The River Pollution Index (RPI) is a comprehensive index used to assess river water quality. The concentrations of four parameters, including dissolved oxygen (DO), biochemical oxygen demand (BOD5), suspended solids (SS) and ammonia nitrogen (NH3-N), are used to calculate the RPI, which determines the level of pollution in river water. The levels of pollution are divided into four categories: non/mildly-polluted, lightly-polluted, moderately-polluted, and severely-polluted.