Integrated Assessment of Heavy Metal Concentrations and Current Pollution Status: A case study of Nworie River, Southeastern Nigeria

Technical Summary: Integrated Assessment of Heavy Metal Concentrations and Current Pollution Status of Nworie River, Southeastern Nigeria

Problem Statement
The Nworie River, a major tributary of the Otamiri River in Owerri, Imo State, serves as a critical water source for domestic, agricultural, and recreational purposes. However, the river faces significant environmental degradation due to rapid urbanization, population growth, and anthropogenic activities. Key stressors include untreated sewage discharge from institutions (e.g., Federal Medical Centre, Alvan Ikoku Federal College of Education), industrial effluents, and domestic waste. A specific concern highlighted in the study is the environmental impact of recent dredging activities conducted by the Imo State Government and the Niger Delta Development Commission (NDDC). While intended to mitigate eutrophication and sedimentation, the dredging process involves the removal and bank-side disposal of sediments and vegetation, which can lead to the re-suspension of pollutants, acidification, and the release of heavy metals into the water column. Despite previous studies (Mgbudem et al., Okere et al., Umunnakwe et al.) assessing the river's quality, continuous monitoring is required to track dynamic changes in contaminant distribution driven by increasing human activity.

Methodology
The study was conducted from January 2022 to January 2023, covering both dry and rainy seasons. Water samples were collected monthly from three distinct stations along the lower river:

1. Egbeada Bridge (Point 1): Upstream.
2. Umezuruike Hospital Bridge (Point 2): Midstream.
3. Akanchawa Bridge (Point 3): Downstream.

Samples were collected 10 cm below the surface, acidified with 10% nitric acid (HNO₃), preserved at 4°C, and filtered using Whatman No. 1 paper. Heavy metal analysis was performed using Atomic Absorption Spectrophotometry (AAS) following USEPA standard methods, involving acid digestion with concentrated HNO₃ and HCl.

To evaluate pollution status, the study employed single pollution indices:

• Contamination Factor (Cf): Calculated as the ratio of measured metal concentration to background concentration ($Cf = C_{metal} / C_{background}$).
• Ecological Risk Factor (Er): Calculated as the product of the toxic-response factor ($Tr$) and the contamination factor ($Er = Tr \times Cf$).
• Ecological Risk Index (ERI): The sum of individual ecological risk factors to assess combined risk.

Data were analyzed using Analysis of Variance (ANOVA) and Fisher's LSD at a 0.05% probability level, with mean separations conducted using Studentized t-tests via SAS software.

Key Results

• Heavy Metal Concentrations:
  • Lead (Pb) and Iron (Fe): Concentrations exceeded World Health Organization (WHO) and Federal Ministry of Environment (FMEnv) permissible limits. The study explicitly found Lead (0.05 mg/l) and Iron (0.71 mg/l) to be above the standard value, a condition attributed to the littered presence of dredging material found along the river banks. Dry season measurements at Egbeada (Point 1) reached 0.082 mg/l for Lead, and Iron concentrations exceeded limits at Points 2 and 3.
  • Mercury (Hg): The study found that Mercury concentrations exceeded the FMEnv permissible limit (0.001 mg/l) at all sampling points in both seasons. While the abstract summarizes a specific value of 0.001 mg/l, the results section clarifies that measured values, such as 0.021 mg/l at Egbeada during the dry season, were above the standard. Consequently, Mercury recorded the highest Contamination Factor (Cf) and Ecological Risk Factor (Er) during specific periods and locations, such as 0.60 Cf and 3.52 Er at Umezuruike and Egbada respectively during the dry season.
  • Cadmium (Cd), Chromium (Cr), and Arsenic (As): These metals were found to be below the WHO/FMEnv standard. Specifically, Cadmium (0.014 mg/l) and Chromium (0.03 mg/l) were found to be below the standard value.
  • Nickel (Ni): Not detected at upstream and midstream stations during the dry season but detected at the downstream station (Point 3) within permissible limits.
• Pollution Indices:
  • Mercury consistently recorded the highest Contamination Factor (Cf) and Ecological Risk Factor (Er) across all stations and seasons, reflecting its elevated risk despite the abstract's summary of a single low value. For instance, at Umezuruike (Point 2), the dry season Cf for Mercury was 0.60.
  • Chromium exhibited the lowest Cf and Er values.
  • Despite some low Cf values, the study notes that these indices depend heavily on the selected background concentration and should not be interpreted as a complete absence of risk.
• Temporal Trends:
  • Comparison with historical data (2010–2019) indicates a deterioration in water quality. Mercury, Arsenic, and Chromium, which were either undetected or below limits in previous studies, were detected in the current study.
  • Lead concentrations in the current study were higher than those reported in previous studies, with the abstract noting they were found above the standard value.
  • Iron concentrations exceeded the permissible limits, with the abstract citing a value of 0.71 mg/l, which is above the standard.

Key Contributions
The study provides an updated baseline of heavy metal concentrations in the Nworie River, specifically addressing the post-dredging period. It quantifies the spatial and seasonal variations of pollutants, identifying Mercury as having the highest ecological risk factors due to its toxicity and elevated concentrations exceeding permissible limits. The research links specific anthropogenic activities—particularly the improper disposal of dredged materials and the presence of abandoned machinery—to elevated levels of Lead and Iron, which were found to exceed standards. Furthermore, it integrates historical data to demonstrate a trend of declining water quality, attributing this to inadequate waste management and the environmental side effects of river engineering projects.

Significance and Claims
The authors assert that the Nworie River is "negatively adversely polluted," a conclusion supported by physicochemical parameters, heavy metal indices, and phytoplankton distribution. The study claims that the findings highlight the urgent need for regular monitoring and the implementation of measures to reduce contamination rates and illegal waste dumping in the watershed. The authors emphasize that while the river remains a vital resource, its quality is compromised by human activities, necessitating strict enforcement of environmental protection laws to prevent indiscriminate dumping and to restrict polluting activities (such as mechanic workshops and agrochemical use) within the river's catchment area. The paper concludes that without intervention, the ecological functions and public health safety associated with the river will continue to degrade.