Assessment of total mercury (Hg) in soil, sediment, and tilapia fish (Oreochromis niloticus) and health risk assessment among residents of Kitwe mining area, Zambia

Abstract

Mercury (Hg) is a heavy metal of global concern because of its persistence in the environment and its ability to bioaccumulate and biomagnify in ecosystems. Despite evidence of extensive environmental pollution in the Copperbelt Province, few studies have investigated Hg contamination in the Kafue River and its tributaries in Kitwe District, Zambia. Total Hg concentrations were determined in soil, sediments, and tilapia by inductively coupled plasma mass spectrometer (ICP-MS) from the mining areas and non-mining areas. There were significant differences in the population means for soil samples (Mean _(mining) = 1.066, Mean _(non-mining) = 0.041, p $\le 0.05$ ) and sediment samples (Mean _(mining) = 1.304, Mean _(non-mining) = 0.034), p $\le 0.05$ ) between mining and non-mining areas. There were also statistically significant differences in the population means for fish samples (Mean _(mining) = 0.015, Mean _(non-mining) = 0.007, p $\le 0.05$ ) between mining and non-mining areas. The levels of Hg in the soil and sediments from the mining area were higher than the United States Environmental Protection Agency (USEPA) reference values of 0.3 mg/kg and 0.2 mg/kg, respectively. There was a weak positive correlation between the size of the fish (length) and Hg accumulation in the Kitwe mining area (r = 0.232, P = 0.1166). The observed correlation between Hg accumulation and length of fish was not statistically significant (P > 0.05). The EDI from the consumption of fish from the mining area was below the USEPA and WHO/FAO maximum tolerable daily intake of 0.1 µg/kg/day and 0.23 µg/kg, respectively. The THQ < 1 was also reported in the current study, suggesting that the exposure level may not cause adverse health effects during a lifetime in the human population. Although the EDI and THQ < 1 in the current study were below the USEPA and WHO/FAO maximum tolerable limit, the presence of Hg in fish in this area must be monitored due to its ability to bioaccumulate in large and predatory fish. The lower EDI value reported in the current study might be attributed to the smaller size of the tilapia fish specimens, resulting in low bioaccumulation of Hg. Since the Hg levels in sediments were above the USEPA limit, we recommend further studies on the bioavailability of Hg in humans and other fish species in the region, particularly carnivorous fish, due to Hg biomagnification to offer a clearer perspective on the environmental and health impacts.

Keywords: Exposure assessment; Mercury pollution; Mining impacts.

Fig. 1

Map of Kitwe District showing sampling points along the three streams (Uchi, Kitwe, and Mindolo) connecting to the Kafue River

Fig. 2

Pearson correlation between size of fish (length and weight) and mercury accumulation in Kitwe mining area