Another way to treat acid mine drainage is to add limestone at the beginning of a narrow AMD stream to precipitate the metals. The first order rate constant is 10/day. Assume the stream is 3ft wide and 1ft deep (assume a rectangular cross section. If the flow throught the stream is 1000 gallons/day and the concentration at the beginning of the stream is 250 ppm, at what distance will the concentration of the metals be 5 ppm