The Fate of Selected Heavy Metals in River Water, Fruits and Vegetables to Potential Human Health Risks: The Case of Rau River in Moshi District, Tanzania

The presence of heavy metals in our environment significantly contributes to pollution in our food supply, particularly in fruits and vegetables. To analyze the levels of Cd, Cu, Fe, Pb, and Zn in green leafy vegetable (Amaranthus sp), fruit Citrullus lanatus (watermelon) and the Rau river water, an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used. Three sampling sites were selected, including one site with the least impact, Mawela and two highly impacted sites, Majengo and Msaranga. The order of heavy metal concentrations in the river water was found to be Fe > Cu > Zn > Pb > Cd, with the majority of the levels exceeding the acceptable standards set by the World Health Organization (WHO). There was significant variation in the concentrations of heavy metals in fruit. For example, the range of concentrations (mean ± standard deviation) observed were as follows: Fe (70.700 ± 1.532 to 221.010 ± 0.661 mg/kg), Cu (1.372 ± 0.047 to 2.540 ± 0.195 mg/kg), Zn (41.072 ± 0.519 to 45.718 ± 0.651 mg/kg), Pb (below detection limit to 0.398 ± 0.012 mg/kg), and Cd (0.031 ± 0.013 to 0.243 ± 0.112 mg/kg). The heavy metal concentration in Citrullus lanatus (watermelon) did not correlate with the levels found in the water samples. This disparity could be attributed to the watermelon's tendency to hyper-accumulate Cd from the river water. The hazard index, which assesses the potential health risks, indicates that the ingestion of vegetables exceeds a value of 1, while that of fruits remains below 1. Despite the heavy metal contamination levels in the tested vegetables and fruits being below the safe limit, there is a significant health concern regarding long-term exposure to even low doses of toxic elements. Therefore, it is crucial to conduct regular monitoring of heavy metals in all types of food items to accurately assess the health risks associated with heavy metal exposure in the human food chain.