Suppose you are a member of an EPA enforcement team investigating the possible dumping of arsenic into Arbor Lake. You take water samples from the middle of the lake and find that the concentration is 90.0 ppb. 10 days later you take another sample and find 82.2 ppb. You furthermore make inquiries at a company on the lake, and an employee confesses to you that they have been dumping 90 g/min for years but that the dumping stopped over 2 weeks ago. If the water in the lake has a residence time of 100 days, the flow rate into the lake is 10 m3/s, the surface area of the lake is 1 x 107 m2, and the rate of evaporation from the lake is 6 cm/wk, how long ago did the dumping stop?