, across three sampling stations situated between Adeoyo Hospital and Zeatech. Nine samples (three from each of three
locations) were analysed for selected potentially toxic elements (PTEs), including Fe, Zn, Cu, Cr, As, Cd, Ni, and Pb. The
results are analysed concerning national and international drinking water quality standards, specifically those established by
the World Health Organisation (WHO) and the Nigerian Standard for Drinking Water Quality (NSDWQ), to evaluate
contamination levels, ecological risks, and public health consequences. The analyses were additionally evaluated using
pollution indices including the Geo-accumulation Index (Igeo), Enrichment Factor (EF), Contamination Factor (CF), and
Pollution Load Index (PLI). Health risk assessments for both non-carcinogenic and carcinogenic risks were performed to
determine the possible public health implications of heavy metal exposure. Results are displayed in tables accompanied by a
comprehensive discussion of national and international standards.

Heavy Metals and Potentially Toxic Elements (PTEs)

Chromium (Cr)

Chromium concentrations were below detection limits at Locations 1 and 2 but measured 0.01 ± 0.00 mg/L at Location 3 as
shown in Table 1, remaining within the WHO and NSDWQ guideline of 0.05 mg/L and the EPA limit of 0.1 mg/L. Although
this level poses no immediate health risk, its isolated detection suggests localized contamination, likely from industrial
effluents, waste disposal, or material corrosion. Chromium’s occurrence is of concern because hexavalent chromium (Cr⁶⁺) is
highly toxic and carcinogenic, associated with oxidative stress, renal, hepatic, and cardiovascular dysfunction (Sazakli et al.,
2024; Pan et al., 2024). Ongoing surveillance and effluent regulation are therefore essential to prevent possible accumulation
and future exceedance of safety thresholds

Table 1: Average Concentration of Heavy Metals in Water Samples Collected at Asese/Mowe Industrial Area

S/N Loc 1 Loc 2 Loc 3 WHO
Guideline

Nigerian Standard EPA Maximum
Contaminant
Level

Chromium BDL BDL 0.01 + 0.00 0.05 0.05 0.1

Cadmium BDL BDL BDL 0.005 0.003 0.005

Arsenic BDL BDL BDL 0.05 0.01 0.01

Magnesium 5.56 + 0.99 8.47 + 0.55 9.93 + 0.24

Mn 0.05 + 0.04 0.10 + 0.01 0.24 + 0.05 0.4 0.4 0.4

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Zinc 0.13 + 0.03 0.14 + 0.03 0.12 + 0.02 5 3 5

Iron 0.04 + 0.01 0.04 +0.01 0.05 + 0.00 0.3 0.3 0.3

Copper 0.01 + 0.00 0.02 + 0.00 0.03 + 0.00 1.0 1 1.3

Nickel 0.27 + 0.15 0.20 + 0.00 0.27 + 0.06 0.07 0.02 0.07

Lead BDL BDL BDL 0.01 0.01 0.01

Cadmium (Cd) and Arsenic (As)

Cadmium was below detection limits (BDL) in all sampling locations (Table 1). This is encouraging, as cadmium is highly
toxic, and even trace exposure has been associated with renal impairment, bone demineralization, and carcinogenic effects
(WHO, 2023; Pan et al., 2024). The recorded values are well below the WHO and EPA guideline of 0.005 mg/L and the more
stringent NSDWQ limit of 0.003 mg/L. The absence of detectable cadmium indicates that industrial effluents from sources
such as battery manufacturing, electroplating, and pigment production are unlikely influencing the study area, suggesting
minimal anthropogenic input.

Arsenic concentrations were below detection limits across all sampling locations (Table 1). This finding is noteworthy, as
arsenic is a highly toxic metalloid associated with skin lesions, carcinogenesis, cardiovascular disorders, and neurotoxicity
following long-term exposure (WHO, 2022; Demissie et al., 2024). The WHO guideline value for arsenic in drinking water is
0.05 mg/L, while the NSDWQ and EPA specify more stringent limits of 0.01 mg/L, reflecting updated health risk assessments.
The absence of detectable arsenic suggests that surface and groundwater within the Ona River catchment are presently
unaffected by geogenic inputs or anthropogenic sources such as pesticide residues or industrial effluents.

