4. Lake Ontario has a volume of 1640 km³ and an initial pollutant concentration of 0.2%. The annual inflow from Lake Erie is 410 km³ with a pollutant concentration of 0.05%. There is an equal yearly outflow of the well-mixed water into the St. Lawrence River. How long will it take to reduce the pollutant concentration in Lake Ontario to 0.08%?

Fig: 1