6. Write the mass balance equation for an idcal 1-D plug-flow stream under steady state conditions with a first-order decaying substance (e.g, BOD). Solve for the concentration as a function of the concentration at x=0, the rate constant kd. longitudinal distance x, and mean velocity u. Assume the rate constant for BOD decay is 0.2d1, the mean velocity of the stream is 0.5ms1, and the initial concentration is 100mgL1 at x0. Plot the resulting concentration with downstream distance x in km