Magnesium (Mg)

Magnesium concentrations ranged from 5.56 ± 0.99 mg/L at Location 1 to 9.93 ± 0.24 mg/L at Location 3, all within acceptable
limits for drinking water (Table 1). As the WHO, NSDWQ, and EPA recognize magnesium as an essential mineral rather than
a contaminant, the observed levels indicate good water quality. The moderate concentrations suggest minimal anthropogenic
influence, with magnesium likely derived from natural geogenic sources such as the dissolution of magnesium-rich rocks and
soil minerals.

Manganese (Mn)

Manganese (Mn) concentrations varied notably across the sampling sites, ranging from 0.05 ± 0.04 mg/L at Location 1 to 0.24
± 0.05 mg/L at Location 3 (Table 1). While the WHO and NSDWQ recommend a limit of 0.2 mg/L and the EPA sets a
secondary standard of 0.05 mg/L, levels at Locations 2 and 3 exceeded the EPA guideline, with Location 3 also surpassing
WHO/NSDWQ limits. The elevated concentration at Location 3 likely reflects stronger geogenic inputs or localized
anthropogenic influence from industrial activities in the Ona River along Adeoyo area. Manganese above recommended limits
may cause water discoloration, metallic taste, and plumbing stains, and prolonged exposure has been linked to neurological
effects, particularly in children. The increasing trend from Locations 1 to 3 highlights the need for regular monitoring and
potential treatment interventions to ensure safe water quality.

Zinc (Zn)

Zinc (Zn) concentrations were relatively uniform across the three sites, ranging from 0.12 ± 0.02 to 0.14 ± 0.03 mg/L (Table
1), all well below permissible limits of 5 mg/L (WHO, EPA) and 3 mg/L (NSDWQ). These low levels indicate minimal zinc
contamination and no associated health or aesthetic risks. As an essential trace element, zinc supports immune and enzymatic
functions but can cause metallic taste and gastrointestinal irritation at high concentrations. Relatively uniform distribution of
zinc across all sites indicates a predominantly geogenic origin, likely resulting from the natural dissolution of zinc-bearing
minerals rather than anthropogenic activities. Overall, zinc exerts minimal influence on the Water Quality Index (WQI),
affirming that the water remains safe concerning this parameter (WHO, 2023; Rahman et al., 2024).

Nickel (Ni)

Nickel (Ni) concentrations ranged from 0.20 ± 0.00 to 0.27 ± 0.15 mg/L across the three sites, surpassing all permissible
limits—0.07 mg/L (WHO, EPA) and 0.02 mg/L (NSDWQ). This identifies nickel as a major contaminant in the Ona River
within Adeoyo. The consistently high levels indicate dominant anthropogenic inputs, likely from industrial effluents, metal
processing, or corrosion of steel infrastructure, rather than natural sources. Elevated nickel poses serious health risks, including
dermatitis, organ toxicity, and carcinogenic effects. Unlike zinc and magnesium, which remained within safe limits, nickel
contamination represents a critical water quality issue requiring urgent remediation and stronger regulatory monitoring (WHO,
2023; Rahman et al., 2024). Nickel levels present a major water quality concern in the study area, unlike zinc and magnesium,
which remained within safe limits. The elevated concentrations indicate that industrial effluents are the primary source of
contamination, emphasizing the need for continuous monitoring and effective regulatory intervention (WHO, 2023; Rahman
et al., 2024).

Lead (Pb)

Lead (Pb) was below detectable limits (BDL) across all sampling locations, indicating an absence of contamination by this
highly toxic metal. Given the permissible limit of 0.01 mg/L set by WHO, NSDWQ, and EPA (Table 1), this result reflects

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good water quality with respect to lead. The non-detection suggests limited industrial activity involving lead, effective waste
management, or geochemical conditions that restrict its mobility. This is notable since chronic lead exposure is linked to
neurological and developmental disorders in children and cardiovascular and renal effects in adults. Nonetheless, as other
metals such as nickel and manganese exceeded safe limits, continued monitoring remains essential to prevent future lead
contamination and ensure long-term water safety (WHO, 2023; Rahman et al., 2024).

