Sulfur hexafluoride, a volatile conservative pollutant, is continuously discharged into a stream having constant hydrologic and geometric characteristics. You measure concentrations of 0.40 and 0.15 ppb at locations 0.5 and 4.0 km downstream from the injection point, respectively. Assuming the concentration of sulfur hexafluoride in the air above the river is negligible, estimate the mass transfer rate constant if the velocity over the stretch is 0.2 m/sec. Enter your value in units of per day, using two significant digits.