Multi-tributary seasonal patterns and exploratory source identification of potentially toxic elements in water and sediments: Ecological risks screening in a gold - mining river systems, Northern, Tanzania

Gold mining in Northern Tanzania raises concerns about potentially toxic elements in rivers draining into Lake Victoria. This study investigated seasonal and spatial distributions of As, U, Pb, Cd, Cu, and Zn in surface water and bed sediment from the Mara River, its tributaries, and the Mori River. 40 water and 42 sediment samples were collected during the 2024 rainy and dry seasons and analysed. Contamination and ecological-risk indices were calculated using Upper Continental Crust (UCC) and local-bedrock backgrounds, alongside sediment quality guidelines and exploratory multivariate analysis. At paired sampling locations, sediment concentrations of studied elements were significantly lower during the rainy season, whereas dissolved concentrations showed no significant seasonal differences. Sediment As and Cd accounted for most guideline exceedances, with 31.0% and 16.7% of samples exceeding Probable Effect Levels, primarily in mining-affected tributaries. Dissolved As and Pb exceeded WHO and Tanzanian guidelines in 17.5% and 15.0% of samples, respectively. Risk assessments were sensitive to background selection: Cd dominated under UCC, whereas As became more influential under local-bedrock backgrounds, although both approaches identified the same priority sites. Multivariate analysis revealed element covariation but did not resolve provenance. These findings establish a baseline-sensitive screening framework and prioritise sites for future monitoring.