A wastewater containing 130mg/L of BOD after preliminary treatment discharges to a river at 75,000m3/ day. The river has a minimum flow rate of 6 m3/s, a BOD of 2mg/L, and a velocity of 2.4km/h. After the wastewater mixes with the river contents, the temperature is 20C and the dissolved oxygen is 75% of saturation. Determine the oxygen sag at the critical point and at distances of half the critical point above and below the critical point. Plot the curve. K is 0.25/ day, and K2 is