A diffuser discharges wastewater into a slow-moving river that has a mean velocity of 3 cm/sec. After initial mixing, the DO concentration in the river is 9.5mg/L and the temperature is 15ºC. If the ultimate BOD of the mixed river water is 30mg/L, the rate constant for BOD at 20ºC is 0.6d-1, the reaeration rate constant at 20ºC is 0.8d-1, and the removal of BOD by sedimentation can be neglected, estimate the minimum DO and the critical location in the river.