Bioaccumulation of heavy metals in Oreochromis niloticus from Ngamo Dam, Zimbabwe

Bioaccumulation of heavy metals

Abstract

Background: Environmental pollution from heavy metals poses a global public health concern, especially in developing countries with intensive mining and poor waste management. In Zimbabwe, freshwater bodies such as Ngamo Dam are at risk of contamination, affecting aquatic organisms and human consumers. This study aimed to investigate the levels of cadmium (Cd), copper (Cu), lead (Pb), manganese (Mn), and zinc (Zn) in Oreochromis niloticus (O. niloticus) and water samples from Ngamo Dam to assess their safety for human consumption.

Methods: Water and fish samples were collected from three sites in Ngamo Dam between January and February 2024. Fish gills and muscles were prepared using wet acid digestion, while water samples were digested with nitric acid. Metal concentrations were analyzed using Atomic Absorption Spectrophotometry (AAS).  Statistical analysis was performed using one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test to determine significant differences among sampling sites (p < 0.05).

Results: Water samples showed heavy metal concentrations exceeding World Health Organization (WHO) and Standards Association of Zimbabwe (SAZ) permissible limits: Pb (5.21–10.30 mg/L), Cu (5.67–8.56 mg/L), Mn (2.96–4.34 mg/L), Zn (2.15–3.67 mg/L), and Cd (1.34–3.23 mg/L). In O. niloticus gills, the order of accumulation was Cu > Zn > Pb > Cd > Mn, with Cu reaching 11.44 mg/kg. Muscles showed lower but significant accumulation, with Cu (5.87–12.62 mg/kg) and Pb (1.39–4.19 mg/kg) being prominent. Pb levels in water exceeded WHO limits by up to 10 times, while Cd exceeded permissible limits by approximately 6 times. Both water and fish samples contained heavy metal concentrations above recommended safety thresholds. A summary of all concentrations is provided in Table 1.

Conclusion: The findings reveal high levels of heavy metal bioaccumulation in Ngamo Dam water and fish, rendering them unsafe for human consumption. Continuous monitoring, implementation of remediation strategies, and strict regulation of anthropogenic activities around the dam are necessary to protect public and ecological health. Authorities should consider bioremediation and improved waste management practices to reduce contamination.