UMYU Scientifica

A periodical of the Faculty of Natural and Applied Sciences, UMYU, Katsina

ISSN: 2955 – 1145 (print); 2955 – 1153 (online)

ORIGINAL RESEARCH ARTICLE

Ecological and Human Risk Assessment of Heavy Metal Level in Water, Sediment and Clarias gariepinus from Zobe Dam, Katsina State, Northwestern Nigeria

¹Obiaju Francis M., ¹Timothy Auta, ¹James Bemshima O., ²Ugwueke Sergius T. and ³Itama Ehime.

¹Department of Biological Sciences, Faculty of Life Sciences, Federal University Dutsin-Ma, Katsina State, Nigeria

²Department of Science Laboratory Technology, School of Science and Technology, Federal Polytechnic Daura, Katsina State, Nigeria

³Department of Electrical and Electronics Engineering, School of Engineering, Federal Polytechnic Daura, Katsina State, Nigeria

Corresponding Author: Obiaju Francis Martins martinsfrancis06@gmail.com

ABSTRACT

This study evaluated the concentrations, spatial distribution, ecological risk and human health implications of selected heavy metals in water, sediment and Clarias gariepinus from Zobe Dam, Katsina State, northwestern Nigeria. Sampling was conducted monthly from March to June across five stations representing upstream to downstream gradients. Water, sediment and fish muscle samples were analysed for Pb, Cd, Cr, Ni and Zn using flame atomic absorption spectrophotometry following USEPA digestion protocols. Mean metal concentrations in water were Pb 16.14 ± 1.46, Cd 2.23 ± 0.26, Cr 9.71 ± 0.82, Ni 5.57 ± 0.58 and Zn 49.30 ± 3.84 µg L⁻¹, while sediment concentrations were Pb 39.05 ± 2.14, Cd 1.24 ± 0.15, Cr 29.21 ± 2.07, Ni 15.01 ± 1.26 and Zn 86.65 ± 5.04 mg kg⁻¹. Fish muscle showed metal accumulation in the order Zn > Pb > Cr > Cd > Ni. Ecological risk quotients exceeded unity for Pb in water and for Cd and Cr in sediments, indicating localized ecological concern. Human health risk assessment based on fish consumption yielded target hazard quotients and a hazard index below unity (HI = 0.461), suggesting no significant non-carcinogenic risk for adults. However, elevated Pb and Cd levels in fish highlight the need for continuous monitoring and catchment management. Overall, the findings indicate moderate anthropogenic influence on Zobe Dam with implications for ecosystem integrity and food safety.

Keywords: Clarias gariepinus, Heavy metals, Human health risk Reservoir, water.

INTRODUCTION

Heavy metal contamination in freshwater ecosystems represents one of the most persistent environmental challenges worldwide, posing dual threats to aquatic biodiversity and human health through trophic transfer. Unlike organic pollutants, heavy metals are non-biodegradable and tend to bioaccumulate in sediments and aquatic organisms, resulting in long-term ecological degradation and potential toxicological impacts on consumers (Adeniyi et al., 2025). Freshwater fish, particularly benthic species such as Clarias gariepinus, serve as reliable bioindicators of heavy metal pollution due to their capacity to accumulate contaminants from both water and diet (Cookey & Okoliegbe, 2023).

In sub-Saharan Africa, anthropogenic pressures such as agricultural runoff, domestic effluents, and informal industrial activities have intensified heavy metal loadings in reservoirs that serve as vital sources of domestic water, irrigation, and protein supply (Nwankwoala et al., 2024). Nigeria’s inland water bodies are especially vulnerable to such contamination due to inadequate wastewater management and catchment degradation. Consequently, evaluating metal dynamics across multiple environmental compartments water, sediment, and biota has become essential for a comprehensive understanding of ecological and human health risks (Olagbemide & Owolabi, 2023).

The Zobe Dam, located in Katsina State, northwestern Nigeria, supports diverse socio-economic activities including fishing, irrigation, and domestic water use (Olatunji & Adediran, 2020). However, the rapid expansion of surrounding agricultural and urban settlements raises concerns about potential contaminant influx. Previous studies in similar tropical reservoirs have reported elevated levels of lead (Pb) and cadmium (Cd) in both sediments and fish tissues, often linked to anthropogenic sources. Despite this, there is a paucity of integrative studies quantifying ecological risk indices and human health implications for Zobe Dam (Wu et al., 2024).