Iron

Iron (Fe) concentrations ranged from 0.04 ± 0.01 to 0.05 ± 0.00 mg/L (Table 1) across the three locations, remaining well below
the 0.3 mg/L permissible limit set by WHO, NSDWQ, and EPA standards. These consistently low values indicate that iron is
not a pollutant of concern in the study area and is primarily of geogenic origin, derived from the natural dissolution of iron-
bearing minerals rather than anthropogenic inputs. The uniform distribution of Fe suggests stable hydrogeochemical conditions
with minimal external influence. Moreover, enrichment factor (EF) analysis classified iron as minimally enriched (EF = 1–2.6),
reinforcing its role as a natural reference element. From a health perspective, the concentrations pose no risk and do not affect
the aesthetic quality of the water, confirming iron as environmentally benign within the River Ona system (WHO, 2022; Adedeji
et al., 2023).

Copper

Copper (Cu) concentrations ranged from 0.01 ± 0.00 to 0.03 ± 0.00 mg/L across the three locations, remaining well below
permissible limits of 1.0 mg/L (WHO, NSDWQ) and 1.3 mg/L (EPA) (Table 1). These values indicate minimal copper
contamination and no immediate health risk, as copper is essential in trace amounts but toxic at higher levels. The slight
downstream increase suggests minor anthropogenic influence, possibly from plumbing corrosion or domestic effluents.
Although dissolved copper levels are low, enrichment factor (EF) analysis revealed extreme enrichment (EF > 400) in
sediments, implying potential long-term accumulation and remobilization risks under changing redox conditions. Overall,
copper levels confirm good water quality but warrant continued monitoring to mitigate future sediment-related impacts (WHO,
2022; Adeyemi et al., 2024).

Heavy Metal Pollution Indices

Geo-Accumulation Index (Igeo)

he Geo-Accumulation Index (Igeo) serves as an effective measure for evaluating heavy metal contamination in sediments, soils,
or water by comparing observed concentrations with natural background values. As shown in Table 2, most metals in the Ona
River (Adeoyo region) exhibited negative Igeo values, indicating an unpolluted condition.

Chromium (Cr), cadmium (Cd), arsenic (As), and lead (Pb) were either below detection limits or highly negative (e.g., Cr at –
13.72 in Location 3), signifying no anthropogenic enrichment or contamination—consistent with earlier reports showing these
metals at trace or non-detectable levels (Rahman et al., 2024). Manganese (Mn) exhibited Igeo values from –14.64 at Location
1 to –12.38 at Location 3, indicating a slight increase but still within the unpolluted range. Although Mn concentrations
marginally exceeded permissible limits at Location 3, the Igeo results suggest this enrichment is geogenic rather than
anthropogenic. Magnesium (Mg) also showed consistently negative Igeo values (–11.98 to –11.15), confirming its natural
geochemical origin and indicating no pollution influence (Rahman et al., 2024).

Table 2: Geo-Accumulation Index (Igeo)

Metal Loc 1 Loc 2 Loc 3

Cr BDL BDL -13.7207

Cd BDL BDL BDL

As BDL BDL BDL

Mn -14.6382 -13.6382 -12.37518

Mg -11.9826 -11.3753 -11.14584

Zn -10.0982 -9.99132 -10.21371

Ni -8.56139 -8.99435 -8.561394

Fe -20.7553 -20.7553 -20.43339

Cu -13.7207 -11.7207 -11.13571

Pb BDL BDL BDL

Zinc (Zn) and nickel (Ni) showed negative Igeo values (–10.10 to –9.99 and –8.56 to –8.99, respectively), indicating minimal
accumulation relative to natural background levels. Although Ni concentrations slightly exceeded drinking water standards, the
Igeo values imply limited enrichment compared to global crustal norms, suggesting localized rather than severe contamination
(Loska and Wiechuła, 2003; Singh et al., 2022).

