An MRI line run across the Pasig River which has an average flow rate of 54 cubic meters per second. The MRI maintenance company sprays the track easement bridge crossing the river with an herbicide concentration in the river of 66 ug/liter. Pasig City is 18.5 km downstream. The flow velocity of the river is 1.25 meters/sec. If the herbicide degrades at a constant rate of 0.65 ug/day, determine the concentration of the herbicide in the river near Pasig city.