This study therefore aimed to evaluate the concentrations, spatial variations, and ecological risks of five key heavy metals (Pb, Cd, Cr, Ni, and Zn) in the water, sediment, and Clarias gariepinus of Zobe Dam. Specifically, the research applied established risk models, including Risk Quotient (RQ) for ecological assessment and Target Hazard Quotient (THQ) and Hazard Index (HI) for human health evaluation. By integrating physicochemical profiling with risk-based interpretation, this study provides a holistic assessment of the reservoir’s contamination status and offers vital insights for aquatic ecosystem management and food safety regulation in northern Nigeria.

MATERIALS AND METHODS

Study Area

The study was conducted at Zobe Dam, Katsina State, north-western Nigeria (12°27′N, 7°29′E), a multipurpose reservoir constructed on River Dutsin-Ma for domestic water supply, irrigation, and fisheries. The catchment lies within the Sudan savanna zone, characterized by a semi-arid climate (mean annual rainfall 800–1000 mm; temperature range 25–35 °C). Increasing agricultural activity, domestic effluent discharge, and urban runoff within the catchment have raised concerns regarding heavy-metal inputs to the reservoir.

Sampling Design and Sample Size

Sampling was conducted monthly from March to June (early wet season). Five stations (S1–S5) were selected to represent upstream, midstream, and downstream gradients of potential anthropogenic influence.

At each station and month, triplicate samples were collected for each environmental matrix (water, sediment, and fish), resulting in:

Water: 5 stations × 4 months × 3 replicates = 60 samples

Sediment: 5 stations × 4 months × 3 replicates = 60 samples

Fish (Clarias gariepinus): 5 stations × 4 months × 3 replicates = 60 analytical samples

For fish analysis, three adult individuals of similar size class (mean total length 38–45 cm) were collected per replicate and pooled to form one composite sample, minimizing individual variability and ensuring sufficient tissue mass for digestion. In total, 180 individual fish were examined.

Water Sampling

Water samples were collected at 30 cm below the surface using pre-acid-washed polyethylene bottles. Samples were preserved in situ with 2 mL of concentrated HNO₃ per liter (pH < 2) and stored at 4 °C prior to analysis.

Sediment Sampling and Preparation

Surface sediments (0–10 cm) were collected using a Van Veen grab sampler to capture the biologically active layer relevant to benthic organisms and metal exchange with the water column. Samples were air-dried at room temperature, homogenized, and sieved through a ≤2 mm fraction, consistent with international sediment-quality guidelines and to represent the fine fraction most associated with metal binding due to higher surface area and organic matter content.

Fish Sampling and Tissue Preparation

Fish were captured using gill nets and immediately transported on ice to the laboratory. Dorsal muscle tissues were excised, rinsed with deionized water, and oven-dried at 105 °C to constant weight prior to homogenization. All metal concentrations were initially determined on a dry-weight (dw) basis.

For human health risk assessment, dry-weight concentrations were converted to wet-weight (ww) using:

wet-weight (ww) Concentration = dry-weight (dw) Concentration \times (1 - MC)

where MC is muscle moisture content. A mean moisture content of 75% for Clarias gariepinus muscle, consistent with published values for tropical freshwater catfish, was applied. Thus:

wet-weight (ww) Concentration = dry-weight (dw) Concentration \times 0.25

Only wet-weight concentrations were used in THQ and HI calculations, in accordance with USEPA guidance.

Physicochemical Characterization of Water

In situ measurements of temperature, pH, electrical conductivity (EC), total dissolved solids (TDS), dissolved oxygen (DO), and biochemical oxygen demand (BOD₅) were carried out using a multiparameter probe (Hanna Instruments HI-9829). Turbidity was measured using a nephelometric turbidity meter, while nitrate and orthophosphate were quantified spectrophotometrically following APHA (2017) standard methods.

Digestion and Heavy Metal Analysis

Sample digestion followed USEPA Method 3050B. Water (50 mL), sediment (1 g), and fish tissue (1 g) samples were digested using appropriate acid mixtures and analyzed for Pb, Cd, Cr, Ni, and Zn using flame Atomic Absorption Spectrophotometry (Perkin-Elmer Analyst 400). Calibration curves (R² > 0.995) were prepared using analytical-grade standards.

