A city discharges 10 m³/d of sewage with an ultimate BOD and DO of 35 mg/L and 1.0 mg/L, respectively, into a stream that has a flow rate of 8.0 m³/s. Immediately upstream of the point of confluence the DO and ultimate BOD in the stream were 9.20 and 5.0 mg/L, respectively. The saturation concentration of DO in the stream is 9.70 mg/L. The decay and reaeration rate constants are 0.30 d¹ and 0.15 d¹, respectively. What is the critical DO deficit? If the average velocity of the stream is 20 km/d, at what distance downstream of the wastewater discharge will the maximum deficit occur? Assume that the wastewater and stream are well mixed at the discharge point.