We have developed a watershed model named "AIST-Standardized Hydrology-based AssessmeNt tool for chemical Exposure Load (AIST-SHANEL)", which can estimate chemical concentrations over 109 Class A river systems in Japan for risk assessment. This model enables us to estimate monthly exposure concentration in a 1 km mesh with emission, climatic, geographical, sewage and industrial statistics in river systems in Japan. In this study, we estimated the linear alkylbenzenesulfonate (LAS) concentration over 109 Class A river systems and evaluated the reliability of this model. The calculated flow rates and LAS concentrations of the rivers were compared with the observed ones. The ratios of the calculation values to the observed values were within a factor of two, suggesting that the model was generally verified. This study suggested that the estimated spatial and temporal LAS concentrations of the rivers are influenced by flow rate, degradation rate and Koc. This model may be used to estimate the chemical concentrations of watershed areas without monitoring chemical concentrations in Japan, using which we can discuss the unknown and latent risk of chemical substances. This model will give us much useful information on aquatic risk assessment and watershed management in Japan.

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