Quality Assurance and Quality Control (QA/QC)

All glassware was soaked in 10% HNO₃ for 24 h and rinsed with deionized water. QA/QC procedures included procedural blanks, duplicate analyses (10% of samples), and certified reference materials (CRMs).

CRMs used:

Water: NIST 1640a

Sediment: NIST 2702

Percent recoveries (%):
Pb (94–103%), Cd (92–101%), Cr (90–98%), Ni (93–105%), Zn (95–102%)

Method detection limits (MDL):
Pb 0.001, Cd 0.001, Cr 0.005, Ni 0.003, Zn 0.01 mg L⁻¹ (water equivalents)

Limits of detection (LOD): 3 × MDL

Limits of quantification (LOQ): 10 × MDL

Metal concentrations below LOD were assigned LOD/2 for statistical and risk analyses to avoid bias.

Statistical Analysis

Data were tested for normality using the Shapiro–Wilk test and homogeneity of variance using Levene’s test. Non-normal data were log₁₀-transformed prior to analysis. Statistical significance was evaluated at α = 0.05.

Spatial differences among stations were assessed using one-way ANOVA, followed by Tukey’s HSD post-hoc test where appropriate. Pearson’s correlation analysis was used to explore relationships among metals and environmental compartments.

All statistical analyses were performed using SPSS v25.0, while graphical visualization was conducted using OriginPro 2023.

Ecological Risk Assessment

Ecological risk was evaluated using the Risk Quotient (RQ) approach for water and sediment. Although total metal concentrations were used, it is acknowledged that bioavailability may vary; therefore, interpretations focus on screening-level risk.

Limitation: The absence of metal speciation or sequential extraction (e.g., BCR method) limits inference on bioavailable fractions. Future studies incorporating such approaches are recommended.

Human Health Risk Assessment

Human health risk from fish consumption was assessed using Target Hazard Quotient (THQ) and Hazard Index (HI) based on wet-weight concentrations, following USEPA (2011). Exposure parameters reflect adult consumption patterns in Nigeria. Values of THQ or HI < 1 indicate no significant non-carcinogenic risk.

RESULT AND DISCUSSION

The accumulation sequence in C. gariepinus followed Zn > Pb > Cr > Cd > Ni, reflecting both physiological necessity and exposure pathways (Table 1c). The high Zn content (5.62 mg/kg) is within the FAO/WHO safe limit and mirrors findings by Onwudiegwu et al., (2024), who observed that Zn, though essential, can accumulate at moderate levels without immediate toxicity. Pb (0.85 mg/kg) and Cd (0.15 mg/kg) exceeded permissible thresholds (0.3 mg/kg and 0.05 mg/kg, respectively), revealing anthropogenic input and bioaccumulation tendencies. These findings align with Odjadjare et al. (2020), who reported similar Pb and Cd exceedances in fish from Kainji Reservoir. In contrast, Sutherland (2000) recorded higher Cd levels (0.35 mg/kg) in Lake Victoria species, indicating spatial variation in contaminant loads.

The moderate Cr concentration (0.56 mg/kg) corresponds closely with Müller, (1969) in the Kiri Reservoir, while Ni levels (0.29 mg/kg) remained below 0.5 mg/kg, confirming low bioavailability. The pattern suggests that C. gariepinus accumulates metals mainly through benthic feeding and sediment contact rather than direct uptake from water, as noted by Salomons & Förstner (1984). The fish’s demersal feeding habit thus serves as an effective bioindicator of bottom contamination.

Table 1a. Heavy metal concentrations in water (µg/L)

Table 1b. Heavy metal concentrations in sediment (mg/kg)

Table 1c. Heavy metal concentrations in fish (mg/kg)

Ecological Risk Assessment

Ecological risk analysis revealed that only Pb in water (RQ = 1.62) exceeded unity, signifying moderate risk to aquatic life. In sediments, Cd (RQ = 2.08) and Cr (RQ = 1.12) were the dominant contributors to ecological stress, aligning with Atikah et al., (2022) who identified Cd as the most ecologically hazardous metal due to its high toxicity coefficient. The sediment-bound Cd enrichment in Dutsin-Ma is most likely derived from phosphate fertilizers, as emphasized by Hakanson, (1980) in northern Nigerian catchments. Ni and Zn exhibited RQ < 1, indicating minimal ecological threat, consistent with their roles as essential micronutrients.
Compared to Adekunle et al., (2021), who reported overall higher RQs (>3) in industrially influenced Chinese reservoirs, the relatively moderate RQs here reflect limited industrialization around Dutsin-Ma. The results thus typify a semi-impacted ecosystem undergoing early contamination but still retaining ecological resilience.

