Abstract
The study is aimed at investigating the contamination of water quality in Omi reservoir. Three sampling stations were selected around the study area while water sample were collected monthly from January to June, 2025 for both rainy and dry season. Water quality parameters such as temp, pH, TDS, BOD, Phosphate, DO and EC were determined. Data were subjected to One-way analysis of variance (ANOVA) using SPSS version 25. The result showed that water quality parameters varied during the study with temperature ranged between (28.93 ± 1.78 - 29.37 ± 1.56 0C), pH (6.80 ± 0.54 - 7.00 ± 0.80), TDS (38.63 ± 6.00 - 40.90 ± 10.44), DO (6.03 ± 2.27 - 6.57 ± 2.53), BOD (0.14 ± 0.01- 0.29 ± 0.03 mg/L), EC (50.51 ± 5.22 - 52.40 ± 6.17 µS/cm) and Phosphate (4.27 ± 2.64 - 4.98 ± 3.06 mg/L) respectively. No significant difference (P< 0.05) was observed in the values of Temperature, pH, TDS, Phosphate, EC and DO. In seasonal variation, water temperature, pH, and Phosphate recorded highest value during dry season while in contrast, TDS, EC and DO were higher in rainy season. BOD recorded the same value throughout the season (both in rainy and dry season). Also, Igeo index, CF, ERI and Cd were assessed to know the level of contamination. All the concentration observed when compared with the heavy metals in shales are within the acceptable baseline level. Geoaccumulation index (Igeo), Contamination factor (CF) Contamination degree (CD), Ecological risk index (ERI) and Integrated ecological risk index (IERI) were employed to established the scale of pollution in Omi reservoir. In Igeo, it was observed that all metals showed no pollution to low pollution except Cd which indicated low to moderate pollution. CF for Cd indicated moderate pollution, CD for Main dam and Ogga station showed low contamination, while Ogbe station showed considerable pollution. In ERI, Cd was the main source of pollution while Pb showed minor scale of pollution. However, monitoring of the reservoir must be given attention to curb water pollution. Continuous discharge of waste, runoff from agriculture fields, and mining sites can lead to water pollution thereby posing serious ecological and health risk to humans and aquatic life who depend largely on the reservoir. A careful management strategy and routine monitoring is required for the improvement and sustenance of the water and fisheries resources in the reservoir.

Key words: Ecological risk, Heavy metals, Water quality, Omi reservoir, Yagba West