1. The problem statement, all variables and given/known data Another way to treat acid mine drainage is to add limestone to the stream to precipitate the metals. The first order reaction rate constant is 10/day. Assume the stream is 3 feet by 1 foot. If the flow through the stream is 1000 gallons a day and the concentration at the beginning of the stream is 250 ppm. At what distance will the concentration be 5 ppm. 2. Relevant equations I'm assuming it is a pfr so the equation, i think, is C(distance) = Ce^-kt and the answer is 17.4 feet, but I keep getting a different answer. Also used t = (distance)(cross-section area)/(flow rate or Q). Any ideas?