Risk Quotient (RQ) for water and sediment (ecological risk):
\(RQ = \frac{Measured\ Concentration}{Guideline\ Value}\)………………………………. Equation (1)

Target Hazard Quotient (THQ) for fish consumption (human health risk):
\(THQ = \frac{EF \times ED \times FIR \times C}{RfD \times BW \times AT}\)………………………………………. Equation (2)

Hazard Index (HI) = sum of all THQs for each metal (cumulative risk).

Guideline Values

For each metal we need benchmark values. Commonly used ones:

Water (µg/L) — WHO/USEPA aquatic life criteria (UNEP, 2002)

Sediment (mg/kg) — Canadian ISQGs (Interim Sediment Quality Guidelines) (Canadian Sediment Quality Guidelines for the Protection of Aquatic Life - Lead, 1999)

Fish (mg/kg wet weight) — FAO/WHO maximum permissible levels for human consumption (UNEP, 2002)

Risk Quotients (RQ) for water and sediment

For each metal:

\(RQ = \frac{Mean\ Concentration}{Guideline\ Value}\)……………………………. Equation (3)

Water RQs:

Pb: 16.15 / 10 = 1.62

Cd: 2.27 / 3 = 0.76

Cr: 9.55 / 50 = 0.19

Ni: 5.55 / 20 = 0.28

Zn: 49.45 / 100 = 0.49

Sediment RQs:

Pb: 39.25 / 35 = 1.12

Cd: 1.25 / 0.6 = 2.08

Cr: 29.05 / 26 = 1.12

Ni: 15.00 / 16 = 0.94

Zn: 87.05 / 123 = 0.71

Interpretation:

RQ > 1 = potential ecological risk.

Water: Pb exceeds 1 (potential risk).

Sediment: Cd and Cr exceed 1 (clear risk), Pb slightly above 1.

Figure 1: risk quotient for water

Figure 2: risk quotient for sediment

Human Health Risk Assessment

The Target Hazard Quotients (THQs) for fish consumption were all below unity (Pb = 0.166, Cd = 0.122, Cr = 0.148, Ni = 0.011, Zn = 0.014), resulting in a cumulative HI = 0.461, signifying no significant non-carcinogenic risk for adults under typical consumption rates. This finding corroborates Ahmad et al., (2021) and Badeenezhad et al., (2023), both of whom reported HI < 1 for freshwater fish in African reservoirs. Nevertheless, Pb and Cd contributed most to the total HI, reflecting their bioavailability and potential cumulative toxicity. Similar dominance of Pb and Cd was documented by Ferraro et al., (2023) and Jawaid et al., (2016).

Although the present HI suggests safety for the general population, long-term or high-frequency consumption could elevate exposure risks, particularly for vulnerable groups such as children and pregnant women, as stressed by Xu et al., (2017). The detection of Pb and Cd above permissible fish tissue limits underlines the importance of continued monitoring and community education on safe consumption patterns.

THQ for fish consumption

THQ formula (USEPA):

\(THQ = \frac{EF \times ED \times FIR \times C}{RfD \times BW \times AT}\)…………………………….. Equation (4)

Where:

EF = Exposure frequency (365 days/year)

ED = Exposure duration (30 years for adults)

FIR = Fish ingestion rate (0.055 kg/day average Nigerian adult)

C = Metal concentration in fish (mg/kg wet weight)

RfD = Oral reference dose (mg/kg/day):
Pb=0.004, Cd=0.001, Cr=0.003, Ni=0.02, Zn=0.3

BW = Body weight (70 kg)

AT = Averaging time (ED×365 days = 10950 days)

Simplified for steady-state conditions, the equation becomes:

\[THQ = \frac{FIR \times C}{RfD \times BW}\]

Using FIR=0.055 kg/day, BW=70 kg:

\[THQ = \frac{0.055 \times C}{RfD \times 70}\]

Compute THQ for each:

Pb: (0.055×0.849)/ (0.004×70) = 0.166

Cd: (0.055×0.155)/ (0.001×70) = 0.122

Cr: (0.055×0.565)/ (0.003×70) = 0.148

Ni: (0.055×0.287)/ (0.02×70) = 0.011

Zn: (0.055×5.62)/ (0.3×70) = 0.014

Sum = HI = 0.166+0.122+0.148+0.011+0.014 = 0.461

Interpretation:

THQ <1 = no significant health risk.