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Iron (Fe) showed extremely low Igeo values (–20.76 to –20.43), confirming it’s purely geogenic origin with no anthropogenic
input. Copper (Cu) also exhibited strongly negative Igeo values (–13.72 to –11.14) (Table 2), with a slight downstream increase
at Location 3 but still within the “uncontaminated” range. Overall, the negative Igeo values for all analyzed metals indicate that
the Ona River is dominated by natural geochemical processes rather than human-induced pollution. This supports the view that,
although certain elements like Ni and Mn exceed drinking water limits, the low Igeo values highlight limited sediment
enrichment and emphasize the importance of integrating both health-based standards and geochemical baselines in water quality
assessment (Islam et al., 2023).

Enrichment Factor (EF)

The Enrichment Factor (EF) serves as an effective geochemical index for distinguishing natural from anthropogenic metal
sources. EF values close to 1 indicate geogenic origin, while those exceeding 10 reflect significant human influence. As
presented in Table 3, EF values varied across metals and sites in the Ona River in along Adeoyo region, revealing distinct
contamination trends. Chromium (Cr) exhibited EF values below detection at Locations 1 and 2 but reached 65.56 at Location
3, indicating strong anthropogenic enrichment likely linked to industrial effluents from metal plating, leather tanning, or
chemical processing activities.

Table 3: Enrichment Factor (EF)

Metal Loc 1 Loc 2 Loc 3

Cr BDL BDL 65.55556

Cd BDL BDL BDL

As BDL BDL BDL

Mn 55.52941 69.41176 166.5882

Mg 349.9093 333.1533 390.58

Zn 1291.789 869.4737 745.2632

Ni 3748.235 1735.294 2342.647

Fe 0.8 0.5 0.625

Cu 209.7778 262.2222 393.3333

Pb BDL BDL BDL

Zinc (Zn) displayed very high EF values (745.26–1291.79), indicating substantial anthropogenic enrichment likely from
galvanization, battery disposal, metal smelting, and industrial effluents typical of industrial zones. Nickel (Ni) showed extreme
EF values (1735.29–3748.24), far exceeding contamination thresholds and reflecting severe anthropogenic input from metal
processing, alloy production, or industrial wastewater consistent with its previously elevated concentrations above drinking
water limits, confirming Ni as the principal pollutant of concern. Cadmium (Cd), arsenic (As), and lead (Pb) were below
detection limits, suggesting no measurable enrichment, while iron (Fe) exhibited low EF values (0.5–0.8), indicating a geogenic
origin suitable for normalization. In contrast, copper (Cu) recorded EF values between 209.78 and 393.33, signifying extreme
enrichment likely linked to corrosion, industrial emissions, and agrochemical runoff.

The Enrichment Factor (EF) results reveal that while elements like magnesium (Mg) are predominantly geogenic, metals such
as nickel (Ni), zinc (Zn), manganese (Mn), and chromium (Cr) exhibit strong anthropogenic enrichment, with Ni emerging as
the principal contaminant. These findings underscore the substantial impact of industrial activities on the water quality of Ona
River along Adeoyo region and highlight the urgent need for regulatory monitoring and remediation to mitigate associated
human and ecological health risks.

Contamination Factor

The Contamination Factor (CF) values for the six analyzed metals reveal varying pollution contributions. Nickel (Ni) recorded
the highest CF values (2.86–3.86) as in Table 4, indicating considerable contamination, as CF > 3 denotes significant
enrichment. This identifies Ni as the major pollutant of concern, likely derived from industrial activities, metal plating, or alloy
corrosion. In contrast, magnesium (Mg) (0.56–0.99), manganese (Mn) (0.25–1.20), zinc (Zn) (0.024–0.028), iron (Fe) (0.13–
0.17), and copper (Cu) (0.008–0.023) all showed CF < 1, reflecting minimal contamination from natural geogenic sources.
Overall, while most metals remain within safe levels, Ni exhibits clear anthropogenic enrichment that warrants monitoring
(Zhang et al., 2024).

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Pollution Index

The Pollution Load Index (PLI) values for the three sampling sites 0.1558, 0.2028, and 0.2741 are all below 1(Table 4),
indicating an overall unpolluted status. Although nickel shows a relatively high CF, the low PLI values suggest minimal
cumulative metal contamination. The gradual rise in PLI from Location 1 to Location 3 points to a localized increase in
pollution, likely linked to industrial or urban runoff influences downstream (Zhang et al., 2024; Ali et al., 2023).

