Nan, a province in northern part of Thailand, is known as an origin of several rivers and tributaries such as Nan River. (Thitiphuree et al. 2013), the provinces topography consists of mostly highlands and mountainous areas. Due to these features, a number of watersheds have been formed due to the flow of several rivers in the region. The longest watershed is of the Nan River which runs through the province. The river bank has become a fertile ground for farming and have engaged in highland rice farming and agricultural activities. Seasonal cultivation in this area involves an intensive utilization of agrochemicals, especially herbicide. The continuous application of herbicide in large amount could lead to environmental contamination and accumulation in aquatic organisms (Uno et al., 2001). In some situations, range of contamination could extend beyond aquatic habitats to bay tributaries (Lehotay et al., 1998) and marine environment (Haynes et al., 2003). It is thus important to monitor an extent of contamination and potential health effects to animals living in the aquatic environment.