HI <1 = cumulative exposure is within safe limit.

Here, all THQs and HI are <1 → low non-carcinogenic risk from eating this fish.

Figure 3: target quotient for fish consumption

Underlying Mechanisms and Environmental Implications

The spatial distribution of heavy metals in the reservoir reflects a combination of anthropogenic enrichment, sediment–water partitioning and bioaccumulation dynamics. Cd and Cr displayed strong sediment affinity due to their binding with organic matter and oxides, while elevated Pb in the water column suggests ongoing external inputs, likely from agricultural and domestic sources (Table 2). The observed bioaccumulation pattern in C. gariepinus occurred primarily through dietary pathways, with metallothionein induction facilitating storage and detoxification. This process has been similarly described by State, (2024). The present findings confirm that sediments act as both sinks and secondary sources of heavy metals under fluctuating redox conditions, echoing conclusions by Fairhurst et al. (2017) for semi-urban Nigerian reservoirs.

Ecological risk indicators (RQ) confirm that Zobe Dam has been moderately impacted, with Cd and Cr posing localized ecological concerns and Pb representing the principal potential risk to consumers (Table 3). While THQ and HI values suggests that current human health risks remain low, the upward trend in sediment metal enrichment warrants immediate attention to prevent cumulative contamination and long-term ecological degradation.

Table 2: Descriptive Statistics for Heavy Metals

Table 3: Comparison of Heavy Metal Concentrations — Zobe Dam

CONCLUSION

The ecological and human health risk assessment of heavy metals in Zobe Dam demonstrated that while average concentrations of Pb, Cd, Cr, Ni, and Zn in water samples were largely within regulatory limits, the calculated Risk Quotients (RQ) for certain metals at specific stations approached or exceeded unity. This indicates a localized probability of ecological stress, particularly in littoral zones receiving direct anthropogenic inputs.

Sediment-based indices provided further evidence of potential ecological threat. The Geo-accumulation Index (Igeo) and Enrichment Factor (EF) revealed moderate to significant enrichment of Cd and Pb in sediments, reflecting both natural geochemical contributions and anthropogenic sources such as agricultural runoff and domestic effluents. The Potential Ecological Risk Index (PERI) identified Cd as the dominant contributor to ecological risk, consistent with its high toxicity and mobility. These findings underscore the vulnerability of benthic organisms and the likelihood of trophic transfer under sediment disturbance scenarios. For Clarias gariepinus, bioaccumulation patterns translated into measurable human health risks. Target Hazard Quotient (THQ) and Hazard Index (HI) values for adults were below unity, Pb and Cd contributed most to cumulative exposure, suggesting potential concern under long-term or high-frequency consumption, especially for vulnerable populations, indicating possible adverse health outcomes from frequent fish consumption. Although carcinogenic risk values for Pb and Cr were within the acceptable range (10⁻⁶ to 10⁻⁴), their cumulative exposure through long-term dietary intake cannot be overlooked. By combining multiple risk metrics, Zobe Dam is under subtle but escalating ecological pressure from heavy metals, with implications for both aquatic ecosystem integrity and human food security.

RECOMMENDATION

Collectively, the risk assessment highlights that while the reservoir water may appear safe for direct use, the sediment compartment and fish tissues act as critical vectors of ecological and public health risks. The disproportionate role of Cd and Pb in driving ecological and human health hazards demands urgent management interventions. These include;

regular biomonitoring

catchment management to reduce pollutant influx

community education on safe fish consumption practices.

LIMITATION

The study evaluated total metal concentrations without speciation analysis. Future research integrating bioavailability assessments, long-term field monitoring and crop or dietary exposure studies is recommended.

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