Table 4: Contamination Factor (CF), Pollution Load Index (PLI), and Ecological Risk Index (ERI) of Heavy Metals in
the Water Samples

Sample Contamination Factor (CF) PLI ERI

Magnesium Manganese Zinc Iron Copper Nickel

Loc 1 0.556 0.25 0.026 0.133333 0.007692 3.857143 0.155791 20.28951

Loc 2 0.847 0.5 0.028 0.133333 0.015385 2.857143 0.202766 15.87097

Loc 3 0.993 1.2 0.024 0.166667 0.023077 3.857143 0.274119 21.78477

Ecological Risk Index

The Ecological Risk Index (ERI), which integrates contamination levels with metal-specific toxic-response factors, provides a
measure of potential ecological harm. ERI values of 20.29, 15.87, and 21.78 for Locations 1, 2, and 3, respectively, all fall
below 40, indicating low ecological risk. This suggests that overall heavy metal levels pose minimal threat to aquatic and soil
ecosystems. However, slightly higher ERI values at Locations 1 and 3 align with elevated nickel concentrations, identifying
localized risk zones. Although the area remains largely safe, continued monitoring particularly for Ni is advisable (Chen et al.,
2023; Zhang et al., 2024).

Human Health Risk Assessment (HHRA)

Table 5: Carcinogenic Risk Assessment

Sample Carcinogenic Risk Assessment

Chromium Arsenic Nickel

ADD 0 0 0.0077143

Loc 1 SF 0.27 32.0 0.91

CR 0 0 0.00702000 ∑ = 0.00702000

Loc 2 ADD 0 0 0.0057143

SF 0.27 32.0 0.91

CR 0 0 0.00520000 ∑ = 0.00520000

Loc 3 ADD 0.0002857 0 0.0077143

SF 0.27 32.0 0.91

CR 0.0000771 0 0.00702000 ∑ = 0.0070971

Carcinogenic Risk

The carcinogenic risk assessment estimates the lifetime probability of cancer from continuous exposure to metals in water,
using the USEPA (1989, 2011) model. CR is calculated as the product of the Average Daily Dose (ADD) and the metal-specific
Slope Factor (SF), which reflects the incremental cancer probability per unit dose. According to USEPA guidelines, acceptable
CR values range from 1 × 10⁻⁶ to 1 × 10⁻⁴; values above this threshold indicate potential public health concerns requiring
intervention.

At Location 1, nickel was the only detected carcinogenic metal, with a CR of 0.00702 (ΣCR = 0.00702; Table 5), exceeding
the USEPA threshold and indicating a high potential cancer risk from long-term exposure. The absence of chromium and arsenic
suggests that nickel, likely originating from industrial effluents or corroded pipelines, is the primary driver of carcinogenicity
at this site.

At Location 2, nickel was the sole contributor to carcinogenic risk, with a total CR of 0.0052 (Table 5), exceeding the acceptable
threshold. Although slightly lower than Location 1, this value indicates potential lifetime cancer risks from prolonged water
consumption, including respiratory and gastrointestinal effects. The reduced risk corresponds to lower nickel concentrations
(0.0057 mg/L) compared to Location 1.

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At Location 3, chromium and nickel contributed to the total carcinogenic risk (ΣCR = 0.0070971; Table 5). Chromium showed
a CR of 7.71 × 10⁻⁵, within the upper acceptable threshold (10⁻⁴), indicating a low-to-moderate risk, whereas nickel exhibited
a substantially higher CR of 0.00702, surpassing safety limits and identifying it as the primary carcinogenic agent. Nickel
contamination consistently elevated total CR values across all sampling sites (0.0052–0.0071), exceeding the USEPA’s
permissible level and suggesting potential lifetime cancer risk from long-term water consumption. Chromium contributed
minimally, while arsenic remained below detection limits. Nonetheless, the high slope factor of arsenic (32.0) indicates that
even trace concentrations could pose significant carcinogenic hazards in future assessments. Overall, the dominance of nickel
underscores industrial and anthropogenic influences as key drivers of carcinogenic risk within the study area.

