The continuous dilution method

The continuous dilution method has been used to estimate the
discharge in a river. The baseline concentration of the tracer in the
stream is 58 ppm. A tracer solution of concentration of 8,000 ppm is
continuously injected into the stream at a rate of 1.8 L/s. At the
sampling point downstream, the observed stable concentration of tracer
is 250 ppm. What is the discharge of the stream?