A river receives a discharge of 0.4 m3 /s at a concentration of 150 mg/L. The river flow upstream is 1.0 m3 /s at zero concentration. For 25 km...

A river receives a discharge of 0.4 m3/s at a concentration of 150 mg/L. The river flow upstream is 1.0 m3/s at zero concentration. For 25 km downstream, the mean velocity is 0.4 m/s. A region of slow moving water is then encountered for the next 35 km where the velocity drops to 0.15 m/s. If the decay rate is 0.2/day, what is the concentration at the point 50 km downstream of the outfall?