Ecological Toxicity of Illegal Mining on Selected Rivers at Obuasi in Ghana Using <i>Daphnia magna</i> and <i>Lemna minor</i>

Illegal mining activities pose a growing threat to aquatic ecosystems through the release of toxic substances which affect the aquatic organisms within the ecosystem. This has become a growing concern because of the rise of illegal mining in certain parts of Ghana. Therefore, this research seeks to assess the effect of illegal mining on selected rivers at Obuasi in the Ashanti Region using Daphnia magna and Lemna minor. Obuasi was selected as the study area because it is an illegal mining hotspot. Three rivers were randomly selected from Obuasi; Kwabrafoso river (KB), Pompom river (PO), and Diewuo river (DE) to assess their level of toxicity to aquatic organisms. The heavy metals were analyzed using the Agilent 4210 Microwave Plasma Atomic Emission Spectrometer (MP-AES) and the physicochemical properties by using the OHAUS Starter 3100 m. Duckweed (Lemna minor) and Daphnia (Daphnia magna) were used as the test organisms following OECD 211 and OECD 202 protocols respectively. The pH of all river samples was alkaline (7.10-7.49) and within the World Health Organization (WHO) permissible limit. All river samples exceeded the WHO standard for Electrical conductivity and Turbidity. Kwabrafoso showed the highest concentrations of Zn (0.46 mg/L), Pb (0.18 mg/L), and As (0.62 mg/L). Bioaccumulation showed Zn and As as the most bioavailable metals to Daphnia. IC₁₀ and EC₅₀ were measured in the toxicity bioassays for duckweed and Daphnia, respectively, with a toxicity gradient of PO < KB < DE ranging from (4.90-11.48 mg/L) for IC₁₀ and DE < PO < KB ranging from (15.15-31.9 mg/L) for EC₅₀. All the river samples showed confirmed toxicity, indicating the impact of illegal mining on aquatic ecosystems. This highlights the need for monitoring and strong regulations to protect aquatic ecosystems.