Recently, usage of the metal-based pesticide especially manganese (Mn) and copper (Cu) have been widely used in the agricultural area around the Lam Takhong Reservoir; the major water resource of Nakhon Ratchasima Province and the Southern I-Sarn. The exposures of these metals can causes neurological and Parkinson diseases. Therefore the objectives of this study were to investigate Mn, Cu, and Fe contaminations in the reservoir as well as the environmental risks by using hazard quotient (HQ). The results showed that the average concentrations of Mn, Cu, and Fe in water column were 0.05 mg/l, 0.01 mg/l, 0.39 mg/l in dry season and 0.08 mg/l, 0.01 mg/l, 0.53 mg/l in wet season, respectively. Mn and Cu concentrations were in normal range except those of Fe that were extremely high. Also, Mn and Cu in sediment were found in the normal range. Interestingly, high concentrations of Fe in sediment were found at many stations with respect of the parent materials in this area. Fortunately, there were no risks of Mn and Cu in both water column and sediment. However, Fe in sediment exceeded the recommended guideline. These results suggested that using Mn/Cu-based synthetic pesticides in this area might be limited with respect of Fe abundance that could enhanced the toxicity of Mn/Cu and might increased the incidents of neurological and Parkinson diseases.