A river initially has an ultimate BOD of 10 [mg/l] in the mixing zone. The initial dissolved oxygen concentration is 8.5 [mg/l]. The deoxygenation rate constant is 0.1 [1/d]. Assume that the velocity of the river is 0.2 [km/min]. A stream has an average stream velocity of 2 [m/s] and an average depth of 5 [m]. Assume the temperature remains 25 C, 5,000 mg/L chloride concentration, and the rivers average velocity and depth remain the same as well.

What is the dissolved oxygen concentration 100 miles downstream after the mixing zone?