Table 6: Non-Carcinogenic Risk Assessment

At Location 3, the total Hazard Index (HI) was 4.11 (Table 6), indicating the highest non-carcinogenic health risk among all
sampling points. Magnesium (HQ = 3.55) and nickel (HQ = 0.39) were the dominant contributors, while manganese and
chromium showed minimal influence. The consistent dominance of magnesium and nickel across sites identifies them as the
key pollutants controlling health risk patterns. All sites exhibited HI values above 1.0, suggesting potential adverse effects from
prolonged ingestion. Elevated magnesium concentrations likely arise from geogenic processes, such as mafic rock weathering,
with additional input from industrial effluents, whereas nickel enrichment reflects industrial discharge and corrosion activities.
The higher HI at Location 3 implies greater contamination or exposure duration, designating it as a critical zone for intervention.
Overall, these findings underscore the need for sustained groundwater monitoring and targeted treatment measures to mitigate
cumulative non-carcinogenic health risks.

Sampl
e

Chromiu
m

Arseni
c

Manganes
e

Magnesiu
m

Zinc Nickle Iron Copper

RFD 0.003 0.0003 0.14 0.08 0.3 0.02 0.30 0.04

IR (L/day) 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

EF (days/year) 365 365 365 365 365 365 365 365

ED (adult) 30 years 30
years

30 years 30 years 30 years 30 years 30 years 30 years

BW 70 kg 70 kg 70 kg 70 kg 70 kg 70 kg 70 kg 70 kg

AT (ED x 365)
(days)

10,950 10,950 10,950 10,950 10,950 10,950 10,950 10,950

Loc 1 C 0 0 0.05 5.56 0.13 0.27 0.04 0.01

AD
D

0 0 0.001429 0.158857 0.00371
4

0.00771
4

0.00114
3

0.00028
6

HQ 0 0 0.010204 1.985714 0.01238
1

0.38571
4

0.00381 0.00714
3

2.40496
6

Loc 2 C 0 0 0.10 8.47 0.14 0.20 0.04 0.02

AD
D

0 0 0.002857 0.242 0.004 0.00571
4

0.00114
3

0.00057
1

HQ 0 0 0.020408 3.025 0.01333
3

0.28571
4

0.00381 0.01428
6

3.36255
1

Loc 3 C 0.01 0 0.24 9.93 0.10 0.27 0.05 0.03

AD
D

0.000286 0 0.006857 0.283714 0.00285
7

0.00771
4

0.00142
9

0.00085
7

HQ 0.095238 0 0.04898 3.546429 0.00952
4

0.38571
4

0.00476
2

0.02142
9

4.11207
5

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Non-carcinogenic risk assessment indicates that long-term consumption of water from the study sites may threaten vulnerable
populations due to elevated magnesium and nickel levels, potentially causing metabolic disorders, kidney stress, and other
chronic health effects in the absence of adequate treatment or pollution control.

Conclusion

The assessment of heavy metals in Ona River water along the Adeoyo–Zeatech stretch indicates generally good water quality
for most elements, with magnesium, iron, copper, zinc, cadmium, arsenic, and lead remaining within permissible limits.
However, nickel and manganese exceed national and international drinking water standards at several locations, highlighting
localized contamination likely from industrial effluents. Pollution indices (Igeo, EF, CF, PLI) confirm that while most metals
are geogenic, nickel exhibits strong anthropogenic enrichment, making it the principal contaminant. Ecological risk remains
low overall, but human health risk assessment reveals that nickel poses significant carcinogenic potential, exceeding USEPA
thresholds. Continuous monitoring, regulatory enforcement, and targeted remediation are recommended to mitigate nickel-
related public health risks and maintain the river’s water quality.

Recommendations

Implement regular monitoring of heavy metals, especially nickel and manganese, along the Ona River. Enforce stricter
regulation of industrial discharges, and promote remediation measures in hotspots. Educate local communities on potential
health risks and restrict direct use of contaminated water for drinking until quality improves.

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