3. Treated wastewater from an industrial facility travels through a 1 km channel before flowing into a natural waterway. The channel has two distinct sections. The upstream section is 600 m long, has a depth of 0.5 m, and channel velocity of 0.75 m/s. The downstream section is 400 m long, has a depth of 0.7 m, and a channel velocity of 1.0 m/s. If the treated wastewater enters the channel with a dissolved oxygen concentration of 4.0 mg/L and a temperature of 22 C, what is the dissolved oxygen concentration where the channel empties into the natural waterway? Assume the water temperature remains constant, the water is not saline, and air pressure is 760 mm Hg (1 atm).